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spinal nerve[′spīn·əl ¦nərv]
a short (approximately 2 cm long) strand of nerve fibers formed segmentally as a result of the fusion of the dorsal (sensory) and ventral (motor) roots of the spinal cord.
Humans have 31 pairs of spinal nerves. Each pair corresponds to a segment of the spinal cord, with eight pairs of cervical nerves, 12 pairs of thoracic nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one pair of coccygeal nerves. Sensory fibers are the processes of the cells of spinal ganglia, and motor fibers are the processes of motoneurons situated in the anterior horns of the gray substance of the spinal cord. Reaching the spinal nerves along with the motor neurons are efferent autonomic branches, which are the processes of nerve cells found in the lateral horns. Myelinated nerve fibers are most numerous in spinal nerves at the cervical spinal swelling (as many as 44,000 fibers in a nerve) and the lumbar spinal swelling (over 55,000 fibers in a nerve).
Spinal nerves emerge through corresponding intervertebral foramina symmetrically on both sides of the spine and are divided into four branches. Departing reversely from each spinal nerve is a delicate meningeal branch that participates in the innervation of the meninges of the spinal cord. The spinal nerves then divide into anterior and posterior somatic branches, which innervate the skin of the trunk and extremities and all body muscles except those of the head. Autonomic sympathetic conductors, called white communicating branches, separate from the spinal nerves or from the anterior branch of spinal nerves and proceed to the ganglia of the sympathetic trunk of the sympathetic nervous system.
The anterior branches of the four upper cervical spinal nerves form the cervical plexus, the last four cervical and the first and second thoracic spinal nerves form the brachial plexus, the twelfth thoracic and the four upper lumbar spinal nerves form the lumbar plexus, the fifth lumbar and the first three sacral spinal nerves form the sacral plexus, and the fourth and fifth sacral and the coccygeal spinal nerves form the coccygeal plexus.
The spinal nerves and the plexuses they form innervate the skin and skeletal muscles.
For injuries to the spinal nerves, seeRADICULITIS; for injuries to the plexuses, seePLEXITIS.
REFERENCESMnogotomnoe rukovodstvo po nevrologii, vol. 1, book 1. Moscow, 1955.
V. V. KUPRIIANOV and V. B. GEL’FAND