Corticonuclear projections of the cerebellum preserve both anteroposteriorand mediolateral pairing patterns

The aim of the present study was to establish whether a diverging arrangeme nt of the corticonuclear cerebellar projections exists and, if so, what rel ation it has with the inferior olivary complex. Iontophoretic injections of a 1 : 1 mixture of tetramethylrhodamine dextran amine and byotinilated dex tran amine into the cerebellar cortex orthogradely labelled fibre terminals in the cerebellar nuclei and retrogradely labelled cell bodies in the infe rior olivary complex. The injections were into A, B, C2, C3, D1 and D2 band s. These injections showed diverging projections to the cerebellar nuclei, with 'primary projections' directed to the nuclear region previously report ed to be specifically connected with the injected band and 'secondary proje ctions' directed to other nuclear regions. Secondary projections from the A , C2 and C3 bands diverged to nuclear regions primarily controlled by corti cal bands lateral to those injected. Secondary projections from the D1, and D2 bands diverged to nuclear regions primarily controlled by cortical band s medial to those injected. Moreover, injections distributed along the D1 a nd D2 bands showed similar sets of nuclear targets, while those distributed along the A, C2 and C3 bands showed two sets of nuclear targets in relatio n to the anteroposterior location of the injected area within these bands. The cortical areas that projected to the same set of nuclear targets were i nnervated from single olivary regions, while those that projected to differ ent sets of nuclear targets were innervated from different subsets of singl e regions of the inferior olive. The results suggest that the olivary bands of the cerebellar cortex project to the cerebellar nuclei with a diverging pattern that is organized in both the mediolateral and the anteroposterior axes.