The olivocerebellar projection in the monkey. Experimental studies with the method of retrograde tracing of horseradish peroxidase

Following injections of horseradish peroxidase (HRP) in various lobes and lobules of the macaque cerebellum the occurrence of retrogradely labeled cells in the inferior olive was mapped. Only cortical areas showing staining of the molecular layer were considered as sites of uptake of HRP. To facilitate comparisons between cases and presentation of findings, a diagram of the macaque inferior olive as imagined unfolded was constructed (Fig. 1). Attempts were made to compare the findings made with data on the olivocerebellar projection in the cat and the pattern of a longitudinal zonal subdivision of the cerebellum. In general there appears to be a remarkably close correspondence between the organization of the olivocerebellar projection in the monkey and the cat. The projection is precisely organized and appears to be purely crossed. Within the projections to some of the cerebellar cortical zones a topical pattern can be demonstrated. Olivary afferents to vermal lobules V, VII, and VIII are derived from the caudal half of the medial accessory olive, projecting to Voogd's zone A. The topical pattern resembles that in the cat (Fig. 8). After injections covering the lateral zone of the anterior lobe vermis (zone B), labeled cells are seen in the caudal part of the dorsal accessory olive. In some cases staining of the intermediate part of the anterior lobe and of the paramedian lobule is followed by labeling of cells in the rostral part of the dorsal accessory olive (zones C 1 and C 3 ) or in the rostral half of the medial accessory olive (zone C 2 ). When the injected area covers lateral parts of the cerebellum, there is labeling in the principal olive (projecting to zones D 1 and D 2 ). Although not entirely decisive, the findings lend support to the view that the ventral lamella of the principal olive supplies zone D 2 , whereas the dorsal lamella supplies zone D 1 . The relatively sparse data in the literature on the afferents to the monkey olive are briefly considered. On many points the projections appear to be as in the cat. However, there is possibly a species difference between cat and monkey as concerns their receipt of afferents from the red nucleus.