Lab 4 - External and Internal Anatomy of the Spinal Cord

Microscopic Sections of the Spinal Cord - Thoracic (continued)

This section was taken at approximately the mid-thoracic level. View and identify the structures in the illustration.

The Gray Matter

Within the posterior horn, locate the nucleus posteromarginalissubstantia gelatinosanucleus proprius, anddorsal nucleus of Clarke. The nucleus posteromarginalis, substantia gelatinosa, and nucleus proprius extend the entire length of the spinal cord. The dorsal nucleus of Clarke is only present at spinal cord levels C8 through L3. It can be identified in myelin-stained sections as the small darkly stained mass located in the neck and base of the posterior horn. The lateral horn of the intermediate gray is well developed at thoracic and upper lumbar levels (T1 - L3) and contains autonomic neurons of the spinal sympathetic system. The anterior horn is much reduced in size in comparison to the cervical cord and reflects the lower density of innervation of trunk musculature as compared with the density of innervation of the limb and digits.

The White Matter

The posterior funiculus is divided by the posterior intermediate sulcus at levels above T6 into the fasciculus gracilis and the fasciculus cuneatus. Below T6, the posterior funiculus consists only of the fasciculus gracilis. Collaterals of posterior root ganglion axons ascend and descend the spinal cord in the tract of LissauerIntersegmental association cord cells send their axons in the fasciculus proprius to cord neurons in lower or higher segments. The fasciculus proprius (propriospinal tract) forms a thin shell of fibers around the gray matter.

Review

The thoracic segments are characterized by the prominent lateral gray horn, which extends into the lateral funiculus. The lateral horn contains the cell bodies of preganglionic sympathetic nerve fibers. The thoracic segments of the spinal cord also display variations, which enable one to identify which level of the thoracic cord is being observed. Lower thoracic (T9 - T12) segments have larger anterior horns than the upper thoracic (T2 - T8) segments because the lower thoracic segments supply axial musculature (back and intercostals) and abdominal musculature, while the upper thoracic segments supply only axial musculature. The posterior intermediate sulcus appears at and above level T6 and subdivides the posterior funiculus into the fasciculus gracilis and fasciculus cuneatus.