A degeneration study of some habenular efferents to the midbrain in a wallaby

In this study, specimens of both sexes of a Western Australian wallaby (Steonix brachyurus) had surgical or electrolytic lesions made in the habenular complex unilaterally. Thirty-five-micron sections, cut coronally and parasagittally, were stained by the Nauta-Gygax ('54) or the Fink-Heimer ('67) technique to demonstrate degenerating fibers. Degeneration was seen in the habenular commissure and bilaterally in the habenula and fasciculus retroflexus, but was most extensive ipsilaterally. Habenular fibers destined for the tegmentum and central gray passed caudally into the midbrain in a continuous fan-like array which extended from the central gray dorsally to the interpeduncular nucleus ventrally. In their posterior course some fibers crossed the midline with the decussating cerebellar fibers. The majority of fibers, with those in the fasciculus retroflexus which passed ventral to the decussation of the superior cerebellar peduncle, turned dorsally. They passed mostly just lateral to the midline nuclei and into the central gray where degeneration was seen throughout the entire extent. Terminal degeneration was seen in the interpeduncular nucleus primarily in the dorsal and posterior parts, in the ventral tegmental nucleus and throughout the central gray, but was more densely clustered in the region of the dorsal tegmental nucleus.