Cross sections of the spinal cord at different levels vary considerably. In the regions of cervical enlargement and lumbar enlargement, the cross sectional area is larger than in the rest of the spinal cord; it is largest at the C4–C5 and L4–L5 levels. In both swellings, the numerous nerves that supply the extremities cause an increase in gray matter. The white matter is most extensive in the cervical region and diminishes gradually in caudal direction; the ascending sensory tracts increase in number from the sacral to the cervical region as more fibers are added, while the descending motor tracts decrease from the cervical to the sacral regions as fibers terminate at various levels.
The butterfly configuration of the gray matter changes in shape at the various levels, and so does the posterolateral tract. The posterior horn is narrowing the cervical spinal cord; its tip ends in the cap-shaped marginal zone (nucleus posteromarginalis). The lateral angle between the posterior and anterior horn is occupied by the reticular formation. The gelatinous substance (Rolando’s substance) contains small, mostly peptidergic neurons where posterior root fibers of various calibers terminate; it also contains descending fibers from the brain stem. Unmyelinated processes of neurons ascend or descend for one to four root levels within the posterolateral tract (Lissauer’s tract) and then reenter into the gelatinous substance. Some of the processes run within the lateral spinothalamic tract to the thalamus. The fibers of proprioceptive sensibility in the muscles (muscle spindles) terminate in the posterior thoracic nucleus (dorsal nucleus of Clarke) where the tracts to the cerebellum begin. The reduced gray matter of the thoracic spinal cord has a slender posterior horn with a prominent dorsal nucleus. In the plump posterior horn of the lumbar and sacral spinal cords, the gelatinous substance is much enlarged and borders dorsally on the narrow band of the marginal zone.
The lateral horn forms in the thoracic spinal cord the lateral intermediate substance. It contains sympathetic nerve fibers mainly for the vasomotor system, the efferent fibers of which emerge via the anterior root. Sympathetic neurons also lie medially in the intermediomedial nucleus. In the sacral spinal cord, parasympathetic neurons form the intermediolateral nucleus und intermediomedial nucleus. The anterior horn expands in the cervical spinal cord and contains several nuclei with large motor neurons, all of which are cholinergic.