Human Physiology, 14th edition (2016)

236 Chapter 8

LEARNING OUTCOMES

After studying this section, you should be able to:

12. Identify the structures of a spinal nerve and describe
    the neural pathways of a reflex arc.

As mentioned in chapter 7, the peripheral nervous system
(PNS) consists of nerves (collections of axons) and their asso-
ciated ganglia (collections of cell bodies). Although this chap-
ter is devoted to the CNS, the CNS cannot function without
the PNS. This section thus serves to complete our discussion
of the CNS and introduces concepts pertaining to the PNS that
will be explored more thoroughly in later chapters (particularly
chapters 9, 10, and 12).

Cranial Nerves

Of the 12 pairs of cranial nerves, 2 pairs arise from neuron
cell bodies located in the forebrain and 10 pairs arise from the
midbrain and hindbrain. The cranial nerves are designated by
Roman numerals and by names. The Roman numerals refer to
the order in which the nerves are positioned from the front of
the brain to the back. The names indicate the structures inner-
vated by these nerves (e.g., facial) or the principal function of
the nerves (e.g., oculomotor). A summary of the cranial nerves
is presented in table 8.6.
Most cranial nerves are classified as mixed nerves. This
term indicates that the nerve contains both sensory and motor
fibers. Those cranial nerves associated with the special senses

(e.g., olfactory, optic), however, consist of sensory fibers only.

The cell bodies of these sensory neurons are not located in the

brain, but instead are found in ganglia near the sensory organ.

Spinal Nerves

There are 31 pairs of spinal nerves. These nerves are grouped

into 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal

according to the region of the vertebral column from which

they arise ( fig. 8.27 ).

Each spinal nerve is a mixed nerve composed of sensory and

motor fibers. These fibers are packaged together in the nerve,

but they separate near the attachment of the nerve to the spinal

8.6 Cranial and Spinal Nerves

The central nervous system communicates with the body

by means of nerves that exit the CNS from the brain (cra-

nial nerves) and spinal cord (spinal nerves). These nerves,

together with aggregations of cell bodies located outside

the CNS, constitute the peripheral nervous system.

| CHECKPOINT

10. Explain why each cerebral hemisphere receives
    sensory input from and directs motor output to the
    contralateral side of the body.
    11a. List the tracts of the pyramidal motor system and
    describe the function of the pyramidal system.
    11b. List the tracts of the extrapyramidal system and
    explain how this system differs from the pyramidal
    motor system.

Lumbar plexus

Sacral plexus

Some peripheral

nerves:

Lateral femoral

cutaneous

Cranial nerves

(12 pairs)

Cervical

(8 pairs)

Thoracic

(12 pairs)

Lumbar

(5 pairs)

Spinal

nerves

Sacral

(5 pairs)

Coccygeal

(1 pair)

Cervical plexus

Brachial plexus

Ulnar

Median

Radial

Femoral

Sciatic

Figure 8.27 Distribution of the spinal nerves. These

interconnect at plexuses (shown on the left) and form specific

peripheral nerves.