Sagittal organization of the olivocerebellonuclear pathway in the rat. II. Connections with the nucleus interpositus

Abstract

The organization of the sagittal Zone C of the cerebellar cortex of the rat was studied with respect to its efferent projections and to its inferior olive (IO) afferent connections. Wheat-germ agglutinin conjugated to horseradish peroxidase was used as a tracer. Zone C has been defined as the cortical region projecting to the nucleus interpositus anterior (NIA) and posterior (NIP). The results show that, in spite of some differences, Zone C of the rat is homologous to that of the cat. Three subzones, C1, C2 and C3, were clearly identified. Subzone C1 appears as a longitudinal band of the cerebellar cortex interrupted at the level of lobules VIb,c and part of lobule VII. It is therefore divided into two sagittal segments, one anterior to lobules I to VIa adjacent to Zone B; and one posterior to lobules VII to VIII adjacent to Zone A. Both segments receive climbing fibres from the lateral aspect of the rostral two-thirds and the medial aspect of the caudal one-third of the dorsal accessory olive (DAO). The Purkinje cell axons from subzone C1 project to both the NIA and the NIP where they occupy the medial one-third of the nucleus. Subzone C2 consists of a continuous sagittal band of the cerebellar cortex and lies between Subzones C1 and C3. It receives climbing fibres from the rostral aspect of the medial accessory olive (MAO) and projects to the central aspect of the NIA and to the lateral half of the NIP. Subzone C3, which is lateral to Subzone C2 and medial to Zone D, appears as a sagittal band of cortex interrupted at the level of lobule VI. It receives climbing fibres from the medial aspect of the DAO and projects to the lateral aspect of the NIA. The orientation of the olivocerebellonuclear circuit is fundamentally sagittal not only in the cerebellar cortex but also in the nuclei and, although less sharply, in the inferior olive.