Ascending projections of the dorsal column in a garter snake (Thamnophis siritalis): a degeneration study.

Ascending axons in the dorsal column of garter snakes were examined following hemisection of the spinal cord at segment levels 2, 3, 4, 11, 13, and 31. After postoperative survival periods of 11 to 28 days, sections of the spinal cord and brain were processed with a silver method to demonstrate degenerated axons and preterminals. The study demonstrated that most ascending degenerated axons are located in the outer half of the dorsal column. The somatotopic pattern of ascending fibers is evident, whereby dorsomedial fibers are primarily of caudal origin and the more dorsolateral axons are from rostral cord segments. Rostral to segment 31, all spinal segments appear to project to a strip of dorsal column adjacent to the dorsal median septum. From the septum, axons descend to terminate somatotopically on cells of the nucleus of Bischoff located caudal to the obex of the medulla. Dorsal column degeneration ascends to the level of the dorsal column nuclei, where most fibers terminate. Degeneration from caudal cord segments terminates on caudo-medial cells of the dorsal column nuclei, while rostral cord segments project to rostro-lateral cells. The dorsal column nuclei consist of an expanded lateral part between tractus descendens trigemini and the vago-solitary complex, and a contiguous, thin medial lamina of cells dorsal and medial to the vagal nuclei. The somatotopic pattern of degeneration in the dorsal column nuclei, probably of dorsal root origin, follows the mammalian organization, which suggests that the garter snake has primitive nuclei gracilis and cuneatus. Other terminal sites of degenerating fibers, although probably of spinal gray origin, are nucleus commissura infima, nucleus descendens vestibuli, and area postrema.