tional divisions, the nervous system operates so
efficiently that most of the time its myriad activi-
ties take place virtually unnoticed. The nervous
system contains two major divisions: the CENTRAL
NERVOUS SYSTEMand 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 cor-
tex also integrates many of the functions of other
divisions of the nervous system and initiates vol-
untary movement.
CEREBRAL DOMINANCE
One or the other cerebral hemisphere is con-
tralaterally 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 domi-
nant: they are right-handed (and usually right-
footed and right-eyed). About 10 percent of
people are right-hemisphere dominant: they are
left-handed (and usually left-footed and left-
eyed). 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 NERVEfibers, 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 occipi-
tal—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 opera-
tions 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 (move-
ment). It receives a constant flow information
from the cerebral cortex, the basal ganglia (a col-
lection 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 relation-
ship to its external environment and sends in
return a constant flow of instructions to seam-
lessly 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 struc-
tures 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, BLOODflow, and many reflexes (automatic,
survival-oriented reactions to environmental stim-
uli). The 2nd through the 12th pairs of CRANIAL
NERVESoriginate 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 BRAINcontains more than
100 billion neurons (NERVEcells) 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 CORDand extend nearly five
feet in an adult.The Nervous System 215