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Brain Facts

A PRIMER ON THE BRAIN AND NERVOUS SYSTEM

SOCIETY FOR NEUROSCIENCE

The Society for Neuroscience is the world’s largest organization of scientists

and physicians dedicated to understanding the brain, spinal cord, and periph-

eral nervous system.

Neuroscientists investigate the molecular and cellular levels of the nervous

system; the neuronal systems responsible for sensory and motor function; and

the basis of higher order processes, such as cognition and emotion. This

research provides the basis for understanding the medical ﬁelds that are con-

cerned with treating nervous system disorders. These medical specialties

include neurology, neurosurgery, psychiatry, and ophthalmology.

Founded in 1969, the Society has grown from 500 charter members to

more than 36,000 members. While a predominantly North American organi-

zation, SfN also has many members who live in Europe, Asia, Latin America,

and Australia/Oceania. The Society has more than 100 regional chapters. With

activities ranging from lectures to networking events and information sharing,

SfN chapters enable individual members to engage their colleagues at the local

level.

The mission of the Society is to:

∫Advance the understanding of the brain and the nervous system by bring-

ing together scientists of diverse backgrounds, by facilitating the integration

of research directed at all levels of biological organization, and by encourag-

ing translational research and the application of new scientiﬁc knowledge to

develop improved disease treatments and cures.

∫Provide professional development activities, information, and educational

resources for neuroscientists at all stages of their careers, including under-

graduates, graduates, and postdoctoral fellows, and increase participation of

scientists from a diversity of cultural and ethnic backgrounds.

∫Promote public information and general education about the nature of

scientiﬁc discovery and the results and implications of the latest neuroscience

research. Support active and continuing discussions on ethical issues relating

to the conduct and outcomes of neuroscience research.

∫Inform legislators and other policymakers about new scientiﬁc knowledge

and recent developments in neuroscience research and their implications for

public policy, societal beneﬁt, and continued scientiﬁc progress.

The exchange of scientiﬁc information occurs at an annual fall meeting

where more than 16,000 reports of new scientiﬁc ﬁndings are presented and

more than 30,000 people attend. This meeting, the largest of its kind in the

world, is the arena for the presentation of new results in neuroscience.

The Society’s weekly journal, The Journal of Neuroscience,contains articles

spanning the entire range of neuroscience research and has subscribers world-

wide. The Society’s ongoing education and professional development e∑orts

reach teachers and help promote the education of Society members. Print and

electronic publications inform members about Society activities.

A major goal of the Society is to inform the public about the progress and

beneﬁts of neuroscience research. The Society accomplishes this goal by pro-

viding information about neuroscience to schoolteachers and encouraging its

members to speak to young people about the human brain and nervous system.

Contents

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

THE NEURON . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Neurotransmitters ∫Second Messengers

BRAIN DEVELOPMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Birth of Neurons and Brain Wiring ∫Paring Back ∫Critical Periods

SENSATION AND PERCEPTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Vision ∫Hearing ∫Taste and Smell ∫Touch and Pain

LEARNING, MEMORY, AND LANGUAGE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

MOVEMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

SLEEP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

The Stu∑ of Sleep ∫Sleep Disorders ∫How is Sleep Regulated?

STRESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

The Immediate Response ∫Chronic Stress

AGING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

Aging Neurons ∫Intellectual Capacity

ADVANCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

Bipolar Disorder ∫Epilepsy ∫Major Depression

Pain ∫Parkinson’s Disease

CHALLENGES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

Addiction ∫Attention Deﬁcit Hyperactivity Disorder

Alzheimer’s Disease ∫Amyotrophic Lateral Sclerosis ∫Anxiety Disorders

Autism ∫Brain Tumors ∫Down Syndrome ∫Huntington’s Disease

Learning Disorders ∫Multiple Sclerosis ∫Neurological AIDS

Neurological Trauma ∫Schizophrenia ∫Stroke ∫Tourette Syndrome

NEW DIAGNOSTIC METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

Imaging Techniques ∫Gene Diagnosis

POTENTIAL THERAPIES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

New Drugs ∫Trophic Factors ∫Engineered Antibodies

Small Molecules and RNAs ∫Cell and Gene Therapy

NEUROETHICS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

NEUROSCIENCE RESOURCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

t sets humans apart from all other species by allowing us to

achieve the wonders of walking on the moon and compos-

ing masterpieces of literature, art, and music. The human

brain—a spongy, three-pound mass of fatty tissue—has

been compared to a telephone switchboard and a super-

computer.

But the brain is much more complicated than either of these

devices, a fact scientists conﬁrm almost daily, with each new dis-

covery.The extent of the brain’s capabilities is unknown, but it is

the most complex living structure known in the universe.

This single organ controls all body activities, ranging from

heart rate and sexual function to emotion, learning, and mem-

ory. The brain is even thought to inﬂuence the immune system’s

response to disease and to determine, in part, how well people

respond to medical treatments. Ultimately, it shapes our

thoughts, hopes, dreams, and imaginations. In short, the brain is

what makes us human.

Neuroscientists have the daunting task of deciphering the

mystery of this most complex of all machines: how as many as a

trillion nerve cells are produced, grow, and organize themselves

into e∑ective, functionally active systems that ordinarily remain

in working order throughout a person’s lifetime.

The motivation of researchers is twofold: to understand

human behavior better—from how we learn to why people have

trouble getting along together—and to discover ways to prevent

or cure many devastating brain disorders.

The more than 1,000 disorders of the brain and nervous sys-

tem result in more hospitalizations than any other disease group,

including heart disease and cancer. Neurological illnesses a∑ect

more than 50 million Americans annually, at costs exceeding $400

billion. In addition, mental disorders, excluding drug and alco-

hol problems, strike 44 million adults a year at a cost of some $148

billion.

However, during the congressionally designated Decade of

the Brain, which ended in 2000, neuroscience made signiﬁcant

discoveries in these areas:

∫Genetics. Disease genes were identiﬁed that are key to several

neurodegenerative disorders—including Alzheimer’s disease,

Huntington’s disease, Parkinson’s disease, and amyotrophic lat-

eral sclerosis. This has provided new insights into underlying dis-

ease mechanisms and is beginning to suggest new treatments.

With the mapping of the human genome, neuroscientists will

be able to make more rapid progress in identifying genes that

either contribute to human neurological disease or that directly

cause disease. Mapping animal genomes will aid the search for

genes that regulate and control many complex behaviors.

∫Brain Plasticity. Scientists began to uncover the molecular

basis of neural plasticity, revealing how learning and memory

occur and how declines might be reversed. These discoveries are

leading to new approaches to the treatment of chronic pain.

∫New Drugs. Researchers gained new insights into the mech-

anisms of molecular neuropharmacology, which provides a new

understanding of the mechanisms of addiction. These advances

also have led to new treatments for depression and obsessive

compulsive disorder.

∫Imaging.Revolutionary imaging techniques, including mag-

netic resonance imaging and positron emission tomography, now

reveal brain systems underlying attention, memory, and emotions

and indicate dynamic changes that occur in schizophrenia.

∫Cell Death. The discovery of how and why neurons die, as

well as the discovery of stem cells, which divide and form new

neurons, has many clinical applications. This has dramatically

improved the outlook for reversing the e∑ects of injury in both

the brain and the spinal cord. The ﬁrst e∑ective treatments for

stroke and spinal cord injury based on these advances have been

brought to clinical practice.

∫Brain Development. New principles and newly discovered

molecules responsible for guiding nervous system development

now give scientists a better understanding of certain disorders of

childhood. Together with the discovery of stem cells, these

advances are pointing to novel strategies for helping the brain or

spinal cord regain functions lost as a result of injury or develop-

mental dysfunction.

Federal neuroscience research funding of more than $5 bil-

lion annually and private support should vastly expand our

knowledge of the brain in the years ahead.

This book only provides a glimpse of what is known about

the nervous system, the disorders of the brain, and some of the

exciting avenues of research that promise new therapies for many

neurological diseases.

Introduction

THE TOLL OF SELECTED BRAIN AND NERVOUS SYSTEM DISORDERS*

Condition Total Cases Costs Per Year

Hearing Loss 28 million $ 56 billion

All Depressive Disorders 20.5 million 44 billion

Alzheimer’s Disease 4.5 million 100 billion

Huntington’s Disease 30,000 2 billion

Stroke 4.7 million 51 billion

Schizophrenia 2 million 32.5 billion

Parkinson’s Disease 1 million 5.6 billion

Traumatic Head Injury 5 million 56.3 billion

Multiple Sclerosis 2.5 million 9.5 billion

Spinal Cord Injury 250,000 10 billion

* Estimates provided by the National Institutes of Health and voluntary organizations.

THE BRAIN. Cerebral cortex

(above). This part of the brain is

divided into four sections: the

occipital lobe, the temporal

lobe, the parietal lobe, and the

frontal lobe. Functions, such as

vision, hearing, and speech, are

distributed in selected regions.

Some regions are associated

with more than one function.

Major internal structures (below).

The (1) forebrain is credited with

the highest intellectual func-

tions—thinking, planning, and

problem-solving. The hippocam-

pus is involved in memory.

The thalamus serves as a relay

station for almost all of the

information coming into the

brain. Neurons in the hypothal-

amus serve as relay stations for

internal regulatory systems by

monitoring information coming

in from the autonomic nervous

system and commanding the

body through those nerves and

the pituitary gland. On the

upper surface of the (2) midbrain

are two pairs of small hills, col-

liculi, collections of cells that

relay specific sensory informa-

tion from sense organs to the

brain. The (3) hindbrain consists

of the pons and medulla

oblongata, which help control

respiration and heart rhythms,

and the cerebellum, which helps

control movement as well as

cognitive processes that require

precise timing.

Aspecialized cell designed to transmit informa-

tion to other nerve cells, muscle, or gland cells,

the neuron is the basic working unit of the

brain. The brain is what it is because of the

structural and functional properties of inter-

connected neurons. It contains between one

billion and one trillion neurons, depending on the species.

The neuron consists of a cell body containing the nucleus,

cytoplasm, and an electrically excitable output ﬁber, the axon.

Most axons also give rise to many smaller branches before ending

at nerve terminals.Synapses,from the Greek word meaning “to

clasp together,” are the contact points where one neuron commu-

nicates with another. Other structures, dendrites,Greek for “tree

branches,” extend from the neuron cell body and receive messages

from other neurons. The dendrites and cell body are covered with

synapses formed by the ends of axons of other neurons.

Neurons signal by transmitting electrical impulses along their

axons, which can range in length from a tiny fraction of an inch

to three or more feet. Many axons are covered with a layered insu-

lating myelin sheath, made of specialized cells called oligoden-

drocytes in the brain and Schwann cells in the peripheral nervous

system, which speeds the transmission of electrical signals along

the axon.

Nerve impulses involve the opening and closing of ion chan-

nels,water-ﬁlled molecular tunnels that pass through the cell

membrane and allow ions—electrically charged atoms—or

small molecules to enter or leave the cell. The ﬂow of these ions

creates an electrical current that produces tiny voltage changes

across the membrane.

The ability of a neuron to ﬁre—that is, to become su≈-

ciently activated by incoming synapses to discharge and com-

municate to its own synaptic target neurons—depends on a

small di∑erence in electrical charge between the inside and out-

side of the cell. When a nerve impulse begins, a dramatic rever-

sal occurs at one point on the cell’s membrane. The change, called

an action potential,then passes along the membrane of the axon

at speeds up to several hundred miles per hour. In this way, a neu-

ron may be able to ﬁre impulses scores of times every second.

Upon reaching the end of an axon, these voltage changes trig-

ger the release of neurotransmitters,the brain’s chemical messen-

gers. Neurotransmitters are released at nerve ending terminals,

di∑use across the intrasynaptic space, and bind to receptors on

the surface of the target neuron.

These receptors act as on and o∑ switches for the next cell.

Each receptor has a distinctly shaped part that selectively recog-

nizes a particular chemical messenger. A neurotransmitter ﬁts

into this region in much the same way as a key ﬁts into a lock.

And when the transmitter is in place, this alters the neuron’s outer

membrane potential (or excitability) and triggers a change, such

as the contraction of a muscle or increased activity of an enzyme

in the cell.

Knowledge of neurotransmitters in the brain and the action

of drugs on these chemicals—gained largely through the study

of animals—is one of the largest ﬁelds in neuroscience. Armed

with this information, scientists hope to understand the circuits

responsible for disorders such as Alzheimer’s disease and Parkin-

son’s disease. Sorting out the various chemical circuits is vital to

understanding how the brain stores memories, why sex is such a

powerful motivation, and what the biological basis of mental ill-

ness is.

Neurotransmitters

Acetylcholine The ﬁrst neurotransmitter, identiﬁed about 75

years ago, was acetylcholine (ACh). This chemical is released by

neurons connected to voluntary muscles (causing them to con-

tract) and by neurons that control the heartbeat. ACh also serves

as a transmitter in many regions of the brain.

ACh is formed at the axon terminals. When an action poten-

tial arrives at the terminal, the electrically charged calcium ion

rushes in, and ACh is released into the synapse and attaches to

ACh receptors. In voluntary muscles, this opens sodium channels

and causes the muscle to contract. ACh is then broken down and

resynthesized in the nerve terminal. Antibodies that block the

receptor for ACh cause myasthenia gravis,a disease characterized

by fatigue and muscle weakness.

Much less is known about ACh in the brain. Recent discov-

eries suggest, however, that it may be critical for normal atten-

tion, memory, and sleep. Since ACh-releasing neurons die in

Alzheimer’s patients, ﬁnding ways to restore this neurotransmit-

ter is one goal of current research.

The neuron

Amino acids Amino acids, widely distributed throughout the

body and the brain, serve as the building blocks of proteins. Cer-

tain amino acids can also serve as neurotransmitters in the brain.

The neurotransmitters glutamate and aspartate act as excita-

tory signals. Glycine and gamma-aminobutyric acid (GABA)

inhibit the ﬁring of neurons. The activity of GABA is increased

by benzodiazepine (Valium) and by anticonvulsant drugs. In

Huntington’s disease, a hereditary disorder that begins during

midlife, the GABA-producing neurons in the brain centers coor-

dinating movement degenerate, thereby causing uncontrollable

movements.

Glutamate or aspartate activates N-methyl-d-aspartate

(NMDA) receptors, one of three major classes of glutamate

receptors, which have been implicated in activities ranging from

learning and memory to development and speciﬁcation of nerve

contacts in a developing animal. The stimulation of NMDA

receptors may promote beneﬁcial changes in the brain, whereas

overstimulation can cause nerve cell damage or cell death in

trauma and stroke.

Key questions remain about this receptor’s precise structure,

regulation, location, and function. For example, developing drugs

to block or stimulate activity at NMDA receptors holds promise

NEURON. A neuron fires by

transmitting electrical signals

along its axon. When signals

reach the end of the axon, they

trigger the release of neuro-

transmitters that are stored in

pouches called vesicles. Neuro-

transmitters bind to receptor

molecules that are present on

the surfaces of adjacent neurons.

The point of virtual contact is

known as the synapse.

for improving brain function and treating neurological disorders.

But this work is still in the early stage.

Catecholamines Dopamine and norepinephrine are widely

present in the brain and peripheral nervous system. Dopamine,

which is present in three circuits in the brain, controls movement,

causes psychiatric symptoms such as psychosis, and regulates

hormonal responses.

The dopamine circuit that regulates movement has been

directly linked to disease. The brains of people with Parkinson’s

disease—with symptoms of muscle tremors, rigidity, and di≈-

culty in moving—have practically no dopamine. Thus, medical

scientists found that the administration of levodopa,a substance

from which dopamine is synthesized, is an e∑ective treatment for

Parkinson’s, allowing patients to walk and perform skilled move-

ments successfully.

Another dopamine circuit is thought to be important for

cognition and emotion; abnormalities in this system have been

implicated in schizophrenia. Because drugs that block dopamine

receptors in the brain are helpful in diminishing psychotic symp-

toms, learning more about dopamine is important to under-

standing mental illness.

In a third circuit, dopamine regulates the endocrine system.

It directs the hypothalamus to manufacture hormones and hold

them in the pituitary gland for release into the bloodstream or to

trigger the release of hormones held within cells in the pituitary.

Nerve ﬁbers containing norepinephrine are present through-

out the brain. Deﬁciencies in this transmitter occur in patients

with Alzheimer’s disease, Parkinson’s disease, and Korsako∑’s syn-

drome,a cognitive disorder associated with chronic alcoholism.

Thus, researchers believe norepinephrine may play a role in both

learning and memory. Norepinephrine is also secreted by the

sympathetic nervous system in the periphery to regulate heart

rate and blood pressure. Acute stress increases the release of nor-

epinephrine.

Serotonin This neurotransmitter is present in many tissues,

particularly blood platelets, the lining of the digestive tract, and

the brain. Serotonin was ﬁrst thought to be involved in high

blood pressure because it is present in blood and induces a very

powerful contraction of smooth muscles. In the brain, serotonin

has been implicated in sleep, mood, depression, and anxiety.

Because serotonin controls the di∑erent switches a∑ecting vari-

ous emotional states, scientists believe these switches can be

manipulated by analogs, chemicals with molecular structures

similar to that of serotonin. Drugs that alter serotonin’s action,

such as ﬂuoxetine (Prozac), have relieved symptoms of depres-

sion and obsessive-compulsive disorder.

Peptides These are chains of amino acids linked together.

Brain peptides called endorphins act like opium to kill pain or

cause sleepiness. (Peptides di∑er from proteins, which are much

larger and more complex combinations of amino acids.)

In 1973, scientists discovered receptors for opiates on neurons

in several regions of the brain, suggesting that the brain must

make substances very similar to opium. Shortly thereafter, scien-

tists made their ﬁrst discovery of an opiate produced by the brain

that resembles morphine, an opium derivative used medically to

kill pain. They named it enkephalin,literally meaning “in the

head.” Soon after, the endorphins—another type of opioid pep-

tide, whose name comes from endogenous morphine—were dis-

covered.

The precise role of the opioid peptides in the body is unclear.

A plausible guess is that they are released by brain neurons in

times of stress to minimize pain and enhance adaptive behavior.

The presence of opioid peptides may explain, for example, why

injuries received during the stress of combat are often not noticed

until hours later.

Opioids and their receptors are closely associated with path-

ways in the brain that are activated by painful or tissue-damag-

ing stimuli. These signals are transmitted to the central nervous

system—the brain and spinal cord—by special sensory nerves,

small myelinated ﬁbers, and tiny unmyelinated C ﬁbers.

Scientists have discovered that some C ﬁbers contain a pep-

tide called substance P that causes the sensation of burning pain.

The active component of chili peppers, capsaicin, causes the

release of substance P.

Trophic factors Researchers have discovered several small

proteins in the brain that are necessary for the development,

function, and survival of speciﬁc groups of neurons. These small

proteins are made in brain cells, released locally in the brain, and

bind to receptors expressed by speciﬁc neurons. Researchers have

also identiﬁed genes that code for receptors and are involved in

the signaling mechanisms of trophic factors. These ﬁndings are

expected to result in a greater understanding of how trophic fac-

tors work in the brain. This information should also prove use-

ful for the design of new therapies for brain disorders of devel-

opment and for degenerative diseases, including Alzheimer’s

disease and Parkinson’s disease.

Hormones After the nervous system, the endocrine system is

the second great communication system of the body. The pan-

creas, kidneys, heart, adrenal glands, gonads, thyroid, thymus,

and pituitary gland are sources of hormones. The endocrine sys-

tem works in large part through the pituitary gland, which

secretes hormones into the blood. Because endorphins are

released from the pituitary gland into the bloodstream, they

might also function as endocrine hormones. Hormones activate

speciﬁc receptors in target organs that release other hormones

into the blood, which then act on other tissues, the pituitary itself,

and the brain. This system is very important for the activation

and control of basic behavioral activities such as sex, emotion,

responses to stress, and the regulation of body functions such as

growth, energy use, and metabolism. Actions of hormones show

the brain to be very malleable and capable of responding to envi-

ronmental signals.

The brain contains receptors for both the thyroid hormone

and the six classes of steroid hormones—estrogens, androgens,

progestins, glucocorticoids,mineralocorticoids,and vitamin D.The

receptors are found in selected populations of neurons in the

brain and relevant organs in the body. Thyroid and steroid hor-

mones bind to receptor proteins that in turn bind to the DNA

genetic material and regulate the action of genes. This can result

in long-lasting changes in cellular structure and function.

In response to stress and changes in our biological clocks,such

as day and night cycles and jet lag, hormones enter the blood and

travel to the brain and other organs. In the brain, hormones alter

the production of gene products that participate in synaptic neu-

rotransmission as well as the structure of brain cells. As a result,

the circuitry of the brain and its capacity for neurotransmission

are changed over a course of hours to days. In this way, the brain

adjusts its performance and control of behavior in response to a

changing environment. Hormones are important agents of pro-

tection and adaptation, but stress and stress hormones can also

alter brain function, including learning. Severe and prolonged

stress can cause permanent brain damage.

Reproduction is a good example of a regular, cyclic process

driven by circulating hormones: The hypothalamus produces

gonadotropin-releasing hormone (GnRH), a peptide that acts on

cells in the pituitary. In both males and females, this causes two

hormones—the follicle-stimulating hormone (FSH) and the

luteinizing hormone (LH)—to be released into the bloodstream.

In males, these hormones are carried to receptors on cells in the

testes, where they release the male hormone testosterone into the

bloodstream. In females, FSH and LH act on the ovaries and

cause the release of the female hormones estrogen and proges-

terone. In turn, the increased levels of testosterone in males and

estrogen in females act back on the hypothalamus and pituitary

to decrease the release of FSH and LH. The increased levels also

induce changes in cell structure and chemistry that lead to an

increased capacity to engage in sexual behavior.

Scientists have found statistically and biologically signiﬁcant

di∑erences between the brains of men and women that are sim-

ilar to sex di∑erences found in experimental animals. These

include di∑erences in the size and shape of brain structures in the

hypothalamus and the arrangement of neurons in the cortex and

hippocampus. Some functions can be attributed to these sex

di∑erences, but much more must be learned in terms of percep-

tion, memory, and cognitive ability. Although di∑erences exist,

the brains of men and women are more similar than they are

di∑erent.

Recently, several teams of researchers have found anatomical

di∑erences between the brains of heterosexual and homosexual

men. Research suggests that hormones and genes act early in life

to shape the brain in terms of sex-related di∑erences in structure

and function, but scientists are still putting together all the pieces

of this puzzle.

Sex di∑erences go well beyond sexual behavior and repro-

duction and a∑ect many brain regions and functions, ranging

from mechanisms for perceiving pain and dealing with stress to

strategies for solving cognitive problems.

Gases Very recently, scientists identiﬁed a new class of neu-

rotransmitters that are gases. These molecules—nitric oxide and

carbon monoxide—do not obey the “laws” governing neuro-

transmitter behavior. Being gases, they cannot be stored in any

structure, certainly not in synaptic storage structures. Instead,

they are made by enzymes as they are needed. They are released

from neurons by di∑usion. And rather than acting at receptor

sites, they simply di∑use into adjacent neurons and act upon

chemical targets, which may be enzymes.

Though only recently characterized, nitric oxide has already

been shown to play important roles. For example, nitric oxide

neurotransmission governs erection in neurons of the penis. In

nerves of the intestine, it governs the relaxation that contributes

to the normal movements of digestion. In the brain, nitric oxide

is the major regulator of the intracellular messenger molecule—

cyclic GMP.In conditions of excess glutamate release, as occurs

in stroke, neuronal damage following the stroke may be attribut-

able in part to nitric oxide. Exact functions for carbon monoxide

have not yet been shown.

Second messengers

Substances that trigger biochemical communication within cells,

after the action of neurotransmitters at their receptors, are called

second messengers; these intracellular e∑ects may be responsible

for long-term changes in the nervous system. They convey the

chemical message of a neurotransmitter (the ﬁrst messenger)

from the cell membrane to the cell’s internal biochemical

machinery. Second messenger e∑ects may endure for a few mil-

liseconds to as long as many minutes.

An example of the initial step in the activation of a second

messenger system involves adenosine triphosphate (ATP), the

chemical source of energy in cells. ATP is present throughout the

cell. For example, when norepinephrine binds to its receptors on

the surface of the neuron, the activated receptor binds G proteins

on the inside of the membrane. The activated G protein causes

the enzyme adenylyl cyclase to convert ATP to cyclic adenosine

monophosphate (cAMP). The second messenger, cAMP, exerts a

variety of inﬂuences on the cell, ranging from changes in the

function of ion channels in the membrane to changes in the

expression of genes in the nucleus, rather than acting as a mes-

senger between one neuron and another. cAMP is called a second

messenger because it acts after the ﬁrst messenger, the transmit-

ter chemical, has crossed the synaptic space and attached itself to

a receptor.

Second messengers also are thought to play a role in the man-

ufacture and release of neurotransmitters, intracellular move-

ments, carbohydrate metabolism in the cerebrum—the largest

part of the brain, consisting of two hemispheres—and the

processes of growth and development. Direct e∑ects of these sub-

stances on the genetic material of cells may lead to long-term

alterations of behavior.

Three to four weeks after conception, one of the

two cell layers of the gelatinlike human embryo,

now about one-tenth of an inch long, starts to

thicken and build up along the middle. As this

ﬂat neural plate grows, parallel ridges, similar to

the creases in a paper airplane, rise across its sur-

face. Within a few days, the ridges fold in toward each other and

fuse to form the hollow neural tube. The top of the tube thickens

into three bulges that form the hindbrain, midbrain, and fore-

brain. The ﬁrst signs of the eyes and then the hemispheres of the

brain appear later.

How does all this happen? Although many of the mecha-

nisms of human brain development remain secrets, neuroscien-

tists are beginning to uncover some of these complex steps

through studies of the roundworm, fruit ﬂy, frog, zebraﬁsh,

mouse, rat, chicken, cat, and monkey.

Many initial steps in brain development are similar across

species, although later steps are di∑erent. By studying these sim-

ilarities and di∑erences, scientists can learn how the human brain

develops and how brain abnormalities, such as mental retarda-

tion and other brain disorders, can be prevented or treated.

Neurons are initially produced along the central canal in the

neural tube. These neurons then migrate from their birthplace to

a ﬁnal destination in the brain. They collect together to form each

of the various brain structures and acquire speciﬁc ways of trans-

mitting nerve messages. Their axons grow long distances to ﬁnd

and connect with appropriate partners, forming elaborate and

speciﬁc circuits. Finally, sculpting action eliminates redundant or

improper connections, honing the speciﬁc purposes of the cir-

cuits that remain. The result is a precisely elaborated adult net-

work of 100 billion neurons capable of body movement, percep-

tion, emotion, and thought.

Knowing how the brain is put together is essential for under-

standing its ability to reorganize in response to external inﬂuences

or injury. Such studies also shed light on brain functions such as

learning and memory. Brain diseases such as schizophrenia and

mental retardation are thought to result from a failure to con-

struct proper connections during development. Neuroscientists

are beginning to discover some general principles to understand

the processes of development, many of which overlap in time.

Birth of neurons and brain wiring

The embryo has three layers that undergo many interactions in

order to grow into organ, bone, muscle, skin, or neural tissue.

Brain development

BRAIN DEVELOPMENT. The human brain and nervous system begin to develop at about three weeks’ gestation with the closing of the

neural tube (left). By four weeks, major regions of the human brain can be recognized in primitive form, including the forebrain, midbrain,

hindbrain, and optic vesicle (from which the eye develops). Irregular ridges, or convolutions, are clearly seen by six months.

Skin and neural tissue arise from one layer, the ectoderm,in

response to signals provided by the next layer, the mesoderm.

A number of molecules interact to determine whether the

ectoderm becomes neural tissue or develops in another way to

become skin. Studies of spinal cord development in frogs show

that one major mechanism depends on speciﬁc molecules that

inhibit the activity of various proteins. If nothing interrupts the

activity of such proteins, the tissue becomes skin. If other mole-

cules, which are secreted from the mesoderm, block protein sig-

naling, then the tissue becomes neural.

Once the ectodermal tissue has acquired its neural fate, more

signaling interactions determine the type of neural cell to which

it gives rise. The mature nervous system contains a vast array of

cell types, which can be divided into two main categories: the

neurons, responsible primarily for signaling, and supporting cells

called glial cells.

Researchers are ﬁnding that the destiny of neural tissue

depends on a number of factors, including position, that deﬁne

the environmental signals to which the cells are exposed. For

example, a key factor in spinal cord development is a secreted

protein called sonic hedgehog that is similar to a signaling protein

found in ﬂies. The protein, initially secreted from mesodermal

tissue lying beneath the developing spinal cord, marks young

neural cells that are directly adjacent to become a specialized class

of glial cells. Cells farther away are exposed to lower concentra-

tions of sonic hedgehog, and they become the motor neurons

that control muscles. An even lower concentration promotes the

formation of interneurons that relay messages to other neurons,

not muscles.

A combination of signals also determines the type of chem-

ical messages, or neurotransmitters, that a neuron will use to

communicate with other cells. For some, such as motor neurons,

the type of neurotransmitter is ﬁxed, but for others it is a matter

of choice. Scientists found that when certain neurons are main-

tained in a dish with no other cell type, they produce the neuro-

transmitter norepinephrine. In contrast, if the same neurons are

maintained with other cells, such as cardiac or heart tissue cells,

they produce the neurotransmitter acetylcholine. Since all neu-

rons have genes containing the information for the production

of these molecules, it is the turning on of a particular set of genes

that begins the production of speciﬁc neurotransmitters. Many

researchers believe that the signal to engage the gene and, there-

fore, the ﬁnal determination of the chemical messengers that a

neuron produces, is inﬂuenced by factors coming from the tar-

gets themselves.

As neurons are produced, they move from the neural tube’s

ventricular zone,or inner surface, to near the border of the mar-

ginal zone,or outer surface. After neurons stop dividing, they

form an intermediate zone where they gradually accumulate as

the brain develops.

The migration of neurons occurs in most structures of the

brain but is particularly prominent in the formation of a large

cerebral cortex in primates, including humans. In this structure,

neurons slither from the place of origin near the ventricular sur-

NEURON MIGRATION.

A cross-sectional view of the

occipital lobe (which processes

vision) of a three-month-old

monkey fetus brain (center)

shows immature neurons migrat-

ing along glial fibers. These

neurons make transient connec-

tions with other neurons before

reaching their destination. A sin-

gle migrating neuron, shown

about 2,500 times its actual size

(right), uses a glial fiber as a

guiding scaffold. To move, it

needs adhesion molecules, which

recognize the pathway, and

contractile proteins to propel it

along.

face, along nonneuronal ﬁbers that form a trail to their proper

destination. Proper neuron migration requires multiple mecha-

nisms, including the recognition of the proper path and the abil-

ity to move long distances. One such mechanism for long-dis-

tance migration is the movement of neurons along elongated

ﬁbers that form transient sca∑olding in the fetal brain. Many

external forces, such as alcohol, cocaine, or radiation, prevent

proper neuronal migration and result in misplacement of cells,

which may lead to mental retardation or epilepsy. Furthermore,

mutations in genes that regulate migration have recently been

shown to cause some rare genetic forms of retardation and

epilepsy in humans.

Once the neurons reach their ﬁnal location, they must make

the proper connections for a particular function, such as vision

or hearing, to occur. They do this through their axons. These

wirelike appendages can stretch out a thousand times longer than

the cell body from which they arise. The journey of most axons

ends when they meet thicker appendages, called dendrites, on

other neurons. These target neurons can be located at a consid-

erable distance, sometimes at opposite sides of the brain. In the

case of a motor neuron, the axon may travel from the spinal cord

all the way down to a foot muscle.

Axon growth is directed by growth cones. These enlarge-

ments of the axon’s tip actively explore the environment as they

seek out their precise destinations. Researchers have discovered

many special molecules that help guide growth cones. Some mol-

ecules lie on the cells that growth cones contact, whereas others

are released from sources found near the growth cone. The

growth cones, in turn, bear molecules that serve as receptors for

the environmental cues. The binding of particular signals with

receptors tells the growth cone whether to move forward, stop,

recoil, or change direction. These molecules include proteins

with names such as cadherin, netrin, semaphorin, ephrin, neu-

ropilin, and plexin. In most cases, these are families of related

molecules; for example, there are at least 15 semaphorins and at

least 10 ephrins.

Perhaps the most remarkable ﬁnding is that most of these

proteins are common to worms, insects, and mammals, includ-

ing humans. Each family is smaller in ﬂies or worms than in mice

or people, but their functions are quite similar. It has therefore

been possible to use the simpler animals to gain knowledge that

can be directly applied to humans. For example, the ﬁrst netrin

was discovered in a worm and shown to guide neurons around

the worm’s “nerve ring.” Later, vertebrate netrins were found to

guide axons around the mammalian spinal cord. Worm receptors

for netrins were then found and proved invaluable in ﬁnding the

corresponding, and again related, human receptors.

Once axons reach their targets, they form synapses, which

permit electric signals in the axon to jump to the next cell, where

they can either provoke or prevent the generation of a new sig-

nal. The regulation of this transmission at synapses, and the inte-

gration of inputs from the thousands of synapses each neuron

receives, are responsible for the astounding information-pro-

cessing capacity of the brain. For processing to occur properly,

the connections must be highly speciﬁc. Some speciﬁcity arises

from the mechanisms that guide each axon to its proper target

area. Additional molecules mediate “target recognition,” whereby

the axon chooses the proper neuron, and often the proper part

of the target, once it arrives at its destination. Few of these mol-

ecules have been identiﬁed. There has been more success, how-

ever, in identifying the ways in which the synapse forms once

contact has been made. The tiny portion of the axon that con-

tacts the dendrite becomes specialized for the release of neuro-

transmitters, and the tiny portion of the dendrite that receives

the contact becomes specialized to receive and respond to the sig-

nal. Special molecules pass between the sending and receiving

cells to ensure that the contact is formed properly and that the

sending and receiving specializations are precisely opposed to

each other so that transmission can be fast and e≈cient.

Paring back

After growth, the network is pared back to create a more sturdy

system. Only about half the neurons generated during develop-

ment survive to function in the adult. Entire populations of neu-

rons are removed through internal suicide programs initiated in

the cells. The programs are activated if a neuron loses its battle

with other neurons to receive life-sustaining nutrients called

trophic factors. These factors are produced in limited quantities

by target tissues. Each type of trophic factor supports the survival

of a distinct group of neurons. For example, nerve growth factor

is important for sensory neuron survival. It has recently become

clear that the internal suicide program is maintained into adult-

hood and constantly held in check. On the basis of this idea,

researchers have found that injuries and some neurodegenerative

diseases kill neurons not directly by the damage they inﬂict but

rather by activating the cells’ own death programs. This discov-

ery—and its implication that death need not inevitably follow

insult—have led to new avenues for therapy.

Brain cells also form too many connections at ﬁrst. For exam-

ple, in primates, the projections from the two eyes to the brain

initially overlap and then sort out to separate territories devoted

only to one eye or the other. Furthermore, in the young primate

cerebral cortex, the connections between neurons are greater in

number and twice as dense as those in an adult primate. Com-

munication between neurons with chemical and electrical signals

is necessary to weed out the connections. The connections that

are active and generating electrical currents survive, whereas

those with little or no activity are lost. Thus, the circuits of the

adult brain are formed, at least in part, by sculpting away incor-

rect connections to leave only the correct ones.

Critical periods

The brain’s reﬁning and building of the network in mammals,

including humans, continues after birth. An organism’s interac-

SPINAL CORD AND

NERVES. The mature central

nervous system (CNS) consists of

the brain and spinal cord. The

brain sends nerve signals to spe-

cific parts of the body through

peripheral nerves, known as the

peripheral nervous system

(PNS). Peripheral nerves in the

cervical region serve the neck

and arms; those in the thoracic

region serve the trunk; those in

the lumbar region serve the legs;

and those in the sacral region

serve the bowels and bladder.

The PNS consists of the somatic

nervous system that connects

voluntary skeletal muscles with

cells specialized to respond

to sensations, such as touch and

pain. The autonomic nervous

system is made of neurons con-

necting the CNS with internal

organs. It is divided into the sym-

pathetic nervous system, which

mobilizes energy and resources

during times of stress and

arousal, and the parasympa-

thetic nervous system, which

conserves energy and resources

during relaxed states.

tions with its surroundings ﬁne-tune connections.

Changes occur during critical periods. These are windows of

time during development when the nervous system must obtain

certain critical experiences, such as sensory, movement, or emo-

tional input, to develop properly.

After a critical period, connections diminish in number and

are less subject to change, but the ones that remain are stronger,

more reliable, and more precise. Injury or sensory or social depri-

vation occurring at a certain stage of postnatal life may a∑ect one

aspect of development, whereas the same injury at a di∑erent

period may a∑ect another aspect.

In one example, a monkey is raised from birth to 6 months of

age with one eyelid closed. The animal permanently loses useful

vision in that eye because of diminished use. This gives cellular

meaning to the saying “use it or lose it.” Loss of vision is caused by

the actual loss of functional connections between that eye and

neurons in the visual cortex. This ﬁnding has led to earlier and bet-

ter treatment for the eye disorders of congenital cataracts and

“crossed eyes” in children.

Research also shows that enriched environments can bolster

brain development during postnatal life. For example, studies

show that animals brought up in toy-ﬁlled surroundings have

more branches on their neurons and more connections than iso-

lated animals. In one recent study, scientists found that enriched

environments resulted in more neurons in a brain area involved in

memory.

Scientists hope that new insights into brain development will

lead to treatments for those with learning disabilities, brain dam-

age, and neurodegenerative disorders, as well as helping us under-

stand aging.

Vision. This wonderful sense allows us to

image the world around us, from the genius of

Michelangelo’s Sistine Chapel ceiling to mist-

ﬁlled vistas of a mountain range. Vision is one

of the most delicate and complicated senses.

It is also the most studied. About one-fourth

of the brain is involved in visual processing, more than for any

other sense. More is known about vision than any other verte-

brate sensory system, with most of the information derived from

studies of monkeys and cats.

Vision begins with the cornea,which does about three-quar-

ters of the focusing, and then the lens,which varies the focus.

Both help produce a clear image of the visual world on the

retina—the sheet of photoreceptors that process vision, and neu-

rons lining the back of the eye.

As in a camera, the image on the retina is reversed: Objects

to the right of center project images to the left part of the retina

and vice versa, and objects above the center project to the lower

part and vice versa. The shape of the lens is altered by the mus-

cles of the iris so that near or far objects can be brought into focus

on the retina.

Visual receptors, about 125 million in each eye, are neurons

specialized to turn light into electrical signals. They occur in two

forms. Rods are most sensitive to dim light and do not convey color.

Cones work in bright light and are responsible for acute detail,

black-and-white vision, and color vision. The human eye contains

three types of cones that are sensitive to red, green, and blue, but

working together they convey information about all visible colors.

Primates, including humans, have well-developed vision

using two eyes. Visual signals pass from each eye along the mil-

lion or so ﬁbers of the optic nerve to the optic chiasm, where

some nerve ﬁbers cross over, so both sides of the brain receive sig-

nals from both eyes. Consequently, the left halves of both retinas

project to the left visual cortex and the right halves project to the

right visual cortex.

The e∑ect is that the left half of the scene you are watching

registers in your right hemisphere. Conversely, the right half of

the scene registers in your left hemisphere. A similar arrangement

applies to movement and touch: Each half of the cerebrum is

responsible for the opposite half of the body.

Scientists know much about the way cells encode visual infor-

mation in the retina, the lateral geniculate nucleus—an interme-

diate point between the retina and visual cortex—and the visual

cortex.These studies give us the best knowledge so far about how

the brain analyzes and processes information.

The retina contains three stages of neurons. The ﬁrst, the layer

of rods and cones, sends its signals to the middle layer, which

relays signals to the third layer. Nerve ﬁbers from the third layer

assemble to form the optic nerve. Each cell in the middle or third

layer typically receives input from many cells in the previous layer,

but the number of inputs varies widely across the retina. Near the

center of gaze, where visual acuity is highest, each cell in the third

layer receives inputs—via the middle layer—from one or a few

cones, thus allowing us to resolve very ﬁne details. Near the mar-

gins of the retina, each cell in the third layer receives signals from

a cluster of rods and cones, explaining why we cannot see ﬁne

details o∑ to either side. Whether large or small, this region of

visual space is called the receptive ﬁeld of the third-layer cell.

About 55 years ago, scientists discovered that the receptive

ﬁeld of such a cell is activated when light hits a tiny region in its

receptive ﬁeld center and is inhibited when light hits the part of

the receptive ﬁeld surrounding the center. If light covers the entire

receptive ﬁeld, the cell reacts only weakly and perhaps not at all.

Thus, the visual process begins with a comparison of the

amount of light striking any small region of the retina and the

amount of light around it. Located in the occipital lobe, the pri-

mary visual cortex—two millimeters thick (a bit larger than a

half-dollar) and densely packed with cells in many layers—

receives messages from the lateral geniculate. In the middle layer,

which also receives input from the lateral geniculate, scientists

found patterns of responsiveness similar to those observed in the

retina and lateral geniculate cells. Cells above and below this

layer responded di∑erently. They preferred stimuli in the shape

of bars or edges. Further studies showed that di∑erent cells pre-

ferred edges at particular angles, edges that moved, or edges

moving in a particular direction.

Although the process is not yet completely understood,

recent ﬁndings suggest that visual signals are fed into at least

three separate processing systems. One system appears to process

information about shape; a second, color; and a third, movement,

Sensation and perception

VISION. The cornea and lens help produce a clear image of the visual world on the retina, the sheet of photoreceptors and neurons lining the

back of the eye. As in a camera, the image on the retina is reversed: Objects to the right of the center project images to the left part of the

retina and vice versa. The eye’s 125 million visual receptors — composed of rods and cones — turn light into electrical signals. Rods are most

sensitive to dim light and do not convey the sense of color; cones work in bright light and are responsible for acute detail, black and white

vision, and color vision. The human eye contains three types of cones that are sensitive to red, green, and blue, but, in combination, convey

information about all visible colors. Rods and cones connect with a middle cell layer and third cell layer (see inset, above). Light passes through

these two layers before reaching the rods and cones. The two layers then receive signals from rods and cones before transmitting the signals

onto the optic nerve, optic chiasm, lateral geniculate nucleus, and, finally, the visual cortex.

location, and spatial organization. These ﬁndings of separate pro-

cessing systems come from monkey anatomical and physiologi-

cal data. They are veriﬁed by human psychological studies show-

ing that the perception of movement, depth, perspective, the

relative size of objects, the relative movement of objects, shading,

and gradations in texture all depend primarily on contrasts in

light intensity rather than in color.

Why movement and depth perception should be carried out

by only one processing system may be explained by a school of

thought called Gestalt psychology. Perception requires various ele-

ments to be organized so that related ones are grouped together.

This stems from the brain’s ability to group the parts of an image

together and also to separate images from one another and from

their individual backgrounds.

How do all these systems combine to produce the vivid

images of solid objects that we perceive? This involves extracting

biologically relevant information at each stage and associating

ﬁring patterns with past experience.

Vision studies also have led to better treatment for visual dis-

orders. Information from research in cats and monkeys has

improved the therapy for strabismus,or squint,a term for “cross-

eye” or wall-eye. Children with strabismus initially have good

vision in each eye. But because they cannot fuse the images in the

two eyes, they tend to favor one eye and often lose useful vision

in the other. Vision can be restored in such cases, but only dur-

ing infancy or early childhood. Beyond the age of 6 or so, the

blindness becomes permanent. But until a few decades ago, oph-

thalmologists waited until children reached the age of 4 before

operating to align the eyes, or prescribing exercises or an eye

patch. Now strabismus is corrected very early in life—before age

4, when normal vision can still be restored.

Hearing

Often considered the most important sense for humans, hearing

allows us to communicate with each other by receiving sounds

and interpreting speech. It also gives us information vital to sur-

vival. For example, the sound of an oncoming train tells us to stay

clear of the railroad track.

Like the visual system, our hearing system distinguishes sev-

eral qualities in the signal it detects. However, our hearing sys-

tem does not blend di∑erent sounds, as the visual system does

when two di∑erent wavelengths of light are mixed to produce

color. We can follow the separate melodic lines of several instru-

ments as we listen to an orchestra or rock band.

From the chirping of crickets to the roar of a rocket engine,

most of the sounds processed by the ear are heard by a mecha-

nism known as air conduction.In this process, sound waves are

ﬁrst funneled through the externally visible part of the ear, the

pinna (or external ear) and the external auditory canal,to the

tympanic membrane (eardrum), which vibrates at di∑erent

speeds. The malleus (hammer), which is attached to the tympanic

membrane, transmits the vibrations to the incus (anvil). This

structure passes them onto the stapes (stirrup), which delivers

them, through the oval window, to the inner ear.

The ﬂuid-ﬁlled spiral passages of each cochlea contain 16,000

hair cells, whose microscopic, hairlike projections respond to the

vibrations produced by sound. The hair cells, in turn, excite the

28,000 ﬁbers of the auditory nerve, which terminate in the

medulla of the brain. Auditory information ﬂows via the thala-

mus to the temporal gyrus,the part of the cerebral cortex involved

in receiving and perceiving sound.

The brain’s analysis of auditory information follows a pattern

similar to that of the visual system. Adjacent neurons respond to

tones of similar frequency. Some neurons respond to only a small

range of frequencies, others react to a wide range; some react only

to the beginning of a sound, others only respond to the end.

Speech sounds, however, may be processed di∑erently than

others. Our auditory system processes all the signals that it

receives in the same way until they reach the primary auditory

cortex in the temporal lobe of the brain. When speech sound is

perceived, the neural signal is funneled to the left hemisphere for

processing in language centers.

Taste and smell

Although di∑erent, the two sensory experiences of taste and smell

are intimately entwined. They are separate senses with their own

receptor organs. However, these two senses act together to allow

us to distinguish thousands of di∑erent ﬂavors. Alone, taste is a

relatively focused sense concerned with distinguishing among

sweet, salty, sour, bitter, and umami (Japanese for savory). The

interaction between taste and smell explains why loss of the sense

of smell apparently causes a serious reduction in the overall taste

experience,which we call ﬂavor.

Tastes are detected within taste buds, special structures of

which every human has some 5,000 to 10,000. Taste buds are

embedded within papillae,or protuberances, located mainly on

the tongue, with others found in the back of the mouth and on

the palate. Taste substances stimulate specialized sensory cells.

Each taste bud consists of 50 to 100 of these cells, which respond

to salts, acidity, sweet substances, bitter compounds, and mono-

sodium glutamate and related amino acids.

Taste signals in the sensory cells are transferred by synapses to

the ends of nerve ﬁbers, which send impulses along cranial nerves

to taste regions in the brain. From here, the impulses are relayed

to other brainstem centers responsible for the basic responses of

acceptance or rejection of the tastes, and to the thalamus and on

to the cerebral cortex for conscious perception of taste.

Specialized smell receptor cells are located in a small patch of

mucus membrane lining the roof of the nose. Axons of these sen-

sory cells pass through perforations in the overlying bone and

enter two elongated olfactory bulbs lying on top of the bone. The

portion of the sensory cell that is exposed to odors possesses hair-

like cilia. These cilia contain the receptor sites that are stimulated

by odorants carried by airborne molecules. These dissolve in the

mucus lining in order to stimulate receptor proteins in the cilia to start the smell response. An odor-

ant acts on many receptors to di∑erent degrees. Similarly, a receptor interacts with many di∑erent

odorants to varying degrees.

The pattern of activity set up in the receptor cells is projected to the olfactory bulb, where it forms

a spatial image of the odor. Impulses created by this stimulation pass to other smell regions, giving

rise to conscious perceptions of odor in the frontal lobe and emotional responses in the limbic sys-

tem of the brain.

Touch and pain

Touch is the sense by which we determine the characteristics of objects: size, shape, and texture. We

do this through touch receptors in the skin. In hairy skin areas, some receptors consist of webs of

sensory nerve cell endings wrapped around the base of hairs. The nerve endings are remarkably sen-

sitive, being triggered by slight movement of the hairs. Other receptors are more common in non-

hairy areas, such as the lips and ﬁngertips, and consist of nerve cell endings that may be free or sur-

rounded by bulblike structures.

Signals from touch receptors pass via sensory nerves to the spinal cord, where they synapse (make

contact) and then travel to the thalamus and sensory cortex. The transmission of this information is

highly topographic, meaning that the body is represented in an orderly fashion at di∑erent levels of

the nervous system. Larger areas of the cortex are devoted to sensations from the hands and lips;

much smaller cortical regions represent less sensitive parts of the body.

Di∑erent parts of the body vary in their sensitivity to touch discrimination and painful stimuli

according to the number and distribution of receptors. The cornea is several hundred times more

sensitive to painful stimuli than are the soles of the feet. The ﬁngertips are good at touch discrimi-

nation, but the chest and back are less sensitive.

HEARING. From the chirping

of crickets to the roar of a

rocket engine, almost all of the

thousands of single tones

processed by the human ear are

heard by a mechanism known

as air conduction. In this

process, sound waves are first

funneled through the external

ear — the pinna and the exter-

nal auditory canal — to the

middle ear — the tympanic

membrane (eardrum)— that

vibrates at different speeds. The

malleus (hammer), which is

attached to the tympanic mem-

brane, transmits the vibrations

to the incus (anvil). The vibra-

tions are then passed onto the

stapes (stirrup) and oval win-

dow that, in turn, pass them

onto the inner ear. In the inner

ear, the fluid-filled spiral pas-

sage of the cochlea contains

cells with microscopic, hairlike

projections that respond to the

vibrations produced by sound.

The hair cells, in turn, excite the

28,000 fibers of the auditory

nerve that end in the medulla in

the brain. Auditory information

flows via the thalamus to the

temporal gyrus, the part of the

cerebral cortex involved in

receiving and perceiving sound.

Not surprisingly, acuity is greatest in the most densely nerve-packed areas of the body. This fea-

ture, in fact, is used to test clinically for the integrity of these somatosensory pathways. For example,

neurologists can run tests by using a two-point threshold.This method involves touching the skin with

calipers at two points. The two-point threshold is the distance between the two points that is neces-

sary for the individual to distinguish two distinct stimuli from one.

Until recently, pain was thought to be a simple message by which neurons sent electrical impulses

from the site of injury directly to the brain. We now know that the process is far more complicated.

Nerve impulses from sites of injury that persist for hours, days, or longer lead to changes in the ner-

vous system that result in an ampliﬁcation and increased duration of the pain. These changes involve

dozens of chemical messengers and receptors. Persistent pain is in many respects a disease of the ner-

vous system, not merely a symptom of some other disease process.

The sensory ﬁbers that respond to stimuli that injure tissue and can cause pain are called noci-

ceptors,special receptors that respond to tissue-damaging stimuli. In addition to directly activating

the nociceptor and evoking a pain sensation, tissue injury causes the release of numerous chemicals

at the site of damage and inﬂammation. One such family of chemicals includes the prostaglandins,

which enhance the sensitivity of receptors to tissue damage and ultimately can induce more intense

pain sensations. Prostaglandins also contribute to the clinical condition in which innocuous stimuli

can produce pain (such as in sunburned skin) because the threshold of the nociceptor is signiﬁcantly

reduced. This phenomenon is called allodynia.

Pain messages are transmitted to the spinal cord via small myelinated ﬁbers and C ﬁbers—very

small unmyelinated ﬁbers. Myelin is a covering sheath around nerve ﬁbers that helps them send their

messages more rapidly. The small myelinated pain-sensitive nerve ﬁbers probably evoke the sharp,

fast pain that is produced by, for example, a pin prick. C ﬁber-induced pain, by contrast, is generally

TASTE AND SMELL . Special-

ized receptors for smell are

located in a patch of mucus

membrane lining the roof of the

nose. Each cell has several fine

hairlike cilia containing receptor

proteins, which are stimulated

by odor molecules in the air,

and a long fiber (axon), which

passes through perforations in

the overlying bone to enter the

olfactory bulb. Stimulated cells

give rise to impulses in the

fibers, which set up patterns in

the olfactory bulb that are

relayed to the brain’s frontal

lobe to give rise to smell per-

ception, and to the limbic sys-

tem to elicit emotional responses.

Tastes are detected by special

structures, taste buds, of which

every human has some 5,000

to 10,000. Taste buds are

embedded within papillae (pro-

tuberances) mainly on the

tongue, with a few located in

the back of the mouth and on

the palate. Each taste bud con-

sists of about 100 receptors that

respond to the four types of

stimuli — sweet, salty, sour, and

bitter — from which all tastes

are formed. A substance is

tasted when chemicals in foods

dissolve in saliva, enter the

pores on the tongue, and come

in contact with taste buds. Here

they stimulate hairs projecting

from the receptor cells and

cause signals to be sent from

the cells, via synapses, to cra-

nial nerves and taste centers

in the brain.

slower in onset, dull, and more di∑use.

In the ascending system,the impulses are relayed from the

spinal cord to several brain structures, including the thalamus and

cerebral cortex, that are involved in the process by which “pain”

messages become conscious experience. The experience of pain is

not just a function of the magnitude of the injury, or even the

intensity of the impulse activity generated by the injury. The set-

ting in which the injury occurs contributes (e.g., the pain of child-

birth or that produced in a car accident). The emotional compo-

nent of the experience is a major contributor to the overall pain.

Pain messages can also be suppressed by a system of neurons

that originate within the gray matter in the brainstem. This

descending system sends messages to the dorsal horn of the spinal

cord,where it suppresses the transmission of pain signals to the

higher brain centers. Some of these descending systems use nat-

urally occurring chemicals similar to opioids. The three major

families of opioid peptides identiﬁed in the brain—enkephalins,

beta-endorphins, and dynorphins—originate from three pre-

cursor proteins encoded by three di∑erent genes. They act at mul-

tiple opioid receptors in the brain and spinal cord. Knowledge of

PAIN. Messages about tissue

damage are picked up by recep-

tors and transmitted to the

spinal cord via small myelinated

fibers and very small unmyeli-

nated fibers. From the spinal

cord, the impulses are carried to

the brainstem, thalamus, and

cerebral cortex and ultimately

perceived as pain. These mes-

sages can be suppressed by a

system of neurons that origi-

nates in the gray matter of the

midbrain. This descending path-

way sends messages to the

spinal cord where it suppresses

the transmission of tissue dam-

age signals to the higher brain

centers. Some of these descend-

ing pathways use naturally

occurring, opiate-like chemicals

called endorphins.

the way pain messages are transmitted has led to new treatments

for pain. For example, scientists began studying the spinal deliv-

ery of opioids when they discovered a dense distribution of opi-

oid receptors in the spinal cord horn. Such treatments were

begun in humans after the method was successfully used in ani-

mals; the technique is now common in treating pain after surgery.

Because the spinal opioid does not interact at all levels of the ner-

vous system, this technique bypasses many potentially negative

opioid side e∑ects.

Many new insights into the pain experience are coming from

studies in which modern imaging tools are used to monitor brain

activity when pain is experienced. One ﬁnding is that there is no

single area in the brain where pain is generated; rather, there are

both emotional and sensory components. Interestingly, when

people are hypnotized so that a painful stimulus is not experi-

enced as unpleasant, activity in only some areas of the brain is

suppressed. As such techniques for brain study improve, it should

be possible to better monitor the changes in the brain that occur

in people with persistent pain and to better evaluate the di∑erent

analgesic drugs being developed.

The conscious memory of a patient known as

H.M. is limited almost entirely to events that

occurred years before his surgery, in which part

of the medial temporal lobe of his brain was

removed to relieve epilepsy. H.M. can remember

recent events for only a few minutes. Talk with

him awhile and then leave the room. When you return, he has no

recollection of ever having seen you.

The medial temporal lobe, which includes the hippocampus

and adjacent brain areas, seems to play a role in converting mem-

ory from a short-term to a long-term, permanent form. The fact

that H.M. retains memories for events that are remote to his

surgery is evidence that the medial temporal region is not the site

of permanent storage but that it plays a role in the formation of

new memories. Other patients like H.M. have also been described.

Additional evidence comes from patients undergoing elec-

troconvulsive therapy (ECT) for depression. ECT is thought to

temporarily disrupt the function of the hippocampus and related

structures. These patients typically have difﬁculty with new learn-

ing and have amnesia for events that occurred during the several

years before treatment. Memory of earlier events is unimpaired.

As time passes after treatment, much of the lost part of memory

becomes available once again.

The hippocampus and the medial temporal region are con-

nected to widespread areas of the cerebral cortex, especially the

vast regions responsible for thinking and language. Whereas the

medial temporal region is important for forming and organizing

memory, cortical areas are important for the long-term storage

of knowledge about facts and events and for how this knowledge

is used in everyday situations.

Working memory,a type of transient, “online” memory that

enables us to retain what someone has said just long enough to

reply, depends in part on the prefrontal cortex. Researchers dis-

covered that certain neurons in this area are inﬂuenced by neu-

rons releasing dopamine and other neurons releasing glutamate.

Although much remains to be discovered about learning and

memory, scientists have already put together important pieces

of the puzzle. For example, the brain appears to process differ-

ent kinds of information in separate ways and then store it dif-

ferently.

Declarative knowledge requires processing in the medial tem-

poral region and parts of the thalamus and can be grouped into

working memory, episodic memory, and semantic memory.

Working memory allows us to keep and use information in our

minds and is mediated by a network of areas in the cerebral cor-

tex. Episodic memory lets us store and replay events in our minds

and depends on the hippocampus. Semantic memory includes

raw facts and data and is stored throughout the cerebral cortex.

The hippocampus may play a role in integrating new episodic

memories into the semantic memory storehouse.

In contrast, nondeclarative knowledge,the knowledge of how

to do something, is expressed in skilled behavior and learned

habits and requires processing by the basal ganglia.

The amygdala appears to play an important role in the emo-

tional aspects of memory. An important factor that inﬂuences

what is stored and how strongly it is stored is whether the action

is followed by reward, punishment, or highly emotional conse-

quences. These consequences help determine what behaviors an

organism will learn and remember.

Memory of motor learning tasks in which precise timing is

involved depends on the cerebellum.

How exactly does memory occur? After years of study, there

is much support for the idea that memory involves a persistent

change in the connection between neurons. In animal studies, sci-

entists found that this occurs in the short term through two bio-

chemical events that affect the strength of the relevant synapses.

The stability of long-term memory is conferred by turning on

genes that may lead to modiﬁcations within neurons that change

the strength and number of synapses. For example, researchers

can correlate speciﬁc chemical and structural changes in the rel-

evant cells with several simple forms of memory exhibited by the

sea slug Aplysia californica.

Another important model for the study of memory is the

phenomenon of long-term potentiation (LTP), a long-lasting

increase in the strength of a synaptic response following stim-

ulation. LTP occurs prominently in the hippocampus, as well as

in other brain areas. Studies of rats suggest that LTP occurs

through changes in synaptic strength at contacts involving

NMDA receptors. It is now possible to study LTP and learning

in genetically modiﬁed mice that have alterations in speciﬁc

Learning, memory, and

language

genes. Examples of these modiﬁed genes can be limited both to

particular brain areas and to speciﬁc times, such as during

learning.

Much of what we have learned about memory comes from

studies of amnesia due to damage to the hippocampus and cor-

tical areas—called the parahippocampal region—in the medial

part of the temporal lobe. Patients with damage in these areas can

remember recent events only while actively engaged in the mate-

rial—yet they often retain childhood memories quite well. This

pattern suggests that the temporal lobe is critical in integrating

early memories into a permanent storehouse that can be accessed

whenever needed.

Several types of memory are spared in amnesia. Reports indi-

cate that the sense of familiarity one has with a face or a scene is

spared, even though the speciﬁc context of the experience may be

lost. The emotional association one might develop with a given

item is also commonly spared in amnesia.

Scientists believe that no single brain center stores memory.

It most likely is stored in distributed collections of cortical pro-

cessing systems that are also involved in the perception, process-

ing, and analysis of the material being learned. In short, each part

of the brain most likely contributes differently to permanent

memory storage.

One of the most prominent intellectual activities dependent

on memory is language. Although the neural basis of language is

not fully understood, scientists have learned a great deal about

this function of the brain from studies of patients who have lost

speech and language abilities due to stroke, and from behavioral

and functional neuroimaging studies of normal people.

A prominent and inﬂuential model, based on studies of these

patients, proposes that the underlying structure of speech com-

prehension arises in part of the left hemisphere of the brain,

called Wernicke’s area. This temporal lobe region is connected

with another region, Broca’s area, in the frontal lobe, where a pro-

gram for vocal expression is created. This program is then trans-

mitted to a nearby area of the motor cortex that activates the

mouth, tongue, and larynx.

This same model proposes that, when we read a word, the

information is transmitted from the primary visual cortex to the

angular gyrus, where the message is somehow matched with the

words when they are spoken. The auditory form of the word is

then processed for comprehension in Wernicke’s area as if the

word had been heard. Writing in response to an oral instruction

requires information to be passed along the same pathways in

the opposite direction—from the auditory cortex to Wernicke’s

area to the angular gyrus. This model accounts for much of the

data from patients and is the most widely used for clinical diag-

nosis and prognosis. Some reﬁnements to this model may be

necessary, however, because of both recent studies with patients

and functional neuroimaging studies in healthy people.

For example, using an imaging technique called positron

emission tomography (PET), scientists have demonstrated that

some reading tasks performed by normal people do not activate

Wernicke’s area or the angular gyrus. These results suggest that,

at least under some conditions, there is a direct reading route that

does not involve speech-sound recoding of the visual stimulus

before the processing of either meaning or speaking. Other stud-

ies with patients also have indicated that it is likely that familiar

words need not be recoded into sound before they can be under-

stood.

Although the understanding of how language is implemented

in the brain is far from complete, there are now several tech-

niques that may be used to gain important insights into this crit-

ical aspect of brain function.

LEARNING AND MEMORY. Different brain areas and systems

mediate distinct forms of memory. The hippocampus, parahip-

pocampal region, and areas of the cerebral cortex (including pre-

frontal cortex) compose a system that supports declarative, or cog-

nitive, memory. Different forms of nondeclarative, or behavioral,

memory are supported by the amygdala, striatum, and cerebellum.

From the stands, we marvel at the perfectly placed

serves of professional tennis players and the light-

ning-fast double plays executed by big league

inﬁelders. But in fact, each of us in our daily activi-

ties performs a host of complex, skilled movements,

such as walking upright, speaking, and writing, that

are just as remarkable. This is made possible by a ﬁnely tuned and

highly complex central nervous system, which controls the

actions of hundreds of muscles. Through learning, the nervous

system can adapt to changing movement requirements to accom-

plish these everyday marvels, and to perform them more skill-

fully with practice.

To understand how the nervous system performs such tricks,

we have to start with the muscles, for these are the body parts

that produce movement under the control of the brain and

spinal cord.

Most muscles attach to points on the skeleton that cross one

or more joints, so they are called skeletal muscles.Activation ofa

given muscle can open or close the joints that it spans, depend-

ing upon whether it is a joint ﬂexor (closer) or extensor (opener).

In addition, if ﬂexors and extensors at the same joint are activated

together, they can “sti∑en” a joint, thus maintaining limb posi-

tion in the face of unpredictable external forces that would oth-

erwise displace it. Muscles that move a joint in an intended direc-

tion are called agonists,and those that oppose this direction of

movement are antagonists.Skilled movements at high speed are

started by agonists and stopped by antagonists, thus placing the

joint or limb at a desired position.

Some muscles act on soft tissue, such as the muscles that

move the eyes and tongue and those that control facial expres-

sion. These muscles are also under control of the central nervous

system, and their principles of operation are similar to those that

attach to bone.

Each skeletal muscle is made up of thousands of individual

muscle ﬁbers, and each of these is controlled by one alpha motor

neuron in either the brain or the spinal cord. On the other hand,

each single alpha motor neuron controls many muscle ﬁbers

(ranging from a few to a hundred or more), forming a functional

unit referred to as a motor unit.These motor units are the criti-

cal link between the brain and muscles. If they die, which can

happen in certain diseases that a∑ect motor neurons directly, a

person is no longer able to move, either voluntarily or through

reﬂexes.

Perhaps the simplest and most fundamental of movements

are reﬂexes. These are relatively ﬁxed, automatic muscle responses

to particular stimuli, such as sudden withdrawal of the foot when

you step on a sharp object, or the slight extension of the leg when

a physician taps your knee with a small rubber hammer. All

reﬂexes involve the activation of small sensory receptors in the

skin, the joints, or even in muscles themselves. For example, the

knee movement referred to above is produced by a slight stretch

of the knee extensor muscles when the physician taps the muscle

tendon at the knee. This slight muscle stretch is “sensed”by recep-

tors in the muscle, called muscle spindles.Innervated by sensory

ﬁbers, the spindles send information to the spinal cord and brain

about the length and speed of shortening or lengthening of a mus-

cle. This information is used in reﬂex control of the joint at which

the muscle acts, and also for control of voluntary movements.

Sudden muscle stretch sends a barrage of impulses into the

spinal cord along the muscle spindle sensory ﬁbers. This, in turn,

activates motor neurons in the stretched muscle, causing a con-

traction called the stretch reﬂex.The same sensory stimulus

causes inactivation, or inhibition, in the motor neurons of the

antagonist muscles through connecting neurons, called inhibitory

neurons,within the spinal cord. Thus, even the simplest of

reﬂexes involves a coordination of activity across motor neurons

that control agonist and antagonist muscles.

Even more amazing is the fact that the brain can control not

only the actions of motor neurons and muscles, but also the

nature of the feedback that it receives from sensory receptors in

the muscles as movements occur. For example, the sensitivity of

the muscle spindle organs is controlled by the brain through a

separate set of gamma motor neurons that control the specialized

muscle ﬁbers and allow the brain to ﬁne-tune the system for

di∑erent movement tasks.

In addition to such exquisite sensing and control of muscle

length by muscle spindles, other specialized sense organs in mus-

cle tendons—the golgi tendon organs—detect the force applied

by a contracting muscle, allowing the brain to also sense and con-

trol the muscular force exerted during movement.

Movement

MOVEMENT. The stretch reflex

(above) occurs when a doctor

taps a muscle tendon to test your

reflexes. This sends a barrage

of impulses into the spinal cord

along muscle spindle sensory

fibers and activates motor neu-

rons to the stretched muscle to

cause contraction (stretch reflex).

The same sensory stimulus

causes inactivation, or inhibition,

of the motor neurons to the

antagonist muscles through con-

nection neurons, called inhibitory

neurons, within the spinal cord.

Afferent nerves carry messages

from sense organs to the spinal

cord; efferent nerves carry motor

commands from the spinal cord

to muscles. Flexion withdrawal

(below) can occur when your

bare foot encounters a sharp

object. Your leg is immediately

lifted (flexion) from the source of

potential injury, but the opposite

leg responds with increased

extension in order to maintain

your balance. The latter event is

called the crossed extension

reflex. These responses occur

very rapidly and without your

attention because they are built

into systems of neurons located

within the spinal cord itself.

We now know that these complex systems are coordinated

and organized to respond di∑erently for tasks that require pre-

cise control of position, such as holding a full teacup, and for

those requiring rapid, strong movement, such as throwing a ball.

You can experience such changes in motor strategy when you

compare walking down an illuminated staircase with the same

task done in the dark.

Another useful reﬂex is the ﬂexion withdrawal that occurs if

your bare foot encounters a sharp object. Your leg is immediately

lifted from the source of potential injury (ﬂexion) but the oppo-

site leg responds with increased extension in order to maintain

your balance. The latter event is called the crossed extension reﬂex.

These responses occur very

rapidly and without your atten-

tion because they are built into

systems of neurons that are

located within the spinal cord

itself. It seems likely that the

same systems of spinal neurons

also participate in controlling

the alternating action of the

legs during normal walking. In

fact, the basic patterns of mus-

cle activation that produce

coordinated walking can be

generated in four-footed ani-

mals within the spinal cord

itself. These spinal mechanisms, which evolved in primitive ver-

tebrates, are likely still present in the human spinal cord.

The most complex movements that we perform, including

voluntary ones that require conscious planning, involve control

of these basic spinal mechanisms by the brain. Scientists are only

beginning to understand the complex interactions that take place

among di∑erent brain regions during voluntary movements,

mostly through careful experiments on animals.

One important brain area in the control of voluntary move-

ment is the motor cortex,which exerts powerful control over the

spinal cord, in part through direct control of its alpha motor

neurons. Some neurons in the motor cortex appear to specify the

coordinated action of many muscles, to produce organized

movement of a limb to a particular place in space. Others appear

to control only two or three functionally related muscles, such

as those of the hand or arm, that are important for ﬁnely tuned,

skilled movement.

In addition to the motor cortex, movement control also

involves the interaction of many other brain regions, including

the basal ganglia, thalamus, cerebellum,and a large number of

neuron groups located within the midbrain and brainstem—

regions that connect cerebral hemispheres with the spinal cord.

The brain regions devoted to such control are large, containing

millions of intricately interconnected neurons.

Scientists know that the basal ganglia and thalamus have

widespread connections with sensory and motor areas of the

cerebral cortex. Loss of regula-

tion of the basal ganglia by

dopamine depletion can cause

serious movement disorders,

such as Parkinson’s disease. Loss

of dopamine neurons in the

substantia nigra of the mid-

brain, which connects with the

basal ganglia, is a major factor

in Parkinson’s disease.

Another brain region that is

crucial for skilled movement

and for the learning of new

movements is the cerebellum. A

disturbance of cerebellar func-

tion, for example, leads to poor coordination of muscle control,

to disorders of balance, and even to di≈culties in speech, one of

the most intricate forms of movement control.

The cerebellum receives direct and powerful sensory infor-

mation from the muscle receptors and the sense organs of the

inner ear, which signal head position and movement, and sig-

nals from the cerebral cortex. The cerebellum apparently acts to

integrate all this information to ensure smooth coordination of

muscle action, enabling us to perform skilled movements more

or less automatically. There is evidence that as we learn to walk,

speak, or play a musical instrument, the necessary, detailed con-

trol information is stored within the cerebellum, where it can be

called upon by commands from the cerebral cortex.

We now know that complex

systems are coordinated and organized

to respond differently for tasks that

require precise control of position,

such as holding a teacup, and for those

requiring rapid, strong movement,

such as throwing a ball.

Sleep remains one of the great mysteries of mod-

ern neuroscience. We spend nearly one-third of

our lives asleep, but the function of sleep still is

not known. Fortunately, over the past few years

researchers have made great headway in under-

standing some of the brain circuitry that con-

trols wake-sleep states.

Scientists now recognize that sleep consists of several di∑-

erent stages; that the choreography of a night’s sleep involves the

interplay of these stages, a process that depends upon a complex

switching mechanism; and that the sleep stages are accompanied

by daily rhythms in bodily hormones, body temperature, and

other functions.

Sleep disorders are among the nation’s most common health

problems, a∑ecting up to 70 million people, most of whom are

undiagnosed and untreated. These disorders are one of the least

recognized sources of disease, disability, and even death, costing

an estimated $100 billion annually in lost productivity, medical

bills, and industrial accidents. Research holds promise for devis-

ing new treatments to allow millions of people to get a good

night’s sleep.

The stuff of sleep

Although sleep appears to be a passive and restful time, it actu-

ally involves a highly active and well-scripted interplay of brain

circuits to produce its various stages.

The stages of sleep were discovered in the 1950s in experi-

ments using electroencephalography (EEG) that examined human

brain waves during sleep. Researchers also measured movements

of the eyes and the limbs during sleep. They found that over the

course of the ﬁrst hour or so of sleep each night, the brain pro-

gresses through a series of stages during which the brain waves

progressively slow down. This period of slow wave sleep is accom-

panied by relaxation of the muscles and the eyes. Heart rate, blood

pressure, and body temperature all fall. If awakened at this time,

most people recall only fragmented thoughts, not an active dream.

Over the next half hour or so, brain activity alters drastically

from the deep slow wave sleep to generate neocortical EEG waves

that are indistinguishable from those observed during waking.

Paradoxically, the fast, waking-like EEG activity is accompanied

by atonia,or paralysis of the body’s muscles (only the muscles

that allow breathing remain active). This state is often called

rapid eye movement (REM) sleep. During REM sleep, there is

active dreaming. Heart rate, blood pressure, and body tempera-

ture become much more variable. Men often have erections dur-

ing this stage of sleep. The ﬁrst REM period usually lasts 10 to 15

minutes.

During the night, these cycles of slow wave and REM sleep

alternate, with the slow wave sleep becoming less deep and the

REM periods more prolonged until waking occurs.

Over the course of a lifetime, the pattern of sleep cycles

changes. Infants sleep up to 18 hours per day, and they spend

much more time in deep slow wave sleep. As children mature,

they spend less time asleep and less time in deep slow wave sleep.

Older adults may sleep only six to seven hours per night, often

complain of early waking that they cannot avoid, and spend very

little time in slow wave sleep.

Sleep disorders

The most common sleep disorder, and the one most people are

familiar with, is insomnia.Some people have di≈culty falling

asleep initially, but other people fall asleep and then awaken part-

way through the night and cannot fall asleep again. Although

there are a variety of short-acting sedatives and sedating antide-

pressant drugs available to help, none of these produces a truly

natural and restful sleep state, because they tend to suppress the

deeper stages of slow wave sleep.

Excessive daytime sleepiness may have many causes. The

most common are disorders that disrupt sleep and result in inad-

equate amounts of sleep, particularly of the deeper stages. These

are usually diagnosed in the sleep laboratory, where the EEG, eye

movements, and muscle tone are monitored electrically as the

individual sleeps. In addition, the heart, breathing, and oxygen

content of the blood can be monitored.

Obstructive sleep apnea causes the airway muscles in the

throat to collapse as sleep deepens. This prevents breathing,

which causes arousal from sleep, and prevents the su∑erer from

entering the deeper stages of slow wave sleep. This condition can

also cause high blood pressure and may increase the risk of heart

attack. The increased daytime sleepiness leads to an increased risk

Sleep

of daytime accidents, especially automobile accidents. Treatment

may include a variety of attempts to reduce airway collapse dur-

ing sleep. Whereas simple things like losing weight, avoiding alco-

hol and sedating drugs prior to sleep, and avoiding sleeping on

one’s back can sometimes help, most people with sleep apnea

require devices that induce continuous positive airway pressure

to keep the airway open. This can be provided by ﬁtting a small

mask over the nose that provides an air stream under pressure

during sleep. In some cases, surgery is needed to correct the air-

way anatomy.

Periodic limb movements of sleep are intermittent jerks of the

legs or arms that occur as the individual enters slow wave sleep

and can cause arousal from sleep. Other people have episodes in

which their muscles fail to be paralyzed during REM sleep, and

they act out their dreams. This REM behavior disorder can also be

very disruptive to a normal night’s sleep. Both disorders are more

common in people with Parkinson’s disease and both can be

treated with drugs that treat Parkinson’s or with a drug called

clonazepam.

Narcolepsy is a relatively uncommon condition—only one

case per 2,500 people—in which the switching mechanism for

REM sleep does not work properly. Narcoleptics have sleep

attacks during the day, in which they suddenly fall asleep. This is

socially disruptive, as well as dangerous, for example, if they are

driving. They tend to enter REM sleep very quickly as well and

may even enter a dreaming state while still partially awake, a con-

dition known as hypnagogic hallucination.They also have attacks

during which they lose muscle tone, similar to what occurs dur-

ing REM sleep, but while they are awake. These attacks of paral-

ysis, known as cataplexy,can be triggered by emotional experi-

ences, even by hearing a funny joke.

Recently, studies into the mechanism of narcolepsy have given

major insights into the processes that control these mysterious

transitions between waking, slow wave, and REM sleep states.

How is sleep regulated?

During wakefulness, the brain is kept in an alert state by the inter-

actions of two major systems of nerve cells. Nerve cells in the

upper part of the pons and in the midbrain, which produce

acetylcholine, send inputs to activate the thalamus. When the

thalamus is activated, it in turn activates the cerebral cortex and

produces a waking EEG pattern. Another important wakefulness

center is in the basal forebrain, whose neurons project directly to

the cerebral cortex. In addition to acetylcholine, other neuro-

transmitters promote wakefulness, including norepinephrine,

serotonin, histamine, and glutamate.

During REM sleep, the cholinergic nerve cells and the thala-

mus and cortex are in a condition similar to wakefulness, but the

brain is not very responsive to external stimuli. The di∑erence is

in the activity of three sets of monoamine nerve cells: the brain-

stem nerve cells in the locus coeruleus that use the neurotrans-

mitter norepinephrine; the dorsal and median raphe groups that

contain serotonin; and, in the hypothalamus, the tuberomammil-

lary cell group that uses histamine. These monoamine neurons

ﬁre most rapidly during wakefulness, but they slow down during

slow wave sleep and stop during REM sleep. These monoamine

neurons act to suppress the occurrence of REM sleep.

The brainstem cell groups that control arousal from sleep are,

in turn, inﬂuenced by two groups of nerve cells in the hypothal-

amus, part of the brain that controls basic body cycles. One group

SLEEP PATTERNS. During a night of sleep, the brain waves of a young adult recorded by the electroencephalogram (EEG) gradually slow

down and become larger as the individual passes into deeper stages of slow wave sleep. After about an hour, the brain re-emerges through

the same series of stages, and there is usually a brief period of REM sleep (on dark areas of graph), during which the EEG is similar to wake-

fulness. The body is completely relaxed; the person is deeply unresponsive and usually is dreaming. The cycle repeats over the course of the

night, with more REM sleep, and less time spent in the deeper stages of slow wave sleep as the night progresses.

THE WAKING AND

SLEEPING BRAIN. Wakeful-

ness is maintained by activity in

two systems of neurons. Neu-

rons that make the neurotrans-

mitter acetylcholine are located

in two main arousal centers,

one in the brainstem and one in

the forebrain (red pathways).

The brainstem arousal center

supplies the acetylcholine for the

thalamus and brainstem, and

the forebrain arousal center

supplies that for the cerebral

cortex. Activation of these cen-

ters alone can create rapid eye

movement sleep. Activation of

other neurons that make

monoamine neurotransmitters

such as norepinephrine, sero-

tonin, and histamine (blue path-

ways) is needed for waking.

of nerve cells, in the ventrolateral preoptic nucleus, contains the

inhibitory neurotransmitters galanin and GABA. When the ven-

trolateral preoptic neurons ﬁre, they are thought to turn o∑ the

arousal systems, causing sleep. Damage to the ventrolateral pre-

optic nucleus produces irreversible insomnia.

A second group of nerve cells in the lateral hypothalamus

inﬂuences and suppresses REM sleep. They contain the neuro-

transmitter orexin, which provides an excitatory signal to the

arousal system, particularly to the monoamine neurons. In exper-

iments in which the gene for the neurotransmitter orexin was

experimentally removed in mice, the animals became narcolep-

tic. Similarly, in two dog species with naturally occurring nar-

colepsy, an abnormality was discovered in the gene for the type 2

orexin receptor. Recent studies show that in humans with nar-

colepsy, the orexin levels in the brain and spinal ﬂuid are abnor-

mally low. Thus, orexin appears to play a critical role in activat-

ing the monoamine system and in preventing abnormal

transitions, particularly into REM sleep.

Two main signals control our need for sleep and its circuitry.

First, there is homeostasis, or the body’s need to seek a natural

equilibrium of rest and sleep followed by wakefulness. Several

mechanisms for the signal of accumulating sleep have been sug-

gested. There is evidence that a chemical called adenosine, which

is linked to brain energy depletion, accumulates in the brain dur-

ing prolonged wakefulness and that it may drive sleep home-

ostasis. Interestingly, the drug ca∑eine, which is widely used to

prevent sleepiness, acts as an adenosine blocker.

If an individual does not get enough sleep, the sleep debt pro-

gressively accumulates and leads to a degradation of mental func-

tion. When the opportunity to sleep again comes, the individual

will sleep much more, to “repay” the debt. The slow wave sleep

debt is usually “paid o∑” ﬁrst.

The other major inﬂuence on sleep cycles is the body’s circa-

dian clock, the suprachiasmatic nucleus. This small group of

nerve cells in the hypothalamus contains clock genes, which go

through a biochemical cycle of about 24 hours, setting the pace

for daily cycles of activity, sleep, hormones, and other bodily

functions. The suprachiasmatic nucleus also receives input

directly from the retina, and the clock can be reset by light, so that

it remains linked to the outside world’s day-night cycle. The

suprachiasmatic nucleus provides signals to the brain areas reg-

ulating sleep and arousal.

The ability to react in response to threatening

events has been with us since the time of our

ancient ancestors. In response to impending dan-

ger, muscles are primed, attention is focused, and

nerves are readied for action—“ﬁght or ﬂight.”

But in today’s complex and fast-paced world, this

response to stress is not enough, and the continued stimulation

of the systems that respond to threat or danger may contribute

to heart disease, obesity, arthritis, and depression, as well as accel-

erating the aging process.

Nearly two-thirds of ailments seen in doctors’ o≈ces are

commonly thought to be stress induced; indeed, stress can both

cause diseases and exacerbate existing ones. Surveys indicate that

60 percent of Americans feel they are under a great deal of stress

at least once a week. Costs due to stress from absenteeism, med-

ical expenses, and lost productivity are estimated at $300 billion

annually.

Stress is di≈cult to deﬁne because its e∑ects vary with each

individual. Specialists now deﬁne stress as any external stimulus

that threatens homeostasis—the normal equilibrium of body

function. Stress also can be induced by the belief that homeosta-

sis might soon be disrupted. Among the most powerful stressors

are psychological and psychosocial stressors that exist between

members of the same species. Lack or loss of control is a partic-

ularly important feature of severe psychological stress that can

have physiological consequences. Most harmful are the chronic

aspects of stress.

During the last six decades, researchers using animals found

that stress both helps and harms the body. When confronted with

a crucial physical challenge, properly controlled stress responses

can provide the extra strength and energy needed to cope. More-

over, the acute physiological response to stress protects the body

and brain and helps to reestablish or maintain homeostasis. But

stress that continues for prolonged periods can repeatedly elevate

the physiological stress responses or fail to shut them o∑ when

not needed. When this occurs, these same physiological mecha-

nisms can badly upset the body’s biochemical balance and accel-

erate disease.

Scientists also believe that the individual variation in

responding to stress is somewhat dependent on a person’s per-

ception of external events. This perception ultimately shapes his

or her internal physiological response. Thus, by controlling your

perception of events, you can do much to avoid the harmful con-

sequences of the sorts of mild to moderate stressors that typically

aΩict Westernized humans.

The immediate response

A stressful situation activates three major communication sys-

tems in the brain that regulate bodily functions. Scientists have

come to understand these complex systems through experiments

primarily with rats, mice, and nonhuman primates, such as mon-

keys. Scientists then veriﬁed the action of these systems in

humans.

The ﬁrst of these systems is the voluntary nervous system,

which sends messages to muscles so that we may respond to sen-

sory information. For example, the sight of a growling bear on a

trail in Yellowstone National Park prompts you to run as quickly

as possible.

The second communication system is the autonomic nervous

system.It combines the sympathetic or emergency branch, which

gets us going in emergencies, and the parasympathetic or calm-

ing branch, which keeps the body’s maintenance systems, such as

digestion, in order and calms the body’s responses to the emer-

gency branch.

Each of these systems has a speciﬁc task. The emergency

branch causes arteries supplying blood to the muscles to relax in

order to deliver more blood, allowing greater capacity to act. At

the same time, the emergency system reduces blood ﬂow to the

skin, kidneys, and digestive tract and increases blood ﬂow to the

muscles. In contrast, the calming branch helps to regulate bodily

functions and soothe the body once the stressor has passed, pre-

venting the body from remaining too long in a state of mobiliza-

tion. Left mobilized and unchecked, these body functions could

lead to disease. Some actions of the calming branch appear to

reduce the harmful e∑ects of the emergency branch’s response to

stress.

The brain’s third major communication process is the neu-

roendocrine system,which also maintains the body’s internal

functioning. Various “stress hormones” travel through the blood

and stimulate the release of other hormones, which a∑ect bodily

Stress

THE STRESS REACTION.

When stress occurs, the sympa-

thetic nervous system is trig-

gered. Norepinephrine is

released by nerves, and epi-

nephrine is secreted by the

adrenal glands. By activating

receptors in blood vessels and

other structures, these sub-

stances ready the heart and

working muscles for action.

Acetylcholine is released in the

parasympathetic nervous sys-

tem, producing calming effects.

The digestive tract is stimulated

to digest a meal, the heart rate

slows, and the pupils of the

eyes become smaller. The neu-

roendocrine system also main-

tains the body’s normal

internal functioning. Corti-

cotrophin-releasing hor-

mone (CRH), a peptide

formed by chains of amino

acids, is released from the

hypothalamus, a collection of

cells at the base of the brain

that acts as a control center for

the neuroendocrine system.

CRH travels to the pituitary

gland, where it triggers the

release of adrenocorticotropic

hormone (ACTH). ACTH travels

in the blood to the adrenal

glands, where it stimulates the

release of cortisol.

processes such as metabolic rate and sexual functions.

Major stress hormones are epinephrine (also known as adre-

naline) and cortisol.When the body is exposed to stressors, epi-

nephrine, which combines elements of hormones and the ner-

vous system, is quickly released into the bloodstream to put the

body into a general state of arousal and enable it to cope with a

challenge.

The adrenal glands secrete glucocorticoids, which are hor-

mones a∑ecting glucose metabolism. In primates, the main glu-

cocorticoid is cortisol (hydrocortisone), whereas in rodents, it is

corticosterone. Some of the actions of glucocorticoids help to

mediate the stress response, while some of the other, slower

actions counteract the primary response to stress and help

reestablish homeostasis. Over the short run, adrenaline mobilizes

energy and delivers it to muscles for the body’s response. With

prolonged exposure, cortisol enhances feeding and helps the

body recover from energy mobilization. Cortisol also raises blood

pressure and increases the risk of adult-onset diabetes, immuno-

suppression, reproductive impairments, and depression, among

other di≈culties.

Acute stress enhances memory of threatening situations and

events, increases activity of the immune system, and helps pro-

tect the body from pathogens. The two major stress hormones,

cortisol and adrenaline, facilitate the movement of immune cells

from the bloodstream and storage organs such as the spleen into

tissue where they are needed to defend against infection.

Glucocorticoids also a∑ect food intake during the sleep-wake

cycle. Cortisol levels peak in the body in the early morning hours

just before waking. This hormone acts as a wake-up signal and

helps to turn on appetite and physical activity. This e∑ect of glu-

cocorticoids may help to explain disorders such as jet lag, which

results when the light-dark cycle is altered by travel over long dis-

tances, causing the body’s biological clock to reset itself more

slowly. Until that clock is reset, cortisol secretion and hunger, as

well as sleepiness and wakefulness, occur at inappropriate times

of day in the new location.

Glucocorticoids do more than help the body respond to

stress. In fact, they are an integral part of daily life and the adap-

tation to environmental change. The adrenal glands help protect

us from stress and are essential for survival.

Chronic stress

When glucocorticoids or adrenaline are secreted in response to

the prolonged psychological stress commonly encountered by

humans, the results are not ideal. Normally, bodily systems gear

up under stress and release hormones to improve memory,

increase immune function, enhance muscular activity, and

restore homeostasis. If you are not ﬁghting or ﬂeeing, but stand-

ing frustrated in a supermarket checkout line or sitting in a

tra≈c jam, you are not engaging in muscular exercise. Yet these

systems continue to be stimulated, and when they are stimulated

chronically, there are di∑erent consequences: Memory is

impaired, immune function is suppressed, and energy is stored

as fat.

Overexposure to cortisol also can lead to weakened muscles

and the suppression of major bodily systems. Elevated epineph-

rine production increases blood pressure. Together, elevated cor-

tisol and epinephrine can contribute to chronic hypertension

(high blood pressure), abdominal obesity, and atherosclerosis

(hardening of the arteries). Adrenaline also increases the activity

of body chemicals that contribute to inﬂammation, and these

chemicals add to the burden of chronic stress, potentially leading

to atherosclerosis, arthritis, and possibly also aging of the brain.

Scientists have identiﬁed a variety of stress-related disorders,

including colitis, high blood pressure, clogged arteries, impotency

and loss of sex drive in males, irregular menstrual cycles in

females, adult-onset diabetes, and possibly cancer. Aging rats

show impairment of neuronal function in the hippocampus—

an area of the brain important for learning, memory, and emo-

tion—as a result of cortisol secretion throughout their lifetimes.

Overexposure to glucocorticoids also increases the number of

neurons damaged by stroke. Moreover, prolonged exposure

before or immediately after birth can cause a decrease in the nor-

mal number of brain neurons and smaller brain size.

The immune system,which receives messages from the ner-

vous system, also is sensitive to many of the circulating hormones

of the body, including stress hormones. Moderate to high levels

of glucocorticoids act to suppress immune function, although

acute elevations of stress hormones actually facilitate immune

function.

Although acute stress-induced immunoenhancement can be

protective against disease pathogens, the glucocorticoid-induced

immunosuppression can also be beneﬁcial. It reduces inﬂamma-

tion and counteracts allergic reactions and autoimmune responses,

which occur when the body’s defenses turn against body tissue.

Synthetic glucocorticoids like hydrocortisone and prednisone are

used often to decrease inﬂammation and autoimmunity. But glu-

cocorticoids may be harmful in the case of increased tumor

growth associated with stress in experiments on animals—an

area of intense research yet to yield any ﬁnal conclusions.

One important determinant of the immune system’s resis-

tance or susceptibility to disease may be a person’s sense of con-

trol as opposed to a feeling of helplessness. This phenomenon

may help explain large individual variations in response to dis-

ease. Scientists are trying to identify how the perception of con-

trol or helplessness inﬂuences physiological processes that regu-

late immune function.

The cardiovascular system receives many messages from the

autonomic nervous system, and stressful experiences have an

immediate and direct e∑ect on heart rate and blood pressure. In

the short run, these changes help in response to stressors. But

when stressors are chronic and psychological, the e∑ect can be

harmful and result in accelerated atherosclerosis and increased

risk for heart attack. Research supports the idea that people hold-

ing jobs that carry high demands and low control, such as tele-

phone operators, waiters, and cashiers, have higher rates of heart

disease than people who can dictate the pace and style of their

working lives.

Behavioral type a∑ects a person’s susceptibility to heart

attack. People at greatest risk are hostile, irritated by trivial things,

and exhibit signs of struggle against time and other challenges.

Researchers found that two groups of men—one with high

hostility scores and the other with low hostility scores—exhib-

ited similar increases in blood pressure and muscle blood ﬂow

when performing a lab test. This ﬁnding conﬁrmed that hostility

scores do not predict the biological response to simple mental

tasks.

Then the researchers added harassment to the test by leading

the subjects to believe that their performances were being

unfairly criticized. Men with high hostility scores showed much

larger increases in muscle blood ﬂow and blood pressure and

showed slower recovery than those with low hostility scores. Sci-

entists found that harassed men with high hostility scores had

larger increases in levels of stress hormones. Thus, if you have

personality traits of hostility, learning to reduce or avoid anger

could be important to avoid cardiovascular damage.

Peuroscientists believe that the brain can remain

relatively healthy and fully functioning as it ages

and that diseases cause the most severe decline in

memory, intelligence, verbal ﬂuency, and other

tasks. Researchers are investigating the normal

changes that occur over time and their e∑ect on

reasoning and other intellectual activities.

It appears that the e∑ects of age on brain function vary

widely. Almost everyone gets a bit forgetful in old age, particu-

larly in forming memories of recent events. For example, once

you reach your 70s, you may start to forget names or phone num-

bers or respond more slowly to conﬂicting information. This is

not disease. However, other individuals develop senile dementia,

the progressive and severe impairment in mental function that

interferes with daily living. The senile dementias include

Alzheimer’s and cerebrovascular diseases and a∑ect about 1 per-

cent of people younger than age 65, with the incidence increas-

ing to nearly 50 percent in those older than 85. In a small, third

group, mental functioning seems una∑ected by age. Many peo-

ple do well throughout life and continue to do well even when

old. The oldest human, Jeanne Calment, kept her wits through-

out her 122-year lifespan.

It’s important to understand that scientiﬁc studies measure

trends and reﬂect what happens to the norm—they don’t tell

what happens to everybody. Some people in their 70s and 80s

function as well as those in their 30s and 40s. The wisdom and

experience of older people often make up for deﬁcits in perfor-

mance.

The belief that pronounced and progressive mental decline

is inevitable was and still is popular for several reasons. For one,

until the 20th century, few people lived past 65. In 1900, when

average life expectancy was about 47 years, 3 million people, or 4

percent of the population, were older than age 65 and were typ-

ically ill. In 2003, when life expectancy was more than 77 years,

nearly 36 million people, or more than 12 percent of the popula-

tion, were older than age 65. A generation ago, frailty was seen

among people in their 60s; today it is more typical among those

in their 80s. Moreover, few people challenged the notion that

aging meant inevitable brain decline because scientists knew lit-

tle about the brain or the aging process. Today’s understanding

of how the normal brain ages comes from studies of the nervous

system that began decades ago and are just now bearing results.

Modern technologies now make it possible to explore the struc-

ture and function of the brain in more depth than ever before

and to ask questions about what actually happens in its aging

cells.

Thus, neuroscientists are increasingly able to distinguish

between the processes of normal aging and disease. Although

some changes do occur in normal aging, they are not as severe as

scientists once thought.

All human behavior is determined by how well the brain’s

communication systems work. Often a failure in the cascade of

one of these systems results in a disturbance of normal function.

Such a failure may be caused by an abnormal biochemical process

or a loss of neurons.

The cause of brain aging still remains a mystery. Dozens of

theories abound. One says that speciﬁc “aging genes”are switched

on at a certain time of life. Another points to genetic mutations

or deletions. Other theories implicate hormonal inﬂuences, an

immune system gone awry, and the accumulation of damage

caused by free radicals,cell byproducts that destroy fats and pro-

teins vital to normal cell function.

Aging neurons

The brain reaches its maximum weight near age 20 and slowly

loses about 10 percent of its weight over a lifetime. Subtle changes

in the chemistry and structure of the brain begin at midlife for

most people. During a lifetime, the brain is at risk for losing some

of its neurons, but widespread neuron loss is not a normal

process of aging. Brain tissue can respond to damage or loss of

neurons in Alzheimer’s disease or after stroke by expanding den-

drites and ﬁne-tuning connections between neurons. A damaged

brain neuron can readjust to damage only if its cell body remains

intact. If it does, regrowth can occur in dendrites and axons.

When neurons are destroyed, nearby surviving neurons can com-

pensate, in part, by growing new dendrites and connections.

Intellectual capacity

From the ﬁrst large studies to monitor the same group of healthy

humans for many years, scientists have uncovered unexpected

Aging

results. They report declines in some mental functions and

improvements in others. In several studies, the speed of carrying

out certain tasks became slower, but vocabulary improved. Sev-

eral studies found less severe declines in the type of intelligence

relying on learned or stored information compared with the type

that uses the ability to deal with new information.

This research is supported by animal studies in which scien-

tists found that changes in mental function are subtle. For exam-

ple, in rodents and primates in which only minor brain abnor-

malities can be detected, certain spatial tasks, such as navigating

to ﬁnd food, tend to become more di≈cult with age.

The aging brain is only as resilient as its circuitry. Scientists

debate whether this circuitry is changed only by neuron atrophy

or whether some neuron loss over time also is inevitable. In any

event, when the circuitry begins to break down, remaining neu-

rons can adapt by expanding their roles.

Learning conditions may dictate what happens to brain cells.

Studies of rats shed light on some of the changes that occur in

brain cells when the animals live in challenging and stimulating

environments. In tests of middle-aged rats exposed to such envi-

ronments, researchers found that dendrites in the cerebral cor-

tex developed more and longer branches than did rats housed in

isolated conditions. Another study showed that brain cells in rats

given acrobatic training had more synapses per cell than rats

given only physical exercise or rats that were inactive. The scien-

tists concluded that motor learning generates new synapses.

Physical exercise, however, improved blood circulation in the

brain. Aerobic exercise can also improve human cognitive per-

formance.

Other scientists report that rats reared in a stimulating envi-

ronment made signiﬁcantly fewer errors in a maze test than did

similar rats kept in an isolated environment. Moreover, the stim-

ulated rats showed an increase in brain weight and cortical thick-

ness compared with animals in the control group.

In response to enriched environments, older rats tend to

form new dendrites and synapses, just as younger animals do. But

the response is more sluggish and not as large. Compared with

younger rats, older rats have less growth of the new blood vessels

that nourish neurons.

Although much has been learned about the aging brain,

many questions remain. For instance, does the production of

proteins decline with age in all brain neurons? In a given neuron,

does atrophy cause a higher likelihood of death? How does aging

a∑ect gene expression in the brain—the organ with the greatest

number of active genes? Are there gender di∑erences in brain

aging that may be due to hormonal changes at menopause?

Neuroscientists speculate that certain genes may be linked to

events leading to death in the nervous system. By understanding

the biology of the proteins produced by genes, scientists hope to

be able to inﬂuence the survival and function of neurons.

THE AGING BRAIN. Studies

of people who have died contra-

dict the popular belief that

adults lose an enormous number

of neurons every day. In fact,

many areas of the brain, pri-

marily in the cortex, maintain

most of their neurons. Examples

include the parietal cortex,

which plays a role in sensory

processes and language, and

the striate cortex, which

processes visual information.

The connectivity between neu-

rons changes with aging, so that

the brain is constantly capable

of being modified or improved.

ipolar disorder. Patients with bipolar disor-

der, previously known as manic-depressive ill-

ness, usually experience episodes of deep depres-

sion and manic highs, with a return to relatively

normal functioning in between. They also have

an increased risk of suicide. Bipolar disorder

a∑ects 1.2 percent of Americans age 18 or older annually, or 2.2

million individuals. Approximately equal numbers of men and

women su∑er from this disorder.

Bipolar disorder tends to be chronic, and episodes can be-

come more frequent without treatment. Because bipolar disor-

der runs in families, e∑orts are underway to identify the respon-

sible gene or genes.

Bipolar patients can beneﬁt from a broad array of treatments.

One of these is lithium. During the 1940s, researchers showed that

lithium injections into guinea pigs made them placid, which

implied mood-stabilizing e∑ects. When given to manic patients,

lithium calmed them and enabled them to return to work and

live relatively normal lives. Regarded as both safe and e∑ective,

lithium is often used to prevent recurrent episodes.

Other useful medications include certain anticonvulsants, such

as valproate or carbamazepine, which can have mood-stabilizing

e∑ects, like lithium, and may be especially useful for di≈cult-to-

treat bipolar episodes. Newer anticonvulsant medications are being

studied to determine how well they work in stabilizing mood cycles.

Epilepsy

Epilepsy, a chronic neurological disorder characterized by sudden,

disorderly discharge of brain cells, is marked by recurrent, unpro-

voked seizures that temporarily alter one or more brain functions.

The disorder a∑ects approximately 1 percent of the population.

Many di∑erent types of epilepsy have been recognized.

Epilepsy can start at any age and can be idiopathic (having an

uncertain cause) or symptomatic. Most idiopathic epilepsies are

likely due to inheriting a mutant gene, more than a dozen of

which have been identiﬁed during the last decade. Symptomatic

epilepsies result from a wide variety of brain diseases or

injuries, including birth trauma, brain infection such as abscess

or meningitis, brain tumors, and stroke.

Seizures are of two types, generalized and partial. Generalized

seizures, which typically result in loss of consciousness, can cause

several behavioral changes, including convulsions or sudden

changes in muscle tone, and arise when there is simultaneous exces-

sive electrical activity over a wide area of the brain. Partial seizures

may occur with maintained consciousness or with altered aware-

ness and behavioral changes. Partial seizures can produce localized

visual, auditory, and skin sensory disturbances; repetitive uncon-

trolled movements; or confused, automatic behaviors. Such seizures

arise from excessive electrical activity in a limited area of the brain.

There are more than a dozen antiseizure medications, approx-

imately half of which have been introduced in the last several

years, available to prevent seizures. The principal targets of anti-

seizure drugs are voltage-gated ion channels permeable to sodium

or calcium and synapses using the transmitter GABA, a naturally

occurring substance in the brain that acts to inhibit electrical dis-

charge. Identiﬁcation of the mutant genes underlying human

epilepsy may provide new targets for the next generation of anti-

seizure drugs. In many instances, epilepsy can be controlled with

a single antiseizure drug that lessens the frequency of seizures, but

sometimes a combination of drugs is necessary. Complete control

of seizures can be achieved in more than 50 percent of patients,

and another 25 percent can be improved signiﬁcantly. It is hoped

that the newly available antiseizure drugs will provide complete

control in additional patients.

Surgery, considered for patients who do not respond to anti-

seizure drugs, should be performed only at specialized medical

centers qualiﬁed to evaluate patients and perform epilepsy

surgery. Epilepsy surgery requires precise location and removal

of the area of the brain where the seizures originate. After surgery,

about 90 percent of properly selected patients experience strik-

ing improvement or complete remission of seizures.

A new form of epilepsy treatment, electrical stimulation ther-

apy, was introduced during the mid-1990s as another option for

hard-to-control seizures. The implantable pacemaker-like device

delivers small bursts of electrical energy to the brain via the vagus

nerve on the side of the neck.

Major depression

This condition, with its harrowing feelings of sadness, hopeless-

ness, pessimism, loss of interest in life, and reduced emotional

Advances

well-being, is one of the most common and debilitating mental

disorders and one of the leading causes of morbidity worldwide.

Depression is as disabling as heart disease or arthritis. Depressed

individuals are 18 times more likely to attempt suicide than peo-

ple with no mental illness.

Annually, major depression a∑ects 5 percent of the popula-

tion, or 9.8 million Americans, aged 18 years and older. Fortu-

nately, 80 percent of patients respond to drugs, psychotherapy, or

a combination of the two. Some severely depressed patients can

be helped with electroconvulsive therapy.

Depression arises from many causes: biological (including

genetic), psychological, environmental, or a combination of

these. Stroke, hormonal disorders, antihypertensives, and birth

control pills also can play a part.

Physical symptoms—disturbances of sleep, sex drive, energy

level, appetite, and digestion—are common. Some of these

symptoms may reﬂect the fact that the disorder a∑ects the deli-

cate hormonal feedback system linking the hypothalamus, the

pituitary gland, and the adrenal glands. For example, many

depressed patients secrete excess cortisol, a stress hormone, and

do not respond appropriately to a hormone that should counter

cortisol secretion. When tested in sleep laboratories, depressed

patients’ electroencephalograms (EEGs) often exhibit abnormal-

ities in their sleep patterns.

The modern era of drug treatment for depression began in

the late 1950s. Most antidepressants a∑ect norepinephrine or

serotonin in the brain, apparently by correcting the abnormal sig-

nals that control mood, thoughts, and other sensations. The tri-

cyclic antidepressants, such as imipramine, primarily block the

reabsorption and inactivation of serotonin and norepinephrine

to varying degrees. Another class of antidepressant medications

is the monoamine oxidase inhibitors (MAOIs). These agents

inhibit monoamine oxidase, an enzyme that breaks down sero-

tonin and norepinephrine, allowing these chemicals to remain

active. MAOIs available for use include isocarboxazid, phenel-

zine, and tranylcypromine.

The popular medication ﬂuoxetine (Prozac) is the ﬁrst of a

class of drugs called selective serotonin reuptake inhibitors, or

SSRIs. SSRIs block the reabsorption and inactivation of serotonin

and keep it active in certain brain circuits. Hence, they are func-

tionally similar to the tricyclic antidepressants, but act selectively

on the serotonin system. There are also several newer antidepres-

sants available, such as bupropion, that are also very e∑ective but

seem to have a di∑erent and as yet unknown mechanism of action.

Pain

If there is a universal experience, pain is it. Each year, more than

97 million Americans su∑er chronic, debilitating headaches or a

bout with a bad back or the pain of arthritis—all at a total cost

of some $100 billion. But it need not be that way. New discover-

ies about how chemicals in the body transmit and regulate pain

messages have paved the way for new treatments for both chronic

and acute pain.

Until the mid-19th century, pain relief during surgery relied

on natural substances, such as opium, alcohol, and cannabis. All

were inadequate and short-lived. Not until 1846 did doctors dis-

HOW PAIN KILLERS

WORK. At the site of injury,

the body produces prosta-

glandins that increase pain sen-

sitivity. Aspirin, which acts

primarily in the periphery, pre-

vents the production of prosta-

glandins. Acetaminophen is

believed to block pain impulses

in the brain itself. Local anes-

thetics intercept pain signals

traveling up the nerve. Opiate

drugs, which act primarily in the

central nervous system, block

the transfer of pain signals from

the spinal cord to the brain.

cover the anesthetic properties of ether, ﬁrst in animals and then

in humans. Soon, the usefulness of chloroform and nitrous oxide

became known and heralded a new era in surgery. The dozens of

drugs used today during surgery abolish pain, relax muscles, and

induce unconsciousness. Other agents reverse these e∑ects.

Local anesthesia is used in a limited area of a person’s body to

prevent pain during examinations, diagnostic procedures, treat-

ments, and surgical operations. The most famous of these agents,

which temporarily interrupt the action of pain-carrying nerve

ﬁbers, is Novocain, which dentists used as a local anesthetic for

many years; lidocaine is more popular today. Very recently, lido-

caine has been used in a slow-release patch to provide long-last-

ing pain in localized, speciﬁc parts of the body.

Analgesia refers to loss of pain sensation without loss of sen-

sitivity to touch. The two main types of analgesics are nonopioids

(aspirin and related nonsteroidal anti-inﬂammatory drugs

[NSAIDs] such as ibuprofen, naproxen, and acetaminophen) and

opioids (morphine, codeine). Nonopioid analgesics are useful for

treating mild or moderate pain, such as headache or toothache.

Because NSAIDs are anti-inﬂammatory, they are e∑ective for

treating such inﬂammatory conditions as arthritis. Moderate

pain also can be treated by combining a mild opioid, such as

codeine, with aspirin. Opioids are the most potent painkillers and

are used for severe pain, such as that occurring after major chest

or abdominal surgery.

Studies of the body’s own pain-control system not only

demonstrated the existence of naturally occurring opioids (the

endorphins) but also identiﬁed the receptors (targets) through

which opioids exert their e∑ects. These ﬁndings led to the use of

injections of morphine and endorphins, and other opioids, into

the cerebrospinal ﬂuid (in which the spinal cord is bathed) with-

out causing paralysis, numbness, or other severe side e∑ects. This

technique came about through experiments with animals that

ﬁrst showed that injecting opioids into the spinal cord could pro-

duce profound pain control. This technique is now commonly

used in humans to treat pain after surgery and is a mainstay for

pain relief after caesarean section.

Although NSAIDs and opioids are quite e∑ective for pain

produced by tissue injury, they are much less e∑ective when the

pain results from injury to the nervous system. These so-called

neuropathic pains include the pain of diabetic neuropathy, post-

therapeutic neuralgia, phantom limb pain, and post-stroke pain.

For these pains, anticonvulsants are more e∑ective, and some

patients can be helped with low doses of antidepressants.

New targets, however, are on the horizon. Molecular biology

has identiﬁed many molecules (ion channels and receptors) that

are predominantly, if not exclusively, expressed by the nociceptor,

which is the ﬁrst-order nerve ﬁber in the pain pathway. Because

adverse side e∑ects of drugs arise from the widespread location

of the molecules targeted by analgesics (e.g., constipation results

from morphine’s action on opioid receptors in the gut), new anal-

gesics that target only the nociceptor may have a better side-e∑ect

proﬁle. Among the many new targets are glutamate receptors,

vanilloid receptors (which are targeted by capsaicin, the active

ingredient in hot peppers), and a variety of acid-sensing ion chan-

nels. Blocking the activity of many of these molecules has proven

e∑ective in animal studies, suggesting that the development of

drugs that target these molecules in humans may have great value

for the treatment of persistent pain.

Parkinson’s disease

This neurologic disorder aΩicts 1 million individuals in the

United States, most of whom are older than 50. Parkinson’s dis-

ease is characterized by symptoms of slowness of movement,

muscular rigidity, tremor, and postural instability.

The discovery in the late 1950s that the level of dopamine was

decreased in the brains of Parkinson’s patients was followed in

the 1960s by the successful treatment of this disorder by admin-

istration of the drug levodopa, which is converted to dopamine

in the brain. The successful treatment of Parkinson’s by replace-

ment therapy is one of the greatest success stories in neurology.

Levodopa is now combined with another drug, carbidopa, that

reduces the peripheral breakdown of levodopa, thus allowing

greater levels to reach the brain and reducing side e∑ects. Also

playing an important role are newer drugs, such as inhibitors of

dopamine breakdown and dopamine agonists, that act directly

on dopamine receptors.

Genetic studies have demonstrated several heritable gene

abnormalities in certain families, but almost all cases of Parkin-

son’s occur sporadically. It is believed that hereditary factors may

render some individuals more vulnerable to environmental fac-

tors such as pesticides. The discovery in the late 1970s that a

chemical substance, MPTP, can cause parkinsonism in drug

addicts stimulated intensive research on the causes of the disor-

der. MPTP was accidentally synthesized by illicit drug designers

seeking to produce a heroin-like compound. MPTP was found to

be converted in the brain to a substance that destroys dopamine

neurons. Parkinson’s is now being intensively studied in primate

MPTP models.

In the past several decades, scientists have shown in primate

models of Parkinson’s that speciﬁc regions in the basal ganglia,

collections of cell bodies deep in the brain, are abnormally over-

active. Most importantly, they found that surgical destruction of

these overactive nuclei—the pallidum and subthalamic

nucleus—can greatly reduce symptoms of Parkinson’s disease.

The past decade has witnessed a resurgence in this surgical pro-

cedure, pallidotomy, and more recently chronic deep-brain stim-

ulation. These techniques are highly successful for treating

patients who have experienced signiﬁcant worsening of symp-

toms and are troubled by the development of drug-related invol-

untary movements. The past decade has also seen further

attempts to treat such patients with surgical implantation of cells,

such as fetal cells, capable of producing dopamine. Replacement

therapy with stem cells also is being explored.

Addiction. Drug abuse is one of the nation’s

most serious health problems. Indeed, 6 per-

cent of Americans, roughly 15 million people,

abuse drugs on a regular basis. Recent esti-

mates show that the abuse of drugs, including

alcohol and nicotine from tobacco, costs the

nation more than $276 billion each year.

If continued long enough, drug abuse—often deﬁned as

harmful drug use—can eventually alter the very structure and

chemical makeup of the brain, producing a true brain disorder.

This disorder is called drug addiction or drug dependence.Drug

addiction is deﬁned as having lost control over drug taking, even

in the face of adverse physical, personal, or social consequences.

People abuse drugs for a simple reason: Drugs produce feel-

ings of pleasure or remove feelings of stress and emotional pain.

Neuroscientists have found that almost all abused drugs produce

pleasure by activating a speciﬁc network of neurons called the

brain reward system.The circuit is normally involved in an

important type of learning that helps us to stay alive. It is acti-

vated when we fulﬁll survival functions, such as eating when we

are hungry or drinking when we are thirsty. In turn, our brain

rewards us with pleasurable feelings that teach us to repeat the

task. Because drugs inappropriately turn on this reward circuit,

people want to repeat drug use.

Neuroscientists also have learned speciﬁcally how drugs a∑ect

neurons to exert their inﬂuence. Neurons release special chemi-

cals, called neurotransmitters,to communicate with each other.

Abused drugs alter the ways neurotransmitters carry their mes-

sages from neuron to neuron. Some drugs mimic neurotrans-

mitters, whereas others block them. Still others alter the way that

the neurotransmitters are released or inactivated. The brain

reward system is inappropriately activated because drugs alter the

chemical messages sent among neurons in this circuit.

Finally, neuroscientists also have learned that addiction

requires more than the activation of the brain reward system. The

process of becoming addicted appears to be inﬂuenced by many

factors. Motivation for drug use is an important one. For exam-

ple, people who take drugs to get high may get addicted, but peo-

ple who use them properly as medicine rarely do. Genetic sus-

ceptibility or environmental factors, such as stress, may also alter

the way that people respond to drugs. In addition, the develop-

ment of tolerance—the progressive need that accompanies

chronic use for a higher drug dose to achieve the same e∑ect—

varies in di∑erent people, as does drug dependence—the adap-

tive physiological state that results in withdrawal symptoms when

drug use stops. Tolerance and dependence are standard responses

of the brain and body to the presence of drugs. However, addic-

tion requires that these occur while a motivational form of depen-

dence—the feeling that a person can’t live without a drug,

accompanied by negative a∑ective states—is also developing.

Together, these insights on abuse and addiction are leading to

new therapies.

Nicotine In 2003, more than 70 million people smoked, at

least occasionally, making nicotine one of the most widely abused

substances. Tobacco kills more than 430,000 U.S. citizens each

year—more than alcohol, cocaine, heroin, homicide, suicide, car

accidents, ﬁre, and AIDS combined. Tobacco use is the leading

preventable cause of death in the United States. Smoking is

responsible for approximately 7 percent of total U.S. health care

costs, an estimated $80 billion each year. The direct and indirect

costs of smoking are estimated at more than $138 billion per year.

Nicotine, the addicting substance in tobacco, acts through the

well-known cholinergic nicotinic receptor. This drug can act as

both a stimulant and a sedative. Immediately after exposure to

nicotine, there is a “kick” caused in part by the drug’s stimulation

of the adrenal glands and resulting discharge of epinephrine. The

rush of adrenaline stimulates the body and causes a sudden

release of glucose as well as an increase in blood pressure, respi-

ration, and heart rate. Nicotine also suppresses insulin output

from the pancreas, which means that smokers are always slightly

hyperglycemic. In addition, nicotine releases dopamine in the

brain regions that control pleasure and motivation. This mecha-

nism is thought to underlie the pleasurable sensations experi-

enced by many smokers.

Much better understanding of addiction, coupled with the

identiﬁcation of nicotine as an addictive drug, has been instru-

mental in the development of treatments. Nicotine gum, the

transdermal patch, nasal spray, and inhalers all appear to be

equally e∑ective in treating more than 1 million people addicted

to nicotine. These techniques are used to relieve withdrawal

Challenges

symptoms and produce less severe physiological alterations than

tobacco-based systems. They generally provide users with lower

overall nicotine levels than they receive with tobacco, as well as

totally eliminating exposure to smoke and its deadly contents.

The ﬁrst non-nicotine prescription drug, bupropion, an antide-

pressant marketed as Zyban, has been approved for use as a phar-

macological treatment for nicotine addiction. Behavioral treat-

ments are important for helping an individual learn coping skills

for both short- and long-term prevention of relapse.

Psychostimulants In 2003, there were an estimated 2.3 mil-

lion chronic cocaine users and 5.9 million occasional cocaine

users in the United States. A popular chemically altered form of

cocaine, crack, is smoked. It enters the brain in seconds, pro-

ducing a rush of euphoria and feelings of power and self-

conﬁdence. The key biochemical factor that underlies the rein-

forcing e∑ects of psychostimulants is the brain chemical

dopamine. We feel pleasure when dopamine-containing neurons

release dopamine into speciﬁc brain areas that include a special

portion of the nucleus accumbens.Cocaine and amphetamines

produce intense feelings of euphoria by increasing the amount

of dopamine that is available to send messages within the brain

reward system.

Cocaine users often go on binges, consuming a large amount

of the drug in just a few days. A crash occurs after this period of

intense drug-taking and includes symptoms of emotional and

physical exhaustion and depression. These symptoms may result

from an actual crash in dopamine function and the activity of

another brain chemical, serotonin, as well as an increased

response of the brain systems that react to stress. Vaccines to

produce antibodies to cocaine in the bloodstream are in clinical

trials.

Opiates Humans have used opiate drugs, such as morphine,

for thousands of years. Monkeys and rats readily self-administer

heroin or morphine and, like humans, will become tolerant and

physically dependent with unlimited access. Withdrawal symp-

toms range from mild ﬂu-like discomfort to major physical ail-

ments, including severe muscle pain, stomach cramps, diarrhea,

and unpleasant mood.

Opiates, like psychostimulants, increase the amount of

dopamine released in the brain reward system and mimic the

e∑ects of endogenous opioids such as opioid peptides. Heroin

injected into a vein reaches the brain in 15 to 20 seconds and

binds to opiate receptors found in many brain regions, including

the reward system. Activation of the receptors in the reward cir-

cuits causes a brief rush of intense euphoria, followed by a cou-

ple of hours of a relaxed, contented state.

Opiates create e∑ects like those elicited by the naturally

occurring opioid peptides. They relieve pain, depress breathing,

cause nausea and vomiting, and stop diarrhea—important med-

ical uses. In large doses, heroin can make breathing shallow or

stop altogether—the cause of death in thousands of people who

have died of heroin overdose.

BRAIN DRUG REWARD SYSTEMS. Scientists are not certain

about all the structures involved in the human brain reward system.

However, studies of rat and monkey brains, and brain imaging stud-

ies in humans, have provided many clues. These illustrations show

what areas are most likely part of the reward systems in the human

brain. A central group of structures is common to the actions of all

drugs. These structures include a collection of dopamine-containing

neurons found in the ventral tegmental area. These neurons are

connected to the nucleus accumbens and other areas, such as the

prefrontal cortex. Cocaine exerts its effects mainly through this sys-

tem. Opiates act in this system and many other brain regions, includ-

ing the amygdala, that normally use opioid peptides. Opioids are

naturally occurring brain chemicals that induce the same actions as

drugs, such as heroin and morphine. Alcohol activates the core

reward system and additional structures throughout the brain because

it acts where GABA and glutamate are used as neurotransmitters.

GABA and glutamate are widely distributed in the brain, including

the cortex, hippocampus, amygdala, and nucleus accumbens.

A standard treatment for opiate addiction involves metha-

done,a long-acting oral opiate that helps keep craving, with-

drawal, and relapse under control. Methadone helps opiate

addicts rehabilitate themselves by preventing withdrawal symp-

toms that are powerful motivators of drug use. A synthetic opi-

ate, known as LAAM, can exert its e∑ects on heroin for up to 72

hours with minimal side e∑ects when taken orally. In 1993, the

Food and Drug Administration approved the use of LAAM for

treating patients addicted to heroin. Its long duration of action

permits dosing just three times per week, eliminating the need

for daily dosing. LAAM will be increasingly available in clinics

that already dispense methadone. Naloxone and naltrexone are

medications that also block the e∑ects of morphine, heroin, and

other opiates. As antagonists, they are especially useful as anti-

dotes. Another medication to treat heroin addiction, buprenor-

phine, causes weaker opiate e∑ects and is less likely to cause over-

dose problems. Buprenorphine is expected to become an

important treatment.

Alcohol Although legal, alcohol is highly addictive. Alcohol

abuse and alcohol addiction—sometimes referred to as alco-

holism or alcohol dependence—are the nation’s major drug

problem, with some people being more susceptible than others.

Nearly 14 million people abuse alcohol or are alcoholic. Fetal alco-

hol syndrome,a∑ecting about 0.5 to 3 of every 1,000 babies born

in the United States, is the leading preventable cause of mental

retardation. Chronic liver diseases, including cirrhosis—the main

HOW CRACK COCAINE AFFECTS THE BRAIN. Crack cocaine takes the same route as nicotine by entering the bloodstream through the

lungs. Within seconds, it is carried by the blood to the brain. The basis for increased pleasure occurs at the gap where the impulses that repre-

sent neural messages are passed from one neuron to another. This gap is called a synapse. Dopamine-containing neurons normally relay their

signals by releasing dopamine into many synapses. Dopamine crosses the synapse and fits into receptors on the surface of the receiving cell.

This triggers an electrical signal that is relayed through the receiver. Then, to end the signal, dopamine molecules break away from the recep-

tors and are pumped back into the nerve terminals that released them. Cocaine molecules block the pump or “transporter,” causing more

dopamine to accumulate in the synapse. Pleasure circuits are stimulated again and again, producing euphoria.

chronic health problem associated with alcohol addiction—are

responsible for more than 25,000 deaths each year. The annual

cost of alcohol abuse and addiction is estimated at $185 billion.

Genetic and environmental factors contribute to alcoholism,

but no single factor or combination of factors enables doctors to

predict who will become an alcoholic.

Ethanol,the active ingredient in alcoholic beverages, reduces

anxiety, tension, and inhibitions. In low doses it may act as a

stimulant, whereas at higher doses, it acts as a depressant. In both

cases, it signiﬁcantly alters mood and behavior. It can also cause

heat loss and dehydration.

The drug, which is easily absorbed into the bloodstream and

the brain, a∑ects several neurotransmitter systems. For example,

alcohol’s interaction with the

GABA receptor can calm anxi-

ety, impair muscle control, and

delay reaction time. At higher

doses, alcohol also decreases the

function of NMDA receptors

that recognize the neurotrans-

mitter glutamate. This interac-

tion can cloud thinking and

eventually lead to coma.

Researchers are developing treatments that interfere with

molecules, such as the opioid peptides, that trigger alcohol’s pos-

itive reinforcing e∑ects. One such drug, naltrexone, recently has

been approved for treating alcoholism.

Marijuana This drug can distort perception and alter the

sense of time, space, and self. In certain situations, marijuana can

produce intense anxiety.

In radioactive tracing studies, scientists found that tetrahy-

drocannabinol (THC), the active ingredient in marijuana, binds

to speciﬁc receptors, many of which coordinate movement. This

may explain why people who drive after they smoke marijuana

are impaired. The hippocampus, a structure involved with mem-

ory storage and learning, also contains many THC receptors. This

may explain why heavy users or those intoxicated on marijuana

have poor short-term memory and problems processing complex

information. Scientists recently discovered that these receptors

normally bind to a natural internal chemical called anandamide,

and they are now working to see how this chemical a∑ects brain

function.

Club Drugs Ecstasy, herbal ecstasy, rohypnol (“rooﬁes”),

GHB, and ketamine are among the drugs used by some teens and

young adults as part of rave and trance events, which are gener-

ally night-long dances, often held in warehouses. The drugs are

rumored to increase stamina and to produce intoxicating highs

that are said to deepen the rave or trance experience. Recent hard

science, however, is uncovering the serious damage that can occur

in several parts of the brain from use of some of these drugs.

Many users tend to experiment with a variety of club drugs

in combination. This practice creates a larger problem, because

combinations of any of these drugs, particularly with alcohol,

can lead to unexpected adverse reactions and even death after

high doses. Physical exhaustion also can enhance some toxicities

and problems.

MDMA, called “Adam,” “ecstasy,” or “XTC” on the street, is a

synthetic, psychoactive drug with hallucinogenic and ampheta-

mine-like properties. Users encounter problems similar to those

found with the use of amphetamines and cocaine. Recent research

also may link ecstasy use to long-term damage to those parts of the

brain critical to thought, memory, and pleasure.

Rohypnol, GHB (gamma hydroxy-butyrate), and ketamine

are predominantly central nervous system depressants. Because

they are often colorless, tasteless, and odorless, they can be eas-

ily added to beverages and

ingested unknowingly. These

drugs have emerged as the so-

called date-rape drugs.When

mixed with alcohol, rohypnol

can incapacitate victims and

prevent them from resisting

sexual assault. Also, rohypnol

may be lethal when mixed with

alcohol and other depressants.

Since about 1990, GHB has been abused in the United States for

euphoric, sedative, and anabolic (body building) e∑ects. It, too,

has been associated with sexual assault. Ketamine is another cen-

tral nervous system depressant abused as a date-rape drug. Ket-

amine, or “Special K,” is a fast-acting general anesthetic. It has

sedative, hypnotic, analgesic, and hallucinogenic properties. It is

marketed in the United States and a number of foreign countries

for use as a general anesthetic in both human and veterinary

medical practice.

Attention deﬁcit hyperactivity disorder

Attention deﬁcit hyperactivity disorder (ADHD) was ﬁrst

described more than 100 years ago. Today it is the focus of hun-

dreds of studies. Characterized by excessively inattentive, hyper-

active, or impulsive behaviors, ADHD a∑ects an estimated 2 mil-

lion children in the United States, or 3 percent to 5 percent of

children. Studies show that 30 percent to 70 percent of these chil-

dren will continue to experience ADHD symptoms as adults.

By deﬁnition, symptoms of ADHD appear before age 7, last

for six months or longer, and impair normal functioning in at

least two areas of a child’s life—at school, among friends, or at

home, for example. Adults must show impairment at home and

at work. Normal children sometimes show similar behavior, and

other conditions, disorders, or environmental triggers—such as

novelty—may also be present with ADHD children; therefore,

diagnosis requires a comprehensive evaluation, using parent and

teacher rating scales, a clinical interview, and testing. Currently,

ADHD is diagnosed solely on the basis of behavioral symptoms.

Some studies show a correlation between ADHD and changes

Attention deficit hyperactivity

disorder (ADHD) was first described

more than 100 years ago. Today it is

the focus of hundreds of studies.

in brain structure, suggesting the possibility of using neuroimag-

ing techniques in the future to help identify targets for treatment

or to help distinguish ADHD symptoms from those stemming

from a learning disability. ADHD also is thought to have a strong

genetic inﬂuence.

In addition to behavioral therapy, ADHD is commonly

treated with medication—largely stimulants. Ritalin, one brand

name under which the stimulant methylphenidate is sold, is one

of the most widely prescribed drugs for treating ADHD. Its use

is controversial. Debate about treatment with Ritalin centers on

the beneﬁts of a more focused child, on the one hand, and doubts

about the long-term risks of exposing children to psychotropic

drugs, on the other.

Alzheimer’s disease

One of the most frightening and

devastating of all neurological

disorders is the dementia that

occurs in the elderly. The most

common cause of this illness is

Alzheimer’s disease (AD). Rare

before age 60 but increasingly

prevalent in each decade there-

after, AD a∑ects an estimated 4

to 5 million Americans. By the

year 2040, it is predicted to

a∑ect approximately 14 million individuals in the United States.

The earliest symptoms are forgetfulness and memory loss;

disorientation to time or place; and di≈culty with concentration,

calculation, language, and judgment. Some patients have severe

behavioral disturbances and may even become psychotic. The ill-

ness is progressive. In the ﬁnal stages, the a∑ected individual is

incapable of self-care. Unfortunately, no e∑ective treatments

exist, and patients usually die from pneumonia or some other

complication. AD, which kills 100,000 people a year, is one of the

leading causes of death in the United States.

In the earliest stages, the clinical diagnosis of possible or

probable AD can be made with greater than 80 percent accuracy.

As the course of the disease progresses, the accuracy of diagno-

sis at Alzheimer’s research centers exceeds 90 percent. The diag-

nosis depends on medical history, physical and neurological

examinations, psychological testing, laboratory tests, and brain

imaging studies. At present, ﬁnal conﬁrmation of the diagnosis

requires examination of brain tissue, usually obtained at autopsy.

The causes and mechanisms of the brain abnormalities are

not yet fully understood, but great progress has been made

through genetics, biochemistry, cell biology, and experimental

treatments. Microscopic examination of AD brain tissue shows

abnormal accumulations of a small ﬁbrillar peptide, termed a

beta amyloid,in the spaces around synapses (neuritic plaques) and

by abnormal accumulations of a modiﬁed form of the protein tau

in the cell bodies of neurons (neuroﬁbrillary tangles). The plaques

and tangles are mostly in brain regions important for memory

and intellectual functions.

In cases of AD, reductions occur in levels of markers for sev-

eral neurotransmitters, including acetylcholine, somatostatin,

monoamine, and glutamate, that allow cells to communicate with

one another. Damage to these neural systems, which are critical

for attention, memory, learning, and higher cognitive abilities, is

believed to cause the clinical symptoms.

Rare individuals with AD have a dominantly inherited form

of the disease. These patients often have early-onset illness.

Recently, scientists have identiﬁed mutations in AD-linked genes

on three chromosomes. The gene encoding the amyloid precur-

sor protein is on chromosome 21. In other families with early-

onset AD, mutations have been

identiﬁed in the presenilin 1 and

2 genes, which are on chromo-

somes 14 and 1, respectively.

Apolipoprotein E (apoE), a chro-

mosome 19 gene, which inﬂu-

ences susceptibility in late life,

exists in three forms, with apoE4

clearly associated with enhanced

risk.

Treatments are available

mostly only for some symptoms

of AD, such as agitation, anxiety,

unpredictable behavior, sleep disturbances, and depression. Three

drugs treat cognitive symptoms in patients with mild to moder-

ate Alzheimer’s. These agents improve memory deﬁcits tem-

porarily. Several other approaches, such as antioxidants, are being

tested.

An exciting area of research is the use of approaches in which

genes are introduced in mice. These transgenic mice carrying

mutant genes linked to inherited AD develop behavioral abnor-

malities and some of the cellular changes that occur in humans.

It is anticipated that these mice models will prove very useful for

studying the mechanisms of AD and testing novel therapies.

Moreover, researchers have begun to knock out genes play-

ing critical roles in the production of amyloid. These enzymes,

termed beta and gamma secretase, which cleave the amyloid pep-

tide from the precursor, are clearly targets for development of

drugs to block amyloid.

Amyotrophic lateral sclerosis

This progressive disorder strikes more than 5,000 Americans

annually, with an average survival time of just three to ﬁve years

from symptom onset. It is the most common disorder within a

group of diseases a∑ecting motor neurons and costs Americans

some $300 million annually.

Commonly known as Lou Gehrig’s disease, amyotrophic lat-

eral sclerosis (ALS) a∑ects neurons that control voluntary muscle

movements such as walking. For reasons that are not completely

One of the most frightening and

devastating of all neurological disorders

is the dementia that occurs in the

elderly. The most common cause of this

illness is Alzheimer's disease.

understood, motor neurons in the brain and spinal cord begin to

disintegrate. Because signals from the brain are not carried by

these damaged nerves to the body, the muscles begin to weaken

and deteriorate from the lack of stimulation and resulting disuse.

The ﬁrst signs of progressive paralysis are usually seen in the

hands and feet. They include weakness in the legs, di≈culty walk-

ing, and clumsiness of the hands when washing and dressing.

Eventually, almost all muscles under voluntary control, including

those of the respiratory system, are a∑ected. Despite the paralysis,

however, the mind and the senses remain intact. Death is usually

caused by respiratory failure or pneumonia.

No speciﬁc test identiﬁes ALS, but muscle biopsies, blood

studies, electrical tests of muscle activity, CT and MRI scans, and

X-rays of the spinal cord help

identify the disease and rule out

other disorders. Still, diagnosis

is often di≈cult because its

causes remain unknown. Poten-

tial causes or contributors to the

disease include glutamate toxic-

ity, oxidative stress, environ-

mental factors, and an autoim-

mune response in which the

body’s defenses turn against

body tissue.

In more than 90 percent of

cases, ALS is sporadic, arising in individuals with no known fam-

ily history of the disorder. In the other 5–10 percent of cases, ALS

is familial—transmitted to family members because of a gene

defect.

Scientists have now identiﬁed several genes that are respon-

sible for some forms of ALS. The most common and well stud-

ied of these are mutations in the gene that codes for superoxide

dismutase,located on chromosome 21, that were linked to the

presence of this disorder. Scientists believe that whatever they

learn from studying this gene and others will have relevance for

understanding the more common sporadic form of motor neu-

ron disease.

Once diagnosed, physical therapy and rehabilitation meth-

ods help strengthen unused muscles. Various drugs can ease

speciﬁc problems, such as twitching and muscle weakness, but

there is no cure. An anti-glutamate drug moderately slows the

disease. Additional drugs are now under study. Protecting or

regenerating motor neurons using nerve growth factors, other

more potent drugs, and stem cells may someday provide signiﬁ-

cant hope for patients.

Anxiety disorders

The most widespread mental illnesses, anxiety disorders annu-

ally a∑ect an estimated 12.6 percent of the adult population, or

24.8 million Americans. They include phobias such as fear of

heights, agoraphobia, and social anxiety disorder; generalized

anxiety disorder; post-traumatic stress disorder; panic disorder;

and obsessive-compulsive disorder (OCD). Some can keep peo-

ple completely housebound or, as in the case of panic disorder,

contribute to suicide. Many of these disorders occur with depres-

sion, and individuals so aΩicted are at high risk of suicide.

In OCD, people become trapped, often for many years, in

repetitive thoughts and behaviors, which they recognize as

groundless but cannot stop, such as repeatedly washing hands, or

checking that doors are locked or stoves turned o∑. The illness is

estimated to a∑ect 3.8 million Americans annually. Social learn-

ing and genetics likely play a role in developing the disorder.

Positron emission tomography (PET) scans reveal abnormalities

in both cortical and deep areas of the brain, suggesting central

nervous system changes in OCD

patients.

Scientists recently discov-

ered that certain breeds of large

dogs that develop acral lick syn-

drome,severely sore paws from

compulsive licking, respond to

the serotonergic antidepressant

clomipramine, which was the

ﬁrst e∑ective treatment devel-

oped for OCD in people.

Serotonergic antidepressants

—especially the tricyclics, such

as clomipramine, and the selective serotonin reuptake inhibitors

(SSRIs), such as sertraline (Zoloft) and paroxetine (Paxil)—are

e∑ective in treating OCD. A specialized type of behavioral inter-

vention, exposure and response prevention,is also e∑ective in many

patients.

Panic disorder, which a∑ects 2.4 million Americans annually,

usually starts “out of the blue.” Patients experience an overwhelm-

ing sense of impending doom, accompanied by sweating, weak-

ness, dizziness, and shortness of breath. With repeated attacks,

patients may develop anxiety in anticipation of another attack and

avoid public settings where attacks might occur. If these patients

are untreated, their lives may constrict until they develop agora-

phobia,becoming virtually housebound.

Phobia is an intense, irrational fear of a particular object or

situation. Individuals can develop phobias of almost anything,

such as dogs, dating, or driving over bridges. Exposure to the

feared object or situation can trigger an extreme fear reaction that

may include a pounding heart, shortness of breath, and sweating.

Experiencing or witnessing a crime or being a victim of sex-

ual abuse can lead to a form of stress that can last a lifetime.

Termed post-traumatic stress disorder, the condition aΩicts 5.2

million Americans aged 18 to 54 each year.

The recent discovery of brain receptors for the benzodi-

azepine antianxiety drugs has sparked research to identify the

brain’s own antianxiety chemical messengers. This ﬁnding may

lead to ways to regulate this brain system and correct its possible

The most widespread

mental illnesses, anxiety disorders

annually affect an estimated

12.6 percent of the adult population,

or 24.8 million Americans.

defects in panic anxiety disorders. PET scans reveal that during

such attacks, the tip of the brain’s temporal lobe is unusually

active compared with controls. When normal people expect to

receive a shock to the ﬁnger, the same general area is activated.

The SSRIs, the serotonin-norepinephrine reuptake inhibitors

(SSRNs), cognitive behavior therapy, or a combination of these

are now the ﬁrst-choice treatments of most anxiety disorders. Tri-

cyclic antidepressants, monoamine oxidase inhibitors (MAOIs),

and high-potency benzodiazepines are also e∑ective for many of

these disorders.

Autism

Almost one in every 166 babies born in the United States, or

about 1.5 million Americans, will develop some form of autism.

The numbers are staggering, considering that in the 1970s, the

estimates were just a few in 10,000 births. The rapid rise in inci-

dence is a key mystery to be solved.

Characterized by communication di≈culties, delayed devel-

opment of language, impaired social skills, communication di≈-

culties, and narrow, obsessive interests or repetitive behaviors,

autism is extremely isolating. There is no cure, but children with

autism respond well to a highly structured environment and spe-

cialized education and language intervention programs. The ear-

lier the interventions begin, the better the outcome.

Currently, autism is diagnosed on the basis of behavioral

symptoms. New research shows that brain imaging data is 95

percent accurate in identifying how the brains of individuals

with autism di∑er from those of typically developing young

children.

Research has also revealed that genetic factors contribute to

the development of autism. Success in identifying the so-called

“vulnerability” genes for autism may allow scientists to develop

an improved diagnostic technique that combines the detection

of behavioral indicators with biological abnormalities to better

identify infants and toddlers at risk for autism.

A clear understanding of the biological abnormalities that

alter brain development in autism could guide the formulation

of new therapies that target the disorder on a molecular level.

These research e∑orts will mean that health-care practitioners

will be better armed with the necessary tools for early diagnosis

and more e∑ective interventions.

Brain tumors

Although brain tumors are not always malignant—a condition

that spreads and becomes potentially lethal—these growths are

always serious because they can cause pressure in the brain and

compression of nearby structures, interfering with normal brain

activity.

Primary brain tumors arise within the brain, whereas sec-

ondary brain tumors spread from other parts of the body

through the bloodstream. For tumors starting in the brain, about

60 percent of which are malignant, the cause is unknown. Tumors

that begin as cancer elsewhere and then spread to the brain are

always malignant.

The incidence of primary brain tumors is about 12 per

100,000 population. About 36,000 new cases occur in the United

States annually. Because of di≈culties in diagnosing and classi-

fying brain tumors, exact statistics on secondary tumors are

unknown.

Symptoms vary according to location and size. The com-

pression of brain tissue or nerve tracts, as well as expansion of

the tumor, can cause symptoms such as seizures, headaches, mus-

cle weakness, loss of vision or other sensory problems, and speech

di≈culties. An expanding tumor can increase pressure within the

skull, causing headache, vomiting, visual disturbances, and

impaired mental functioning. Brain tumors are diagnosed with

MRI (magnetic resonance imaging) and CT (computed tomog-

raphy) scanning.

Surgery is a common treatment if the tumor is accessible and

vital structures will not be disturbed. Radiation is used to stop a

tumor’s growth or cause it to shrink. Chemotherapy destroys

tumor cells that may remain after surgery and radiation. Steroid

drugs relieve swelling and other symptoms.

Available treatments are primarily palliative and at best pro-

long life by a few weeks. A number of promising experimental

therapies, however, are currently being explored. These include

antiangiogenic therapy,in which the tumor’s blood supply is

restricted; immunotherapy,which uses the body’s own immune

system against the tumor; gene therapy,in which bioengineered

genes are delivered to the cancer cells to kill them; and several

approaches for a targeted delivery of antibodies, toxins, or

growth-inhibiting molecules that attach speciﬁcally to the can-

cer cells and kill them.

Down syndrome

Down syndrome, the most frequently occurring chromosomal

abnormality, appears in one of every 800 to 1,000 babies. It occurs

when an extra copy of chromosome 21—or part of its long

arm—is present in the egg or, less commonly, in the sperm, at

the time of conception. It is not known why this error occurs, and

it is not linked to any environmental or behavioral factors, either

before or during pregnancy, but the risk is markedly increased

with the age of the mother. At age 35, the risk is about one in 365

births; at age 40, it is one in 110. It is important to note, however,

that the average age of women who give birth to children with

Down syndrome is 28, because younger women give birth more

often. Prenatal screening tests, such as the Triple Screen and

Alpha-fetaprotein Plus, can accurately detect Down syndrome in

about 60 percent of fetuses.

Down syndrome is associated with approximately 50 physical

and developmental characteristics. An individual with Down syn-

drome is likely to possess, to various degrees, some of these char-

acteristics: mild to moderate mental retardation; low muscle tone;

an upward slant to the eyes; a ﬂat facial proﬁle; an enlarged tongue;

and an increased risk of congenital heart defects, respiratory

problems, and digestive tract obstruction. All people with Down

syndrome show the neuropathological changes of Alzheimer’s

disease by age 40, and most show cognitive decline by age 60.

Babies with Down syndrome develop much like typical chil-

dren, but at a somewhat slower rate. They will learn to sit, walk,

talk, and toilet train, just like their peers. Early intervention pro-

grams can begin shortly after birth and can help foster an infant’s

development.

Thanks to medical advances and a greater understanding of

the potential of those with this condition, people with Down syn-

drome have been able to have longer and fuller lives. Individuals

with Down syndrome are being

educated in their neighborhood

schools, participating in com-

munity activities, and ﬁnding

rewarding employment and

relationships.

Although there is no cure

for or means of preventing

Down syndrome, scientists are

moving closer to understanding

the role that the genes on chro-

mosome 21 play in a person’s

development. Once this mystery

is understood, they hope to decode the biochemical processes

that occur in Down syndrome and learn to treat or cure this dis-

order.

Huntington’s disease

A∑ecting some 30,000 Americans and placing another 200,000 at

risk, Huntington’s disease (HD) is now considered one of the

most common hereditary brain disorders. The disease, which

killed folk singer Woody Guthrie in 1967, progresses slowly over

a 10- to 20-year period and eventually robs the a∑ected individ-

ual of the ability to walk, talk, think, and reason. HD usually

appears between the ages of 30 and 50. It a∑ects both the basal

ganglia, which control coordination, and the brain cortex, which

serves as the center for thought, perception, and memory.

The most recognizable symptoms include involuntary jerk-

ing movements of the limbs, torso, and facial muscles. These are

often accompanied by mood swings, depression, irritability,

slurred speech, and clumsiness. As the disease progresses, com-

mon symptoms include di≈culty swallowing, unsteady gait, loss

of balance, impaired reasoning, and memory problems. Eventu-

ally, the individual becomes totally dependent on others for care,

with death often due to pneumonia, heart failure, or another

complication.

Diagnosis consists of a detailed clinical examination and

family history. Brain scans may be helpful. The identiﬁcation in

1993 of the gene that causes HD has simpliﬁed genetic testing,

which can be used to help conﬁrm a diagnosis. However, HD

researchers and genetic counselors have established speciﬁc pro-

tocols for predictive testing to ensure that the psychological and

social consequences of a positive or negative result are under-

stood. Predictive testing is available only for adults, though chil-

dren under 18 may be tested to conﬁrm a diagnosis of juvenile-

onset HD. Prenatal testing may be performed. The ethical issues

of testing must be considered, and the individual must be ade-

quately informed, because there is no e∑ective treatment or cure.

The HD mutation is an expanded triplet repeat in the HD

gene—a kind of molecular stutter in the DNA. This abnormal

gene codes for an abnormal protein called huntingtin.The hunt-

ingtin protein, whose normal function is still unknown, is widely

distributed in the brain and

appears to be associated with

proteins involved in transcrip-

tion, protein turnover, and

energy production. But the

cause of HD probably involves

the gain of a new and toxic func-

tion. Cell and transgenic animal

models can replicate many fea-

tures of the disease and are now

being used to test new theories

and therapies. Clinical and

observational trials are being

conducted. Any of these may yield an e∑ective treatment that

would slow the progression of or delay onset of the disease while

researchers continue working toward a cure.

Learning disorders

An estimated 10 percent of the population, as many as 25 million

Americans, have some form of learning disability involving

di≈culties in the acquisition and use of listening, speaking, read-

ing, writing, reasoning, or mathematical abilities. These chal-

lenges often occur in people with normal or high intelligence.

Dyslexia,or speciﬁc reading disability, is the most common

and most carefully studied of the learning disabilities. It a∑ects

80 percent of all of those identiﬁed as learning-disabled.

Dyslexia is characterized by an unexpected di≈culty in reading

in children and adults who otherwise possess the intelligence,

motivation, and schooling considered necessary for accurate and

ﬂuent reading. Studies indicate that dyslexia is a persistent,

chronic condition. It does not represent a transient “develop-

mental lag.”

There is now a strong consensus that the central di≈culty in

most forms of dyslexia reﬂects a deﬁcit within the language sys-

tem—and more speciﬁcally, in a component of the language sys-

tem called phonology. This is illustrated in di≈culty transform-

ing the letters on the page to the sounds of language. A current

debate exists as to whether this di≈culty reﬂects a general sound-

processing deﬁcit or a problem speciﬁc to sounds of language,

phonemes, and conscious awareness of these sounds.

Affecting some 30,000 Americans and

placing another 200,000 at risk,

Huntington’s disease is now considered

one of the most common hereditary

brain disorders.

As children approach adolescence, one manifestation of

dyslexia may be a very slow reading rate. Children may learn to

read words accurately, but they will not be ﬂuent or automatic,

reﬂecting the lingering e∑ects of a phonologic deﬁcit. Because

they can read words accurately—albeit very slowly—dyslexic

adolescents and young adults may mistakenly be assumed to have

“outgrown” their dyslexia. The ability to read aloud accurately,

rapidly, and with good expression, as well as facility with spelling,

may be most useful clinically in distinguishing students who are

average from those who are poor readers.

A range of investigations indicates that there are di∑erences

in brain regions between dyslexic and nonimpaired readers,

especially the temporo-parieto-

occipital and frontal opercular

regions. Recent data using func-

tional brain imaging indicate

that dyslexic readers demon-

strate a functional disruption in

an extensive system in the pos-

terior portion of the brain. The

disruption occurs within the

neural systems linking visual

representations of the letters to

the phonologic structures they

represent. The speciﬁc cause of

the disruption of neural sys-

tems in dyslexia is thought to

result from developmental missteps relating to neuronal migra-

tion to the cerebral cortex. It is clear that dyslexia runs in fami-

lies, and active research aims to identify what appear to be sev-

eral dyslexia-susceptibility genes.

Interventions to help children with dyslexia focus on teach-

ing the child that words can be segmented into smaller units of

sound and that these sounds are linked with speciﬁc letter pat-

terns. In addition, children with dyslexia require practice in read-

ing stories, both to allow them to apply their newly acquired

decoding skills to reading words in context and to experience

reading for meaning.

Multiple sclerosis

The most common central nervous system disease of young adults

after epilepsy, multiple sclerosis (MS) is a lifelong ailment of

unknown origin that a∑ects more than 400,000 Americans. MS is

diagnosed mainly in individuals between the ages of 20 and 50,

with two of three cases occurring in women. MS results in earn-

ing losses of about $2 billion annually for families with MS.

Although a cause has yet to be found, MS is thought to be an

autoimmune disease in which the body’s natural defenses act

against the myelin and nerve ﬁbers in the central nervous system

as though they were foreign tissue. Some nerve ﬁbers are actually

cut in association with the loss of myelin. In MS, when brain tis-

sue is destroyed, it is replaced by scars of hardened sclerotic patches

of tissue. Such lesions are called plaques and appear in multiple

places within the central nervous system. These e∑ects are com-

parable to the loss of insulating material around an electrical wire,

which interferes with the transmission of signals.

Siblings of people with MS are 10 to 15 times more likely

than others to be diagnosed with the disorder, whereas the risk

for disease concordance for identical twins is about 30 percent.

In addition, the disease is as much as ﬁve times more prevalent

in temperate zones, such as the northern United States and

northern Europe, than it is in the tropics. Thus, both genetic

and environmental factors are probably involved in the cause.

An infection acquired during the ﬁrst 15 years of life may be

responsible for triggering the

disease in a genetically suscep-

tible individual.

The most common symp-

toms are blurred vision, awk-

ward gait, numbness, and fatigue.

These can occur singly or in

combination, vary in intensity,

and last from several weeks to

months. In some patients, symp-

toms include slurred speech,

weakness, loss of coordination,

uncontrollable tremors, loss of

bladder control, memory prob-

lems, depression, and paralysis.

Muscle spasticity can a∑ect balance and coordination, causing

pain and involuntary jerking movement—and, if untreated, can

create contractures,or the “freezing” of a joint that prevents

movement.

MS cannot be cured at present, but several medications con-

trol relapsing forms of MS. A wide range of medications and ther-

apies are available to control symptoms such as spasticity, pain,

fatigue, and mood swings, as well as bladder, bowel, or sexual dys-

functions. Steroids, which have been used to treat MS for more

than three decades, may e∑ectively shorten attacks and speed

recovery from MS-related acute attacks. Promising new agents to

control MS or to alleviate its symptoms are in clinical trials.

Neurological AIDS

In 2003, about 4.8 million people became infected with human

immunodeﬁciency virus (HIV), the largest number since the onset

of the AIDS epidemic; 38 million are now living with HIV. The

epidemic is still the most intense in sub-Saharan Africa but is

gaining speed in Asia and Eastern Europe. The impact of AIDS

in the United States has been muted because of life-prolonging

drugs, but in developing countries only 400,000 of 6 million peo-

ple are receiving such treatment. Women now represent nearly

half of all worldwide cases.

Although the principal target of HIV is the immune system,

the nervous system also may be profoundly a∑ected. Some 20

An estimated 10 percent of the

population, as many as 25 million

Americans, have some form of learning

disability involving difficulties

in the acquisition and use of listening,

speaking, reading, writing,

reasoning, or mathematical abilities.

percent to 40 percent of patients with full-blown AIDS also

develop clinically signiﬁcant dementia that includes movement

impairment, with a smaller percentage still su∑ering from an

overt dementia. Those a∑ected have mental problems ranging

from mild di≈culty with concentration or coordination to pro-

gressive, fatal dementia.

Despite advances in treating other aspects of the disease,

AIDS dementia remains incompletely understood. Most current

hypotheses center on an indirect e∑ect of HIV infection related

to secreted viral products or cell-coded signal molecules called

cytokines.Nonetheless, HIV infection appears to be the prime

mover in this disorder because antiviral treatment may prevent

or reverse this condition in some patients.

Experts believe that serious neurologic symptoms are

uncommon early in HIV/AIDS infection. Later, however, patients

develop di≈culty with concentration and memory and experi-

ence general slowing of their mental processes. At the same time,

patients may develop leg weakness and a loss of balance. Imag-

ing techniques, such as computed tomography (CT) and mag-

netic resonance imaging (MRI), show that the brains in these

patients have undergone some shrinkage. The examination of

brain cells under a microscope suggests that abnormalities are

present principally in subcortical areas. Neurons in the cortex

also may be altered or lost, however.

Recent studies indicate that highly active combination anti-

retroviral treatment—cocktails of three or more drugs active

against HIV—is e∑ective in reducing the incidence of AIDS

dementia. Such treatment also can e∑ectively reverse the cogni-

tive abnormalities attributed to brain HIV infection.

Peripheral neuropathy is also a major neurological problem

seen commonly in HIV patients. It is believed that the virus trig-

gers a distal sensory neuropathy through neurotoxic mechanisms.

This has often been unmasked or exacerbated by certain of the

antiretroviral drugs that have mitochondrial toxicity and tend to

make the neuropathies more frequent and serious. In current

cohorts of advanced patients, more than half have neuropathy,

making it a major area for preventive and symptomatic thera-

peutic trials.

Despite remarkable progress in developing therapy, some

patients develop these problems and fail to respond to treatment,

thus requiring additional approaches to prevention and treatment

of these symptoms. In addition, because of immunodeﬁciency in

HIV patients, encountering otherwise rare opportunistic infec-

tions and malignancies is still relatively common.

Neurological trauma

No magic bullet has yet been found, but doctors have discovered

several methods to stave o∑ severe neurological damage caused

by head and spinal cord injuries and to improve neurological

function. These treatments include better imaging techniques,

methods to understand and improve the brain’s ability to regen-

erate and repair itself, and improved rehabilitation techniques.

Some 750,000 people su∑er traumatic head injuries requir-

ing hospitalization each year, and roughly 100,000 die—many

before reaching the hospital. Economic costs approach $25 bil-

lion annually.

Greater access to and use of computed tomography (CT) and

magnetic resonance imaging (MRI) o∑er physicians the oppor-

tunity to diagnose the extent of trauma and to avoid secondary

injury related to edema, or swelling, and a reduction in blood

ﬂow to the brain (ischemia). In general, patients who arrive in

the emergency room and are diagnosed with a severe head injury

have a pressure-monitoring device inserted into their brain, usu-

ally within the lateral ventricle. As swelling progresses, the CT or

MRI images of the brain show the surface of the brain being

pressed against the inside of the skull. This pressure inside the

skull increases and can become life-threatening. Patients so

injured are not allowed to lie ﬂat on their backs in bed. Rather,

they are positioned in a modiﬁed sitting position, which raises

the head to reduce pressure e∑ects within the skull.

Treatments for increases in intracranial pressure include the

removal of cerebrospinal ﬂuid, moderate hyperventilation to

decrease blood volume, and the administration of drugs to

reduce cellular metabolism or to remove water from the injured

tissue. Treatments for the injury-induced reduction of cerebral

blood ﬂow include the administration of drugs that increase

mean arterial blood pressure. In combination with the reduction

in intracranial pressure, this results in an increase in blood ﬂow,

allowing more blood to reach vital areas.

In addition to helping the physician avoid cerebral edema

and reductions in cerebral blood ﬂow following traumatic brain

injury, imaging can reveal mass lesions produced by the initial

injury. These mass lesions can consist of bleeding on the surface

or within the brain as well as the formation of contusions. Once

blood leaves its respective vessels and comes into direct contact

with brain tissue, it can add focal pressure, thereby reducing cere-

bral blood ﬂow, or it can by itself be toxic to brain cells. As a con-

sequence, once detected, it is usually surgically removed. Contu-

sions can also be troubling, because they can increase pressure as

well as contribute to the development of post-traumatic epilepsy.

Depending on the location and type, they are also candidates for

surgical removal.

An estimated 250,000 individuals are living with spinal cord

injury in the United States. Some 11,000 new injuries are reported

annually and are caused mostly by motor vehicle accidents, vio-

lence, and falls. Economic costs approach $10 billion a year.

Researchers have found that people who su∑er spinal cord

injuries become less severely impaired if they receive high intra-

venous doses of a commonly used steroid drug, methylpred-

nisolone, within eight hours of the injury. Building on this knowl-

edge, researchers hope to decipher the precise order of chemical

reactions that lead to damage and to develop new therapies to

block these reactions.

Scientists have known that, after a spinal cord injury, animals

can regain the ability to bear their weight and walk at various

speeds on a treadmill belt. More recently, scientists have recog-

nized that the level of this recovery depends to a large degree on

whether these tasks are practiced—that is, trained for—after

injury. It appears that humans with spinal cord injury also

respond to training interventions.

Recently, scientists have discovered that new nerve cells can

be born in the adult brain, but these new cells do not seem capa-

ble of helping the injured brain regenerate. Studies are underway

to determine how to “jumpstart” the pathway that stimulates

neurogenesis,the birth of new nerve cells. Researchers are also try-

ing to decipher how certain environmental cues can be used or

overcome to attract these new

cells—or transplanted stem or

progenitor cells—to areas of

brain injury to facilitate regen-

eration and repair.

These and other recent dis-

coveries are pointing the way

toward new therapies to pro-

mote nerve regeneration after

brain and spinal cord injury.

Although these new therapies

have not yet reached the clinic,

several approaches are on the path to clinical trials.

Schizophrenia

Marked by disturbances in thinking, emotional reactions, and

social behavior, schizophrenia usually results in chronic illness

and personality change. Delusions, hallucinations, and thought

disorder are common.

A∑ecting about 1 percent of the population, or 2 million

Americans each year, schizophrenia is disabling and costly. On a

given day, these patients occupy up to 100,000 hospital beds.

Annual costs total about $32.5 billion.

Schizophrenia is thought to reﬂect changes in the brain, pos-

sibly caused by disease or injury at the time of birth, and a genetic

disposition that may be exacerbated by environmental stress.

Recently, several genes have been identiﬁed that appear to

increase the risk of developing schizophrenia. Brain systems using

the chemicals dopamine, glutamate, and GABA appear to be par-

ticularly involved in the pathogenesis of the disorder. Brain scans

and postmortem studies show abnormalities in some people with

schizophrenia, such as enlarged ventricles (ﬂuid-ﬁlled spaces)

and reduced size of certain brain regions. Functional neuro-

imaging scans such as positron emission tomography (PET) and

functional magnetic resonance imaging (fMRI) taken during intel-

lectual tasks show abnormal functioning in speciﬁc brain areas

of people with this illness.

The disorder usually begins between the ages of 15 and 25.

Some patients fully recover following treatment, but most con-

tinue to have moderate or severe symptoms, particularly in

response to stress. About 15 percent of patients return to normal

life after a single episode, 60 percent will have intermittent

episodes throughout their lives, and another 25 percent will not

recover their ability to live as independent adults. Deﬁcits in cog-

nition, particularly involving attention and memory, are fre-

quent, lifelong manifestations in most patients, even in those

who show good recovery from acute symptoms.

After a long search for an e∑ective antipsychotic medication,

scientists synthesized the drug chlorpromazine during the late

1940s. By the 1950s, it was found to be useful in treating psychotic

states and later became a mainstay of drug treatment.

Since that time, many agents similar to chlorpromazine have

been developed. When given as

long-acting injections, these

drugs reduce some symptoms

and aid patients’ readiness for

adjustment back into their com-

munities. Chronic use of the

drugs, however, may cause ab-

normal muscle movements and

tremors in some patients. Safer

treatments are being sought.

Thus far, most drugs are

successful in treating hallucina-

tions and thought disorder. Clozapine acts somewhat di∑erently

from other antipsychotic drugs. It treats the approximately 30

percent of patients who are not helped by conventional med-

ications. The drug can, however, induce a potentially fatal blood

disorder, agranulocytosis,in about 1 percent of patients. To pre-

vent this disorder, patients must take regular weekly to biweekly

blood tests, a precaution that makes using the drug very costly.

Several new antipsychotics—risperidone, olanzapine, quetiap-

ine, ziprasadone,and aripiprazole—o∑er some of the beneﬁts

of clozapine without risk of angranulocytosis, but their long-

term side e∑ect proﬁles are not fully known.

Stroke

Until recently, if you or a loved one had a stroke, your doctor

would tell your family there was no treatment. In all likelihood,

the patient would live out the remaining months or years with

severe neurological impairment.

This dismal scenario is now brightening. For one, use of the

clot-dissolving bioengineered drug, tissue plasminogen activator

(tPA), is now a standard treatment in many hospitals. This

approach rapidly opens blocked vessels to restore circulation

before oxygen loss causes permanent damage. Given within three

hours of a stroke, it often can help in limiting the ensuing brain

damage. Also, attitudes about the nation’s third leading cause of

death are changing rapidly. Much of this has come from new and

better understandings of the mechanisms that lead to the death

of neurons following stroke and devising ways to protect these

neurons.

Schizophrenia is thought to reflect

changes in the brain, possibly caused by

disease or injury at the time of birth,

and a genetic disposition that may be

exacerbated by environmental stress.

STROKE. A stroke occurs when a blood vessel bringing oxygen and nutrients to the brain bursts or is clogged by a blood clot (1). This lack of

blood leads to a cascade of neurochemical abnormalities that can cause cell death within minutes. Free radicals are released, causing dam-

age to endothelial cells (2) and the mitochondria (3) of neurons. Normally the body readily disarms free radicals (4), but in stroke, endothelial

cell damage allows many more than can be controlled to move into brain tissue. Depending on its location, a stroke can have different symp-

toms such as paralysis on one side of the body or a loss of speech.

Stroke a∑ects roughly 700,000 Americans a year—150,000 of

whom die; total annual costs are estimated at $51.2 billion.

A stroke occurs when a blood vessel bringing oxygen and

nutrients to the brain bursts or is clogged by a blood clot or some

other particle. This deprives the brain of blood, causing the death

of neurons within minutes. Depending on its location, a stroke

can cause many permanent disorders, such as paralysis on one

side of the body and loss of speech.

Stroke often occurs in individuals over 65 years of age, yet a

third are younger. Stroke tends to occur more in males and blacks

and in those with diabetes, high blood pressure, heart disease,

obesity, high cholesterol, and a family history of stroke.

In addition to tPA, increased use of preventive measures is

battling the disorder. Controlling risk factors such as obesity,

blood pressure, diabetes, and high cholesterol can help prevent

stroke. Other speciﬁc treatments involving surgery can clear clogs

in the arteries of the neck region and help prevent a cuto∑ of

blood supply.

Treatments that target the heart’s blood ﬂow can prevent

stroke. Surgery can help repair damaged heart valves. Anticoag-

ulant drugs can reduce the chance of clots forming, traveling to

the brain and causing a stroke.

Other experimental therapies under investigation may lead

to even bigger payo∑s for patients in the future. Some strategies

target mechanisms inside the neuron. In this way, the vicious

cycle of local damage followed by a widening fringe of biochem-

ical-induced neuronal death can be slowed. A number of classes

of drugs have been shown to be e∑ective in animal studies.

Another promising possibility is the use of neural stem cells.

Some animal studies have shown that an injection of stem cells

aids recovery even if administered several days after the injury.

Administration of growth factors may further enhance the

beneﬁts of stem cell transplantation.

Tourette syndrome

One of the most common and least understood neurobiological

disorders, Tourette syndrome (TS) is an inherited disorder that

a∑ects an estimated one in 500 Americans, roughly 200,000 peo-

ple. Males are a∑ected three to four times as often as females.

Symptoms usually appear between the ages of four and eight,

but in rare cases may emerge in the late teenage years. The symp-

toms include motor and vocal tics—repetitive, involuntary

movements or utterances that are rapid and sudden and persist

for more than one year. The types of tics may change frequently

and increase or decrease in severity over time. Generally, this dis-

order lasts a lifetime, but one-third of patients may experience a

remission or decrease in symptoms as they get older. Most peo-

ple with TS do not require medication; their symptoms are mild

and do not a∑ect functioning.

A high percentage of people with TS also have associated

conditions such as problems with learning, di≈culties with atten-

tion span, and obsessive behaviors. Sometimes these manifesta-

tions are more troublesome to individuals than the tics them-

selves, so physicians must consider them when choosing a

treatment regimen.

The disorder seems to result from a hypersensitivity of dopa-

mine receptors. Another neurotransmitter, serotonin, also has

been implicated. The most e∑ective drugs for control of move-

ments, such as haloperidol, act by blocking the overactive system.

Other symptoms, such as obsessive-compulsive traits and atten-

tion deﬁcit disorder, often require treatment with other classes of

drugs that act on serotonin.

The neuroleptic drugs haloperidol and pimozide have been

the mainstays of treatment for the movements and vocalizations,

but not for the associated conditions that so many people with

TS experience. The neuroleptics are not perfect medications and

are prescribed only when the movements are fairly severe and

interfere with functioning. They can cause disturbing side

e∑ects—abnormal involuntary movements, sti∑ness of the face

and limbs, or sedation and weight gain—in some patients.

Recently, newer medications, such as low doses of selective sero-

tonin reuptake inhibitors like risperidone, have been found

e∑ective in some patients. Other classes of medications are pre-

scribed to reduce the symptoms of the comorbid conditions.

any of the recent advances in understand-

ing the brain are due to the development of

techniques that allow scientists to directly

monitor neurons throughout the body.

Electrophysiological recordings trace

brain electrical activity in response to a

speciﬁc external stimulus. In this method, electrodes placed in

speciﬁc parts of the brain—depending on which sensory system

is being tested—make recordings that are then processed by a

computer. The computer makes an anaylsis based on the time

lapse between stimulus and response. It then extracts this infor-

mation from background activity.

Following the discovery that material is transported within

neurons, methods have been developed to visualize activity and

precisely track ﬁber connections within the nervous system. This

can be done by injecting a radioactive amino acid into the brain

of an experimental animal; the animal is killed a few hours later,

and then the presence of radioactive cells is visualized on ﬁlm. In

another technique, the enzyme horseradish peroxidase is injected

and taken up by nerve ﬁbers that can be later identiﬁed under a

microscope.

These and other methods have resulted in many advances in

knowledge about the workings of the nervous system and are still

useful today. New methods, safely applicable to humans, promise

to give even more precise information.

Imaging techniques

Positron emission tomography (PET) PET is one of the most

important techniques for measuring blood ﬂow or energy con-

sumption in the brain. This method of measuring brain function

is based on the detection of radioactivity emitted when positrons,

positively charged particles, undergo radioactive decay in the

brain. Small amounts of radiation are introduced into the blood,

which is then taken up into di∑erent brain areas in proportion to

how hard the neurons are working. Computers build three-

dimensional images of the brain based on the amount of radia-

tion emitted in these di∑erent areas.

PET studies have helped scientists understand more about

how drugs a∑ect the brain and what happens during learning,

when using language, and in certain brain disorders—such as

stroke, depression, and Parkinson’s disease. Within the next few

years, PET could enable scientists to identify the biochemical

nature of neurological and mental disorders and determine how

well therapy is working in patients. PET has revealed marked

changes in the depressed brain. Knowing the location of these

changes helps researchers understand the causes of depression

and monitor the e∑ectiveness of speciﬁc treatments.

Another technique, single photon emission computed tomog-

raphy (SPECT), is similar to PET, but its pictures are not as

detailed. SPECT is much less expensive than PET because the

tracers it uses have a longer half-life and do not require an accel-

erator nearby to produce them.

Magnetic resonance imaging (MRI) Providing a high-qual-

ity, three-dimensional image of organs and structures inside the

body without X-rays or other radiation, MRIs are unsurpassed in

anatomical detail and may reveal minute changes that occur over

time.

MRIs tell scientists when structural abnormalities ﬁrst appear

in the course of a disease, how they a∑ect subsequent develop-

ment, and precisely how their progression correlates with men-

tal and emotional aspects of a disorder.

During the 15-minute MRI procedure, a patient lies inside a

massive, hollow, cylindrical magnet and is exposed to a power-

ful, steady magnetic ﬁeld. Di∑erent atoms in the brain resonate

to di∑erent frequencies of magnetic ﬁelds. In MRI, a background

magnetic ﬁeld lines up all the atoms in the brain. A second mag-

netic ﬁeld, oriented di∑erently from the background ﬁeld, is

turned on and o∑ many times a second; at certain pulse rates,

particular atoms resonate and line up with this second ﬁeld.

When the second ﬁeld is turned o∑, the atoms that were lined up

with it swing back to align with the background ﬁeld. As they

swing back, they create a signal that can be picked up and con-

verted into an image. Tissue that contains a lot of water and fat

produces a bright image; tissue that contains little or no water,

such as bone, appears black.

MRI images can be constructed in any plane, and the tech-

nique is particularly valuable in studying the brain and spinal

cord.It reveals the precise extent of tumors rapidly and vividly.

And MRI provides early evidence of potential damage from

stroke, allowing physicians to administer proper treatments early.

New diagnostic methods

Magnetic resonance spectroscopy (MRS), a technique related to

MRI, uses the same machinery but measures the concentration of

speciﬁc chemicals—such as neurotransmitters—in di∑erent

parts of the brain instead of blood ﬂow. MRS also holds great

promise: by measuring the molecular and metabolic changes that

occur in the brain, this technique has already provided new infor-

mation on brain development and aging, Alzheimer’s disease,

schizophrenia, autism, and stroke. Because it is noninvasive, MRS

is ideal for studying the natural course of a disease or its response

to treatment.

Functional magnetic resonance imaging (fMRI) Among the

most popular neuroimaging techniques today is fMRI. This tech-

nique compares brain activity under resting and activated condi-

tions. It combines the high-spatial-resolution, noninvasive imag-

ing of brain anatomy o∑ered by standard MRI with a strategy for

detecting increases in blood oxygen levels when brain activity

brings fresh blood to a particular area of the brain. This technique

allows for more detailed maps of brain areas underlying human

mental activities in health and disease. To date, fMRI has been

applied to the study of various functions of the brain, ranging

from primary sensory responses to cognitive activities.

Magnetoencephalography (MEG) MEG is a recently devel-

oped technique that reveals the source of weak magnetic ﬁelds

emitted by neurons. An array of cylinder-shaped sensors moni-

tors the magnetic ﬁeld pattern near the patient’s head to deter-

mine the position and strength of activity in various regions of

the brain. In contrast with other imaging techniques, MEG can

characterize rapidly changing patterns of neural activity—down

to millisecond resolution—and can provide a quantitative mea-

sure of the strength of this activity in individual subjects. More-

over, by presenting stimuli at various rates, scientists can deter-

mine how long neural activation is sustained in the diverse brain

areas that respond.

One of the most exciting developments in imaging is the com-

bined use of information from fMRI and MEG. The former pro-

vides detailed information about the areas of brain activity in a

particular task, whereas MEG tells researchers and physicians

when certain areas become active. Together, this information leads

to a much more precise understanding of how the brain works in

health and disease.

Optical imaging techniques Optical imaging relies on shin-

ing weak lasers through the skull to visualize brain activity. These

techniques are inexpensive and relatively portable. They are also

silent and safe: Because only extremely weak lasers are used, they

can be used even to study young infants. In a technique called near

infrared spectroscopy (NIRS), technicians shine lasers through the

skull at near-infrared frequencies, which renders the skull trans-

parent. Blood with oxygen in it absorbs di∑erent frequencies of

light than blood in which the oxygen has been consumed. By

observing how much light is reﬂected back from the brain at each

frequency, researchers can track blood ﬂow. Di∑use optical tomog-

raphy is then used to create maps of brain activity. A related tech-

nique, the event-related optical signal,records how light is scat-

tered in response to cellular changes that arise when neurons ﬁre

and potentially can assess very fast—well under a second—

changes in neural activity.

Gene diagnosis

The inherited blueprint for all human characteristics, genes con-

sist of short sections of deoxyribonucleic acid (DNA), the long, spi-

raling, double-helix structure found on the 23 pairs of chromo-

somes in the nucleus of every human cell.

New gene diagnosis techniques now make it possible to ﬁnd

the chromosomal location of genes responsible for neurologic and

psychiatric diseases and to identify structural changes in these

genes that are responsible for causing disease.

This information is useful for identifying individuals who

carry faulty genes and thereby improving diagnosis, for under-

standing the precise cause of diseases in order to improve meth-

ods of prevention and treatment, and for evaluating the malig-

nancy of and susceptibility to certain tumors.

So far, scientists have identiﬁed defective genes for more than

50 neurological disorders and the chromosomal location of the

defect in up to 100. Prenatal or carrier tests exist for many of the

most prevalent of these illnesses.

Scientists have tracked down the gene on chromosome 4 that

goes awry in Huntington’s patients. The defect is an expansion of

a CAG repeat. CAG is the genetic code for the amino acid gluta-

mine, and the expanded repeat results in a long string of gluta-

mines within the protein. This expansion appears to alter the pro-

tein’s function. Scientists have found that the size of the expanded

repeat in an individual is predictive of Huntington’s disease. Other

neurodegenerative disorders have been attributed to expanded

CAG repeats in other genes. The mechanisms by which these

expansions caused adult-onset neurodegeneration is the focus of

intense research.

Sometimes patients with single-gene disorders are found to

have a chromosomal abnormality—a deletion or break in the

DNA sequence of the gene—that can lead scientists to a more

accurate position of the disease gene. This is the case with some

abnormalities found on the X chromosome in patients with

Duchenne muscular dystrophy and on chromosome 13 in patients

with inherited retinoblastoma,a rare, highly malignant childhood

eye tumor that can lead to blindness and death.

Gene mapping has led to the localization on chromosome 21

of the gene coding for the beta amyloid precursor protein that is

abnormally cut to form the smaller peptide, beta amyloid. It is this

peptide that accumulates in the senile plaques that clog the brains

of patients with Alzheimer’s disease. This discovery shed light on

why individuals with Down syndrome invariably accumulate

amyloid deposits: they make too much amyloid due to having

three rather than two copies of this gene (trisomy 21). Mutations

in this gene have been shown to underlie Alzheimer’s in a distinct

subset of these patients.

Several other genetic factors have been identiﬁed in Alz-

heimer’s disease, including genes for two proteins, presenilin 1

and presenilin 2, located on chromosomes 14 and 1, respectively.

A risk factor for late-onset Alzheimer’s is the gene for the apo-

lipoprotein E protein located on chromosome 19.

Gene mapping has enabled doctors to diagnose fragile X men-

tal retardation,the most common cause of inherited mental

retardation. Scientists have now identiﬁed this gene, which is

important for neuronal communication. Several groups of sci-

entists are investigating whether there are genetic components to

schizophrenia, bipolar disorder, and alcoholism, but their ﬁnd-

ings are not yet conclusive.

Overall, the characterizations of the structure and function

of individual genes causing diseases of the brain and nervous sys-

tem are in the early stages. Factors that determine variations in

the genetic expression of a single-gene abnormality—such as

what contributes to the early or late start or severity of a disor-

der—are still largely unknown.

Scientists also are studying the genes in mitochondria, struc-

tures found outside the cell nucleus that have their own DNA and

are responsible for the production of energy used by the cell.

Recently, di∑erent mutations in mitochondrial genes were found

to cause several rare neurological disorders. Some scientists spec-

ulate that an inheritable variation in mitochondrial DNA may

play a role in diseases such as Alzheimer’s, Parkinson’s, and some

childhood diseases of the nervous system.

CHROMOSOMES, GENES, AND PROTEINS. Every trait and chemical process in the body is controlled by a gene or group of genes on

23 paired chromosomes in the nucleus of every cell (1). Each gene is a discrete segment along the two tightly coiled strands of DNA that make

up these chromosomes. DNA strands bear four different types of coding molecules — adenine (A), cytosine (C), guanine (G), and thymine (T)—

the sequence of which contains the instructions for making all the proteins necessary for life (2). During protein production, a gene uses a mol-

ecule called mRNA to send a message with instructions for the amino acids needed to manufacture a protein (3).

Iew drugs. Most medicines used today were

developed using trial-and-error techniques,

which often do not reveal why a drug produces a

particular e∑ect. But the expanding knowledge

gained from the new methods of molecular biol-

ogy—the ability to determine the structure of

receptors or other proteins—makes it possible to design safer

and more e∑ective drugs.

In a test tube, the potency of an agent can be determined by

how well it attaches to a receptor or other protein target. A sci-

entist then can vary the drug’s structure to enhance its action on

the desired target. Thus, subsequent generations of drugs can be

designed to interact more selectively with the target or, in many

cases, speciﬁc subtypes of the target, producing better therapeu-

tic e∑ects and fewer side e∑ects.

While this “rational drug design” holds promise for develop-

ing drugs for conditions ranging from stroke and migraine

headaches to depression and anxiety, it will take considerable

e∑ort to clarify the role of the di∑erent potential drug targets in

these disorders.

Other promising candidates for drug therapies include

trophic factors, antibodies engineered to speciﬁcally modify the

interactions and toxicity of misfolded proteins, small molecules

that take advantage of speciﬁc biochemical pathways, interfering

RNAs (RNAi) that reduce toxic levels of individual proteins, and

stem cells that could replace dead or dying neurons.

Trophic factors

One result of basic neuroscience research is the discovery of

numerous growth factors or trophic factors in the brain, which

control the development and survival of speciﬁc groups of neu-

rons. Once the speciﬁc actions of these molecules and their recep-

tors are identiﬁed and their genes cloned, procedures can be

developed to modify trophic factor-regulated functions in ways

that might be useful in the treatment of neurological disorders.

Once a trophic factor for a particular cell is found, copies of

the factor might be genetically targeted to the area of the brain

where this type of cell has died. The treatment may not cure a

disease, but could improve symptoms and delay progression.

Already, researchers have demonstrated the possible value of

at least one of these factors, nerve growth factor (NGF). Infused

into the brains of rats, NGF prevented cell death and stimulated

the regeneration and sprouting of damaged neurons that are

known to die in Alzheimer’s disease. When aged animals with

learning and memory impairments were treated with NGF, sci-

entists found that these animals were able to remember a maze

task as well as healthy aged rats. NGF, which slows the destruc-

tion of neurons that use acetylcholine, also holds promise for

slowing the memory deﬁcits associated with normal aging.

Recently, several new factors have been identiﬁed. They are

potentially useful for therapy, but scientists must ﬁrst understand

how they may inﬂuence neurons. Alzheimer’s disease, Parkinson’s

disease, and amyotrophic lateral sclerosis (ALS) may be treated

in the future with trophic factors or their genes.

In an interesting twist on growth factor therapy, researchers

demonstrated that a neutralization of inhibitory molecules can

help repair damaged nerve ﬁber tracts in the spinal cord. Using

antibodies to Nogo-A, a protein that inhibits nerve regeneration,

Swiss researchers succeeded in getting some nerves of damaged

spinal cords to regrow in rats. Treated rats showed large improve-

ments in their ability to walk after spinal cord damage.

In these experiments, scientists cut one of the major groups

of nerve ﬁber tracts in the spinal cord that connect it to the brain.

When an antibody directed against the factor Nogo-A was

administered to the spinal cords or brains of adult rats, enhanced

sprouting of nerve ﬁbers occurred where the spinal cord had been

cut. Within two to three weeks, some ﬁbers grew to the lower level

of the spinal cord and, in some animals, along its whole length.

In untreated spinal cord-injured rats, the maximum distance of

nerve regrowth rarely exceeded one-tenth of an inch. This

research could eventually have clinical implications for spinal

cord- or brain-damaged people.

Engineered antibodies

The immune system has evolved to very speciﬁcally identify and

modify factors both inside and outside of cells. It is sometimes

possible to trick the body into attacking proteins that cause neu-

rological diseases by “vaccinating” patients against these proteins.

This approach has shown some promise in Alzheimer’s disease,

although it also carries risks. Another new approach combines

Potential therapies

CELL AND GENE THERAPY. In potential therapy techniques, scientists plan to insert genetic material for a beneficial neurotransmitter or

trophic factor into stem cells or a virus. The cells or virus are then put into a syringe and injected into the patient where they will produce the

beneficial molecule and, it is hoped, improve symptoms.

genetic engineering with immunology to engineer antibodies or

fragments of antibodies that can bind to and alter the disease

characteristics of speciﬁc proteins. These therapies can be deliv-

ered either as proteins or as genes.

Promising preliminary results have been obtained for Hunt-

ington’s, Parkinson’s, Alzheimer’s, and prion diseases. For exam-

ple, fruit ﬂies (Drosophila) that get Huntington’s disease (HD)

because they have been modiﬁed to carry the mutant human gene

are generally too weak and uncoordinated to break out of their

pupal case the way normal insects do. However, when they also

express the gene for an anti-HD antibody, all of them can emerge

as young adults. Furthermore, these treated ﬂies live longer than

the untreated ones that do man-

age to emerge, and the treated

ones show less pathology in

their brains.

Small molecules and

RNAs

Clarifying the processes that

underlie brain damage will open

up the potential to use small

molecule drugs to alter these

processes. Some success has occurred in developing animal mod-

els using approaches based on known mechanisms of drugs.

Examples include drugs such as antibiotics and anti-tumor drugs,

which appear to reduce the neuronal damage in ALS, HD, and

Parkinson’s disease. Thousands of small molecule drug candidates

can be tested using high throughput screening to alter a cellular

property that represents an important part of a disease process.

Because many neurodegenerative diseases involve proteins that

misfold and clump abnormally, lasers are used to measure

whether proteins are clumped inside cells that have been roboti-

cally distributed into tiny wells, along with the small molecules to

be tested. A machine then scans the wells and reports whether par-

ticular drugs have changed the protein clumping, so that these

drugs can be tested further. New leads for Alzheimer’s and prion

drugs have recently been described using these methods.

Several neurodegenerative diseases are caused by the accu-

mulation of abnormal proteins. If the cells made much less of such

proteins to begin with, then the disease would progress much

more slowly. A new class of potential drugs is based on removing

the RNAs that code for the proteins that are causing damage.

Mouse models of HD and ALS appear to have responded posi-

tively to such treatments, which are delivered via gene therapies.

Cell and gene therapy

Researchers throughout the world are pursuing a variety of new

ways to repair or replace neurons and other cells in the brain.

These experimental approaches are still being worked out in ani-

mals and cannot be considered therapies for humans at this time.

Scientists have identiﬁed em-

bryonic neuronal stem cells—

unspecialized cells that give rise

to cells with speciﬁc functions—

in the brain and spinal cord of

embryonic and adult mice. Stem

cells can continuously produce

all three major cell types of the

brain: neurons; astrocytes,the

cells that nourish and protect

neurons; and oligodendrocytes,

the cells that surround axons and allow them to conduct their sig-

nals e≈ciently. The production abilities of stem cells may some-

day be useful for replacing brain cells lost to disease. A more lim-

ited type of stem cell also has been discovered in the adult nervous

system in various kinds of tissue, raising the possibility that these

adult stem cells might be pharmacologically directed to replace

damaged neurons.

In other work, researchers are studying a variety of viruses

that may ultimately be used to act as “Trojan horses,” carrying

therapeutic genes to the brain to correct nervous system diseases.

The viruses include herpes simplex type 1 virus (HSV),adenovirus,

lentivirus,adeno-associated virus,and others naturally attracted

to neurons. It has been found that all can be modiﬁed to carry

new genes to cells in tissue culture and in the rodent central ner-

vous system. HSV and adenovirus vectors have also been evalu-

ated in early-stage human trials for treating brain tumors.

Thousands of small molecule drug

candidates can be tested using high

throughput screening to alter a cellular

property that represents an important

part of a disease process.

Breaking a conﬁdence. Going along to get along. Telling a “white lie” to protect a friend.

Everyone faces ethical dilemmas—in school, at home, and nearly everywhere in

everyday life. And this is no di∑erent for neuroscientists. With the tremendous

advances in the ﬁeld, many scientists and nonscientists alike have sensed a critical

turning point. Researchers are now on the threshold of understanding some funda-

mental principles of brain function, and are even beginning to develop treatments for

some of the most devastating neurological diseases and conditions. At the same time, much of this

knowledge and implications for treatments and diagnostics raise ethical questions.

For example, some recent brain imaging studies have sought to deﬁne areas responsible for phe-

nomena such as deception. The post-9/11 era has created much interest in lie detection for security

purposes in screening immigrants. How should privacy be balanced with national security? Is the

technology accurate enough to provide useful data upon which to base decisions? Pursuing these

lines of scientiﬁc inquiry in a responsible way requires neuroscientists to examine how what they do

a∑ects the world beyond the laboratory or clinic.

This self-examination makes up a ﬁeld known as neuroethics.Scientists and ethicists are begin-

ning to reﬂect on the implications of neuroscience in areas such as moral reasoning, decision-mak-

ing, and behavior. Much discussion will focus on neuroethics in the coming years.

Neuroethics is the part of bioethics that considers the intended and unintended consequences of

neuroscience in medical practice, research, and society at large. Neuroethics also deals with issues

that touch no other area of science—our sense of self, our personalities, and our behavior. And brain

science can change these aspects in signiﬁcant ways. Neuroethics has been the subject of several con-

ferences that have attracted a wide range of thinkers, basic and clinical neuroscientists, economists,

philosophers, journalists, sociologists, lawyers, and others. Some major topics include the subjects

listed below.

Need for ethical framework It is very likely that the potential application of new knowledge to

human behavior will generate a great deal of ethical and public policy concern. Neuroethics spans

ﬁelds as diverse as forensic psychiatry, athletics, education, college admissions, corporate hiring, polic-

ing, admission to seminaries, and the law. Information about why people, and categories of people,

behave as they do will lead those in these ﬁelds to eagerly use new knowledge about the human brain.

As the brain sciences advance, researchers are working toward developing a paradigm of morals that

might help guide the use of the new knowledge.

Morality Learning right from wrong enhances skills in cooperative behavior and helps people

know when to modify their social interactions. Discoveries in neuroscience indicate that learning

right from wrong may depend on the development of brain tissue during the adolescent years. Given

this possibility, researchers are considering the implications of how problems in brain development

may a∑ect moral behaviors such as self-control. For example, could some individuals have a biolog-

ical handicap that impairs their ability to obey the law? Researchers also are investigating the ethical

issues related to using drugs that inﬂuence behaviors like self-control.

Social behavior Neurobiological factors may play a role in disturbances in social behavior.

Patients who have malfunctions caused by disease in selected brain regions exhibit antisocial behav-

Neuroethics

ioral change. Because modern neuroscience can investigate the

mechanisms behind disturbed behaviors, society must ponder

the manner in which it manages individuals who violate its rules

by taking into account those with medical conditions. This situ-

ation raises questions regarding the punishment and treatment

of such individuals.

Social policy Since neuroscience has the potential to trans-

form our understanding of human nature, we may be able to

make predictions about an individual’s future, including the risk

for ill health and cognitive impairment, potential success in school

or employment, and violent behavior or addiction to drugs. Of

particular concern is the potential role for neuroscience in delin-

eating the boundary between

what society views as normal

and what is deemed patho-

logical. For example, many boys

are being prescribed medicines

for hyperactivity, such as Ri-

talin, for conditions that many

worry may not be clear-cut

brain diseases. Troubling ques-

tions related to this phenome-

non include: Who should have

access to such agents? If learn-

ing can be sped up and atten-

tion enhanced through phar-

maceuticals, should such drugs

be available to all, or should re-

sources be devoted to trans-

forming only the environment

of the classroom?

Genetics Sequencing the

human genome—identifying all the approximately 20,000 to

25,000 genes in humans—went hand-in-hand with an invest-

ment in studying the ethical, legal, and social implications of

human genetics. Neuroethics may play an important role in

issues arising from genetics, such as the social consequences of

using DNA to predict the future. Should patients with a family

history of a neurological or psychiatric disorder be genetically

tested even though no treatment is available?

Brain injury Since traumatic brain injury may cause a per-

son to experience signiﬁcant cognitive, personality, emotional,

and behavioral changes, su∑erers may become legally incompe-

tent. Traumatic brain injury may excuse or mitigate a person’s

responsibility for acts that otherwise would be classiﬁed as

crimes. But the medical profession sometimes can have trouble

determining a person’s preinjury behavior compared with postin-

jury behavior. This poses serious problems for clinical evaluators

as well as for the legal system.

Neurological disorders With the likelihood of developing

e∑ective treatments for disorders such as Alzheimer’s disease

(AD), in which brain cells die at a progressive rate, it will be

important to identify individuals who are at risk in order to pre-

vent brain damage at the earliest stage. Yet there is little prece-

dence for such early intervention. Moreover, AD is a genetically

complex disorder with multiple genes and degrees of risk associ-

ated with each. Thus, the medical profession faces challenges in

ensuring that candidates for treatment understand their risk for

severity of AD and evaluating the potential risks and beneﬁts of

treatments that may be harmful.

Informed consent in research Special care must be taken in

the informed-consent process and throughout the research pro-

tocol when individuals have

thinking or emotional impair-

ments that might a∑ect their

decision-making capacity. Con-

sent is an ongoing process that

should involve education of the

potential research participant

and, when appropriate, family

members. Researchers are dis-

cussing potential needs to exer-

cise greater scrutiny, ensure

safeguards, and enhance partic-

ipants’ grasp of a study, includ-

ing risks and benefits.

Environmental inﬂuences

Many are concerned about the

possible implications of research

indicating that most of the brain

gets built after birth and that it

uses experiences from the out-

side world to form its circuits. With this as background, scientists

are discussing whether society has a moral obligation to ensure

that millions of children no longer grow up in violent and im-

poverished environments that can stunt their brains.

At this stage, the ﬁeld of neuroethics raises more questions

than answers. It poses challenges to scientists and to the public

to work through the social implications of new discoveries. The

issues are too broad-based to expect that scientists alone will sup-

ply the answers. But neuroscientists are well-positioned to help

shape and contribute to the debate and discussion.

One of the hallmarks of the ﬁeld has always been the drive

toward integrating information from disparate ﬁelds and special-

izations to increase knowledge. Sorting through the complex issues

captured under the umbrella of neuroethics provides an impor-

tant opportunity for informed and rich discussions among scien-

tists and with the public. Continuing study of neuroethics will help

all segments of society deal with the challenges posed by emerging

technologies that investigate the brain and how it works.

Researchers are now on the threshold

of understanding some fundamental

principles of brain function, and are

even beginning to develop treatments for

some of the most devastating neuro-

logical diseases and conditions. At the

same time, much of this knowledge and

implications for treatments and

diagnostics raise ethical questions.

ACETYLCHOLINE A neurotransmitter active both in the brain,

where it regulates memory, and in the peripheral nervous system,

where it controls the actions of skeletal and smooth muscle.

ACTION POTENTIAL An electrical charge travels along the axon

to the neuron’s terminal, where it triggers the release of a neuro-

transmitter. This occurs when a neuron is activated and tem-

porarily reverses the electrical state of its interior membrane from

negative to positive.

ADRENAL CORTEX An endocrine organ that secretes corticos-

teroids for metabolic functions; for example, in response to stress.

ADRENAL MEDULLA An endocrine organ that secretes epi-

nephrine and norepinephrine in concert with the activation of the

sympathetic nervous system; for example, in response to stress.

AGONIST A neurotransmitter, drug, or other molecule that

stimulates receptors to produce a desired reaction.

ALZHEIMER’S DISEASE The major cause of dementia most

prevalent in the elderly, it inﬂicts enormous human ﬁnancial cost

on society. The disease is characterized by death of neurons in the

hippocampus, cerebral cortex, and other brain regions.

AMINO ACID TRANSMITTERS The most prevalent neuro-

transmitters in the brain, these include glutamate and aspartate,

which have excitatory actions on nerve cells, and glycine and

gamma-amino butyric acid (GABA), which also have inhibitory

actions on nerve cells.

AMYGDALA A structure in the forebrain that is an important

component of the limbic system and plays a central role in emo-

tional learning, particularly within the context of fear.

ANDROGENS Sex steroid hormones, including testosterone,

found in higher levels in males than females. They are responsi-

ble for male sexual maturation.

ANTAGONIST A drug or other molecule that blocks receptors.

Antagonists inhibit the e∑ects of agonists.

APHASIA Disturbance in language comprehension or produc-

tion, often as a result of a stroke.

APOPTOSIS Programmed cell death induced by specialized bio-

chemical pathways, often serving a speciﬁc purpose in the devel-

opment of the animal.

Glossary

AUDITORY NERVE A bundle of nerve ﬁbers extending from the

cochlea of the ear to the brain that contains two branches: the

cochlear nerve, which transmits sound information, and the

vestibular nerve, which relays information related to balance.

AUTONOMIC NERVOUS SYSTEM A part of the peripheral ner-

vous system responsible for regulating the activity of internal

organs. It includes the sympathetic and parasympathetic nervous

systems.

AXON The ﬁberlike extension of a neuron by which it sends

information to target cells.

BASAL GANGLIA Clusters of neurons, which include the cau-

date nucleus, putamen, globus pallidus, and substantia nigra,

located deep in the brain that play an important role in the initi-

ation of movements. Cell death in the substantia nigra contributes

to Parkinson’s disease.

BRAINSTEM The major route by which the forebrain sends infor-

mation to and receives information from the spinal cord and

peripheral nerves. The brainstem controls, among other things,

respiration and the regulation of heart rhythms.

BROCA’S AREA The brain region located in the frontal lobe of

the left hemisphere that is important for the production of speech.

CATECHOLAMINES The neurotransmitters dopamine, epineph-

rine, and norepinephrine, which are active in both the brain and

the peripheral sympathetic nervous system. These three molecules

have certain structural similarities and are part of a larger class of

neurotransmitters known as monoamines.

CEREBELLUM A large structure located at the roof of the hind-

brain that helps control the coordination of movement by mak-

ing connections to the pons, medulla, spinal cord, and thalamus.

It also may be involved in aspects of motor learning.

CEREBRAL CORTEX The outermost layer of the cerebral hemi-

spheres of the brain. It is largely responsible for all forms of con-

scious experience, including perception, emotion, thought, and

planning.

CEREBRAL HEMISPHERES The two specialized halves of the

brain. For example, in right-handed people, the left hemisphere is

specialized for speech, writing, language, and calculation; the right

hemisphere is specialized for spatial abilities, visual face recogni-

tion, and some aspects of music perception and production.

CEREBROSPINAL FLUID A liquid found within the ventricles of

the brain and the central canal of the spinal cord.

CHOLECYSTOKININ A hormone released from the lining of the

stomach during the early stages of digestion that acts as a pow-

erful suppressant of normal eating. It also is found in the brain.

CIRCADIAN RHYTHM A cycle of behavior or physiological

change lasting approximately 24 hours.

CLASSICAL CONDITIONING Learning in which a stimulus that

naturally produces a speciﬁc response (unconditioned stimulus)

is repeatedly paired with a neutral stimulus (conditioned stimu-

lus). As a result, the conditioned stimulus can come to evoke a

response similar to that of the unconditioned stimulus.

COCHLEA A snail-shaped, ﬂuid-ﬁlled organ of the inner ear

responsible for transducing motion into neurotransmission to

produce an auditory sensation.

COGNITION The process or processes by which an organism

gains knowledge or becomes aware of events or objects in its

environment and uses that knowledge for comprehension and

problem-solving.

CONE A primary receptor cell for vision located in the retina. It

is sensitive to color and is used primarily for daytime vision.

CORPUS CALLOSUM The large bundle of nerve ﬁbers linking

the left and right cerebral hemispheres.

CORTISOL A hormone manufactured by the adrenal cortex. In

humans, cortisol is secreted in the greatest quantities before

dawn, readying the body for the activities of the coming day.

DEPRESSION A mental disorder characterized by depressed

mood and abnormalities in sleep, appetite, and energy level.

DENDRITE A tree-like extension of the neuron cell body. The

dendrite is the primary site for receiving and integrating infor-

mation from other neurons.

DOPAMINE A catecholamine neurotransmitter known to have

varied functions depending on where it acts. Dopamine-con-

taining neurons in the substantia nigra of the brainstem project

to the caudate nucleus and are destroyed in Parkinson’s victims.

Dopamine is thought to regulate key emotional responses such

as reward and plays a role in schizophrenia and drug abuse.

DORSAL HORN An area of the spinal cord where many nerve

ﬁbers from peripheral pain receptors meet other ascending and

descending nerve ﬁbers.

DRUG ADDICTION Loss of control over drug intake or com-

pulsive seeking and taking of drugs, despite adverse conse-

quences.

ENDOCRINE ORGAN An organ that secretes a hormone

directly into the bloodstream to regulate cellular activity of cer-

tain other organs.

ENDORPHINS Neurotransmitters produced in the brain that

generate cellular and behavioral e∑ects like those of morphine.

EPILEPSY A disorder characterized by repeated seizures, which

are caused by abnormal excitation of large groups of neurons in

various brain regions. Epilepsy can be treated with many types of

anticonvulsant medications.

EPINEPHRINE A hormone, released by the adrenal medulla and

specialized sites in the brain, that acts with norepinephrine to

a∑ect the sympathetic division of the autonomic nervous system.

Sometimes called adrenaline.

ESTROGENS A group of sex hormones found more abundantly

in females than males. They are responsible for female sexual

maturation and other functions.

EVOKED POTENTIALS A measure of the brain’s electrical activ-

ity in response to sensory stimuli. This is obtained by placing

electrodes on the surface of the scalp (or more rarely, inside the

head), repeatedly administering a stimulus, and then using a

computer to average the results.

EXCITATION A change in the electrical state of a neuron that is

associated with an enhanced probability of action potentials.

FOLLICLE-STIMULATING HORMONE A hormone released by

the pituitary gland that stimulates the production of sperm in the

male and growth of the follicle (which produces the egg) in the

female.

FOREBRAIN The largest part of the brain, which includes the

cerebral cortex and basal ganglia. The forebrain is credited with

the highest intellectual functions.

FRONTAL LOBE One of the four divisions (the other lobes are

the parietal, temporal, and occipital) of each hemisphere of the

cerebral cortex. The frontal lobe has a role in controlling move-

ment and in the planning and coordinating of behavior.

GAMMA-AMINO BUTYRIC ACID (GABA) An amino acid

transmitter in the brain whose primary function is to inhibit the

ﬁring of nerve cells.

GLIA Specialized cells that nourish and support neurons.

GLUTAMATE An amino acid neurotransmitter that acts to excite

neurons. Glutamate stimulates N-methyl-d-aspartate (NMDA)

and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid

(AMPA). AMPA receptors have been implicated in activities

ranging from learning and memory to development and

speciﬁcation of nerve contacts in developing animals. Stimula-

tion of NMDA receptors may promote beneﬁcial changes,

whereas overstimulation may be a cause of nerve cell damage or

death in neurological trauma and stroke.

GONAD Primary sex gland: testis in the male and ovary in the

female.

GROWTH CONE A distinctive structure at the growing end of

most axons. It is the site where new material is added to the axon.

HIPPOCAMPUS A seahorse-shaped structure located within the

brain and considered an important part of the limbic system. One

of the most studied areas of the brain, it functions in learning,

memory, and emotion.

HORMONES Chemical messengers secreted by endocrine glands

to regulate the activity of target cells. They play a role in sexual

development, calcium and bone metabolism, growth, and many

other activities.

HUNTINGTON’S DISEASE A movement disorder caused by the

death of neurons in the basal ganglia and other brain regions. It

is characterized by abnormal movements called chorea—sudden,

jerky movements without purpose.

HYPOTHALAMUS A complex brain structure composed of many

nuclei with various functions, including regulating the activities

of internal organs, monitoring information from the autonomic

nervous system, controlling the pituitary gland, and regulating

sleep and appetite.

INHIBITION A synaptic message that prevents a recipient neu-

ron from ﬁring.

IONS Electrically charged atoms or molecules.

LIMBIC SYSTEM A group of brain structures—including the

amygdala, hippocampus, septum, basal ganglia, and others—that

help regulate the expression of emotion and emotional memory.

LONG-TERM MEMORY The ﬁnal phase of memory, in which

information storage may last from hours to a lifetime.

MANIA A mental disorder characterized by excessive excitement,

exalted feelings, elevated mood, psychomotor overactivity, and

overproduction of ideas. It may be associated with psychosis, for

example, delusions of grandeur.

MELATONIN Produced from serotonin, melatonin is released by

the pineal gland into the bloodstream. Melatonin a∑ects physio-

logical changes related to time and lighting cycles.

MEMORY CONSOLIDATION The physical and psychological

changes that take place as the brain organizes and restructures

information to make it a permanent part of memory.

METABOLISM The sum of all physical and chemical changes that

take place within an organism and all energy transformations that

occur within living cells.

MIDBRAIN The most anterior segment of the brainstem. With

the pons and medulla, the midbrain is involved in many functions,

including regulation of heart rate, respiration, pain perception,

and movement.

MITOCHONDRIA Small cylindrical organelles inside cells that

provide energy for the cell by converting sugar and oxygen into

special energy molecules, called ATP.

MONOAMINE OXIDASE (MAO) The brain and liver enzyme

that normally breaks down the catecholamines norepinephrine,

dopamine, and epinephrine, as well as other monoamines such as

serotonin.

MOTOR NEURON A neuron that carries information from the

central nervous system to muscle.

MYASTHENIA GRAVIS A disease in which acetylcholine recep-

tors on muscle cells are destroyed so that muscles can no longer

respond to the acetylcholine signal to contract. Symptoms include

muscular weakness and progressively more common bouts of

fatigue. The disease’s cause is unknown but is more common in

females than in males; it usually strikes between the ages of 20 and

50.

MYELIN Compact fatty material that surrounds and insulates the

axons of some neurons.

NECROSIS Cell death due to external factors, such as lack of oxy-

gen or physical damage, that disrupt the normal biochemical

processes in the cell.

NERVE GROWTH FACTOR A substance whose role is to guide

neuronal growth during embryonic development, especially in the

peripheral nervous system. Nerve growth factor also probably

helps sustain neurons in the adult.

NEURON A nerve cell specialized for the transmission of infor-

mation and characterized by long, ﬁbrous projections called axons

and shorter, branchlike projections called dendrites.

NEUROPLASTICITY A general term used to describe the adaptive

changes in the structure or function of nerve cells or groups of

nerve cells in response to injuries to the nervous system or alter-

ations in patterns of their use and disuse.

NEUROTRANSMITTER A chemical released by neurons at a

synapse for the purpose of relaying information to other neurons

via receptors.

NOCICEPTORS In animals, nerve endings that signal the sensa-

tion of pain. In humans, they are called pain receptors.

NOREPINEPHRINE A catecholamine neurotransmitter, pro-

duced both in the brain and in the peripheral nervous system.

Norepinephrine is involved in arousal and in regulation of sleep,

mood, and blood pressure.

OCCIPITAL LOBE One of the four subdivisions of the cerebral

cortex.The occipital lobe plays a role in processing visual infor-

mation.

ORGANELLES Small structures within a cell that maintain the

cell and do the cell’s work.

PARASYMPATHETIC NERVOUS SYSTEM A branch of the auto-

nomic nervous system concerned with the conservation of the

body’s energy and resources during relaxed states.

PARIETAL LOBE One of the four subdivisions of the cerebral cor-

tex. The parietal lobe plays a role in sensory processes, attention,

and language.

PARKINSON’S DISEASE A movement disorder caused by death

of dopamine neurons in the substantia nigra, located in the mid-

brain. Symptoms include tremor, shuΩing gait, and general

paucity of movement.

PEPTIDES Chains of amino acids that can function as neuro-

transmitters or hormones.

PERIPHERAL NERVOUS SYSTEM A division of the nervous sys-

tem consisting of all nerves that are not part of the brain or spinal

cord.

PHOSPHORYLATION A process that modiﬁes the properties of

neurons by acting on an ion channel, neurotransmitter receptor,

or other regulatory protein. During phosphorylation, a phos-

phate molecule is placed on a protein and results in the activa-

tion or inactivation of the protein. Phosphorylation is believed

to be a necessary step in allowing some neurotransmitters to act

and is often the result of second-messenger activity.

PINEAL GLAND An endocrine organ found in the brain. In

some animals, the pineal gland serves as a light-inﬂuenced bio-

logical clock.

PITUITARY GLAND An endocrine organ closely linked with the

hypothalamus. In humans, the pituitary gland is composed of

two lobes and secretes several di∑erent hormones that regulate

the activity of other endocrine organs throughout the body.

PONS A part of the hindbrain that, with other brain structures,

controls respiration and regulates heart rhythms. The pons is a

major route by which the forebrain sends information to and

receives information from the spinal cord and peripheral nervous

system.

PSYCHOSIS A severe symptom of mental disorders character-

ized by an inability to perceive reality. Psychosis can occur in

many conditions, including schizophrenia, mania, depression,

and drug-induced states.

RECEPTOR CELL A specialized sensory cell, designed to pick up

and transmit sensory information.

RECEPTOR MOLECULE A speciﬁc protein on the surface of or

inside a cell with a characteristic chemical and physical structure.

Many neurotransmitters and hormones exert their e∑ects by

binding to receptors on cells.

REUPTAKE A process by which released neurotransmitters are

absorbed for later reuse.

ROD A sensory neuron located in the periphery of the retina.

The rod is sensitive to light of low intensity and is specialized for

nighttime vision.

SCHIZOPHRENIA A chronic mental disorder characterized by

psychosis (e.g., hallucinations and delusions), ﬂattened emotions,

and impaired cognitive function.

SECOND MESSENGERS Substances that trigger communica-

tions among di∑erent parts of a neuron. These chemicals play a

role in the manufacture and release of neurotransmitters, intra-

cellular movements, carbohydrate metabolism, and processes of

growth and development. The messengers’ direct e∑ects on the

genetic material of cells may lead to long-term alterations of

behavior, such as memory and drug addiction.

SEROTONIN A monoamine neurotransmitter believed to play

many roles, including, but not limited to, temperature regulation,

sensory perception, and the onset of sleep. Neurons using sero-

tonin as a transmitter are found in the brain and gut. Several anti-

depressant drugs are targeted to brain serotonin systems.

SHORT-TERM MEMORY A phase of memory in which a limited

amount of information may be held for several seconds or min-

utes.

STIMULUS An environmental event capable of being detected

by sensory receptors.

STROKE The third-largest cause of death in the United States,

stroke is an impeded blood supply to the brain. Stroke can be

caused by a rupture of a blood vessel wall, an obstruction of

blood ﬂow caused by a clot or other material, or pressure on a

blood vessel (as by a tumor). Deprived of oxygen, which is car-

ried by blood, nerve cells in the a∑ected area cannot function and

die. Thus, the part of the body controlled by those cells cannot

function either. Stroke can result in loss of consciousness and

death.

SYMPATHETIC NERVOUS SYSTEM A branch of the autonomic

nervous system responsible for mobilizing the body’s energy and

resources during times of stress and arousal.

SYNAPSE A physical gap between two neurons that functions

as the site of information transfer from one neuron to another.

TEMPORAL LOBE One of the four major subdivisions of each

hemisphere of the cerebral cortex. The temporal lobe functions

in auditory perception, speech, and complex visual perceptions.

THALAMUS A structure consisting of two egg-shaped masses of

nerve tissue, each about the size of a walnut, deep within the

brain. The key relay station for sensory information ﬂowing into

the brain, the thalamus ﬁlters out information of particular

importance from the mass of signals entering the brain.

VENTRICLES Comparatively large spaces ﬁlled with cere-

brospinal ﬂuid. Of the four ventricles, three are located in the

forebrain and one in the brainstem. The lateral ventricles, the two

largest, are symmetrically placed above the brainstem, one in each

hemisphere.

WERNICKE’S AREA A brain region responsible for the com-

prehension of language and the production of meaningful

speech.

Acetylcholine 6

Action potential 6

Addiction 3639

Aging 3132

and intellectual capacity 3132

AIDS 4445

Alcohol 37,3839

Alcoholism 3839

Alpha motor neurons 2223,24

Alzheimer’s disease 5, 40

Amino acid transmitters 78

Amphetamines 37

Amyloid protein 40

Amyotrophic lateral sclerosis (ALS)

4041

Analgesia 35

Androgen 8

Anxiety disorders 4142

Attention deficit hyperactivity disorder

(ADHD) 3940

Autoimmune response 30

Autonomic nervous system 13,28, 29

Axon 67

Basal ganglia 24

Biological clock 9

Bipolar disorder 33

Brain

aging 3132

anatomical organization 5

development 1013

diseases 4, 5

tumors 42

Catecholamines 8

Central nervous system 8, 13

Cerebellum 5,21,24

Cerebral cortex 5,11, 19,21,27

Club drugs 39

Cocaine 37,38

Cortisol 2930

Costs of brain diseases 45

Crossed extension reflex 2324

Declarative knowledge 20

Dementia 31, 40, 45

Dendrite 67

Depression

major 3334

Dopamine 8, 37,38

Down syndrome 4243

Drug reward system 37

Endocrine system 89, 28, 29,30

Endorphins 8, 19

Epilepsy 33

Epinephrine 2930

Estrogen 8

Fetal alcohol syndrome 38

Firing of neurons 67

Flexion withdrawal 2324

Fluoxetine 8

Forebrain 5,10,27

Fragile X mental retardation 51

Functional magnetic resonance imaging

(fMRI) 46, 50

Gammaaminobutyric acid (GABA) 7, 37

Gamma motor neurons 22

Gene 5051

diagnosis 50

therapy 53,54

Glucocorticoids 9, 2930

Glutamate 7, 37

Hearing 1617

Heroin 3738

Hippocampus 5,2021,37

Huntington’s disease 5, 43

Hypothalamus 5,8,29

Immune system 30

Information processing 12

and hearing 1617

and learning and memory 2021

and movement 22, 2324

and pain 1719

and taste and smell 1618

and vision 14, 1516

Inhibitory neurons 2223

Ion channels 6

Language 2021

Learning 2021

Learning disorders 4344

Levodopa 8

Limbic system 17

Longterm potentiation 2021

Lou Gehrig’s disease 4041

Magnetic resonance imaging (MRI) 49

Magnetic resonance spectroscopy (MRS)

Magnetoencephalography (MEG) 50

Marijuana 39

Memory 2021

Methylprednisolone 45

Midbrain 5,10,19,24

Mitochondria 47,51

Monoamine oxidase inhibitors (MAOIs)

Morphine 8, 35, 3738

Motor cortex 5, 24

Motor neuron 11, 2223,24

Motor unit 22

Movement 2223,24

MPTP 35

Multiple sclerosis 5, 44

Myasthenia gravis 6

Myelin 67

Index

Numbers in bold refer to illustrations.

Narcolepsy 26

Nerve growth factor (NGF) 52

Nerve impulse 67

Neuroethics 5556

Neurofibrillary tangles 40

Neurological trauma 4546

Neuron 67

birth 1011

migration 1011

pathfinding 1112

survival 1213

Neurotransmitters 69

Nicotine 3637

NMDA receptors 7

Nondeclarative knowledge 2021

Norepinephrine 8

Obsessivecompulsive disorder (OCD)

Occipital lobe 5,11,14

Olfactory bulbs 1618

Opiates 3738

Optical imaging 50

Pain 1719,3435

Panic disorder 4142

Parietal lobe 5

Parkinson’s disease 5, 35

Peptides 8

Peripheral nervous system 13

Phobias 4142

Pituitary gland 89, 29

Pons 5,27

Positron emission tomography (PET)

21, 49

Primary visual cortex 14, 15

Prostaglandins 18, 34

Psychostimulants 37

Receptive field 14

Receptors 67

Reflex 2223

Regeneration 46, 52

Reproduction 9

Schizophrenia 5, 46

Second messengers 9

Serotonin 8

Single photon emission computed

tomography (SPECT) 49

Sleep 25, 26,27

REM sleep 2526,27

stages 26

disorders 2526

Smell 1617, 18

Spinal cord 5, 13,19,23,4546

Strabismus 16

Stress 2830

in arousal 2829

chronic 30

and endocrine system 2830

and schizophrenia 46

Stroke 5, 4647,48

Substance P 8

Synapse 67

Taste 1617, 18

Tempor a l lobe 5,2021

Testosterone 9

Thalamus 5

Touch 1719

Tourette syndrome 48

Tricyclic antidepressants 34

Trophic factors 8, 52

Vision 14, 1516

Working memory 20

National Institutes of

Health

National Eye Institute

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National Institute on Aging

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National Institute on Alcohol

Abuse and Alcoholism

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