Spinal Nerves


The upper cervical spinal nerves lie horizontally, but all the others assume an increasingly oblique and downward direction as they proceed to their foramina of exit. In the adult, the lumbar, sacral, and coccygeal cord segments lie opposite the last three thoracic and first lumbar vertebrae, and their attached nerve roots descend as a sheath around the filum terminale to constitute the cauda equina.


The spinal nerves are connected with adjacent sympathetic trunk ganglia by rami communicantes. These rami contribute efferent and afferent sympathetic fibers to the spinal nerves, which consist primarily of efferent and afferent somatic fibers derived from the ventral and dorsal nerve roots.


Shortly after emerging from the intervertebral foramina, the spinal nerves give off small recurrent meningeal branches, which supply the meninges and their vessels; they also supply filaments to adjacent articular and ligamentous structures. They then divide into ventral (anterior) and dorsal (posterior primary) rami, which contain fibers from both nerve roots and a variable number of sympathetic fibers.


The ventral rami supply the anterior and lateral parts of the neck and trunk and make up the nerves of the perineum and limbs. Except in the thoracic region, where they retain their separate identities as intercostal and subcostal nerves, the ventral rami divide and reunite in differing patterns to form the following nerve plexuses: the cervical plexus, from the ventral rami of the first four cervical nerves; the brachial plexus, from the ventral rami of the lower four cervical and first thoracic nerves; the lumbar plexus, from the ventral rami of the first three lumbar nerves and from most of the ventral ramus of the fourth lumbar nerve; the sacral plexus, from the remainder of the ventral ramus of the fourth lumbar nerve and from the ventral rami of the fifth lumbar and first three sacral nerves; and the small sacrococcygeal plexus, from the ventral rami of the fourth and fifth sacral nerves and from the coccygeal nerve. (The plexuses and their branches are described in detail in Plates 4-12 and 4-13.


The dorsal rami turn dorsally and are distributed to cutaneous, muscular, and other structures of the back of the neck and trunk. Although some dorsal rami join to form loops, their branches do not form true plexuses as do branches derived from the ventral rami. Also, the dorsal rami (with the exception of those from the first and second cervical nerves) are generally smaller than the corresponding ventral rami. All the dorsal rami, except those from the first cervical, fourth and fifth sacral, and coccygeal nerves, divide into larger medial and smaller lateral branches. Most medial branches supply the muscles and the skin, whereas the lateral branches end in the muscles. However, the lateral branches tend to increase in size from above downward, so that those from the last thoracic, five lumbar, and five sacral nerves provide both muscular and cutaneous filaments.


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Sep 2, 2016 | Posted by in NEUROLOGY | Comments Off on Spinal Nerves

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