spinal cord

Major component of the central nervous system in vertebrates. It consists of bundles of nerves enveloped in three layers of membrane (the meninges) and is bathed in cerebrospinal fluid. The spinal cord is encased and protected by the vertebral column, lying within the vertebral canal formed by the posterior arches of successive vertebrae.

In humans, the spinal cord is about 45 cm/18 in long, extending from the bottom of the skull, where it is continuous with the medulla oblongata, to about waist level. It consists of nerve cell bodies (grey matter) and their myelinated processes or nerve fibres (white matter). In cross-section, the grey matter is arranged in an H-shape around the central canal of the spinal cord, and it is surrounded in turn by the white matter.

Paired spinal nerves arise from the cord at each vertebra. Each is a mixed nerve, consisting of both sensory and motor nerve fibres. The sensory fibres enter the spinal cord at a dorsal root and the motor fibres enter at a ventral root. This arrangement enables the spinal cord to relay impulses coming in and out at the same level, to relay impulses going up and down the cord to other levels, and relay impulses to and from the brain. The first of these involves a reflex arc, by which a sensory impulse can create a very rapid, involuntary response to a particular stimulus.