Most of us can probably remember being told,

when we were children, not to touch the stove or some
other source of potential harm. Because children are
curious, such warnings often go unheeded. The
result? Touching a hot stove brings about an immedi-
ate response of pulling away and a vivid memory of
painful fingers. This simple and familiar experience
illustrates the functions of the nervous system:

1. To detect changes and feel sensations


2. To initiate appropriate responses to changes


3. To organize information for immediate use and
   store it for future use
   The nervous system is one of the regulating sys-
   tems (the endocrine system is the other and is dis-
   cussed in Chapter 10). Electrochemical impulses of
   the nervous system make it possible to obtain infor-
   mation about the external or internal environment
   and do whatever is necessary to maintain homeostasis.
   Some of this activity is conscious, but much of it hap-
   pens without our awareness.

NERVOUS SYSTEM DIVISIONS

The nervous system has two divisions. The central
nervous system (CNS) consists of the brain and
spinal cord. The peripheral nervous system(PNS)
consists of cranial nerves and spinal nerves. The PNS
includes the autonomic nervous system (ANS).
The peripheral nervous system relays information
to and from the central nervous system, and the brain
is the center of activity that integrates this informa-
tion, initiates responses, and makes us the individuals
we are.

NERVE TISSUE

Nerve tissue was briefly described in Chapter 4, so we
will begin by reviewing what you already know and
then add to it.
Nerve cells are called neurons, or nerve fibers.
Whatever their specific functions, all neurons have the
same physical parts. The cell body contains the
nucleus (Fig. 8–1) and is essential for the continued
life of the neuron. As you will see, neuron cell bodies
are found in the central nervous system or close to it

in the trunk of the body. In these locations, cell bodies

are protected by bone. There are no cell bodies in the

arms and legs, which are much more subject to injury.

Dendritesare processes (extensions) that transmit

impulses toward the cell body. The one axonof a neu-

ron transmits impulses away from the cell body. It is

the cell membrane of the dendrites, cell body, and

axon that carries the electrical nerve impulse.

In the peripheral nervous system, axons and den-

drites are “wrapped” in specialized cells called

Schwann cells (see Fig. 8–1). During embryonic

development, Schwann cells grow to surround the

neuron processes, enclosing them in several layers of

Schwann cell membrane. These layers are the myelin

sheath; myelin is a phospholipid that electrically insu-

lates neurons from one another. Without the myelin

sheath, neurons would short-circuit, just as electrical

wires would if they were not insulated (see Box 8–1:

Multiple Sclerosis).

The spaces between adjacent Schwann cells, or seg-

ments of the myelin sheath, are called nodes of

Ranvier (neurofibril nodes). These nodes are the parts

of the neuron cell membrane that depolarize when an

electrical impulse is transmitted (see “The Nerve

Impulse” section, on pages 171–172).

The nuclei and cytoplasm of the Schwann cells are

wrapped around the outside of the myelin sheath and

are called the neurolemma, which becomes very

important if nerves are damaged. If a peripheral nerve

is severed and reattached precisely by microsurgery,

the axons and dendrites may regenerate through the

tunnels formed by the neurolemmas. The Schwann

cells are also believed to produce a chemical growth

factor that stimulates regeneration. Although this re-

generation may take months, the nerves may eventu-

ally reestablish their proper connections, and the

person may regain some sensation and movement in

the once-severed limb.

In the central nervous system, the myelin sheaths

are formed by oligodendrocytes, one of the neu-

roglia(glial cells), the specialized cells found only in

the brain and spinal cord. Because no Schwann cells

are present, however, there is no neurolemma, and

regeneration of neurons does not occur. This is why

severing of the spinal cord, for example, results in per-

manent loss of function. Another kind of neuroglia are

microglia,which are constantly moving, phagocytiz-

ing cellular debris, damaged cells, and pathogens.

166 The Nervous System