Sagittal organization of the olivocerebellonuclear pathway in the rat. I. Connections with the nucleus fastigii and the nucleus vestibularis lateralis.

The organization of the afferent and efferent connections of the sagittal Zones A and B of the cerebellar cortex of the rat have been studied using wheat-germ agglutinin conjugated to horseradish peroxidase as a tracer. A single injection of this tracer into the cerebellar cortex allowed us to study, simultaneously, the crossed olivocortical connections (revealed by the retrograde transport) and the direct corticonuclear connections (revealed by the anterograde transport). The results demonstrate that the olivocerebellonuclear pathway is organized in a longitudinal direction so that for a given small injection of the tracer in one lobule of the cortex, a long sagittal band of the retrograde-labelled cells is obtained in the inferior olive, and a long sagittal band of the labelled terminals is obtained in the cerebellar nuclei. Zone A and Zone B have been arbitrarily defined as the cortical regions projecting, respectively, to the nucleus fastigii (NF) and the nucleus vestibularis lateralis (NVL). Zone A of the rat runs parasagitally from lobules I to IX as described in the cat, but in the posterior lobe it extends much more laterally than in the other mammals to include the lobulus paramedianus and crus I regions. The projections of Zone A to the NF recognize a mediolateral as well as a dorsoventral organization. Zone A receives climbing fibres exclusively from the caudal half of the medial accessory olive (MAO) with a further topographical organization in 4 distinct connections. Zone B of the rat is a narrow strip of the cortex lying adjacent to Zone A and extending from lobule I to VI. It receives climbing fibres from the caudolateral half of the dorsal accessory olive (DAO) and projects to the ipsilateral NVL with no other detectable organization. The majority of the labelled terminals end in the dorsal aspect of the NVL, but a non-negligible quantity also end in the ventral aspect.