Organization of projections from the inferior olive to the cerebellar nuclei in the rat

The detailed organization of projections from the inferior olive to the cer ebellar nuclei of the rat was studied by using anterograde tracing. The pre sence of a collateral projection to the cerebellar nuclei could be confirme d, and a detailed organization was recognized at the nuclear and subnuclear level. Olivary projections to the different parts of the medial cerebellar nucleus arise from various parts of the caudal half of the medial accessor y olivary nucleus. The interstitial cell groups receive olivary afferents f rom the intermediate part of the medial accessory olive and from the dorsom edial cell column. A mediolateral topography was noted in the projections f rom the rostral half of the medial accessory olive to the posterior interpo sed nucleus. Olivary projections to the lateral cerebellar nucleus are deri ved from the principal olive according to basically inversed rostrocaudal t opography. Projections from the dorsomedial group of the principal olive to the dorsolateral hump were found to follow a basically rostrocaudal topogr aphy. The anterior interposed nucleus receives olivary afferents from the d orsal accessory olive. Its rostromedial parts are directed to the lateral p art of the anterior interposed nucleus and its caudolateral part reach the medial anterior interposed nucleus. No terminal arborizations in the cerebe llar nuclei were found to originate from (1) the dorsal fold of the dorsal accessory olive, which resulted in projections to the lateral vestibular nu cleus and (2) the dorsal cap of Kooy. It was noted that the olivary project ion to the cerebellar nuclei is strictly reciprocal to the nucleo-olivary p rojection as described by Ruigrok and Voogd (1990). Moreover, it is suggest ed that the olivonuclear projection adheres to the organization of the clim bing fiber projection to the cerebellar cortex and to the corticonuclear pr ojection, thus, establishing and extending the detailed micromodular organi zation of the connections between inferior olive and cerebellum. (C) 2000 W iley-Liss, Inc.