Anatomy and Physiology of the Spinal Cord

Abstract

Gross Anatomy The spinal cord is part of the central nervous system (CNS), which extends caudally and is protected by the bony structures of the vertebral column. It is covered by the three membranes of the CNS, i.e., the dura mater, arachnoid and the innermost pia mater. In most adult mammals it occupies only the upper two-thirds of the vertebral canal as the growth of the bones composing the vertebral column is proportionally more rapid than that of the spinal cord. According to its rostrocaudal location the spinal cord can be divided into four parts: cervical, thoracic, lumbar and sacral, two of these are marked by an upper (cervical) and a lower (lumbar) enlargement. Alongside the median sagittal plane the anterior and the posterior median fissures divide the cord into two symmetrical portions, which are connected by the transverse anterior and posterior commissures. On either side of the cord the anterior lateral and posterior lateral fissures represent the points where the ventral and dorsal rootlets (later roots) emerge from the cord to form the spinal nerves. Unlike the brain, in the spinal cord the grey matter is surrounded by the white matter at its circumference. The white matter is conventionally divided into the dorsal, dorsolateral, lateral, ventral and ventrolateral funiculi. Each half of the spinal grey matter is crescent-shaped, although the arrangement of the grey matter and its proportion to the white matter vary at different rostrocaudal levels. The grey matter can be divided into the dorsal horn, intermediate grey, ventral horn and a centromedial region surrounding the central canal (central grey matter) The white matter gradually ceases towards the end of the spinal cord and the grey matter blends into a single mass (conus terminalis) where parallel spinal roots form the so-called cauda equina. The dorsal roots leave the dorsal horn and dorsolateral white matter, coalesce into two bundles and enter the dorsal root ganglion (DRG) in the intervertebral foramen. Immediately distal to the ganglion, the dorsal and ventral roots unite and form a trunk, the spinal nerve. The spinal nerves, which are now outside the vertebral column, converge and form plexuses and from these emerge the peripheral nerves. The number of spinal nerves and spinal segments largely corresponds to the number of vertebrae with a few exceptions: there are eight cervical, 12 thoracic, five lumbar, five sacral and one coccygeal spinal segments in humans. The number of these segments varies slighdy in different species.