The pontocerebellar system in the opossum, Didelphis virginiana. A horseradish peroxidase study.

The method of retrograde axonal transport of horseradish peroxidase was employed to demonstrate certain organizational features of the pontocerebellar system in adult opossums. The spinal cerebellum (anterior lobe, pyramis and paramedian lobule) receives input from neurons situated in ventral and lateral regions throughout the rostro-caudal extent of the pons. The projection to the anterior lobe and pyramis was primarily contralateral, whereas the projection to the paramedian lobule included a substantial ipsilateral contribution. The pontine projection to the vermal visual-auditory area was also found to include significant bilateral components which were observed to be organized in mirror image locations in medial, ventral and lateral regions. The paraflocculus was found to receive input from a relatively large number of pontine neurons, the medial injection producing a more bilateral distribution of labeled neurons while the lateral injection resulted in primarily contralateral labeling. Crus I and crus II of the cerebellar hemispheres received a bilateral projection which also included neurons distributed in mirror image locations in both halves of the pontine gray. Taken together such observations indicate that the pontocerebellar system includes a more substantial ipsilateral contribution than has heretofore been recognized. In addition, comparison of the locations of various groups of labeled neurons resulting from spatially separate injection sites suggests the possibility that some pontocerebellar neurons might diverge to reach more than one cerebellar zone, i.e. cells in a similar ventrolateral pontine region were labeled following injections in the anterior lobe, pyramis and crus I. Evidence for convergence of multiple pontine areas to single cerebellar foci was not as compelling.