Spinal cord

The spinal cord is a part of the central nervous system and is located in the spinal column. It is continuous with the brain, and acts as a relay station between the rest of the body and the brain. If sensory impulses are generated at the periphery of the body, they travel via sensory nerves to the spinal cord. There the impulses can be relayed to the brain where perception of the stimulus takes place. The brain can send motor impulses down the cord, from which they can be relayed to the motor neurons and travel to the periphery via the motor nerves. In some instances immediate action has to be taken following a stimulus.

The information from the periphery is then relayed to the spinal cord and travels directly to motor nerve cells there. This enables our body to react without thinking to certain stimuli. The mechanism is called a reflex. An example is burning a finger and immediately withdrawing it. The burning pain is felt after the withdrawal reflex. The spinal cord begins as a continuation of the lowest part of the brain, the medulla oblongata, where it passes through the large hole in the base of the skull called the foramen magnum. It is surrounded by three protective membranes, the meninges, the outer two of which are fairly loose and contain cerebrospinal fluid continuous with that surrounding the brain.

Housed inside the bony canal formed by the arches of the vertebrae, the cord is embedded in fat and blood vessels. These cushion the delicate nervous tissue from movements of the spine. The strong bone of the vertebrae and the tough ligaments that bind them together provide further support and protection. There are 31 pairs of spinal nerves. Each nerve arises during embryonic development by the fusion of a sensory (afferent) nerve root, which deals with incoming messages, with a motor (efferent) root which carries outgoing messages. Each spinal nerve divides, one division going to supply the skin and muscles of the back, and the other division supplying not only the skin and muscles of the front of the trunk, but also meeting in groups, or plexi, from which nerves travel to the limbs. The spinal nerves emerge from holes between the vertebrae, from which they travel to the periphery. Between the vertebrae are cartilageous discs. If these discs degenerate they can protrude. In that case the nerves that emerge between the vertebrae can get pinched, causing pain along the course of the nerve. This condition is called a herniated (or slipped) disc.

In the embryo the spinal cord runs down to the lowest vertebrae. As the body grows much quicker than the nervous system, in the adult the spinal cord reaches down only to the first or second lumbar vertebrae. There it tapers to a thin thread, the filium terminale, that continues down to the coccyx where it is attached. At the end of the spinal cord there is a bundle of nerves called the cauda equina. These nerves travel down andemerge at a lower level between the vertebrae. Thus between the second lumbar vertebra and the coccyx there is no spinal cord.