A

aging, neurologic changes that occur with A
rudimentary NERVOUS SYSTEM is among the first
structures to form as a new life begins. The CEN-
TRAL NERVOUS SYSTEM begins to form about two
weeks after CONCEPTION, arising from a cluster of
specialized cells called the ectoderm. Its physical
and functional development is about 60 percent
complete by birth, 80 percent complete by age
three, and finally reaches completion at the end of
ADOLESCENCE. Though the full complement of neu-
rons is in place by early childhood, the BRAINcon-
tinues to establish new pathways for NEURON
communication for most of life.

The Prenatal Nervous System
The first recognizable neurologic structure is
apparent 21 days after conception when the cells
of the ectoderm grow and divide to form the neu-
ral tube, a primitive structure of NERVEtissue. Over
the following four weeks the neural tube elon-
gates and closes at each end to form the SPINAL
CORD and the brain, a process called neuronal
migration. This is one of the most sensitive times
in embryonic development. NEURAL TUBE DEFECTS—
such as SPINA BIFIDAand anencephaly, which result
when one or the other end of the neural tube fails
to close—are among the most common BIRTH
DEFECTSinvolving the nervous system.
The brain grows by 50,000 neurons a second
during most of this migration period, forming the
brain’s three major divisions—prosencephalon
(forebrain), mesencephalon (midbrain), and
rhombencephalon (hindbrain)—which themselves
grow and divide to form the core structures of the
brain. From the spinal cord, the SPINAL NERVESand
PERIPHERAL NERVES begin to tendril out to the
organs and structures of the body, establishing
motor and sensory innervations that will carry

nerve impulses to and from the body throughout

life.

The Early Childhood Nervous System

The brain continues to add new cells at an aston-

ishing rate, doubling in size from birth by the time

a child reaches 18 months of age. Intense learning

takes place during this period, during which the

brain acquires the foundations of language, sen-

sory interpretation, and motor skills. Interruptions

of these processes can have serious and lifelong

consequences. Studies with animals show, for

example, that depriving the brain of visual input

during the time the brain is establishing the path-

ways for interpreting visual signals results in per-

manent blindness even though the structural

components of vision—the EYE, OPTIC NERVE, and

brain regions—are intact.

Throughout childhood brain neurons continue

to expand the connections they make with each

other, laying down the hundreds of thousands of

pathways necessary for learning and remember-

ing. These neuronal networks provide shortcuts

that allow the brain to carry out familiar functions

with great speed and efficiency. The foundations

of language and motor movement develop and

evolve during this period of expansion. Though

researchers agree it is never to late for the brain to

learn, it is during the years of childhood that the

brain is most receptive.

The Adolescent Nervous System

During adolescence (between the ages of 12 and

20) axons continue to grow and branch, most

notably from the neurons of the frontal lobes,

which are responsible for many of the functions of

cognition and behavior, and to lesser extent from

neurons in other areas of the brain. This is the

220