Sagittal organisation of the olivocerebellonuclear pathway in the rat. III. Connections with the nucleus dentatus.

The organization of olivary afferents and nuclear efferents of the D-zone of the rat cerebellum was studied by means of tracing with wheat-germ agglutinin-coupled peroxidase using tetramethylbenzidine as a chromagen. The tracer was injected iontophoretically within the cerebellar cortex. This allowed us to study both afferent and efferent pathways of the cerebellar lobules concerned with retrograde and anterograde tracing, respectively. Retrograde cellular labelling in the inferior olive was restricted to the principal olive (PO). Anterograde terminal labelling was found only within the various subdivisions of the nucleus lateralis or dentatus (ND). For any one of our small cortical injections there was a corresponding sagittal band of retrogradely labelled cells in the contralateral PO, and a sagittal band of terminal labelling through the ND. Based on both their olivary and nuclear connections, 3 sagittal subzones can be distinguished within the D-zone of the rat. From medial to lateral, we call them D0, D1 and D2. The 3 subzones run through part of the anterior and posterior lobes. D1 and D2 run continuously from their rostral to their caudal extents whereas D0 is discontinuous. It is interrupted through lobule VIc (crus I). The olivary projections to D0 arise within the medial half of the ventral lamella of the PO, including the dorsomedial cell column. Those to D1 arise within the dorsal lamella of the PO. Those to D2 arise within the lateral half of the ventral lamella of the PO. Rostrocaudally, widely distant cells of the same subdivision of the PO project to the same cerebellar lobule. This indicates extensive convergence of the olivary afferents within each of the 3 hemispheric compartments, D0, D1 and D2. Each of the 3 hemispheric subzones specifically projects to one of the 3 subdivisions distinguished within the ND of the rat, without apparent mediolateral overlapping. The medialmost D0 projects onto the dorsolateral hump; D1 projects more laterally onto the main, magnocellular part of the ND, and D2 projects ventrally onto the parvicellular subdivision of the ND. Thus the sagittal partition of the hemispheric cortex is reflected at the nuclear level. In contrast, Purkinje cell axons from individual lobules appear to branch extensively in the rostrocaudal direction. Therefore, within each of the 3 compartments D0, D1 as well as D2, the nuclear projection of the anterior lobe and the posterior lobe are largely coextensive.