matter is shaped roughly like the letter H. It establishes
sensory communication between the brain and the
spinal nerves, conducting sensory impulses from the
The spinal cord may be thought of as an electric
cable containing many wires (nerves) that connect
parts of the body with each other and with the brain.
Sensations received by a sensory nerve are brought to
the spinal cord, and the impulse is transferred either to
the brain or to a motor nerve. The majority of impulses
go to the brain for action. However, a system exists for
quickly handling emergency situations. It is called the
If you touch a hot stove, you must remove your
hand from the heat source immediately or the skin will
burn very quickly. But the passage of a sense impulse
to the brain and back again to a motor nerve takes too
much time. The reflex arc responds instantaneously to
emergency situations like the one just described. The
sensation of heat travels to the spinal cord on a sensory
nerve. When the sensation reaches the spinal cord, it is
picked up by an interneuron in the gray matter. This
reception then triggers the appropriate nerve to
stimulate a muscle reflex drawing the hand away. An
illustrated example of the reflex arc is shown in figure
The reflex arc works well in simple situations
requiring no action of the brain. Consider, however,
what action is involved if the individual touching the
stove pulls back and, in so doing, loses balance and has
to grab a chair to regain stability. Then the entire spinal
cord is involved. Additional impulses must travel to
the brain, then down to the muscles of the legs and arms
to enable the individual to maintain balance and to hold
on to a steadying object. While all this activity is going
on, the stimulus is relayed through the sympathetic
autonomic nerve fibers to the adrenal glands, causing
adrenalin to flow, which stimulates heart action. The
stimulus then moves to the brain, making the
individual conscious of pain. In this example, the
spinal cord has functioned not only as a center for
spinal relaxes, but also as a conduction pathway for
other areas of the spinal cord to the autonomic nervous
system and to the brain.
PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system (PNS) consists of
the nerves that branch out from the CNS and connect it
to the other parts of the body. The PNS includes 12
pairs of cranial nerves and 31 pairs of spinal nerves.
Cranial and spinal nerves carry both voluntary and
The 12 pairs of cranial nerves are sensory, motor,
or mixed (sensory and motor). Table 1-3 shows the 12
cranial nerves and parts of the body they service.
There are 31 pairs of spinal nerves that originate
from the spinal cord. Although spinal nerves are not
named individually, they are grouped according to the
level from which they arise, and each nerve is
numbered in sequence. Thus, there are 8 pairs of
cervical nerves, 12 pairs of thoracic nerves, 5 pairs of
lumbar nerves, 5 pairs of sacral nerves, and 1 pair of
coccygeal nerves. See figure 1-46.
Spinal nerves (mixed) send fibers to sensory
surfaces and muscles of the trunk and extremities.
Nerve fibers are also sent to involuntary smooth
muscles and glands of the gastrointestinal tract,
urogenital system, and cardiovascular system.
Figure 1-45.Cross section of the spinal cord and reflex
arcarrows and numbers show impulse pathway.