Chapter 6 The Nervous System 217
Spinal Nerves and Nerve
Plexuses
Thirty-one pairs of spinal nerves branch out
from the left and right sides of the spinal cord.
Each pair is named for the vertebral level from
which it originates. As you learned in chapter 4,
the vertebral levels include the cervical, thoracic,
and lumbar regions, as well as the sacrum. All of
the spinal nerves are mixed nerves, carrying both
afferent and efferent information.
The spinal nerve cell bodies are located
within the gray matter of the spinal cord. The
axons of spinal nerve cells extend out of the
spinal cord and eventually connect with muscles.
As shown earlier in Figure 6.15, dorsal (posterior)
and ventral (anterior) spinal nerve roots unite to
form the left and right spinal nerves that exit at
each spinal level.
The spinal nerves are only about one-half
inch long, immediately dividing into a dorsal
ramus (DOR-sal RAY-mus) and ventral ramus
(VEHN-tral RAY-mus) (Figure 6.19). The dorsal
and ventral rami carry nerve impulses to the
muscle and skin of the trunk.
Figure 6.16 Structure of a nerve showing the
protective, fibrous tissue sheaths. How does the
structure of a nerve help to protect it from injury?
Axon
Myelin sheath
Endoneurium
Perineurium
Fascicle
Blood
vessels
Epineurium
Figure 6.17 Functions of the Cranial Nerves
Nerve # System Function
Olfactory I sensory smell
Optic II sensory sight
Oculomotor III both eye movements
Trochlear IV both eye movements
Trigeminal V both facial sensation, jaw motion
Abducens VI both eye movements
Facial VII both facial movements, taste
Vestibulocochlear VIII sensory hearing, balance
Glossopharyngeal IX both throat muscle movements, taste
Vagus X both autonomic control of heart, lungs, digestion, taste, communica-
tion between brain and organs
Accessory XI mostly motor trapezius movements, sternocleidomastoid movements
Hypoglossal XII both tongue muscle movements, tongue sensation