Thys ordur, unyte, and concord, whereby the partys of thys body are, as hyt were, wyth senewys and neruys knyt togyddur. STARKEY England II . i.158, 1538
Development of the peripheral nervous system
Formation of the neural tube: neuroblast formation
Neural tube and crest
The neural tube is formed by fusion of the edges of the neural plate. Cells from the edges of the plate that remain dorsal to the tube during fusion create the neural crest ( Fig. 4-1 A). The tube becomes the central nervous system of the brain and spinal cord, whereas the crest forms part of the peripheral nervous system of the cranial, spinal, and autonomic ganglia and nerves.
Neuroblasts and nerve roots
Neuroepithelial cells expand in the wall of the neural tube, forming the ependymal layer of gray matter from which all the neurons and microglial cells of the cord arise. Outside this layer, other neuroepithelial cells form a marginal zone that will become the white matter after invasion by the axons of nerve cell bodies lying in the cord or dorsal root ganglia.
Some cells in the ependymal layer develop into neuroblasts, which after developing axons become neurons. As the cord develops, a limiting groove forms on each side, indicating the division into alar dorsal and basal ventral plates ( Fig. 4-1 B). In the alar plates , the posterior gray columns (horns) develop, composed of cell bodies destined to form the afferent nuclei. From each alar plate, a dorsal spinal nerve root leads to the spinal ganglion containing sensory neuroblasts that were derived from the neural crest. In the basal plates , the lateral and anterior gray columns develop from cell bodies that send out bundles of axons from the motor neuroblasts to form the ventral spinal nerve roots .
Migration of neural crest neuroblasts
Neural crest cells at first lie as a strip on either side of the neural plate. As the neural tube forms, they are carried to a dorsal position in the cord and then migrate extensively to the primitive spinal ganglia , the lateral vertebral chain ganglia (sympathetic), the preaortic ganglia , and the visceral ganglia ( Fig. 4-2 ). They also travel to the adrenal cortical area, where they form the pheochromocytes of the adrenal medulla and to the primitive gonad to provide paraganglion cells (see Table 4-1 ).
Neural crest cells develop into the sensory neurons of the dorsal root ganglia of the spinal nerves, both somatic and sympathetic, into the main sympathetic and parasympathetic postganglionic neurons in the sympathetic chains, and into the mesenteric, renal, and vesical plexuses. The neural crest cells also form part of the amine precursor uptake and decarboxylation system, the diffuse neuroendocrine system that includes the adrenal medulla, the paraganglia, the para-aortic bodies, and other aberrant chromaffin tissue (see Chapter 12 ; Fig 12-39 ).
Autonomic nervous system
The neuroblasts from which the autonomic system are derived come from the neural crest. The central axons of these neurons enter the spinal cord from the dorsal root ganglia as the dorsal roots of the spinal nerves and either end locally in the gray matter or ascend centrally to the brain in the dorsal white columns. Their peripheral processes run in the spinal nerves and are distributed through the sympathetic ganglia and sympathetic trunk of the sympathetic chain to the viscera through ganglia such as the preaortic ganglion .
The cells from the basal plate of the neural tube have their cell bodies in the lateral horn of the spinal cord at the T1 to T12 and L1 to L2 levels and are distributed by way of white rami communicantes to the splanchnic nerves (see Fig. 4-5 ).
Divisions of the spinal nerves
The spinal nerves are attached to the spinal cord through dorsal and ventral roots ( Fig. 4-3 ).
Dorsal roots
The neural crest neuroblasts, in migrating from their position beside the neural tube, form the spinal ganglia (dorsal root ganglia), which contain the cell bodies of the sensory neurons and form sympathochromaffin cells. The dorsal primary division of the spinal nerves supplies the dorsal part of the body; the larger ventral primary division supplies the ventral part, including the arms and legs. The third division is made up of the rami communicantes, which connect the spinal nerves to the sympathetic ganglia.
Ventral roots
Neuroblasts in the intermediate zone of the cord pass through the ventral roots into the myotomes of the mesodermal somites.
Nerve supply of the genitourinary system
Spinal cord
Structure of the lower spinal cord, arteries, coverings, and veins
Meninges and venous drainage
Meninges.
The coverings of the spinal cord occur in three layers within the vertebral canal: (1) the dura mater, (2) arachnoid membrane, and (3) pia mater ( Fig. 4-4 A).
The dura mater is a layer of collagen mixed with elastic fibers. At the exit site of a nerve, the dura becomes continuous with the perineurium. The delicate arachnoid membrane lies beneath the dura and is partially adherent to it, leaving only a narrow space, the subdural space , which has little or no fluid within it. The arachnoid envelops the cord and the nerves up to their point of exit from the vertebral canal. It encloses the subarachnoid space , which contains the cerebrospinal fluid and the major blood vessels supplying the cord. A vascularized membrane, the pia mater , closely covers the cord in two layers—an outer epipia, carrying blood vessels, and an inner pia intima, lying over the glial capsule that actually covers the cord. The pia mater extends over the exiting nerves and joins their sheaths.
Venous drainage.
Two sets of veins drain the vertebral column—(1) the external and (2) the internal vertebral venous plexuses—each of which has a posterior and an anterior portion (for details see Figs. 2-18 and 2-20 ).
The external vertebral vein and plexus is divided into two parts: (1) an anterior external venous plexus situated about the vertebral body and (2) a posterior intervertebral plexus distributed about the laminae, spines, and transverse processes of the vertebra. The two portions of the external plexus come together at their junction with the ascending lumbar vein , which, in turn, is connected through the lumbar veins to the anterior external venous plexus that is associated with the lumbar azygos vein , to drain into the inferior vena cava.
The internal vertebral plexus is located outside the dura within the vertebral canal. The anterior internal venous plexus is adjacent to the vertebral body, and the posterior internal venous plexus lies next to the vertebral arches.
There is free communication between the internal and external plexuses throughout the length of the vertebral canal. The two plexuses connect with each other through the basivertebral veins in the vertebral body and through the intervertebral veins in the intervertebral foramina . Blood from the vertebral system is carried to the lumbar veins as well as to the posterior intercostal veins. The intervertebral veins lack valves, so reverse flow probably occurs during abdominal straining, thus allowing pelvic neoplasms to spread to the spine.
Spinal cord.
The cord extends from the atlas to the first lumbar intervertebral disk. It may only reach the 12th thoracic vertebra, or it may extend one vertebra lower. Enlargements occur in the cervical and lumbar regions where large nerves emerge ( Fig. 4-4 B). The ventral surface of the cord has an anterior medial fissure, and the dorsal surface has a posterior median sulcus that is connected to a posterior median septum that extends well into the cord. A posterolateral sulcus indicates the site of entry of the dorsal roots.
The conus medullaris of the cord ends in the filum terminale , which is covered by the dura around a large subarachnoid space (suitable for spinal puncture) except for a part covered only by adherent dura. Dorsal and ventral roots of spinal nerves emerging along the cord pass through the dura individually to unite as paired roots.
At the midlevel of the sacrum, which contains the cauda equina and filum terminale , the subarachnoid and subdural spaces become obliterated. Here the lower spinal nerve roots and the filum terminale pass through the arachnoid and the dura. Both the filum terminale and the 5th sacral spinal nerve emerge from the sacral hiatus .
Arterial supply.
The intercostal and lumbar arteries give off spinal branches to the cord in the trunk as anterior and posterior radicular arteries that enter along the ventral and dorsal nerve roots. The supply is supplemented by contributions from the anterior and the paired posterior spinal arteries. Longitudinal branches ascend and descend within the cord.
Somatic nervous system
Organization of the somatic nervous system
Somatic motor nerves
Somatic motor functions are performed by a single neuron, the somatic motor neuron (white line, Fig. 4-5 ). The neuron is composed of a central cell body in the anterior gray column (anterior horn) and an axon extending to a muscle. The axon exits through the ventral root and passes along the spinal nerve to a motor end plate on muscle fibers . Somatic motor neurons can stimulate but not inhibit contraction of striated muscle, in contrast to autonomic motor neurons, which can both stimulate and inhibit smooth muscle contraction.
