Lab 4 - External and Internal Anatomy of the Spinal Cord

Microscopic Sections of Spinal Cord - Cervical (continued)

This section was taken through the cervical spinal cord at a level where the anterior horn is enlarged to contain the cells innervating the musculature of the arms, hands and digits.

The Gray Matter

The posterolateral tip of the posterior horn contains a thin layer of relatively large cells called the nucleus posteromarginalis. Below this nucleus is the substantia gelatinosa, which appears as a light area forming the cap of the posterior horn. The nucleus proprius is a poorly defined cell column located in the head and neck of the posterior horn. It can be identified in myelin-stained sections as the darkly stained area beneath the substantia gelatinosa. The difference in coloration of the substantia gelatinosa and nucleus proprius is indicative of the different types of afferent fibers (unmyelinated and myelinated, respectively) that terminate in these two areas.

At this level of the spinal cord (i.e., above T1), the intermediate gray is poorly developed and does not contain autonomic neurons. In contrast, the anterior horn is highly developed at most cervical levels. Recall that the anterior horn contains large (alpha) motor neurons and small (gamma) motor neurons whose axons innervate the skeletal musculature.

The White Matter

The posterior funiculus lies between the posterior median sulcus and the posterior root entry zone at the posterolateral sulcus. Above T6 the posterior funiculus is subdivided by the posterior intermediate sulcus into the fasciculus gracilis medially and the fasciculus cuneatus laterally. The posterior funiculus contains large myelinated nerve fibers of posterior root ganglion cells. The peripheral processes of these ganglion cells form or innervate sensory receptors in the skin and deep tissues of the body. The lateral funiculus lies between the posterior root entry zone and the site of emergence of the anterior root fibers at the anterolateral sulcus. The lateral funiculus contains both ascending and descending nerve fiber tracts. The anterior funiculus lies between the emergence of the anterior root fibers and the anterior median fissure. The anterior funiculus also contains both ascending and descending nerve fiber tracts.

Note that sensory information from segments below T6 travels in the fasciculus gracilis and that above T6 travels in fasciculus cuneatus. Also as you travel superiorly through the cord the sensory information coming in from each successive spinal nerve stacks just lateral to the sensory information carried by the nerve just below it so that sensory information is topographically organized.

Review

The cervical segments are characterized by their large size, extensive white matter, and the increase in gray matter in the segments which supply the efferent and afferent innervation to the brachial plexus. The fasciculus cuneatus of the posterior funiculus at cervical level is much larger than at thoracic levels. There are no autonomic neurons and no dorsal nucleus of Clarke at cervical levels of the spinal cord, while all descending tracts are present.