DISTRIBUTION OF SPINAL NERVES
To review, each of 31 spinal nerves are attached to the spinal cord via the dorsal and ventral roots. The dor-sal root is enlarged to form the dorsal root ganglion. Close to the intervertebral foramen, the dorsal and ventral roots are bound together to form the spinal nerve.
As the spinal nerve continues to the periphery, it is surrounded by layers of connective tissue. The outer-most layer, or epineurium,consists of a dense net-work of collagen fibers. The nerve is divided into many bundles of axons by the perineurium, which consists of collagen fibers that extend inward from the epineurium. Individual axons are surrounded by delicate connective tissue fibers, theendoneurium. The blood vessels travel along the connective tissue layers, delivering nutrients and oxygen to individual axons, Schwann cells, and connective tissue and re-moving waste products.
Figure 5.17 shows the distribution of a typical spinal nerve. The first branch of the spinal nerve in the thoracic and upper lumbar regions (T1–L2) car-ries preganglionic axons of the sympathetic nervous system . These myelinated fibers appear white, and the branch is known as the white ramus. The axons synapse with postganglionic neurons lo-cated in the sympathetic ganglion that runs along the side of the vertebral column. From the sympathetic ganglion, axons of postganglionic fibers that inner-vate glands and smooth muscles in the body wall and limbs join the spinal nerve again as the gray ramus.Other postganglionic fibers of the sympathetic sys-tem that innervate the organs in the thoracic and ab-dominal cavity form networks and nerves in the tho-rax and abdomen.
The spinal nerve then branches into the dorsal ra-mus and ventral ramus, distal to the white and grayrami. The dorsal ramus of each spinal nerve carries sensory and motor innervation to the skin and the muscles of the back. The larger ventral ramus sup-plies the structures on the body wall, the limbs, and the ventrolateral areas of the body.