The projection of cervical primary fibers to the DCN of the squirrel, Sciurus niger: fiber sorting in the dorsal columns.

The course and terminal distribution of cervical primary afferents in the dorsal column nuclei were studied in the fox squirrel, Sciurus niger. Unilateral rhizotomies were performed at cervical levels C3, C4, C5, C6 and C8. For all spinal levels studied except C8, degeneration in the medullary cuneate fasciculus was present within two distinct groups. They included a large oblique lateral lamina and a small superficial group of degeneration. The superficial group was horizontal, located medial to the large lamina and appeared to be formed by the medial shifting of fibers away from the initial larger group. Fibers in the large primary lamina appeared to terminate primarily in the ventrolateral areas of the cuneate nucleus. Fibers from the superficial group, however, appeared to project mostly to dorsal cuneate nuclear areas. Hence, the dual transverse terminal patterns for cervical fibers in the cuneate nucleus appeared to be related directly to a fiber sorting process that involved the formation of two often separate ascending fiber groups in the cuneate fasciculus. The results of this study are compared with the mechanosensory mapping of the dorsal column nuclei in the squirrel.