The spinal cord is the part of the central nervous system that is found within the spinal canal of the vertebral column. The cord extends from the corticomedullary junction at the foramen magnum of the skull down to the tip of the conus medullaris within the lumbar cistern. It is lined by spinal pia mater and contained by the other spinal meninges in the thecal sac.
The spinal cord measures approximately 42-45 cm in length, ~1 cm in diameter and 35 g in weight.
Like the brain, it is composed of grey and white matter, however, opposite to the brain, the grey matter is on the internal aspect of the cord and the white matter tracts are external. Throughout its length paired dorsal and ventral nerve roots enter its dorsolateral and ventrolateral surface respectively.
The spinal cord is divided into cervical, thoracic and lumbar parts and terminates at the conus medullaris at approximately the L1 vertebral body level in adults.
The spinal cord is segmented by the nerve roots that emerge from it. There are 31 nerve roots in total:
- 8 cervical
- 12 thoracic
- 5 lumbar
- 5 sacral
- 1 coccygeal
A transverse section of the spinal cord shows a peripheral mass of white matter surrounding an ‘H’ or butterfly-shaped central mass of grey matter with a small ependyma-lined central canal filled with CSF . The cord is incompletely divided into left and right halves by the posterior median sulcus (shallow) and the anterior median fissure (deep) .
The grey matter contains the cell bodies of neurons and glia and is enlarged in the cervical and lumbosacral regions to provide fibers to the large nerve plexuses. It is divided into anterior, dorsal and lateral horns and periependymal grey matter :
- anterior horns
- contain motor neurons for skeletal muscle
- send efferent fibers via the ventral nerve roots
- lateral horns
- contain autonomic neurons for pelvic and visceral organs
- only present in the thoracic region
- dorsal horns
- contain somatosensory neurons
- receive primary afferents from the dorsal roots of the spinal nerves
- periependymal grey matter
- divided into ventral and dorsal grey matter commissures
The white matter contains nerve fibers or tracts and is divided into anterior, dorsal and lateral columns (also known as funiculi) as well as the anterior spinal commissure .
- anterior columns primarily contain the spinothalamic tracts which are responsible for pain, temperature, coarse (non-discriminative) touch and pressure sensations
- dorsal columns contain ascending fibers which are responsible for vibration, conscious proprioception, and fine (discriminative) touch sensations
- lateral columns primarily contain the corticospinal tracts which are the principal motor pathway connecting the cerebral cortex to spinal motor neurons
- anterior spinal commissure is located between the posterior-most extent of the anterior median fissure anteriorly and the ventral grey matter commissure posteriorly .
In general, fibers found posteriorly process and relay sensory information, fibers found laterally are preganglionic visceral motor neurons and somatic motor fibers are found anteriorly .
Blood supply is discussed separately. See spinal cord blood supply.