Input‐output connections of the “hindlimb” region of the inferior olive in cats

The aim of the present study was to establish whether gracile afferents to the inferior olive are topographically organized and whether such inputs coincide with the location of cells that output to the hindlimb C1 zone in the cerebellar posterior lobe. Small injections (n = 15) of the anterograde tracers Fluoro‐Ruby or biotinylated dextran amine were made into gracile, resulting in anterograde labelling often distributed in partially separate, rostrocaudally directed columns within the lateral half of the contralateral rostral dorsal accessory olive (rDAO). In 12 cases, anterograde labelling was also located within the caudolateral medial accessory olive. Evidence for a topographical organization was obtained, suggesting that medial gracile targets only the most lateral part of rDAO, whereas lateral gracile also targets more medial parts. Gracile injections centred dorsally also resulted in more extensive terminal fields in rDAO than similar sized injections centred ventrally which may relate to the representation of the distal hindlimb in dorsal gracile (e.g., Millar and Basbaum, 1975). Injections of retrograde tracer (green beads) into the C1 zone in the ipsilateral caudal paramedian lobule (7 cases), resulted in retrograde cell labelling in the middle one‐third of contralateral DAO in two columns that fused caudally. The proportion of labelled cells that overlapped with anterograde labelled terminals was related to the dorsoventral locus of the gracile injection site: gracile injections centred dorsally produced the largest degree of overlap and therefore, potentially, the greatest functional convergence. In summary, the results suggest that a topographical organization is present within the gracilo‐olivo‐cerebellar projection which is discussed in relation to the functional organization of the olivocerebellar system. J. Comp. Neurol. 399:513–529, 1998. © 1998 Wiley‐Liss, Inc.