Projections of caudal fasciculus gracilis to nucleus gracilis and other medullary structures, and Clarke's nucleus in the rat.

The extent of the projection of fasciculus gracilis from low thoracic (T12--13) spinal cord, to nucleus gracilis (NG) was studied by the Nauta--Gygax method in the rat in order to: (1) help define projection fields of fasciculus gracilis hindlimb and tail afferents along the longitudinal extent of NG; and (2) broaden the comparative data on the projection of the lower body to NG via fasciculus gracilis, already known in cat and monkey. Results showed that: (1) degenerated fasciculus gracilis axons are situated mainly dorsal to the corticospinal fibers, in the dorsal funiculus; (2) terminal projection fields were located at all levels of ipsilateral NG--caudal to the obex, terminal debris was confined to the medial two-thirds of NG--rostrad of the obex, in oral NG, degeneration was of finer caliber and projection fields more diffuse; and (3) ipsilateral degeneration was also seen at the level of a portion of Clarke's nucleus, and, in commissural and lateral cuneate nuclei and medial reticular formation in medulla.