spinal cord

the cord of nerve tissue extending through the spinal canal of the spinal column.

[1830-40]

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In vertebrates, the body's major nerve tract.

In humans it is about 18 in. (45 cm) long, running from the base of the brain through the vertebral column. It is covered by the meninges and cushioned by cerebrospinal fluid. It connects the peripheral nervous system (outside the brain and spinal cord) to the brain. The spinal cord and the brain constitute the central nervous system. Sensory impulses reach the brain via the spinal cord, and impulses from the brain travel down the spinal cord to motor neurons, which reach the body's muscles and glands via the peripheral nerves. The peripheral nerves are connected to the spinal cord via the spinal nerves. In humans there are 31 pairs of spinal nerves containing both sensory and motor fibres, which originate in the spinal cord and pass out between the vertebrae. These nerves branch and relay motor impulses to all parts of the body. Injury to the spinal cord may result in loss of communication between the brain and outlying parts and cause paralysis, loss of sensation, or weakness in the parts of the body served by areas below the injured region. Because nerve cells and fibres are unable to regenerate themselves, the effects are usually permanent.

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▪ anatomy

      major nerve tract of vertebrates, extending from the base of the brain through the canal of the spinal column. It is composed of fibres that mediate reflex actions and that transmit impulses to and from the brain.

      Like the brain, the spinal cord is covered by three connective-tissue envelopes called the meninges. The space between the outer and middle envelopes is filled with cerebrospinal fluid, a clear, colourless fluid that cushions the spinal cord.

 A cross section of the spinal cord reveals white matter arranged around a butterfly-shaped area of gray matter. The white matter consists of myelinated fibres, or axons (axon), that form nerve tracts ascending to and descending from the brain. The white matter is grouped into discrete sectors called funiculi. The gray matter contains cell bodies, unmyelinated motor-neuron fibres, and interneurons connecting the two sides of the cord. Gray-matter cells form projections called horns. Fibres exiting the spinal cord from the dorsal and ventral horns join in paired tracts to form the spinal nerves (spinal nerve). Information travels up the ascending tracts of neurons and is sorted by the brain. Responses are induced by nerve impulses traveling down the descending tracts that stimulate motor neurons or that initiate glandular secretion. (See also nervous system, human.)

 

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