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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 peripheral 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 concerned 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 organization, 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 encouraging 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 undergraduates, 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 worldwide. 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 providing 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

Introduction

t sets humans apart from all other species by allowing us to
achieve the wonders of walking on the moon and composing 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 supercomputer.
But the brain is much more complicated than either of these
devices, a fact scientists conﬁrm almost daily, with each new discovery. 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 memory. 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 system 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 alcohol 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 lateral sclerosis. This has provided new insights into underlying dis4

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 mechanisms 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 magnetic 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 developmental dysfunction.
Federal neuroscience research funding of more than $5 billion 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.

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 functions — thinking, planning, and
problem-solving. The hippocampus is involved in memory.
The thalamus serves as a relay
station for almost all of the
information coming into the
brain. Neurons in the hypothalamus 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, colliculi, collections of cells that
relay specific sensory information 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

THE TOLL OF SELECTED BRAIN AND NERVOUS SYSTEM DISORDERS*

Condition

Total Cases

Hearing Loss

28 million

All Depressive Disorders
Alzheimer’s Disease
Huntington’s Disease
Stroke
Schizophrenia

control movement as well as

Costs Per Year

cognitive processes that require

precise timing.

56 billion

20.5 million

44 billion

4.5 million

100 billion

30,000

2 billion

4.7 million

51 billion

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 neuron

specialized cell designed to transmit information 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 interconnected 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 communicates 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 insulating myelin sheath, made of specialized cells called oligodendrocytes 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 channels, 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 communicate to its own synaptic target neurons — depends on a
small di∑erence in electrical charge between the inside and outside of the cell. When a nerve impulse begins, a dramatic reversal 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 neuron may be able to ﬁre impulses scores of times every second.
Upon reaching the end of an axon, these voltage changes trigger the release of neurotransmitters, the brain’s chemical messen6

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 recognizes 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 Parkinson’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 illness 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 contract) 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 potential 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 discoveries suggest, however, that it may be critical for normal attention, memory, and sleep. Since ACh-releasing neurons die in
Alzheimer’s patients, ﬁnding ways to restore this neurotransmitter is one goal of current research.

Amino acids Amino acids, widely distributed throughout the
body and the brain, serve as the building blocks of proteins. Certain amino acids can also serve as neurotransmitters in the brain.
The neurotransmitters glutamate and aspartate act as excitatory 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 coordinating 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 neurotransmitters that are stored in
pouches called vesicles. Neurotransmitters 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 movements 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 symptoms, learning more about dopamine is important to understanding 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 throughout the brain. Deﬁciencies in this transmitter occur in patients
with Alzheimer’s disease, Parkinson’s disease, and Korsako∑’s syndrome, 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 norepinephrine.
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 various 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 depression 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
8

make substances very similar to opium. Shortly thereafter, scientists 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 peptide, whose name comes from endogenous morphine — were discovered.
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 pathways in the brain that are activated by painful or tissue-damaging 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 peptide 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 factors work in the brain. This information should also prove useful for the design of new therapies for brain disorders of development 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 pancreas, kidneys, heart, adrenal glands, gonads, thyroid, thymus,
and pituitary gland are sources of hormones. The endocrine system 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 environmental 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 hormones 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 neurotransmission 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 protection 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 progesterone. 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 similar 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 perception, 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 reproduction 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 neurotransmitters that are gases. These molecules — nitric oxide and
carbon monoxide — do not obey the “laws” governing neurotransmitter 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 attributable 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 milliseconds 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 messenger between one neuron and another. cAMP is called a second
messenger because it acts after the ﬁrst messenger, the transmitter chemical, has crossed the synaptic space and attached itself to
a receptor.
Second messengers also are thought to play a role in the manufacture and release of neurotransmitters, intracellular movements, 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 substances on the genetic material of cells may lead to long-term
alterations of behavior.
9

Brain development

hree 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 surface. 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 forebrain. The ﬁrst signs of the eyes and then the hemispheres of the
brain appear later.
How does all this happen? Although many of the mechanisms of human brain development remain secrets, neuroscientists 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 similarities and di∑erences, scientists can learn how the human brain
develops and how brain abnormalities, such as mental retardation 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 transmitting 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 circuits that remain. The result is a precisely elaborated adult network of 100 billion neurons capable of body movement, perception, emotion, and thought.
Knowing how the brain is put together is essential for understanding 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 construct 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. 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.

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 migrating along glial fibers. These
neurons make transient connections with other neurons before
reaching their destination. A single 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.

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 molecules, which are secreted from the mesoderm, block protein signaling, 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 concentrations 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 chemical 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 maintained in a dish with no other cell type, they produce the neurotransmitter 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 neurons 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, therefore, the ﬁnal determination of the chemical messengers that a
neuron produces, is inﬂuenced by factors coming from the targets themselves.
As neurons are produced, they move from the neural tube’s
ventricular zone, or inner surface, to near the border of the marginal 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 sur11

face, along nonneuronal ﬁbers that form a trail to their proper
destination. Proper neuron migration requires multiple mechanisms, including the recognition of the proper path and the ability to move long distances. One such mechanism for long-distance 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 considerable 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 enlargements 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 molecules 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, neuropilin, 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, including 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 signal. The regulation of this transmission at synapses, and the integration of inputs from the thousands of synapses each neuron
12

receives, are responsible for the astounding information-processing 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 molecules have been identiﬁed. There has been more success, however, in identifying the ways in which the synapse forms once
contact has been made. The tiny portion of the axon that contacts the dendrite becomes specialized for the release of neurotransmitters, and the tiny portion of the dendrite that receives
the contact becomes specialized to receive and respond to the signal. 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 development survive to function in the adult. Entire populations of neurons 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 adulthood 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 discovery — 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 example, 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. Communication 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 incorrect 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 specific 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 connecting the CNS with internal
organs. It is divided into the sympathetic nervous system, which
mobilizes energy and resources
during times of stress and
arousal, and the parasympathetic 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 emotional 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 deprivation 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 better 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 isolated 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 damage, and neurodegenerative disorders, as well as helping us understand aging.
13

Sensation and perception

ision. This wonderful sense allows us to
image the world around us, from the genius of
Michelangelo’s Sistine Chapel ceiling to mistﬁlled 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 vertebrate sensory system, with most of the information derived from
studies of monkeys and cats.
Vision begins with the cornea, which does about three-quarters 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 neurons 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 muscles 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 million or so ﬁbers of the optic nerve to the optic chiasm, where
some nerve ﬁbers cross over, so both sides of the brain receive signals 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.
14

Scientists know much about the way cells encode visual information in the retina, the lateral geniculate nucleus — an intermediate 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 margins 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 primary 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 preferred 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,

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 processing systems come from monkey anatomical and physiological data. They are veriﬁed by human psychological studies showing 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 elements 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 disorders. Information from research in cats and monkeys has
improved the therapy for strabismus, or squint, a term for “crosseye” 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 during infancy or early childhood. Beyond the age of 6 or so, the
blindness becomes permanent. But until a few decades ago, ophthalmologists 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 survival. 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 several qualities in the signal it detects. However, our hearing system 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 instruments 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 mechanism 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
16

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 thalamus 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 monosodium 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 sensory 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 hairlike cilia. These cilia contain the receptor sites that are stimulated
by odorants carried by airborne molecules. These dissolve in the

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 external 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-

mucus lining in order to stimulate receptor proteins in the cilia to start the smell response. An odorant 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 system of the brain.

brane, transmits the vibrations
to the incus (anvil). The vibrations are then passed onto the
stapes (stirrup) and oval window that, in turn, pass them
onto the inner ear. In the inner
ear, the fluid-filled spiral passage of the cochlea contains

Touch and pain

cells with microscopic, hairlike

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 sensitive, being triggered by slight movement of the hairs. Other receptors are more common in nonhairy areas, such as the lips and ﬁngertips, and consist of nerve cell endings that may be free or surrounded 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 discrimination, but the chest and back are less sensitive.

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.

TASTE AND SMELL . Specialized 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 perception, and to the limbic system 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 (protuberances) mainly on the
tongue, with a few located in
the back of the mouth and on
the palate. Each taste bud consists 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 cranial nerves and taste centers
in the brain.

Not surprisingly, acuity is greatest in the most densely nerve-packed areas of the body. This feature, 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 necessary 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 nervous 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 nervous 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 nociceptors, 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

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 setting in which the injury occurs contributes (e.g., the pain of childbirth or that produced in a car accident). The emotional component 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 naturally occurring chemicals similar to opioids. The three major
families of opioid peptides identiﬁed in the brain — enkephalins,
beta-endorphins, and dynorphins — originate from three precursor proteins encoded by three di∑erent genes. They act at multiple opioid receptors in the brain and spinal cord. Knowledge of

the way pain messages are transmitted has led to new treatments
for pain. For example, scientists began studying the spinal delivery of opioids when they discovered a dense distribution of opioid receptors in the spinal cord horn. Such treatments were
begun in humans after the method was successfully used in animals; the technique is now common in treating pain after surgery.
Because the spinal opioid does not interact at all levels of the nervous 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 experienced 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.
PAIN. Messages about tissue
damage are picked up by receptors and transmitted to the
spinal cord via small myelinated
fibers and very small unmyelinated fibers. From the spinal
cord, the impulses are carried to
the brainstem, thalamus, and
cerebral cortex and ultimately
perceived as pain. These messages can be suppressed by a
system of neurons that originates in the gray matter of the
midbrain. This descending pathway sends messages to the
spinal cord where it suppresses
the transmission of tissue damage signals to the higher brain
centers. Some of these descending pathways use naturally
occurring, opiate-like chemicals
called endorphins.

Learning, memory, and
language

he 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 memory 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 electroconvulsive 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 learning 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 connected 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 discovered that certain neurons in this area are inﬂuenced by neurons 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 different kinds of information in separate ways and then store it differently.
20

Declarative knowledge requires processing in the medial temporal 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 cortex. 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 emotional 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 consequences. 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, scientists found that this occurs in the short term through two biochemical 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 relevant 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 stimulation. 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 AND MEMORY. Different brain areas and systems
mediate distinct forms of memory. The hippocampus, parahippocampal region, and areas of the cerebral cortex (including prefrontal cortex) compose a system that supports declarative, or cognitive, memory. Different forms of nondeclarative, or behavioral,
memory are supported by the amygdala, striatum, and cerebellum.

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 cortical 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 material — 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 indicate 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 processing systems that are also involved in the perception, processing, 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 comprehension 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 program for vocal expression is created. This program is then transmitted 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 diagnosis 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 studies with patients also have indicated that it is likely that familiar
words need not be recoded into sound before they can be understood.
Although the understanding of how language is implemented
in the brain is far from complete, there are now several techniques that may be used to gain important insights into this critical aspect of brain function.
21

Movement

rom the stands, we marvel at the perfectly placed
serves of professional tennis players and the lightning-fast double plays executed by big league
inﬁelders. But in fact, each of us in our daily activities 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 accomplish these everyday marvels, and to perform them more skillfully 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 of a
given muscle can open or close the joints that it spans, depending 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 position in the face of unpredictable external forces that would otherwise displace it. Muscles that move a joint in an intended direction 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 expression. 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 critical link between the brain and muscles. If they die, which can
22

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 receptors 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 muscle. 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 contraction 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 muscle tendons — the golgi tendon organs — detect the force applied
by a contracting muscle, allowing the brain to also sense and control the muscular force exerted during 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 neurons 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 connection 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.

to control only two or three functionally related muscles, such
We now know that these complex systems are coordinated
as those of the hand or arm, that are important for ﬁnely tuned,
and organized to respond di∑erently for tasks that require preskilled movement.
cise control of position, such as holding a full teacup, and for
In addition to the motor cortex, movement control also
those requiring rapid, strong movement, such as throwing a ball.
involves the interaction of many other brain regions, including
You can experience such changes in motor strategy when you
the basal ganglia, thalamus, cerebellum, and a large number of
compare walking down an illuminated staircase with the same
neuron groups located within the midbrain and brainstem —
task done in the dark.
regions that connect cerebral hemispheres with the spinal cord.
Another useful reﬂex is the ﬂexion withdrawal that occurs if
your bare foot encounters a sharp object. Your leg is immediately
The brain regions devoted to such control are large, containing
lifted from the source of potential injury (ﬂexion) but the oppomillions of intricately interconnected neurons.
site leg responds with increased extension in order to maintain
Scientists know that the basal ganglia and thalamus have
your balance. The latter event is called the crossed extension reﬂex.
widespread connections with sensory and motor areas of the
These responses occur very
cerebral cortex. Loss of regularapidly and without your attention of the basal ganglia by
We now know that complex
tion because they are built into
dopamine depletion can cause
systems of neurons that are
serious movement disorders,
systems are coordinated and organized
located within the spinal cord
such as Parkinson’s disease. Loss
itself. It seems likely that the
of dopamine neurons in the
to respond differently for tasks that
same systems of spinal neurons
substantia nigra of the midalso participate in controlling
brain, which connects with the
require precise control of position,
the alternating action of the
basal ganglia, is a major factor
legs during normal walking. In
in Parkinson’s disease.
such as holding a teacup, and for those
fact, the basic patterns of musAnother brain region that is
cle activation that produce
crucial for skilled movement
requiring rapid, strong movement,
coordinated walking can be
and for the learning of new
generated in four-footed animovements is the cerebellum. A
such as throwing a ball.
mals within the spinal cord
disturbance of cerebellar funcitself. These spinal mechanisms, which evolved in primitive vertion, for example, leads to poor coordination of muscle control,
tebrates, are likely still present in the human spinal cord.
to disorders of balance, and even to di≈culties in speech, one of
The most complex movements that we perform, including
the most intricate forms of movement control.
voluntary ones that require conscious planning, involve control
The cerebellum receives direct and powerful sensory inforof these basic spinal mechanisms by the brain. Scientists are only
mation from the muscle receptors and the sense organs of the
beginning to understand the complex interactions that take place
inner ear, which signal head position and movement, and sigamong di∑erent brain regions during voluntary movements,
nals from the cerebral cortex. The cerebellum apparently acts to
mostly through careful experiments on animals.
integrate all this information to ensure smooth coordination of
One important brain area in the control of voluntary movemuscle action, enabling us to perform skilled movements more
ment is the motor cortex, which exerts powerful control over the
or less automatically. There is evidence that as we learn to walk,
spinal cord, in part through direct control of its alpha motor
speak, or play a musical instrument, the necessary, detailed conneurons. Some neurons in the motor cortex appear to specify the
trol information is stored within the cerebellum, where it can be
coordinated action of many muscles, to produce organized
called upon by commands from the cerebral cortex.
movement of a limb to a particular place in space. Others appear

Sleep

leep remains one of the great mysteries of modern 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 understanding some of the brain circuitry that controls 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 devising 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 actually 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 experiments 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 progresses through a series of stages during which the brain waves
progressively slow down. This period of slow wave sleep is accompanied 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 temperature become much more variable. Men often have erections during 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 partway through the night and cannot fall asleep again. Although
there are a variety of short-acting sedatives and sedating antidepressant 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 inadequate 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
25

of daytime accidents, especially automobile accidents. Treatment
may include a variety of attempts to reduce airway collapse during sleep. Whereas simple things like losing weight, avoiding alcohol 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 airway 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 condition known as hypnagogic hallucination. They also have attacks
during which they lose muscle tone, similar to what occurs during REM sleep, but while they are awake. These attacks of paralysis, 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 interactions 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 neurotransmitters promote wakefulness, including norepinephrine,
serotonin, histamine, and glutamate.
During REM sleep, the cholinergic nerve cells and the thalamus 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 brainstem nerve cells in the locus coeruleus that use the neurotransmitter norepinephrine; the dorsal and median raphe groups that
contain serotonin; and, in the hypothalamus, the tuberomammillary 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 hypothalamus, 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 wakefulness. 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. Wakefulness is maintained by activity in
two systems of neurons. Neurons that make the neurotransmitter 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 centers alone can create rapid eye
movement sleep. Activation of
other neurons that make
monoamine neurotransmitters
such as norepinephrine, serotonin, and histamine (blue pathways) is needed for waking.

of nerve cells, in the ventrolateral preoptic nucleus, contains the
inhibitory neurotransmitters galanin and GABA. When the ventrolateral preoptic neurons ﬁre, they are thought to turn o∑ the
arousal systems, causing sleep. Damage to the ventrolateral preoptic nucleus produces irreversible insomnia.
A second group of nerve cells in the lateral hypothalamus
inﬂuences and suppresses REM sleep. They contain the neurotransmitter orexin, which provides an excitatory signal to the
arousal system, particularly to the monoamine neurons. In experiments in which the gene for the neurotransmitter orexin was
experimentally removed in mice, the animals became narcoleptic. Similarly, in two dog species with naturally occurring narcolepsy, an abnormality was discovered in the gene for the type 2
orexin receptor. Recent studies show that in humans with narcolepsy, the orexin levels in the brain and spinal ﬂuid are abnormally low. Thus, orexin appears to play a critical role in activating 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 suggested. There is evidence that a chemical called adenosine, which
is linked to brain energy depletion, accumulates in the brain during prolonged wakefulness and that it may drive sleep homeostasis. 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 progressively accumulates and leads to a degradation of mental function. 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 circadian 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 regulating sleep and arousal.
27

Stress

he ability to react in response to threatening
events has been with us since the time of our
ancient ancestors. In response to impending danger, 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 accelerating 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, medical 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 homeostasis 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 particularly 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. Moreover, 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 mechanisms can badly upset the body’s biochemical balance and accelerate 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 consequences of the sorts of mild to moderate stressors that typically
aΩict Westernized humans.

The immediate response
A stressful situation activates three major communication systems 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 monkeys. 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 sensory 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 calming branch, which keeps the body’s maintenance systems, such as
digestion, in order and calms the body’s responses to the emergency 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, preventing the body from remaining too long in a state of mobilization. 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 neuroendocrine 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

THE STRESS REACTION.
When stress occurs, the sympathetic nervous system is triggered. Norepinephrine is
released by nerves, and epinephrine is secreted by the
adrenal glands. By activating
receptors in blood vessels and
other structures, these substances ready the heart and
working muscles for action.
Acetylcholine is released in the
parasympathetic nervous system, 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 neuroendocrine system also maintains the body’s normal
internal functioning. Corticotrophin-releasing hormone (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 adrenaline) and cortisol. When the body is exposed to stressors, epinephrine, which combines elements of hormones and the nervous 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 hormones a∑ecting glucose metabolism. In primates, the main glucocorticoid 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, immunosuppression, 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 pro29

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 glucocorticoids may help to explain disorders such as jet lag, which
results when the light-dark cycle is altered by travel over long distances, 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 adaptation 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 standing 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 epinephrine production increases blood pressure. Together, elevated cortisol 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 emotion — 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 normal number of brain neurons and smaller brain size.
30

The immune system, which receives messages from the nervous 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 inﬂammation 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 glucocorticoids 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 resistance or susceptibility to disease may be a person’s sense of control as opposed to a feeling of helplessness. This phenomenon
may help explain large individual variations in response to disease. Scientists are trying to identify how the perception of control or helplessness inﬂuences physiological processes that regulate 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 holding jobs that carry high demands and low control, such as telephone 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 — exhibited 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. Scientists 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.

Aging

N
P

euroscientists 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, particularly in forming memories of recent events. For example, once
you reach your 70s, you may start to forget names or phone numbers 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 percent of people younger than age 65, with the incidence increasing to nearly 50 percent in those older than 85. In a small, third
group, mental functioning seems una∑ected by age. Many people do well throughout life and continue to do well even when
old. The oldest human, Jeanne Calment, kept her wits throughout 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 performance.
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 typically ill. In 2003, when life expectancy was more than 77 years,
nearly 36 million people, or more than 12 percent of the population, 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 little 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 structure 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 proteins 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 dendrites 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 compensate, 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
31

THE AGING BRAIN. Studies
of people who have died contradict the popular belief that
adults lose an enormous number
of neurons every day. In fact,
many areas of the brain, primarily 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 neurons changes with aging, so that
the brain is constantly capable
of being modified or improved.

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. Several 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 scientists found that changes in mental function are subtle. For example, in rodents and primates in which only minor brain abnormalities 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 neurons 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 environments, researchers found that dendrites in the cerebral cortex 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 scien32

tists concluded that motor learning generates new synapses.
Physical exercise, however, improved blood circulation in the
brain. Aerobic exercise can also improve human cognitive performance.
Other scientists report that rats reared in a stimulating environment made signiﬁcantly fewer errors in a maze test than did
similar rats kept in an isolated environment. Moreover, the stimulated rats showed an increase in brain weight and cortical thickness 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.

Advances
ipolar disorder. Patients with bipolar disorder, previously known as manic-depressive illness, usually experience episodes of deep depression 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 become more frequent without treatment. Because bipolar disorder runs in families, e∑orts are underway to identify the responsible 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-totreat bipolar episodes. Newer anticonvulsant medications are being
studied to determine how well they work in stabilizing mood cycles.

B
Epilepsy

Epilepsy, a chronic neurological disorder characterized by sudden,
disorderly discharge of brain cells, is marked by recurrent, unprovoked 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 excessive electrical activity over a wide area of the brain. Partial seizures
may occur with maintained consciousness or with altered awareness and behavioral changes. Partial seizures can produce localized
visual, auditory, and skin sensory disturbances; repetitive uncontrolled 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, approximately half of which have been introduced in the last several
years, available to prevent seizures. The principal targets of antiseizure 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 discharge. Identiﬁcation of the mutant genes underlying human
epilepsy may provide new targets for the next generation of antiseizure 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 antiseizure 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 striking improvement or complete remission of seizures.
A new form of epilepsy treatment, electrical stimulation therapy, 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, hopelessness, pessimism, loss of interest in life, and reduced emotional
33

HOW PAIN KILLERS
WORK. At the site of injury,
the body produces prostaglandins that increase pain sensitivity. Aspirin, which acts
primarily in the periphery, prevents the production of prostaglandins. Acetaminophen is
believed to block pain impulses
in the brain itself. Local anesthetics 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.

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 people with no mental illness.
Annually, major depression a∑ects 5 percent of the population, or 9.8 million Americans, aged 18 years and older. Fortunately, 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 delicate 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 abnormalities 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 signals that control mood, thoughts, and other sensations. The tri34

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 serotonin and norepinephrine, allowing these chemicals to remain
active. MAOIs available for use include isocarboxazid, phenelzine, 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 functionally similar to the tricyclic antidepressants, but act selectively
on the serotonin system. There are also several newer antidepressants 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 discoveries 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-

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, treatments, 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, lidocaine has been used in a slow-release patch to provide long-lasting pain in localized, speciﬁc parts of the body.
Analgesia refers to loss of pain sensation without loss of sensitivity 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) without 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 produce 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, posttherapeutic 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 analgesics 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 channels. 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 disease 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 administration of the drug levodopa, which is converted to dopamine
in the brain. The successful treatment of Parkinson’s by replacement 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 Parkinson’s occur sporadically. It is believed that hereditary factors may
render some individuals more vulnerable to environmental factors 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 disorder. 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 overactive. 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 procedure, pallidotomy, and more recently chronic deep-brain stimulation. These techniques are highly successful for treating
patients who have experienced signiﬁcant worsening of symptoms and are troubled by the development of drug-related involuntary 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.
35

Challenges

ddiction. Drug abuse is one of the nation’s
most serious health problems. Indeed, 6 percent of Americans, roughly 15 million people,
abuse drugs on a regular basis. Recent estimates 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 feelings 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 activated 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 chemicals, called neurotransmitters, to communicate with each other.
Abused drugs alter the ways neurotransmitters carry their messages from neuron to neuron. Some drugs mimic neurotransmitters, 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 example, people who take drugs to get high may get addicted, but people who use them properly as medicine rarely do. Genetic susceptibility or environmental factors, such as stress, may also alter
36

the way that people respond to drugs. In addition, the development 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 adaptive 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, addiction requires that these occur while a motivational form of dependence — 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, respiration, 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 mechanism is thought to underlie the pleasurable sensations experienced by many smokers.
Much better understanding of addiction, coupled with the
identiﬁcation of nicotine as an addictive drug, has been instrumental 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

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 studies 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 system. Opiates act in this system and many other brain regions, including 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.

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 antidepressant marketed as Zyban, has been approved for use as a pharmacological treatment for nicotine addiction. Behavioral treatments 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 million 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, producing a rush of euphoria and feelings of power and selfconﬁdence. The key biochemical factor that underlies the reinforcing 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 symptoms range from mild ﬂu-like discomfort to major physical ailments, 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 circuits causes a brief rush of intense euphoria, followed by a couple 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 medical 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.
37

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 represent 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 receptors 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.

A standard treatment for opiate addiction involves methadone, a long-acting oral opiate that helps keep craving, withdrawal, and relapse under control. Methadone helps opiate
addicts rehabilitate themselves by preventing withdrawal symptoms that are powerful motivators of drug use. A synthetic opiate, 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
38

other opiates. As antagonists, they are especially useful as antidotes. Another medication to treat heroin addiction, buprenorphine, causes weaker opiate e∑ects and is less likely to cause overdose 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 alcoholism 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 alcohol 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

chronic health problem associated with alcohol addiction — are
combinations of any of these drugs, particularly with alcohol,
responsible for more than 25,000 deaths each year. The annual
can lead to unexpected adverse reactions and even death after
cost of alcohol abuse and addiction is estimated at $185 billion.
high doses. Physical exhaustion also can enhance some toxicities
Genetic and environmental factors contribute to alcoholism,
and problems.
but no single factor or combination of factors enables doctors to
MDMA, called “Adam,” “ecstasy,” or “XTC” on the street, is a
predict who will become an alcoholic.
synthetic, psychoactive drug with hallucinogenic and amphetaEthanol, the active ingredient in alcoholic beverages, reduces
mine-like properties. Users encounter problems similar to those
anxiety, tension, and inhibitions. In low doses it may act as a
found with the use of amphetamines and cocaine. Recent research
stimulant, whereas at higher doses, it acts as a depressant. In both
also may link ecstasy use to long-term damage to those parts of the
cases, it signiﬁcantly alters mood and behavior. It can also cause
brain critical to thought, memory, and pleasure.
heat loss and dehydration.
Rohypnol, GHB (gamma hydroxy-butyrate), and ketamine
The drug, which is easily absorbed into the bloodstream and
are predominantly central nervous system depressants. Because
the brain, a∑ects several neurotransmitter systems. For example,
they are often colorless, tasteless, and odorless, they can be easalcohol’s interaction with the
ily added to beverages and
GABA receptor can calm anxiingested unknowingly. These
Attention
deficit
hyperactivity
ety, impair muscle control, and
drugs have emerged as the sodelay reaction time. At higher
called date-rape drugs. When
disorder (ADHD) was first described
mixed with alcohol, rohypnol
doses, alcohol also decreases the
can incapacitate victims and
function of NMDA receptors
more than 100 years ago. Today it is
prevent them from resisting
that recognize the neurotranssexual assault. Also, rohypnol
mitter glutamate. This interacthe focus of hundreds of studies.
may be lethal when mixed with
tion can cloud thinking and
alcohol and other depressants.
eventually lead to coma.
Since about 1990, GHB has been abused in the United States for
Researchers are developing treatments that interfere with
euphoric, sedative, and anabolic (body building) e∑ects. It, too,
molecules, such as the opioid peptides, that trigger alcohol’s poshas been associated with sexual assault. Ketamine is another cenitive reinforcing e∑ects. One such drug, naltrexone, recently has
tral nervous system depressant abused as a date-rape drug. Ketbeen approved for treating alcoholism.
Marijuana This drug can distort perception and alter the
amine, or “Special K,” is a fast-acting general anesthetic. It has
sense of time, space, and self. In certain situations, marijuana can
sedative, hypnotic, analgesic, and hallucinogenic properties. It is
produce intense anxiety.
marketed in the United States and a number of foreign countries
In radioactive tracing studies, scientists found that tetrahyfor use as a general anesthetic in both human and veterinary
drocannabinol (THC), the active ingredient in marijuana, binds
medical practice.
to speciﬁc receptors, many of which coordinate movement. This
Attention deficit hyperactivity disorder
may explain why people who drive after they smoke marijuana
Attention deﬁcit hyperactivity disorder (ADHD) was ﬁrst
are impaired. The hippocampus, a structure involved with memdescribed more than 100 years ago. Today it is the focus of hunory storage and learning, also contains many THC receptors. This
dreds of studies. Characterized by excessively inattentive, hypermay explain why heavy users or those intoxicated on marijuana
active, or impulsive behaviors, ADHD a∑ects an estimated 2 milhave poor short-term memory and problems processing complex
lion children in the United States, or 3 percent to 5 percent of
information. Scientists recently discovered that these receptors
children. Studies show that 30 percent to 70 percent of these chilnormally bind to a natural internal chemical called anandamide,
and they are now working to see how this chemical a∑ects brain
dren will continue to experience ADHD symptoms as adults.
function.
By deﬁnition, symptoms of ADHD appear before age 7, last
Club Drugs Ecstasy, herbal ecstasy, rohypnol (“rooﬁes”),
for six months or longer, and impair normal functioning in at
GHB, and ketamine are among the drugs used by some teens and
least two areas of a child’s life — at school, among friends, or at
young adults as part of rave and trance events, which are generhome, for example. Adults must show impairment at home and
ally night-long dances, often held in warehouses. The drugs are
at work. Normal children sometimes show similar behavior, and
rumored to increase stamina and to produce intoxicating highs
other conditions, disorders, or environmental triggers — such as
that are said to deepen the rave or trance experience. Recent hard
novelty — may also be present with ADHD children; therefore,
science, however, is uncovering the serious damage that can occur
diagnosis requires a comprehensive evaluation, using parent and
in several parts of the brain from use of some of these drugs.
teacher rating scales, a clinical interview, and testing. Currently,
Many users tend to experiment with a variety of club drugs
ADHD is diagnosed solely on the basis of behavioral symptoms.
in combination. This practice creates a larger problem, because
Some studies show a correlation between ADHD and changes
39

and tangles are mostly in brain regions important for memory
and intellectual functions.
In cases of AD, reductions occur in levels of markers for several 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 precursor protein is on chromosome 21. In other families with earlyonset AD, mutations have been
Alzheimer’s disease
identiﬁed in the presenilin 1 and
One of the most frightening and
2 genes, which are on chromoOne of the most frightening and
devastating of all neurological
somes 14 and 1, respectively.
Apolipoprotein
E (apoE), a chrodisorders is the dementia that
devastating of all neurological disorders
mosome 19 gene, which inﬂuoccurs in the elderly. The most
ences susceptibility in late life,
common cause of this illness is
is the dementia that occurs in the
exists in three forms, with apoE4
Alzheimer’s disease (AD). Rare
clearly associated with enhanced
before age 60 but increasingly
elderly. The most common cause of this
risk.
prevalent in each decade thereTreatments are available
after, AD a∑ects an estimated 4
illness is Alzheimer's disease.
mostly only for some symptoms
to 5 million Americans. By the
of AD, such as agitation, anxiety,
year 2040, it is predicted to
unpredictable behavior, sleep disturbances, and depression. Three
a∑ect approximately 14 million individuals in the United States.
drugs treat cognitive symptoms in patients with mild to moderThe earliest symptoms are forgetfulness and memory loss;
ate Alzheimer’s. These agents improve memory deﬁcits temdisorientation to time or place; and di≈culty with concentration,
porarily. Several other approaches, such as antioxidants, are being
calculation, language, and judgment. Some patients have severe
tested.
behavioral disturbances and may even become psychotic. The illAn exciting area of research is the use of approaches in which
ness is progressive. In the ﬁnal stages, the a∑ected individual is
genes are introduced in mice. These transgenic mice carrying
incapable of self-care. Unfortunately, no e∑ective treatments
mutant genes linked to inherited AD develop behavioral abnorexist, and patients usually die from pneumonia or some other
malities and some of the cellular changes that occur in humans.
complication. AD, which kills 100,000 people a year, is one of the
It is anticipated that these mice models will prove very useful for
leading causes of death in the United States.
studying the mechanisms of AD and testing novel therapies.
In the earliest stages, the clinical diagnosis of possible or
Moreover, researchers have begun to knock out genes playprobable AD can be made with greater than 80 percent accuracy.
ing critical roles in the production of amyloid. These enzymes,
As the course of the disease progresses, the accuracy of diagnotermed beta and gamma secretase, which cleave the amyloid pepsis at Alzheimer’s research centers exceeds 90 percent. The diagtide from the precursor, are clearly targets for development of
nosis depends on medical history, physical and neurological
drugs to block amyloid.
examinations, psychological testing, laboratory tests, and brain
imaging studies. At present, ﬁnal conﬁrmation of the diagnosis
Amyotrophic lateral sclerosis
requires examination of brain tissue, usually obtained at autopsy.
This progressive disorder strikes more than 5,000 Americans
The causes and mechanisms of the brain abnormalities are
annually, with an average survival time of just three to ﬁve years
not yet fully understood, but great progress has been made
from symptom onset. It is the most common disorder within a
through genetics, biochemistry, cell biology, and experimental
group of diseases a∑ecting motor neurons and costs Americans
treatments. Microscopic examination of AD brain tissue shows
some $300 million annually.
abnormal accumulations of a small ﬁbrillar peptide, termed a
beta amyloid, in the spaces around synapses (neuritic plaques) and
Commonly known as Lou Gehrig’s disease, amyotrophic latby abnormal accumulations of a modiﬁed form of the protein tau
eral sclerosis (ALS) a∑ects neurons that control voluntary muscle
in the cell bodies of neurons (neuroﬁbrillary tangles). The plaques
movements such as walking. For reasons that are not completely
in brain structure, suggesting the possibility of using neuroimaging 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.

understood, motor neurons in the brain and spinal cord begin to
anxiety disorder; post-traumatic stress disorder; panic disorder;
disintegrate. Because signals from the brain are not carried by
and obsessive-compulsive disorder (OCD). Some can keep peothese damaged nerves to the body, the muscles begin to weaken
ple completely housebound or, as in the case of panic disorder,
and deteriorate from the lack of stimulation and resulting disuse.
contribute to suicide. Many of these disorders occur with depresThe ﬁrst signs of progressive paralysis are usually seen in the
sion, and individuals so aΩicted are at high risk of suicide.
hands and feet. They include weakness in the legs, di≈culty walkIn OCD, people become trapped, often for many years, in
ing, and clumsiness of the hands when washing and dressing.
repetitive thoughts and behaviors, which they recognize as
Eventually, almost all muscles under voluntary control, including
groundless but cannot stop, such as repeatedly washing hands, or
those of the respiratory system, are a∑ected. Despite the paralysis,
checking that doors are locked or stoves turned o∑. The illness is
however, the mind and the senses remain intact. Death is usually
estimated to a∑ect 3.8 million Americans annually. Social learncaused by respiratory failure or pneumonia.
ing and genetics likely play a role in developing the disorder.
No speciﬁc test identiﬁes ALS, but muscle biopsies, blood
Positron emission tomography (PET) scans reveal abnormalities
studies, electrical tests of muscle activity, CT and MRI scans, and
in both cortical and deep areas of the brain, suggesting central
X-rays of the spinal cord help
nervous system changes in OCD
identify the disease and rule out
patients.
other disorders. Still, diagnosis
Scientists recently discovThe most widespread
is often di≈cult because its
ered that certain breeds of large
causes remain unknown. Potendogs that develop acral lick synmental illnesses, anxiety disorders
drome, severely sore paws from
tial causes or contributors to the
compulsive licking, respond to
disease include glutamate toxicannually affect an estimated
the serotonergic antidepressant
ity, oxidative stress, environclomipramine, which was the
mental factors, and an autoim12.6 percent of the adult population,
ﬁrst e∑ective treatment develmune response in which the
oped for OCD in people.
body’s defenses turn against
or 24.8 million Americans.
Serotonergic antidepressants
body tissue.
— especially the tricyclics, such
In more than 90 percent of
as clomipramine, and the selective serotonin reuptake inhibitors
cases, ALS is sporadic, arising in individuals with no known fam(SSRIs), such as sertraline (Zoloft) and paroxetine (Paxil) — are
ily history of the disorder. In the other 5–10 percent of cases, ALS
e∑ective in treating OCD. A specialized type of behavioral interis familial — transmitted to family members because of a gene
defect.
vention, exposure and response prevention, is also e∑ective in many
patients.
Scientists have now identiﬁed several genes that are responPanic disorder, which a∑ects 2.4 million Americans annually,
sible for some forms of ALS. The most common and well studusually starts “out of the blue.” Patients experience an overwhelmied of these are mutations in the gene that codes for superoxide
dismutase, located on chromosome 21, that were linked to the
ing sense of impending doom, accompanied by sweating, weakpresence of this disorder. Scientists believe that whatever they
ness, dizziness, and shortness of breath. With repeated attacks,
learn from studying this gene and others will have relevance for
patients may develop anxiety in anticipation of another attack and
understanding the more common sporadic form of motor neuavoid public settings where attacks might occur. If these patients
ron disease.
are untreated, their lives may constrict until they develop agoraphobia, becoming virtually housebound.
Once diagnosed, physical therapy and rehabilitation methPhobia is an intense, irrational fear of a particular object or
ods help strengthen unused muscles. Various drugs can ease
situation. Individuals can develop phobias of almost anything,
speciﬁc problems, such as twitching and muscle weakness, but
such as dogs, dating, or driving over bridges. Exposure to the
there is no cure. An anti-glutamate drug moderately slows the
feared object or situation can trigger an extreme fear reaction that
disease. Additional drugs are now under study. Protecting or
may include a pounding heart, shortness of breath, and sweating.
regenerating motor neurons using nerve growth factors, other
Experiencing or witnessing a crime or being a victim of sexmore potent drugs, and stem cells may someday provide signiﬁual abuse can lead to a form of stress that can last a lifetime.
cant hope for patients.
Termed post-traumatic stress disorder, the condition aΩicts 5.2
Anxiety disorders
million Americans aged 18 to 54 each year.
The most widespread mental illnesses, anxiety disorders annuThe recent discovery of brain receptors for the benzodially a∑ect an estimated 12.6 percent of the adult population, or
azepine antianxiety drugs has sparked research to identify the
24.8 million Americans. They include phobias such as fear of
brain’s own antianxiety chemical messengers. This ﬁnding may
heights, agoraphobia, and social anxiety disorder; generalized
lead to ways to regulate this brain system and correct its possible
41

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. Tricyclic 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 incidence is a key mystery to be solved.
Characterized by communication di≈culties, delayed development 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 specialized education and language intervention programs. The earlier 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 secondary 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
42

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 classifying brain tumors, exact statistics on secondary tumors are
unknown.
Symptoms vary according to location and size. The compression of brain tissue or nerve tracts, as well as expansion of
the tumor, can cause symptoms such as seizures, headaches, muscle 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 tomography) 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 prolong 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 cancer 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 syndrome is likely to possess, to various degrees, some of these characteristics: 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
researchers and genetic counselors have established speciﬁc proproblems, and digestive tract obstruction. All people with Down
tocols for predictive testing to ensure that the psychological and
syndrome show the neuropathological changes of Alzheimer’s
social consequences of a positive or negative result are underdisease by age 40, and most show cognitive decline by age 60.
stood. Predictive testing is available only for adults, though chilBabies with Down syndrome develop much like typical children under 18 may be tested to conﬁrm a diagnosis of juveniledren, but at a somewhat slower rate. They will learn to sit, walk,
onset HD. Prenatal testing may be performed. The ethical issues
talk, and toilet train, just like their peers. Early intervention proof testing must be considered, and the individual must be adegrams can begin shortly after birth and can help foster an infant’s
quately informed, because there is no e∑ective treatment or cure.
development.
The HD mutation is an expanded triplet repeat in the HD
Thanks to medical advances and a greater understanding of
gene — a kind of molecular stutter in the DNA. This abnormal
the potential of those with this condition, people with Down syngene codes for an abnormal protein called huntingtin. The huntingtin protein, whose normal function is still unknown, is widely
drome have been able to have longer and fuller lives. Individuals
distributed in the brain and
with Down syndrome are being
appears to be associated with
educated in their neighborhood
proteins involved in transcripschools, participating in comAffecting some 30,000 Americans and
tion, protein turnover, and
munity activities, and ﬁnding
energy production. But the
rewarding employment and
placing another 200,000 at risk,
cause of HD probably involves
relationships.
the gain of a new and toxic funcAlthough there is no cure
Huntington’s disease is now considered
tion. Cell and transgenic animal
for or means of preventing
models can replicate many feaDown syndrome, scientists are
one of the most common hereditary
tures of the disease and are now
moving closer to understanding
being used to test new theories
the role that the genes on chrobrain disorders.
and therapies. Clinical and
mosome 21 play in a person’s
observational trials are being
development. Once this mystery
conducted. Any of these may yield an e∑ective treatment that
is understood, they hope to decode the biochemical processes
would slow the progression of or delay onset of the disease while
that occur in Down syndrome and learn to treat or cure this disresearchers continue working toward a cure.
order.

Huntington’s disease

Learning disorders

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 individual 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 jerking 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, common symptoms include di≈culty swallowing, unsteady gait, loss
of balance, impaired reasoning, and memory problems. Eventually, 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

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, reading, writing, reasoning, or mathematical abilities. These challenges 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 “developmental lag.”
There is now a strong consensus that the central di≈culty in
most forms of dyslexia reﬂects a deﬁcit within the language system — and more speciﬁcally, in a component of the language system called phonology. This is illustrated in di≈culty transforming the letters on the page to the sounds of language. A current
debate exists as to whether this di≈culty reﬂects a general soundprocessing deﬁcit or a problem speciﬁc to sounds of language,
phonemes, and conscious awareness of these sounds.
43

of tissue. Such lesions are called plaques and appear in multiple
As children approach adolescence, one manifestation of
dyslexia may be a very slow reading rate. Children may learn to
places within the central nervous system. These e∑ects are comread words accurately, but they will not be ﬂuent or automatic,
parable to the loss of insulating material around an electrical wire,
reﬂecting the lingering e∑ects of a phonologic deﬁcit. Because
which interferes with the transmission of signals.
they can read words accurately — albeit very slowly — dyslexic
Siblings of people with MS are 10 to 15 times more likely
adolescents and young adults may mistakenly be assumed to have
than others to be diagnosed with the disorder, whereas the risk
“outgrown” their dyslexia. The ability to read aloud accurately,
for disease concordance for identical twins is about 30 percent.
rapidly, and with good expression, as well as facility with spelling,
In addition, the disease is as much as ﬁve times more prevalent
may be most useful clinically in distinguishing students who are
in temperate zones, such as the northern United States and
average from those who are poor readers.
northern Europe, than it is in the tropics. Thus, both genetic
A range of investigations indicates that there are di∑erences
and environmental factors are probably involved in the cause.
in brain regions between dyslexic and nonimpaired readers,
An infection acquired during the ﬁrst 15 years of life may be
especially the temporo-parietoresponsible for triggering the
occipital and frontal opercular
disease in a genetically suscepAn estimated 10 percent of the
regions. Recent data using functible individual.
tional brain imaging indicate
The most common symppopulation, as many as 25 million
that dyslexic readers demontoms are blurred vision, awkstrate a functional disruption in
ward gait, numbness, and fatigue.
Americans, have some form of learning
an extensive system in the posThese can occur singly or in
terior portion of the brain. The
combination, vary in intensity,
disability involving difficulties
disruption occurs within the
and last from several weeks to
neural systems linking visual
months. In some patients, sympin the acquisition and use of listening,
representations of the letters to
toms include slurred speech,
speaking, reading, writing,
the phonologic structures they
weakness, loss of coordination,
represent. The speciﬁc cause of
uncontrollable tremors, loss of
reasoning, or mathematical abilities.
the disruption of neural sysbladder control, memory probtems in dyslexia is thought to
lems, depression, and paralysis.
result from developmental missteps relating to neuronal migraMuscle spasticity can a∑ect balance and coordination, causing
tion to the cerebral cortex. It is clear that dyslexia runs in famipain and involuntary jerking movement — and, if untreated, can
lies, and active research aims to identify what appear to be sevcreate contractures, or the “freezing” of a joint that prevents
movement.
eral dyslexia-susceptibility genes.
MS cannot be cured at present, but several medications conInterventions to help children with dyslexia focus on teachtrol relapsing forms of MS. A wide range of medications and thering the child that words can be segmented into smaller units of
apies are available to control symptoms such as spasticity, pain,
sound and that these sounds are linked with speciﬁc letter patfatigue, and mood swings, as well as bladder, bowel, or sexual dysterns. In addition, children with dyslexia require practice in readfunctions. Steroids, which have been used to treat MS for more
ing stories, both to allow them to apply their newly acquired
than three decades, may e∑ectively shorten attacks and speed
decoding skills to reading words in context and to experience
recovery from MS-related acute attacks. Promising new agents to
reading for meaning.
control MS or to alleviate its symptoms are in clinical trials.

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 earning 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 tissue is destroyed, it is replaced by scars of hardened sclerotic patches
44

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 people 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

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 progressive, 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 experience general slowing of their mental processes. At the same time,
patients may develop leg weakness and a loss of balance. Imaging techniques, such as computed tomography (CT) and magnetic 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 antiretroviral 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 cognitive 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 triggers 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 therapeutic 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 infections 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 regenerate and repair itself, and improved rehabilitation techniques.

Some 750,000 people su∑er traumatic head injuries requiring hospitalization each year, and roughly 100,000 die — many
before reaching the hospital. Economic costs approach $25 billion annually.
Greater access to and use of computed tomography (CT) and
magnetic resonance imaging (MRI) o∑er physicians the opportunity 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, usually 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 cerebral blood ﬂow, or it can by itself be toxic to brain cells. As a consequence, once detected, it is usually surgically removed. Contusions 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, violence, 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 intravenous doses of a commonly used steroid drug, methylprednisolone, within eight hours of the injury. Building on this knowledge, 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
45

can regain the ability to bear their weight and walk at various
response to stress. About 15 percent of patients return to normal
speeds on a treadmill belt. More recently, scientists have recoglife after a single episode, 60 percent will have intermittent
nized that the level of this recovery depends to a large degree on
episodes throughout their lives, and another 25 percent will not
whether these tasks are practiced — that is, trained for — after
recover their ability to live as independent adults. Deﬁcits in coginjury. It appears that humans with spinal cord injury also
nition, particularly involving attention and memory, are frerespond to training interventions.
quent, lifelong manifestations in most patients, even in those
Recently, scientists have discovered that new nerve cells can
who show good recovery from acute symptoms.
be born in the adult brain, but these new cells do not seem capaAfter a long search for an e∑ective antipsychotic medication,
ble of helping the injured brain regenerate. Studies are underway
scientists synthesized the drug chlorpromazine during the late
1940s. By the 1950s, it was found to be useful in treating psychotic
to determine how to “jumpstart” the pathway that stimulates
neurogenesis, the birth of new nerve cells. Researchers are also trystates and later became a mainstay of drug treatment.
ing to decipher how certain environmental cues can be used or
Since that time, many agents similar to chlorpromazine have
overcome to attract these new
been developed. When given as
cells — or transplanted stem or
long-acting injections, these
Schizophrenia is thought to reflect
progenitor cells — to areas of
drugs reduce some symptoms
brain injury to facilitate regenand aid patients’ readiness for
changes in the brain, possibly caused by
eration and repair.
adjustment back into their comThese and other recent dismunities. Chronic use of the
disease or injury at the time of birth,
coveries are pointing the way
drugs, however, may cause abtoward new therapies to pronormal muscle movements and
and a genetic disposition that may be
mote nerve regeneration after
tremors in some patients. Safer
brain and spinal cord injury.
treatments are being sought.
exacerbated by environmental stress.
Although these new therapies
Thus far, most drugs are
have not yet reached the clinic,
successful in treating hallucinaseveral approaches are on the path to clinical trials.
tions and thought disorder. Clozapine acts somewhat di∑erently
from other antipsychotic drugs. It treats the approximately 30
Schizophrenia
percent of patients who are not helped by conventional medMarked by disturbances in thinking, emotional reactions, and
ications. The drug can, however, induce a potentially fatal blood
social behavior, schizophrenia usually results in chronic illness
disorder, agranulocytosis, in about 1 percent of patients. To prevent this disorder, patients must take regular weekly to biweekly
and personality change. Delusions, hallucinations, and thought
blood tests, a precaution that makes using the drug very costly.
disorder are common.
Several new antipsychotics — risperidone, olanzapine, quetiapA∑ecting about 1 percent of the population, or 2 million
ine, ziprasadone, and aripiprazole — o∑er some of the beneﬁts
Americans each year, schizophrenia is disabling and costly. On a
of clozapine without risk of angranulocytosis, but their longgiven day, these patients occupy up to 100,000 hospital beds.
term side e∑ect proﬁles are not fully known.
Annual costs total about $32.5 billion.
Schizophrenia is thought to reﬂect changes in the brain, posStroke
sibly caused by disease or injury at the time of birth, and a genetic
Until recently, if you or a loved one had a stroke, your doctor
disposition that may be exacerbated by environmental stress.
would tell your family there was no treatment. In all likelihood,
Recently, several genes have been identiﬁed that appear to
the patient would live out the remaining months or years with
increase the risk of developing schizophrenia. Brain systems using
severe neurological impairment.
the chemicals dopamine, glutamate, and GABA appear to be parThis dismal scenario is now brightening. For one, use of the
ticularly involved in the pathogenesis of the disorder. Brain scans
clot-dissolving bioengineered drug, tissue plasminogen activator
and postmortem studies show abnormalities in some people with
(tPA), is now a standard treatment in many hospitals. This
schizophrenia, such as enlarged ventricles (ﬂuid-ﬁlled spaces)
approach rapidly opens blocked vessels to restore circulation
and reduced size of certain brain regions. Functional neurobefore oxygen loss causes permanent damage. Given within three
imaging scans such as positron emission tomography (PET) and
functional magnetic resonance imaging (fMRI) taken during intelhours of a stroke, it often can help in limiting the ensuing brain
lectual tasks show abnormal functioning in speciﬁc brain areas
damage. Also, attitudes about the nation’s third leading cause of
of people with this illness.
death are changing rapidly. Much of this has come from new and
The disorder usually begins between the ages of 15 and 25.
better understandings of the mechanisms that lead to the death
Some patients fully recover following treatment, but most conof neurons following stroke and devising ways to protect these
tinue to have moderate or severe symptoms, particularly in
neurons.
46

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 damage 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 symptoms 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. Anticoagulant 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 biochemical-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 people. 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 symptoms 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 disorder lasts a lifetime, but one-third of patients may experience a
remission or decrease in symptoms as they get older. Most people 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 attention span, and obsessive behaviors. Sometimes these manifestations are more troublesome to individuals than the tics themselves, so physicians must consider them when choosing a
treatment regimen.
The disorder seems to result from a hypersensitivity of dopamine receptors. Another neurotransmitter, serotonin, also has
been implicated. The most e∑ective drugs for control of movements, such as haloperidol, act by blocking the overactive system.
Other symptoms, such as obsessive-compulsive traits and attention 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 serotonin reuptake inhibitors like risperidone, have been found
e∑ective in some patients. Other classes of medications are prescribed to reduce the symptoms of the comorbid conditions.

New diagnostic methods

any of the recent advances in understanding 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 information 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 consumption 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 threedimensional images of the brain based on the amount of radiation 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 tomography (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 accelerator nearby to produce them.
Magnetic resonance imaging (MRI) Providing a high-quality, 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 development, and precisely how their progression correlates with mental 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 powerful, 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 magnetic 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 converted 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 technique 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.
49

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 information 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 technique compares brain activity under resting and activated conditions. It combines the high-spatial-resolution, noninvasive imaging 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 developed technique that reveals the source of weak magnetic ﬁelds
emitted by neurons. An array of cylinder-shaped sensors monitors the magnetic ﬁeld pattern near the patient’s head to determine 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 measure of the strength of this activity in individual subjects. Moreover, by presenting stimuli at various rates, scientists can determine how long neural activation is sustained in the diverse brain
areas that respond.
One of the most exciting developments in imaging is the combined use of information from fMRI and MEG. The former provides 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 shining 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 transparent. 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 tomography is then used to create maps of brain activity. A related tech50

nique, the event-related optical signal, records how light is scattered 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 consist of short sections of deoxyribonucleic acid (DNA), the long, spiraling, double-helix structure found on the 23 pairs of chromosomes 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 understanding the precise cause of diseases in order to improve methods of prevention and treatment, and for evaluating the malignancy 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 glutamine, and the expanded repeat results in a long string of glutamines within the protein. This expansion appears to alter the protein’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 Alzheimer’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 apolipoprotein E protein located on chromosome 19.
Gene mapping has enabled doctors to diagnose fragile X mental retardation, the most common cause of inherited mental
retardation. Scientists have now identiﬁed this gene, which is
important for neuronal communication. Several groups of scientists are investigating whether there are genetic components to
schizophrenia, bipolar disorder, and alcoholism, but their ﬁndings are not yet conclusive.
Overall, the characterizations of the structure and function

of individual genes causing diseases of the brain and nervous system 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 disorder — are still largely unknown.
Scientists also are studying the genes in mitochondria, structures 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 speculate 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 molecule called mRNA to send a message with instructions for the amino acids needed to manufacture a protein (3).

Potential therapies

N
I

ew 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 biology — 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 scientist 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 therapeutic e∑ects and fewer side e∑ects.
While this “rational drug design” holds promise for developing 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 neurons. Once the speciﬁc actions of these molecules and their receptors 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
52

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, scientists found that these animals were able to remember a maze
task as well as healthy aged rats. NGF, which slows the destruction 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 improvements 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 neurological 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

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
mulation of abnormal proteins. If the cells made much less of such
fragments of antibodies that can bind to and alter the disease
proteins to begin with, then the disease would progress much
characteristics of speciﬁc proteins. These therapies can be delivmore slowly. A new class of potential drugs is based on removing
ered either as proteins or as genes.
the RNAs that code for the proteins that are causing damage.
Promising preliminary results have been obtained for HuntMouse models of HD and ALS appear to have responded posiington’s, Parkinson’s, Alzheimer’s, and prion diseases. For examtively to such treatments, which are delivered via gene therapies.
ple, fruit ﬂies (Drosophila) that get Huntington’s disease (HD)
Cell and gene therapy
because they have been modiﬁed to carry the mutant human gene
Researchers throughout the world are pursuing a variety of new
are generally too weak and uncoordinated to break out of their
ways to repair or replace neurons and other cells in the brain.
pupal case the way normal insects do. However, when they also
These experimental approaches are still being worked out in aniexpress the gene for an anti-HD antibody, all of them can emerge
mals and cannot be considered therapies for humans at this time.
as young adults. Furthermore, these treated ﬂies live longer than
Scientists have identiﬁed emthe untreated ones that do manbryonic neuronal stem cells —
age to emerge, and the treated
Thousands of small molecule drug
unspecialized cells that give rise
ones show less pathology in
to cells with speciﬁc functions—
their brains.
candidates can be tested using high
in the brain and spinal cord of
Small molecules and
embryonic and adult mice. Stem
throughput screening to alter a cellular
RNAs
cells can continuously produce
Clarifying the processes that
all three major cell types of the
property that represents an important
underlie brain damage will open
brain: neurons; astrocytes, the
cells
that nourish and protect
up the potential to use small
part of a disease process.
neurons; and oligodendrocytes,
molecule drugs to alter these
the cells that surround axons and allow them to conduct their sigprocesses. Some success has occurred in developing animal modnals e≈ciently. The production abilities of stem cells may someels using approaches based on known mechanisms of drugs.
day be useful for replacing brain cells lost to disease. A more limExamples include drugs such as antibiotics and anti-tumor drugs,
ited type of stem cell also has been discovered in the adult nervous
which appear to reduce the neuronal damage in ALS, HD, and
system in various kinds of tissue, raising the possibility that these
Parkinson’s disease. Thousands of small molecule drug candidates
adult stem cells might be pharmacologically directed to replace
can be tested using high throughput screening to alter a cellular
damaged neurons.
property that represents an important part of a disease process.
In other work, researchers are studying a variety of viruses
Because many neurodegenerative diseases involve proteins that
that may ultimately be used to act as “Trojan horses,” carrying
misfold and clump abnormally, lasers are used to measure
therapeutic genes to the brain to correct nervous system diseases.
whether proteins are clumped inside cells that have been robotiThe viruses include herpes simplex type 1 virus (HSV), adenovirus,
cally distributed into tiny wells, along with the small molecules to
lentivirus, adeno-associated virus, and others naturally attracted
be tested. A machine then scans the wells and reports whether parto neurons. It has been found that all can be modiﬁed to carry
ticular drugs have changed the protein clumping, so that these
new genes to cells in tissue culture and in the rodent central nerdrugs can be tested further. New leads for Alzheimer’s and prion
vous system. HSV and adenovirus vectors have also been evaludrugs have recently been described using these methods.
ated in early-stage human trials for treating brain tumors.
Several neurodegenerative diseases are caused by the accu-

Neuroethics

reaking 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 fundamental 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 phenomena 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 beginning to reﬂect on the implications of neuroscience in areas such as moral reasoning, decision-making, 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 conferences 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, policing, 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 biological 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 behav55

ioral change. Because modern neuroscience can investigate the
e∑ective treatments for disorders such as Alzheimer’s disease
mechanisms behind disturbed behaviors, society must ponder
(AD), in which brain cells die at a progressive rate, it will be
the manner in which it manages individuals who violate its rules
important to identify individuals who are at risk in order to preby taking into account those with medical conditions. This situvent brain damage at the earliest stage. Yet there is little preceation raises questions regarding the punishment and treatment
dence for such early intervention. Moreover, AD is a genetically
of such individuals.
complex disorder with multiple genes and degrees of risk associSocial policy Since neuroscience has the potential to transated with each. Thus, the medical profession faces challenges in
form our understanding of human nature, we may be able to
ensuring that candidates for treatment understand their risk for
make predictions about an individual’s future, including the risk
severity of AD and evaluating the potential risks and beneﬁts of
for ill health and cognitive impairment, potential success in school
treatments that may be harmful.
Informed consent in research Special care must be taken in
or employment, and violent behavior or addiction to drugs. Of
the informed-consent process and throughout the research proparticular concern is the potential role for neuroscience in delintocol when individuals have
eating the boundary between
thinking or emotional impairwhat society views as normal
Researchers are now on the threshold
ments that might a∑ect their
and what is deemed pathodecision-making capacity. Conlogical. For example, many boys
of understanding some fundamental
sent is an ongoing process that
are being prescribed medicines
should involve education of the
for hyperactivity, such as Riprinciples of brain function, and are
potential research participant
talin, for conditions that many
and, when appropriate, family
worry may not be clear-cut
even beginning to develop treatments for
members. Researchers are disbrain diseases. Troubling quescussing potential needs to exertions related to this phenomesome of the most devastating neurocise greater scrutiny, ensure
non include: Who should have
safeguards, and enhance particaccess to such agents? If learnlogical diseases and conditions. At the
ipants’ grasp of a study, including can be sped up and attening risks and benefits.
tion enhanced through pharsame time, much of this knowledge and
Environmental inﬂuences
maceuticals, should such drugs
Many are concerned about the
be available to all, or should reimplications for treatments and
possible implications of research
sources be devoted to transindicating that most of the brain
forming only the environment
diagnostics raise ethical questions.
gets built after birth and that it
of the classroom?
Genetics Sequencing the
uses experiences from the outhuman genome — identifying all the approximately 20,000 to
side world to form its circuits. With this as background, scientists
25,000 genes in humans — went hand-in-hand with an investare discussing whether society has a moral obligation to ensure
ment in studying the ethical, legal, and social implications of
that millions of children no longer grow up in violent and imhuman genetics. Neuroethics may play an important role in
poverished environments that can stunt their brains.
issues arising from genetics, such as the social consequences of
At this stage, the ﬁeld of neuroethics raises more questions
using DNA to predict the future. Should patients with a family
than answers. It poses challenges to scientists and to the public
history of a neurological or psychiatric disorder be genetically
to work through the social implications of new discoveries. The
tested even though no treatment is available?
issues are too broad-based to expect that scientists alone will supBrain injury Since traumatic brain injury may cause a perply the answers. But neuroscientists are well-positioned to help
son to experience signiﬁcant cognitive, personality, emotional,
shape and contribute to the debate and discussion.
and behavioral changes, su∑erers may become legally incompeOne of the hallmarks of the ﬁeld has always been the drive
tent. Traumatic brain injury may excuse or mitigate a person’s
toward integrating information from disparate ﬁelds and specialresponsibility for acts that otherwise would be classiﬁed as
izations to increase knowledge. Sorting through the complex issues
crimes. But the medical profession sometimes can have trouble
captured under the umbrella of neuroethics provides an impordetermining a person’s preinjury behavior compared with postintant opportunity for informed and rich discussions among scienjury behavior. This poses serious problems for clinical evaluators
tists and with the public. Continuing study of neuroethics will help
as well as for the legal system.
all segments of society deal with the challenges posed by emerging
Neurological disorders With the likelihood of developing
technologies that investigate the brain and how it works.

Glossary

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

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.

to the neuron’s terminal, where it triggers the release of a neurotransmitter. This occurs when a neuron is activated and temporarily reverses the electrical state of its interior membrane from
negative to positive.

AUTONOMIC NERVOUS SYSTEM A part of the peripheral nervous system responsible for regulating the activity of internal
organs. It includes the sympathetic and parasympathetic nervous
systems.

ADRENAL CORTEX An endocrine organ that secretes corticos-

AXON The ﬁberlike extension of a neuron by which it sends

teroids for metabolic functions; for example, in response to stress.

information to target cells.

ADRENAL MEDULLA An endocrine organ that secretes epinephrine and norepinephrine in concert with the activation of the
sympathetic nervous system; for example, in response to stress.

BASAL GANGLIA Clusters of neurons, which include the caudate nucleus, putamen, globus pallidus, and substantia nigra,
located deep in the brain that play an important role in the initiation of movements. Cell death in the substantia nigra contributes
to Parkinson’s disease.

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 emotional learning, particularly within the context of fear.
ANDROGENS Sex steroid hormones, including testosterone,

found in higher levels in males than females. They are responsible 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 production, often as a result of a stroke.
APOPTOSIS Programmed cell death induced by specialized bio-

chemical pathways, often serving a speciﬁc purpose in the development of the animal.

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 making 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 conscious 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 recognition, 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 powerful 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 stimulus). 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 information from other neurons.
DOPAMINE A catecholamine neurotransmitter known to have

varied functions depending on where it acts. Dopamine-containing 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 consequences.
ENDOCRINE ORGAN An organ that secretes a hormone
directly into the bloodstream to regulate cellular activity of certain other organs.
58

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 movement 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. Stimulation 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-

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 receptors 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

IONS Electrically charged atoms or molecules.

neuronal growth during embryonic development, especially in the
peripheral nervous system. Nerve growth factor also probably
helps sustain neurons in the adult.

LIMBIC SYSTEM A group of brain structures—including the

NEURON A nerve cell specialized for the transmission of infor-

amygdala, hippocampus, septum, basal ganglia, and others—that
help regulate the expression of emotion and emotional memory.

mation and characterized by long, ﬁbrous projections called axons
and shorter, branchlike projections called dendrites.

LONG-TERM MEMORY The ﬁnal phase of memory, in which

NEUROPLASTICITY A general term used to describe the adaptive

information storage may last from hours to a lifetime.

changes in the structure or function of nerve cells or groups of
nerve cells in response to injuries to the nervous system or alterations in patterns of their use and disuse.

ron from ﬁring.

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

NEUROTRANSMITTER A chemical released by neurons at a

synapse for the purpose of relaying information to other neurons
via receptors.

the pineal gland into the bloodstream. Melatonin a∑ects physiological changes related to time and lighting cycles.

NOCICEPTORS In animals, nerve endings that signal the sensa-

MEMORY CONSOLIDATION The physical and psychological

NOREPINEPHRINE A catecholamine neurotransmitter, pro-

changes that take place as the brain organizes and restructures
information to make it a permanent part of memory.

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.

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,

tion of pain. In humans, they are called pain receptors.

OCCIPITAL LOBE One of the four subdivisions of the cerebral

cortex. The occipital lobe plays a role in processing visual information.
ORGANELLES Small structures within a cell that maintain the
cell and do the cell’s work.
PARASYMPATHETIC NERVOUS SYSTEM A branch of the autonomic 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 midbrain. 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 phosphate molecule is placed on a protein and results in the activation 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 biological 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-

role in the manufacture and release of neurotransmitters, intracellular 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 serotonin as a transmitter are found in the brain and gut. Several antidepressant 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 minutes.
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 carried 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.

ized by an inability to perceive reality. Psychosis can occur in
many conditions, including schizophrenia, mania, depression,
and drug-induced states.

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.

RECEPTOR CELL A specialized sensory cell, designed to pick up
and transmit sensory information.

THALAMUS A structure consisting of two egg-shaped masses of

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
60

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 cerebrospinal 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 comprehension of language and the production of meaningful
speech.

Index
Numbers in bold refer to illustrations.

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
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)
50
Magnetoencephalography (MEG) 50
Marijuana 39
Memory 2021
Methylprednisolone 45
Midbrain 5, 10, 19, 24
Mitochondria 47, 51
Monoamine oxidase inhibitors (MAOIs)
34
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

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)
41
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
Temporal 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

Neuroscience Resources

National Institutes of
Health

National Eye Institute
31 Center Drive, MSC 2510
Bethesda, MD 20892–2510
(301) 496–2234
http://www.nei.nih.gov
National Institute on Aging
31 Center Drive, MSC 2292
Bethesda, MD 20892–2292
(301) 496–9265
http://www.nia.nih.gov
National Institute on Alcohol
Abuse and Alcoholism
5635 Fishers Lane, MSC 9304
Bethesda, MD 20892–9304
(301) 443–3885
http://www.niaaa.nih.gov
National Institute of
Biomedical Imaging and Bioengineering
6707 Democracy Blvd., Ste.
202, MSC 5469
Bethesda, MD 20892–5469
(301) 451–6768
http://www.nibib.nih.gov
National Institute of Child
Health and Human Development
Public Information and
Communications Branch
31 Center Drive, MSC 2425
Bethesda, MD 20892–2425

National Institute on Deafness
and Other Communication
Disorders
O≈ce of Health Communication and Public Liaison
31 Center Drive, MSC 2320
Bethesda, MD 20892–2320

National Institute of General
Medical Sciences
45 Center Drive, MSC 6200
Bethesda, MD 20892–6200

(301) 402–0900
http://www.nidcd.nih.gov

National Institute of Mental
Health
O≈ce of Communications
6001 Executive Blvd.,
MSC 9663
Bethesda, MD 20892–9663

National Center for Complementary and Alternative
Medicine
31 Center Drive, MSC 2182
Bethesda, MD 20892–2182

(301) 443–3673
http://www.nimh.nih.gov

(301) 435–6826
http://nccam.nih.gov

National Institute of Dental
and Craniofacial Research
Public Information and
Liaison Branch
45 Center Drive, MSC 6400
Bethesda, MD 20892–6400
(301) 496–3571
http://www.nidcr.nih.gov
National Institute on Drug
Abuse
6001 Executive Blvd., Rm. 5213
Bethesda, MD 20892–9561
(301) 443–6480
http://www.nida.nih.gov
National Institute of
Environmental Health
Sciences
P.O. Box 12233
Research Triangle Park, NC
27709–2233
(919) 541–3201
http://www.niehs.nih.gov

(301) 594–2172
http://www.nigms.nih.gov

National Institute of
Neurological Disorders and
Stroke
P.O. Box 5801
Bethesda, MD 20824–5801
(301) 496–9746
http://www.ninds.nih.gov

National Center for Research
Resources
6701 Democracy Boulevard,
MSC 4874
Bethesda, MD 20892–4874
(301) 435–0888
http://www.ncrr.nih.gov

Other Resources

The Society for Neuroscience
1121 14th Street, NW,
Suite 1010
Washington, DC 20005
http://www.sfn.org

National Institute of Nursing
Research
31 Center Drive
Bldg. 31, Rm. 5B10
Bethesda, MD 20892–2178

Dana Alliance for Brain
Initiatives
745 Fifth Ave., Ste. 900
New York, NY 10151

(301) 496–8230
http://ninr.nih.gov/ninr

dabiinfo@dana.org
http://www.dana.org

National Library of Medicine
8600 Rockville Pike
Bethesda, MD 20894
(301) 496–8834
http://www.nlm.nih.gov

(301) 496–3454
http://www.nichd.nih.gov
63

Copyright ©2005, 2006 Society for Neuroscience
1121 14th Street, NW, Suite 1010
Washington, DC 20005 USA
www.sfn.org
All rights reserved. No portion of this publication may be reproduced,
stored in a retrieval system, or transmitted in any from or by any means,
electronic, mechanical, photocopying, recording, or otherwise without
permission of the Society for Neuroscience.
To acquire additional copies of this book, please visit our Web site
www.sfn.org, go to Publications, and click on Brain Facts.

The Society gratefully acknowledges the contributions of the many neuroscientists who volunteered their expertise and guidance in the development
of this book.
s o c i et y for n eu r o s c i e n ce
Editor: Joseph Carey, Senior Director, Communications and Public A∑airs
Managing Editor: Dawn McCoy
Designer: Marc Alain Meadows, Meadows Design O≈ce Inc.,
Washington, DC www.mdomedia.com
Illustrator: Lydia V. Kibiuk, Baltimore, Maryland
Printed and bound in Canada
Fifth Edition, revised 2006
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