The Peripheral Nervous System and Reﬂex Activity
(Posterior view.) The spinal nerves are
shown at right; their ventral rami are shown at left. Most ventral
rami form nerve plexuses (cervical, brachial, lumbar, and sacral).
Composition of Cranial Nerves
In the last chapter, we described how ventral (motor) and dorsal
(sensory) roots fuse to form spinal nerves. Cranial nerves, on
the other hand, vary markedly in their composition.
Most cranial nerves are mixed nerves, as shown in Figure 13.6b.
However, two nerve pairs (the olfactory and optic) associated with
special sense organs are generally considered purely sensory. Te
cell bodies of the sensory neurons in the olfactory and optic nerves
their respective special sense organs. In other
sensory neurons contributing to cranial nerves (V, VII, IX, and X),
the cell bodies are located in
cranial sensory ganglia
the brain. Some cranial nerves have a single sensory ganglion, oth-
ers have several, and still others have none.
Several of the mixed cranial nerves contain both somatic and
autonomic motor ﬁbers and hence serve both skeletal muscles
and visceral organs. Except for some autonomic motor neurons
located in ganglia, the cell bodies of motor neurons contribut-
ing to the cranial nerves are located in the ventral gray matter
regions (nuclei) of the brain stem.
Remembering the primary functions of the cranial nerves
oth) can be a challenge. Tis sentence
might help: “
Check Your Understanding
Name the cranial nerve(s) most involved in each of the following:
moving your eyeball; sticking out your tongue; controlling your
heart rate and digestive activity; shrugging your shoulders.
For answers, see Appendix H.
Describe the general structure of a spinal nerve and the
general distribution of its rami.
Deﬁne plexus. Name the major plexuses and describe the
distribution and function of the peripheral nerves arising from
Tirty-one pairs of
, each containing thousands of
nerve ﬁbers, arise from the spinal cord and supply all parts of the body
except the head and some areas of the neck. All are mixed nerves.
Spinal nerves are named according to where they issue from
the spinal cord
. Te spinal nerves include:
8 pairs of cervical spinal nerves (C
12 pairs of thoracic nerves (±
5 pairs of lumbar nerves (L
5 pairs of sacral nerves (S
1 pair of tiny coccygeal nerves (Co
Notice that there are eight pairs of cervical nerves but only
seven cervical vertebrae. Tis “discrepancy” is easily explained.
Te ﬁrst seven pairs exit the vertebral canal
to the ver-
tebrae for which they are named, but C
seventh cervical vertebra (between C
). Below the cervical
level, each spinal nerve leaves the vertebral column
As we mentioned in Chapter 12, each spinal nerve connects to
the spinal cord by a dorsal root and a ventral root
Each root forms from a series of
that attach along the
length of the corresponding spinal cord segment (Figure 13.8a).
(eﬀerent) ﬁbers that arise from
ventral horn motor neurons and extend to and innervate the
skeletal muscles. (In Chapter 14, we describe autonomic ner-
vous system eﬀerents that are also contained in the ventral roots.)
(aﬀerent) ﬁbers that arise from
sensory neurons in the dorsal root ganglia and conduct im-
pulses from peripheral receptors to the spinal cord.
Te spinal roots pass laterally from the cord and unite just
distal to the dorsal root ganglion to form a spinal nerve be-
fore emerging from the vertebral column via their respective