Spinal Nerves

Figure 68 shows the origin and function of the twelve cranial nerves:

SPINAL NERVES

Some nerves are not connected at all to the brain but are instead connected to the spinal cord.

This is a more regular arrangement, with a nerve exiting between each of the vertebrae. All of

these nerves combine both sensation and motor function, with axons that separate into two

nerve roots. The sensory component involves axons that enter the spinal cord via the dorsal

nerve root, while the somatic and autonomic motor nerve leave the spinal cord as the ventral

nerve root.

There are 31 spinal nerves, labeled according to the vertebral level. There are eight pairs of

cervical spinal nerves, labeled C1 to C8, 12 pairs of thoracic spinal nerves, labeled T1 to T12, five

pairs of lumbar spinal nerves, labeled L1 to L5, five pairs of sacral nerves, labeled S1 to S5, and a

single pair of coccygeal nerves. The first spinal nerve, C1, exits between the occipital bone and

the first cervical vertebra; the others follow, exiting just above the vertebra it is named after.

The exception is C8, which emerges between the seventh cervical vertebra and the first

thoracic vertebra.

The actual nerves in the periphery involve the reorganization of axons in the spinal nerves to

make a nerve that follows a specific course. Axons from different spinal nerves will come

together to form a systemic nerve. They do this reorganization at four different areas along the

spinal cord in places called a “nerve plexus.” There are no cell bodies in nerve plexuses—only

axons that are getting reorganized.

• The cervical plexus incorporates axons from the C1-C5 nerve root level to make head/neck nerves and the phrenic nerve, supplying the diaphragm.

• The brachial plexus incorporates C4-T1 to make the nerves of the arms. Two large

branches are the radial nerve and the axillary nerve. There are two sub-branches to the

radial nerve: the ulnar and median nerves. Figure 69 shows the brachial plexus: