SPINAL NERVES: Distribution, features, structural arrangements, classification, plexuses, intercostal nerves, cervical nerves, brachial nerves, thoracic nerves & kumbar nerves (features & distribution in detail).

Friday 6 March 2015

SPINAL NERVES: Distribution, features, structural arrangements, classification, plexuses, intercostal nerves, cervical nerves, brachial nerves, thoracic nerves & kumbar nerves (features & distribution in detail).

Spinal nerves and the nerves that branch from them are part of the peripheral nervous system (PNS). They connect the CNS to sensory receptors, muscles, and glands in all parts of the body. The 31 pairs of spinal nerves are named and numbered according to the region and level of the vertebral column from which they emerge. Not all spinal cord segments are aligned with their corresponding vertebrae.

Recall that the spinal cord ends near the level of the superior border of the second lumbar vertebra, and that the roots of the lumbar, sacral, and coccygeal nerves descend at an angle to reach their respective foramina before emerging from the vertebral column. This arrangement constitutes the cauda equina. The ﬁrst cervical pair of spinal nerves emerges between the occipital bone and the atlas (ﬁrst cervical vertebra). Most of the remaining spinal nerves emerge from the vertebral column through the intervertebral foramina between adjoining vertebrae. Spinal nerves C1–C7 exit the vertebral canal above their corresponding vertebrae.

Spinal nerve C8 exits the vertebral canal between vertebrae C7 and T1. Spinal nerves T1–L5 exit the vertebral canal below their corresponding vertebrae. From the spinal cord, the roots of the sacral spinal nerves (S1–S5) and the coccygeal spinal nerves (Co1) enter the sacral canal, the part of the vertebral canal in the sacrum. Subsequently, spinal nerves S1–S4 exit the sacral canal via the four pairs of anterior and posterior sacral foramina and spinal nerves S5 and Co1 exit the sacral canal via the sacral hiatus. As noted earlier, a typical spinal nerve has two connections to the cord: a posterior root and an anterior root. The posterior and anterior roots unite to form a spinal nerve at the intervertebral foramen. Because the posterior root contains sensory axons and the anterior root contains motor axons, a spinal nerve is classiﬁed as a mixed nerve. The posterior root contains a posterior root ganglion in which cell bodies of sensory neurons are located.

Connective Tissue Coverings of Spinal Nerves

Each spinal nerve and cranial nerve consists of many individual axons and contains layers of protective connective tissue coverings. Individual axons within a nerve, whether myelinated or unmyelinated, are wrapped in endoneurium, the innermost layer. Groups of axons with their endoneurium are arranged in bundles called fascicles, each of which is wrapped in perineurium, the middle layer. The outermost covering over the entire nerve is the epineurium. Extensions of the epineurium also occur between fascicles. The dura mater of the spinal meninges fuses with the epineurium as the nerve passes through the intervertebral foramen. Note the presence of many blood vessels, which nourish nerves, within the perineurium and epineurium. You may recall that the connective tissue coverings of skeletal muscles endomysium, perimysium, and epimysium are similar in organization to those of nerves.

Distributions of spinal nerves

Branches

A short distance after passing through its intervertebral foramen, a spinal nerve divides into several branches. These branches are known as rami. The posterior (dorsal) ramus serves the deep muscles and skin of the posterior surface of the trunk. The anterior (ventral) ramus serves the muscles and structures of the upper and lower limbs and the skin of the lateral and anterior surfaces of the trunk. In addition to posterior and anterior rami, spinal nerves also give off a meningeal branch. This branch reenters the vertebral cavity through the intervertebral foramen and supplies the vertebrae, vertebral ligaments, blood vessels of the spinal cord, and meninges. Other branches of a spinal nerve are the rami communicantes, components of the autonomic nervous system.

Plexuses

Axons from the anterior rami of spinal nerves, except for thoracic nerves T2–T12, do not go directly to the body structures they supply. Instead, they form networks on both the left and right sides of the body by joining with various numbers of axons from anterior rami of adjacent nerves. Such a network of axons is called a plexus ( braid or network). The principal plexuses are the cervical plexus, brachial plexus, lumbar plexus, and sacral plexus. A smaller coccygeal plexus is also present. Refer to the diagram to see their relationships to one another. Emerging from the plexuses are nerves bearing names that are often descriptive of the general regions they serve or the course they take. Each of the nerves in turn may have several branches named for the speciﬁc structures they innervate. Exhibits summarize the principal plexuses. The anterior rami of spinal nerves T2–T12 are called intercostal nerves and will be discussed next.

Intercostal Nerves

The anterior rami of spinal nerves T2–T12 do not enter into the formation of plexuses and are known as intercostal or thoracic nerves. These nerves directly connect to the structures they supply in the intercostal spaces. After leaving its intervertebral foramen, the anterior ramus of nerve T2 innervates the intercostal muscles of the second intercostal space and supplies the skin of the axilla and posteromedial aspect of the arm. Nerves T3–T6 extend along the costal grooves of the ribs and then to the intercostal muscles and skin of the anterior and lateral chest wall. Nerves T7–T12 supplies the intercostal muscles and abdominal muscles, along with the overlying skin. The posterior rami of the intercostal nerves supply the deep back muscles and skin of the posterior aspect of the thorax.

origin, distribution of all spinal nerves are depicted in the diagrams...