Olivocerebellar projections to the pyramis and copula pyramidis in the rat: differential projections to parasagittal zones.

The organization of the olivocerebellar projection to the pyramis and copula pyramidis in the rat has been studied with the use of microinjections of horseradish peroxidase (HRP). These injections were made in different mediolateral positions along the width of this lobule. The objective was to determine the subnuclear origin of parasagittal zones of olivary innervation. The results indicate that six zones of varying width can be distinguished, each of which receives a different olivary projection. Injections in the pyramis in succeedingly more lateral positions resulted in retrograde labeling of the lateral part of the caudal medial accessory olive (MAO), nucleus beta of the MAO, and a slightly more rostral part of the lateral MAO. Injections in the copula pyramidis result in retrograde labeling of the lateral part of the dorsal accessory olive (DAO), the intermediate part of the MAO, and the caudal tip of the principal olive. In very few of the experiments was there labeling of more than one subnuclear locus within the inferior olivary complex. These results suggest that parasagittal zones within the pyramis and copula pyramidis of the rat cerebellum receive highly ordered and differential projections from the inferior olivary complex.