b. Peripheral Nervous System.
(1) Structure. The peripheral nervous system consists of the cranial and
spinal nerves and their branches (see figure 1-14). Twelve pairs of cranial nerves have
their roots in the brain. These nerves give off branches to the structures of the head
and face. Thirty-one pairs of spinal nerves have their roots in the spinal cord. They
give off branches to the structures of the body from the neck down. Each nerve
supplies a specific body area or structure (called innervations). Because many of the
various nerve branches join together (anastomose), the result is an overlapping of nerve
supply to certain parts. Even though some peripheral nerves are composed entirely of
either motor fibers or sensory fibers, most contain both.
(2) Function. The peripheral nervous system primarily involves conscious
activity of the body. The sensory nerves carry impulses (such as touch, pain, and sight)
to the brain. The brain normally evaluates the impulse and sends out, through motor
nerves, impulses which cause a bodily response. Normally, all conscious body
movements result from interaction of the peripheral and central nervous systems.
However, there is an exception called a reflex action. Reflex action results when some
sensation or stimulation passes over a reflex arc to a peripheral organ. The organ is
thus stimulated to act without the aid of consciousness. For example, jerking your hand
away from a hot stove that you accidentally touched.
c. Autonomic Nervous System. This system controls the action of cardiac and
smooth muscles, sweat glands, digestive glands, some endocrine glands, and dilation
and contraction of blood vessels. The control of this system is almost entirely
involuntary and subconscious.