| National Institute of Neurological Disorders and Stroke | |
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Get Web page suited for printing The brain is the most complex part of the human body. This three-pound organ is the
seat of intelligence, interpreter of the senses, initiator of body movement, and
controller of behavior. Lying in its bony shell and washed by protective fluid, the brain
is the source of all the qualities that define our humanity. The brain is the crown jewel
of the human body. For centuries, scientists and philosophers have been fascinated by the brain, but until
recently they viewed the brain as nearly incomprehensible. Now, however, the brain is
beginning to relinquish its secrets. Scientists have learned more about the brain in the
last 10 years than in all previous centuries because of the accelerating pace of research
in neurological and behavioral science and the development of new research techniques. As
a result, Congress named the 1990s the Decade of the Brain. At the forefront of research
on the brain and other elements of the nervous system is the National Institute of
Neurological Disorders and Stroke (NINDS), which conducts and supports scientific studies
in the United States and around the world. This fact sheet is a basic introduction to the human brain. It may help you understand
how the healthy brain works, how to keep it healthy, and what happens when the brain is
diseased or dysfunctional. <top> The brain is like a committee of experts. All the parts of the brain work together, but
each part has its own special properties. The brain can be divided into three basic units:
the forebrain, the midbrain, and the hindbrain. The hindbrain includes the upper part of the spinal cord, the brain stem, and a
wrinkled ball of tissue called the cerebellum (1). The
hindbrain controls the body’s vital functions such as respiration and heart rate. The
cerebellum coordinates movement and is involved in learned rote movements. When you play the piano or hit a tennis ball you are activating the cerebellum. The uppermost part of the brainstem is the midbrain, which controls some reflex actions and is part of the circuit involved in the control of eye movements and other voluntary movements. The forebrain is the largest and most highly developed part of the human brain:
it consists primarily of the cerebrum (2) and the
structures hidden beneath it (see "The Inner Brain"). When people see pictures of the brain it is usually the cerebrum that they notice. The
cerebrum sits at the topmost part of the brain and is the source of intellectual
activities. It holds your memories, allows you to plan, enables you to imagine and think.
It allows you to recognize friends, read books, and play games. The cerebrum is split into two halves (hemispheres) by a deep fissure. Despite the
split, the two cerebral hemispheres communicate with each other through a thick tract of
nerve fibers that lies at the base of this fissure. Although the two hemispheres seem to
be mirror images of each other, they are different. For instance, the ability to form
words seems to lie primarily in the left hemisphere, while the right hemisphere seems to
control many abstract reasoning skills. For some as-yet-unknown reason, nearly all of the signals from the brain to the body
and vice-versa cross over on their way to and from the brain. This means that the right
cerebral hemisphere primarily controls the left side of the body and the left hemisphere
primarily controls the right side. When one side of the brain is damaged, the opposite
side of the body is affected. For example, a stroke in the right hemisphere of the brain
can leave the left arm and leg paralyzed. The Forebrain ------- The
Midbrain -------- The Hindbrain <top> Each cerebral hemisphere can be divided into sections, or lobes, each of which
specializes in different functions. To understand each lobe and its specialty we will take
a tour of the cerebral hemispheres, starting with the two frontal lobes (3), which lie directly behind the forehead. When you plan a
schedule, imagine the future, or use reasoned arguments, these two lobes do much of the work. One
of the ways the frontal lobes seem to do these things is by acting as short-term storage
sites, allowing one idea to be kept in mind while other ideas are considered. In the
rearmost portion of each frontal lobe is a motor area (4),
which helps control voluntary movement. A nearby place on the left frontal lobe called Broca’s
area (5) allows thoughts to be transformed into words. When you enjoy a good meal—the taste, aroma, and texture of the food—two
sections behind the frontal lobes called the parietal lobes (6) are at work. The forward parts of these lobes, just behind
the motor areas, are the primary sensory areas (7).
These areas receive information about temperature, taste, touch, and movement from the
rest of the body. Reading and arithmetic are also functions in the repertoire of each
parietal lobe. As you look at the words and pictures on this page, two areas at the back of the brain
are at work. These lobes, called the occipital lobes (8),
process images from the eyes and link that information with images stored in memory.
Damage to the occipital lobes can cause blindness. The last lobes on our tour of the cerebral hemispheres are the temporal lobes (9), which lie in front of the visual areas and nest under the
parietal and frontal lobes. Whether you appreciate symphonies or rock music, your brain
responds through the activity of these lobes. At the top of each temporal lobe is an area
responsible for receiving information from the ears. The underside of each temporal lobe
plays a crucial role in forming and retrieving memories, including those associated with
music. Other parts of this lobe seem to integrate memories and sensations of taste, sound,
sight, and touch. <top> Coating the surface of the cerebrum and the cerebellum is a vital layer of tissue the
thickness of a stack of two or three dimes. It is called the cortex, from the Latin word
for bark. Most of the actual information processing in the brain takes place in the
cerebral cortex. When people talk about "gray matter" in the brain they are
talking about this thin rind. The cortex is gray because nerves in this area lack the
insulation that makes most other parts of the brain appear to be white. The folds in the
brain add to its surface area and therefore increase the amount of gray matter and the
quantity of information that can be processed. <top> Deep within the brain, hidden from view, lie structures that are the gatekeepers
between the spinal cord and the cerebral hemispheres. These structures not only determine
our emotional state, they also modify our perceptions and responses depending on that
state, and allow us to initiate movements that you make without thinking about them. Like
the lobes in the cerebral hemispheres, the structures described below come in pairs: each
is duplicated in the opposite half of the brain. The hypothalamus (10), about the size of a pearl,
directs a multitude of important functions. It wakes you up in the morning, and gets the
adrenaline flowing during a test or job interview. The hypothalamus is also an important
emotional center, controlling the molecules that make you feel exhilarated, angry, or
unhappy. Near the hypothalamus lies the thalamus (11),
a major clearinghouse for information going to and from the spinal cord and the cerebrum. An arching tract of nerve cells leads from the hypothalamus and the thalamus to the hippocampus
(12). This tiny nub acts as a memory indexer—sending
memories out to the appropriate part of the cerebral hemisphere for long-term storage and
retrieving them when necessary. The basal ganglia (not shown) are clusters of nerve
cells surrounding the thalamus. They are responsible for initiating and integrating
movements. Parkinson’s disease, which results in tremors, rigidity, and a stiff,
shuffling walk, is a disease of nerve cells that lead into the basal ganglia. <top> The brain and the rest of the nervous system are composed of many different types of
cells, but the primary functional unit is a cell called the neuron. All sensations,
movements, thoughts, memories, and feelings are the result of signals that pass through
neurons. Neurons consist of three parts. The cell body (13)
contains the nucleus, where most of the molecules that the neuron needs to survive and
function are manufactured. Dendrites (14) extend out
from the cell body like the branches of a tree and receive messages from other nerve
cells. Signals then pass from the dendrites through the cell body and may travel away from
the cell body down an axon (15) to another neuron, a
muscle cell, or cells in some other organ. The neuron is usually surrounded by many
support cells. Some types of cells wrap around the axon to form an insulating sheath (16). This sheath can include a fatty molecule called myelin,
which provides insulation for the axon and helps nerve signals travel faster and farther.
Axons may be very short, such as those that carry signals from one cell in the cortex to
another cell less than a hair’s width away. Or axons may be very long, such as those
that carry messages from the brain all the way down the spinal cord. Scientists have learned a great deal about neurons by studying the synapse—the
place where a signal passes from the neuron to another cell. When the signal reaches the
end of the axon it stimulates tiny sacs (17). These
sacs release chemicals known as neurotransmitters (18)
into the synapse (19). The neurotransmitters cross
the synapse and attach to receptors (20) on the
neighboring cell. These receptors can change the properties of the receiving cell. If the
receiving cell is also a neuron, the signal can continue the transmission to the next
cell. <top> Acetylcholine is called an excitatory neurotransmitter because it
generally makes cells more excitable. It governs muscle contractions and causes glands to
secrete hormones. Alzheimer’s disease, which initially affects memory formation, is
associated with a shortage of acetylcholine. GABA (gamma-aminobutyric acid) is called an inhibitory neurotransmitter because it
tends to make cells less excitable. It helps control muscle activity and is an important
part of the visual system. Drugs that increase GABA levels in the brain are used to treat
epileptic seizures and tremors in patients with Huntington’s disease. Serotonin is an inhibitory neurotransmitter that constricts blood vessels and brings on
sleep. It is also involved in temperature regulation. Dopamine is an inhibitory
neurotransmitter involved in mood and the control of complex movements. The loss of
dopamine activity in some portions of the brain leads to the muscular rigidity of
Parkinson’s disease. Many medications used to treat behavioral disorders work by
modifying the action of dopamine in the brain. <top> When the brain is healthy it functions quickly and automatically. But when problems
occur, the results can be devastating. Some 50 million people in this country—one in
five—suffer from damage to the nervous system. The NINDS supports research on more
than 600 neurological diseases. Some of the major types of disorders include: neurogenetic
diseases (such as Huntington’s disease and muscular dystrophy), developmental
disorders (such as cerebral palsy), degenerative diseases of adult life (such as
Parkinson’s disease and Alzheimer’s disease), metabolic diseases (such as
Gaucher’s disease), cerebrovascular diseases (such as stroke and vascular dementia),
trauma (such as spinal cord and head injury), convulsive disorders (such as epilepsy),
infectious diseases (such as AIDS dementia), and brain tumors. <top> Since its creation by Congress in 1950, the NINDS has grown to become the leading
supporter of neurological research in the United States. Most research funded by the NINDS
is conducted by scientists in public and private institutions such as universities,
medical schools, and hospitals. Government scientists also conduct a wide array of
neurological research in the more than 20 laboratories and branches of the NINDS itself.
This research ranges from studies on the structure and function of single brain cells to
tests of new diagnostic tools and treatments for those with neurological disorders. For
more information, write or call the Institute's Brain Resources and Information Network (BRAIN) at: BRAIN Prepared by Reviewed July 1, 2001 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
