Cerebellar cortical efferent fibers in the North American opossum, Didelphis virginiana. I. The anterior lobe.

The distribution of cerebellar corticonuclear and corticovestibular fibers from the anterior lobe of the North American opossum, Didelphis virginiana, was studied using the Fink and Heimer ('67) technique. Corticonuclear fibers from medial areas of anterior lobe project into the medial cerebellar nucleus (NM) in a topographically organized manner. Fibers from lobules II and III enter rostral and rostrodorsal NM, while those from lobules IV and V terminate in progressively more caudal parts of the nucleus. Collectively the terminal fields in NM for axons from lobules II-V occupy about the rostral two-thirds of the nucleus. Those areas of lobules II-V of opossum that project into NM presumably correspond to zone A of cat and primate. Cerebellar corticovestibular fibers originate from cortex located immediately lateral to areas projecting to NM. The predominance of corticovestibular projections into the lateral vestibular nucleus suggests the presence of a B zone and identifies its points of interface with the A zone. The results further suggest that zones A and B overlap at their respective margins. In contrast to other mammals studied to date, zones A and B of opossum anterior lobe are comparatively wide. Corticonuclear fibers to anterior and posterior interposed nuclei and to the lateral cerebellar nucleus (NL) originate from relatively narrow lateral portions of anterior lobe. These results also suggest that the intermediate cortex of opossum anterior lobe is not clearly divisible into individual zones C1,C2 or C3. The cortical area that innervates NL is very narrow and presumably corresponds to zone D of other forms.