Spinal nerves

Spinal nerves (n. spinales) are paired, metamerically located nerve trunks. In humans, there are 31-33 pairs of spinal nerves: 8 pairs of cervical, 12 pairs of thoracic, 5 pairs of lumbar, 5 pairs of sacral and 1-3 pairs of coccygeal, corresponding to 31-33 segments of the spinal cord. Each spinal nerve by origin corresponds to a certain segment of the body and innervates the area of skin (derived from the dermatome) that developed from this segment, muscles (from the myotome) and bones (from the sclerotome).

The spinal nerve begins with motor and sensory roots. The anterior (motor) root (radix ventralis, s. anterior, s. motoria) of the spinal nerve is formed by axons of motor neurons, the bodies of which are located in the anterior horns of the spinal cord. The posterior (sensory) root (radix dorsalis, s. posterior, s. sensoria) is formed by the central processes of pseudounipolar cells, the bodies of which form the spinal ganglion. The peripheral processes of pseudounipolar neurons go to the periphery, where their perceiving apparatus - receptors - are located in organs and tissues. The level at which the roots exit the spinal cord does not coincide with the location of the intervertebral openings, since the spinal cord does not fill the entire spinal canal. The roots, starting from the lower cervical ones, go to their intervertebral openings in a descending direction. The roots of the lower lumbar and sacral spinal nerves form the "cauda equina".

Each posterior root has an extension - a spinal ganglion (ganglion spinale). The number of neurons that form a spinal ganglion is very large. The cervical and lumbar spinal ganglions contain about 50,000 nerve cells, the thoracic ganglions - 25,000, and the sacral ganglions - 35,000 neurons in one ganglion. The spinal ganglions are located near the intervertebral openings. The spinal ganglions of the first and second cervical spinal nerves are located above and below the atlas arch, respectively. Each spinal ganglion is surrounded by a connective tissue capsule. Thin bundles of connective tissue fibers penetrate from the capsule into the ganglion parenchyma, forming the framework of the ganglion and containing blood vessels. Neurons in the spinal ganglions are located in groups, occupying mainly the periphery of the ganglion. The center of the spinal ganglion consists mainly of processes of nerve cells. The neurons of the node are surrounded by glial cells - mantle gliocytes.

At the exit through the intervertebral foramen from the spinal canal, the anterior and posterior roots join, forming the trunk of the spinal nerve. It is short (0.5-1.5 cm long) and does not completely fill the intervertebral foramen, leaving space for the passage of blood vessels. Each spinal nerve contains both motor and sensory fibers. The anterior roots emerging from the VIII cervical, all thoracic and upper two lumbar segments always contain vegetative (sympathetic) preganglionic fibers coming from the neurons of the lateral horns of the spinal cord.

The spinal nerve after exiting the intervertebral foramen divides into several branches: anterior, posterior, meningeal, and also the white connecting branch (in the thoracolumbar region). The white connecting branch is present only from the VIII cervical to the II lumbar spinal nerves. The anterior and posterior branches of the spinal nerves are mixed. The white connecting branches contain preganglionic sympathetic fibers that go to the nodes of the sympathetic trunk.

The meningeal branches of the spinal nerves also penetrate through the corresponding intervertebral openings in the spinal canal; they innervate the walls of the spinal canal and the membranes of the spinal cord.

Gray communicating branches (rr. communicantes grisei) pass from the sympathetic trunk to all spinal nerves. They are represented by sympathetic nerve fibers coming from all nodes of the sympathetic trunk. As part of all spinal nerves and their branches, postganglionic sympathetic fibers are directed to blood and lymphatic vessels, skin, skeletal muscles and other tissues, which ensures their functions and metabolic processes (trophic innervation).

The posterior branches of the spinal nerves (rr. dorsales, s. posteriores) give off lateral and medial branches (rr. laterales et mediales), which innervate the deep (proper) muscles of the back, the muscles of the back of the head and the skin of the back of the head and trunk. Having separated from the trunks of the spinal nerves, the posterior branches go backwards (between the transverse processes of the vertebrae), bending around the articular processes. The posterior branches of the sacral spinal nerves exit through the dorsal sacral openings. There are branches of the cervical, thoracic, lumbar, sacral and coccygeal nerves.

The posterior branch of the first spinal nerve (CI) is called the suboccipital nerve (n. suboccipitalis). It runs backwards between the occipital bone and the atlas, passing along the upper surface of the posterior arch of the atlas. This nerve is almost entirely motor, it innervates the superior and inferior oblique muscles of the head, the posterior major and minor rectus muscles of the head. A small number of sensory fibers in its composition innervate the joints between the atlas and the axial vertebra, as well as the capsule of the atlanto-occipital joint. There is a constant connection of the suboccipital nerve with the posterior branch of the second cervical spinal nerve.

The posterior branch of the second cervical spinal nerve (CII) - the great occipital nerve (n. occipitalis major) - is thick, departs from the second cervical spinal nerve at the lower edge of the inferior oblique muscle (of the head). Then the nerve goes between the inferior oblique and semispinalis capitis muscles to the lateral surface of the nuchal ligament. This nerve gives off short muscular branches and a long cutaneous branch. The muscular branches innervate the semispinalis capitis and longus capitis muscles, the splenius muscles of the head and neck. The long branch of the nerve pierces the semispinalis capitis muscle and the trapezius muscle, accompanies the occipital artery. Together with this artery, the nerve ascends upward and innervates the skin of the occipital region. The posterior branches of the remaining cervical spinal nerves innervate the skin of the back of the neck.

The posterior rami of the spinal nerves branch into the muscles and skin of the back that they innervate.

The posterior branches of the lumbar spinal nerves innervate the deep muscles of the back and the skin of the lumbar region. The three upper lateral branches run downward and laterally to the skin of the lateral half of the gluteal region and the greater trochanter, forming the superior nerves of the buttocks (nn. cluneum superiores).

The posterior branches of the sacral and coccygeal spinal nerves consist mainly of sensory fibers. The posterior branches of the four upper sacral spinal nerves pass through the dorsal sacral openings, give off branches to the sacroiliac joint, innervate the skin of the posterior surface of the sacrum, and also form the middle nerves of the buttocks (nn. cluneum medii). These nerves pierce the gluteus maximus muscle and innervate the skin in the middle and lower gluteal regions. The posterior branches of the fifth sacral and coccygeal spinal nerves pass near (or pierce) the sacrococcygeal ligament, join with the anococcygeal nerve (see "Coccygeal plexus") and innervate the skin in the coccyx and anus.

The anterior branches of the spinal nerves (rr. ventrales, s. anteriores) innervate the muscles and skin of the anterior and lateral parts of the neck, chest, abdomen and limbs. Only the branches of the thoracic spinal nerves retain a metameric structure. The anterior branches of the cervical, lumbar, sacral and coccygeal spinal nerves form plexuses. These plexuses are formed by connecting adjacent spinal nerves with each other. In the plexuses, an exchange of fibers belonging to adjacent segments of the spinal cord occurs. Due to the redistribution of sensory fibers in the plexuses, a connection is established between one area of the skin and adjacent segments of the spinal cord, therefore, when external factors act on the skin, response signals are transmitted to many muscles. As a result, the reliability of peripheral innervation increases and complex reflex reactions of the body are ensured. The cervical, brachial, lumbar, sacral and coccygeal plexuses are distinguished.

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