Spinal Membranes and Nerve Roots


The external, tough, fibrous dura mater continues downward as far as the second sacral vertebra, where it ends blindly. It is separated from the wall of the vertebral canal by an epidural space containing fatty areolar tissue and a plexus of veins. The dura ensheathes the anterior and posterior spinal nerve roots, which lie close together when they pierce it; then the roots unite almost immediately to form a spinal nerve, and the dural sheath fuses with the epineurium. Between the dura mater and arachnoid is a potential subdural space, which normally contains the merest film of lymphlike fluid.


The spinal arachnoid is loose and tenuous and also ends at the level of the second sacral vertebra. It is separated from the pia mater by the subarachnoid space, which is traversed by delicate mesothelial septa and contains cerebrospinal fluid. The spinal nerve roots, up to the points at which they penetrate the dura mater, are loosely enclosed in arachnoid.


The pia mater is a thin layer of vascular connective tissue that intimately invests the spinal cord and its nerve roots. Below the conus medullaris, it is continuous with the slender filum terminale that descends in the midst of the cauda equina, pierces the terminal parts of the dura and arachnoid, and ends by blending with the connective tissue behind the first segment of the coccyx. On each side, the pia is attached to the dura by 22 pointed processes, the denticulate ligaments.


Nerve Roots. The spinal cord is a segmented structure, and this is indicated by the regular attachments of the pairs of spinal nerves. As explained earlier, the cord and vertebral segments coincide in early embryonic life, but the vertebral canal eventually becomes longer than the cord so that most of the spinal nerves run obliquely downward to their points of exit.


The nerve filaments, or rootlets, are attached to the cord along its anterolateral and posterolateral regions. The anterior (ventral) filaments emerge in two or three irregular rows. They are composed predominantly of efferent fibers, which are the axons of cells in the anterior columns, or horns, of gray matter, and they carry motor impulses to the voluntary muscles. In the thoracic and upper lumbar regions, the filaments also contain preganglionic sympathetic fibers, which are the axons of lateral columnar, or cornual, cells. The posterior (dorsal) filaments are attached in a regular series along a shallow groove, the posterolateral sulcus, and are collections of the central processes of nerve cells located in the spinal ganglia of the related dorsal nerve roots. The lateral cell processes pass on in spinal nerves and their branches to peripheral receptors, and they convey afferent impulses back to the spinal cord from somatic, visceral, and vascular sources.


The spinal cord shows an anterior median fissure and a shallow posterior median sulcus from which a median septum of neuroglia extends forward for 4 to 6 mm. The cord is divided into symmetric halves by the fissure, sulcus, and septum. The lines of attachment of the anterior and posterior nerve filaments are used to demarcate the white matter in each half of the cord into anterior, lateral, and posterior columns, or funiculi.


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Sep 2, 2016 | Posted by in NEUROLOGY | Comments Off on Spinal Membranes and Nerve Roots

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