The organization of olivo-cerebellar projections in the opossum, Didelphis virginiana, as revealed by the retrograde transport of horseradish peroxidase.

The retrograde transport of horseradish peroxidase was utilized to map olivo-cerebellar projections in the Virginia opossum. The spinal cerebellum (anterior lobe, paramedian lobule and pyramis) receives input from several separate regions in the dorsal accessory nucleus, the medial accessory nucleus and portions of the principal nucleus. Evidence is present for a topographical organization whereby specific regions of the olive project to restricted longitudinal zones. The visual-auditory region of the posterior vermis receives input from small areas within the caudal part of the medial accessory nucleus. From a distinctly separate region of the caudal medial accessory nucleus (as well as the principal nucleus), axons project to the uvula. The vestibulo-cerebellum is the recipient of axons from the cap of Kooy and from two spatially separate regions of the medial accessory nucleus. The cerebellar hemisphere (Crus I and II, lobus simplex) is the target of axons from parts of all three olivary nuclei and it is possible that the projections from the different nuclei are targeted upon separate zones. The paraflocculus was found to receive an input from the rostral part of the medial accessory nucleus and from the principal nucleus. The present results suggest that a distinct olivary region may project to several widely separate areas of the cerebellum, and that one cerebellar region may receive input from several areas of the olive. The organization of the olivocerebellar projection is highly complex, but when considered in light of known inputs to the olive, certain patterns emerge.