Cross Sections of the Spinal Cord
Cross sections at different levels (left, myelin stain; right, cellular stain) vary con-siderably. 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 numer-ous nerves that supply the extremities cause an increase in gray matter.
The white matter is most extensive in the cer-vical 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 (Lissauer’stract) (A–D1).
The posterior horn is narrow in the cervical spinal cord; its tip ends in the cap-shaped marginal zone (nucleus posteromarginalis) (A2). The lateral angle between the poste-rior and anterior horn is occupied by the re-ticular formation (AD3). The gelatinous sub-stance (Rolando’s substance) (A – D4) con-tains small, mostly peptidergic neurons where posterior root fibers of various cali-bers terminate; it also contains descending fibers from the brain stem (raphe nuclei, p. 108, B28; reticular formation, p. 146). Un-myelinated 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 sensi-bility in the muscles (muscle spindles) ter-minate in the posterior thoracic nucleus (dor-sal nucleus of Clarke) (AB5) where the tractsto 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 hornof the lumbar and sacral spinal cords, the gelatinous substance (CD4) is much en-larged and borders dorsally on the narrow band of the marginal zone (CD2).
The lateral horn forms in the thoracic spi-nal cord the lateral intermediate substance (B6). It contains sympathetic nerve fibers mainly for the vasomotor system, the effer-ent fibers of which emerge via the anterior root. Sympathetic neurons also lie medially in the intermediomedial nucleus (B7). In the sacral spinal cord, parasympathetic neurons form the intermediolateral nucleus und in-termediomedial nucleus (D8).
The anterior horn expands in the cervical spinal cord and contains several nuclei with large motor neurons, all of which are cholin-ergic.
Medial group of nuclei
Anteromedial nucleus (A9)
Posteromedial nucleus (A10)
Lateral group of nuclei
Anterolateral nucleus (A11)
Posterolateral nucleus (A12)
Retroposterolateral nucleus (A13)
In the region supplying the upper limbs, the anterior horn is far more differentiated than in the thoracic spinal cord where only a few cell groups can be identified. The expanded, plump anterior horn of the lumbar and sacral spinal cords, which supplies the lower limbs, again contains several groups of nuclei.