Spinal cord

The spinal cord is extremely important to the overall function of the nervous system. It is the communication link between the brain and the peripheral nervous system inferior to the head, integrating incoming information and producing responses through reflex mechanisms.

The spinal cord extends from the foramen magnum to the level of the second lumbar vertebra. It is shorter than the vertebral column because it does not grow as rapidly as the vertebral column during embryonic development. It is composed of cervical, thoracic, lumbar, and sacral segments, which are named according to the area of the vertebral column from which their nerves enter and exit. Because the spinal cord is shorter than the vertebral column, the nerves do not always exit the vertebral column at the same level that they exit the spinal cord. Thirty-one pairs of the spinal nerves exit the spinal cord and pass out of the vertebral column through the intervertebral foramina.

The spinal cord is not uniform in diameter throughout its length. There is a general decrease in diameter superiorly to inferiorly, and there are two enlargements where nerves supplying the limbs enter and leave the cord. The cervical enlargement in the inferior cervical region corresponds to the location at which nerves that supply the upper limbs enter or exit the cord, and the lumbosacral enlargement in the inferior thoracic and superior lumbar regions is the site at which the nerves that supply the lower limbs enter or exit.

Immediately inferior to the lumbar enlargement the spinal cord tapers to form a cone-like region called the conus medullaris. Its tip is at the level of the second lumbar vertebra and is the inferior end of the spinal cord. A connective tissue filament, the filum terminale, extends inferiorly from the apex of the conus medullaris to the coccyx and functions to anchor the cord to the coccyx. The nerves supplying the legs and other inferior structures of the body (L2 to S5) exit the lumbar enlargement and conus medullaris, course inferiorly through the vertebral canal, and exit through the intervertebral foramina from L2 to S5. The conus medullaris and the numerous nerves extending inferiorly from it resemble a horse's tail and are therefore called the cauda equina.

NOTE:

S – sacral L – lumbar