Encyclopedia of Health and Medicine

The Nervous System 215
tional divisions, the nervous system operates so
efficiently that most of the time its myriad activities
take place virtually unnoticed. The nervous
system contains two major divisions: the CENTRAL
NERVOUS SYSTEM and the PERIPHERAL NERVOUS SYSTEM.
The brain and spinal cord make up the central
nervous system. The brain’s main divisions are the
cerebral cortex (cerebrum), cerebellum, and
brainstem. The cerebral cortex is the largest and
most complex part of the brain, accounting for 85
percent of the brain’s mass and conducting all
functions of consciousness and voluntary action.
The cerebral cortex filters, sorts, and manages
information about the body’s experiences in its
external environment. It is the center of thought,
reason, intellect, emotion, judgment, personality,
mood, behavior, and movement. The cerebral cortex
also integrates many of the functions of other
divisions of the nervous system and initiates voluntary
movement.
CEREBRAL DOMINANCE
One or the other cerebral hemisphere is contralaterally
dominant in nearly everyone. The
most prominent feature of cerebral dominance is
handedness. In about 85 percent of people, the
left hemisphere of the cerebral cortex is dominant:
they are right-handed (and usually rightfooted
and right-eyed). About 10 percent of
people are right-hemisphere dominant: they are
left-handed (and usually left-footed and lefteyed).
About 5 percent of people appear to use
either side of the body with equal ease: they are
ambidextrous.
Structurally the cerebral cortex consists of two
matched halves: the right hemisphere and the left
hemisphere. A band of NERVE fibers, the corpus
callosum, connects the two hemispheres at the
bottom of the deep fissure that separates them.
The functions of the hemispheres are contralateral
to the body—that is, the right hemisphere controls
the left side of the body and the left hemisphere
controls the right side of the body. Matching pairs
of lobes—frontal, temporal, parietal, and occipital—form
the structure of each hemisphere, with
each lobe specializing in certain functions. Though
each pair of lobes handles similar operations, the
right and left lobes conduct different aspects of
those operations. Right hemisphere lobe operations
tend to be spatial and conceptual, whereas
left hemisphere lobe operations tend to be linear
and logical. Regions of the frontal lobes work
together to collect, assimilate, and integrate the
results.
The cerebellum, a small structure at the back of
the brain, coordinates motor function (movement).
It receives a constant flow information
from the cerebral cortex, the basal ganglia (a collection
of nerve fibers on the basal, or bottom,
surface of the cerebral cortex, where the planning
and initiation of motor function takes place), the
brainstem, and the body about the body’s relationship
to its external environment and sends in
return a constant flow of instructions to seamlessly
carry out tasks ranging from threading a
needle to running a marathon. Like the cerebral
cortex, the cerebellum has two hemispheres but
operates ipsilaterally. The cerebellum manages
balance, coordination, speed, direction, and the
smoothness of movements.
The brainstem, an elongated, bulbous structure
between the cerebral cortex and the spinal cord,
maintains the functions of survival and connects
the brain with the spinal cord. The primary structures
of the brainstem are the pons, and midbrain,
medulla oblongata. The pons functions as a bridge
connecting the cerebellum, the cerebral cortex, and
the spinal cord. The medulla oblongata is the segue
from the brain to the spinal cord. It is responsible
for the beating of the HEART, BREATHING, BLOOD PRES-
SURE, BLOOD flow, and many reflexes (automatic,
survival-oriented reactions to environmental stimuli).
The 2nd through the 12th pairs of CRANIAL
NERVES originate in the brainstem. Injuries to the
brainstem can be debilitating or fatal.
BILLIONS AND BILLIONS OF BRAIN CELLS
At the completion of its structural development
just before birth, the BRAIN contains more than
100 billion neurons (NERVE cells) and about 50
times as many glial cells (cells that support the
neurons). The longest neuronal axons, the
threadlike fibers that carry nerve impulses away
from the NEURON, reach from the brain to the
base of the SPINAL CORD and extend nearly five
feet in an adult.