Pontocerebellar projections to the pyramis and copula pyramidis in the rat: evidence for a mediolateral topography.

In this study, using the horseradish peroxidase (HRP) retrograde tracing technique, the pontocerebellar projection to the pyramis and copula pyramidis in the rat has been analyzed in a series of experiments which had been performed to study the olivocerebellar projection (Eisenman, '81). Microinjections of HRP were made in different mediolateral positions along the width of the pyramis and copula pyramidis. Injections situated within the pyramis but in different loci resulted in the retrograde labeling of primarily two bilateral (much heavier contralaterally) groups of pontine cells: a medial and a ventrolateral group, both situated in the caudal pontine nuclei. With injections in the intermediate region, i.e., the border between the pyramis and copula pyramidis, a different pattern of labeled cells is seen. In these cases the intermediate part of the caudal pontine nuclei contains labeled cells. Injections placed in the lateral parts of the copula pyramidis resulted in still another pattern of retrograde labeling. Injections in this region resulted in the presence of labeled cells in four pontine areas, three caudal groups, and one group in the rostral pontine nuclei. Two of the caudal groups overlap with, but extend more rostrally than, the regions containing labeled cells after pyramis injections. These data suggest the presence in the pyramis and copula pyramidis of three parasagittal zones of differing widths, each of which receives a mainly differential projection from the pontine nuclei.