Thirty-one pairs of spinal nerves originate from the spinal cord. They are mixed nerves, and they provide two-way communication between the spinal cord and parts of the upper and lower limbs, neck, and trunk.
Spinal nerves are not named individually but are grouped by the level from which they arise, with each nerve numbered in sequence (fig. 11.30). Thus, there are eight pairs of cervical nerves (numbered C1 to C8), twelve pairs of thoracic nerves (numbered T1 to T12), five pairs of lumbar nerves (numbered L1 to L5), five pairs of sacral nerves (numbered S1 to S5), and one pair of coccygeal nerves (Co).
The nerves arising from the superior part of the spinal cord pass outward almost horizontally, whereas those from the inferior portions of the spinal cord descend at sharp angles. This arrangement is a consequence of growth. In early life, the spinal cord extends the entire length of the vertebral column, but with age, the column grows more rapidly than the cord. Thus, the adult spinal cord ends at the level between the first and second lumbar vertebrae, so the lumbar, sacral, and coccygeal nerves descend to their exits beyond the end of the cord. These descending nerves form a structure called the cauda equina (horse's tail) (fig. 11.30).
Each spinal nerve emerges from the cord by two short branches, or roots, which lie within the vertebral column. The dorsal root (posterior, or sensory, root) can be identified by an enlargement called the dorsal root ganglion. This ganglion contains the cell bodies of the sensory neurons whose dendrites conduct impulses inward from the peripheral body parts. The axons of these neurons extend through the dorsal root and into the spinal cord, where they form synapses with dendrites of other neurons.
An area of skin that the sensory nerve fibers of a particular spinal nerve innervate is called a dermatome. Dermatomes are highly organized, but they vary considerably in size and shape, as figure 11.31 indicates. A map of the dermatomes is often useful in localizing the sites of injuries to dorsal roots or the spinal cord.
The ventral root (anterior, or motor, root) of each spinal nerve consists of axons from the motor neurons whose cell bodies are located within the gray matter of the cord. A ventral root and a dorsal root unite to form a spinal nerve, which extends outward from the vertebral canal through an intervertebral foramen. Just beyond its foramen, each spinal nerve branches. One of these parts, the small meningeal branch, reenters the vertebral canal through the intervertebral foramen and supplies the meninges and blood vessels of the cord, as well as the intervertebral ligaments and the vertebrae.
As figure 11.32 shows, a posterior branch (posterior ramus) of each spinal nerve turns posteriorly and innervates the muscles and skin of the back. The main portion of the nerve, the anterior branch (anterior ramus), continues forward to supply muscles and skin on the front and sides of the trunk and limbs.
The spinal nerves in the thoracic and lumbar regions have a fourth, or visceral branch, which is part of the autonomic nervous system. Except in the thoracic re
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