spinal cord

spinal cord The spinal cord extends down from the brain stem at the base of the skull, enclosed in the vertebral canal; brain and spinal cord in continuity comprise the central nervous system. Like the brain, the cord is ensheathed by membranes (meninges), and bathed by cerebrospinal fluid. In the spinal cord are tracts of white matter, nerve fibres carrying information to and from the brain as well as between different levels of the cord itself; and a core of grey matter, containing nerve cells and synapses that mediate motor, sensory, and reflex functions. The substance of the cord is continuous, but functional segments are marked by the series of nerve roots at intervals down its length. At each level, two nerve roots (dorsal or posterior carrying ingoing nerve impulses; ventral or anterior carrying outgoing impulses) join to form a spinal nerve on each side. The uppermost emerges between the skull and the uppermost cervical vertebra; the rest emerge between two adjacent vertebrae, and between the segments of the sacrum. There are 8 cervical nerves, and below this the nerves are named according to the vertebra above their point of exit: thus there are 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal nerve. The spinal canal is longer, however, than the spinal cord, which ends in the lumbar part of the canal. Therefore the distance that a spinal nerve must travel to reach its point of exit increases from above downwards, from zero for the first cervical nerve to about 20 cm for the lowest sacral and coccygeal. In the canal below the end of the cord, there is therefore a sheaf of descending spinal nerves that becomes progressively smaller as the nerves leave; this is known as the horse's tail — the cauda equina. This arrangement has consequences for the effects of spinal injury at different vertebral levels. Anywhere above the second lumbar vertebra, it is the spinal cord that is damaged; below this, it is spinal nerves. Spinal cord damage leaves uncontrolled motor neurons below the level of the lesion; voluntary movement is lost, but after recovery from an initial period of spinal shock, the muscles can and do contract, spontaneously and reflexly: a spastic paralysis. Damage to the spinal nerves in the cauda equina, by contrast, separates the affected muscles from their spinal motor neurons; voluntary movement is lost and the muscles remain relaxed: a flaccid paralysis followed by wasting. In either case paralysis is accompanied by loss of sensation.

Sheila Jennett

See nervous system.See also central nervous system; meninges; motor neurons; paralysis; reflexes; spinal shock.