The Organization of the Spinal Cord

This chapter introduces the general anatomy and organization of the spinal cord. The spinal cord is the part of the central nervous system that controls the voluntary muscles of the limbs and trunk, and which receives sensory information from these regions. It also controls most of the viscera and blood vessels of the thorax, abdomen, and pelvis. The cord is described as being made up of a series of segmental components, but in the adult the cord is actually a continuous cylinder of central nervous tissue. The segments are determined by the pattern of emergence of spinal nerves; the nerve rootlets arising from the cord are bundled together so that one pair of spinal nerves emerges from each segment. The spinal cord and its meninges (dura mater, arachnoid, and pia mater) lie within the vertebral canal. The spinal cord is cylindrical, but slightly flattened dorsoventrally. The spinal white matter occupies most of the cross-sectional area of the cervical enlargement, while the spinal cord gray matter is greatly enlarged in the regions where the nerves of the limbs (the brachial and lumbosacral plexuses) arise. The human spinal cord has 12 thoracic segments, 5 lumbar segments, 5 sacral segments, and 1 coccygeal segment, making a total of 31 segments. Pairs of spinal nerves arise from the spinal cord and leave the vertebral column through the intervertebral foramina. The central canal of the spinal cord is a remnant of the embryological ventricular system and is present throughout the whole length of the spinal cord. The spinal cord is supplied by a single ventral spinal artery and two dorsal spinal arteries.