The inferior olivary connections to the cerebellum in the rat studied by retrograde axonal transport of horseradish peroxidase.

Olivocerebellar projections were investigated in the rat using retrograde axonal transport of horseradish peroxidase. Discrete cell groups of the inferior olive were labelled, subsequent to injections in the paravermal region, the vermis, or the caudolateral hemisphere. Injections in the mid-rostrocaudal third of the paravermal area resulted in labelling of cells in the medial accessory olive (MAO), in cell group "b" at caudal levels, and in its lateral portion at mid-rostrocaudal levels. The rostral pole of the principal olive (PO), the dorsal accessory olive (DAO), and the dorsomedial cell column, were heavily labelled. By comparison, caudal paravermal injections resulted in labelling in the medial part of the mid-rostrocaudal levels of the MAO, but not in its caudal portion. The PO lamellae were labelled in their lateral half, excluding the lateral bend connecting them. Injections slightly lateral within this paravermal area gave no caudal MAO labelling, but did label cells in segments of both PO lamellae, medial to those in the previous case. From vermal injections, cell groups "b" and "c" of the caudal MAO were labelled, but no labelled cells were present in the PO. Subsequent to injections in the paramedian lobule, cells in the dorsal lamella of the PO were labelled. No cells of the MAO were labelled. These results are discussed in terms of specific labelling patterns and the general concepts of organization presently held for the olivocerebellar system.