Ascending somatosensory projections to the medial accessory portion of the inferior olive: a retrograde study in cats.

The cells in the dorsal column nuclei, lumbosacral spinal cord, lateral cervical nucleus, and nucleus z that project to the medial accessory portion of the inferior olive of cats were identified with retrograde tracing techniques. Injections of wheat germ agglutinin complexed to horseradish peroxidase were made in the caudal portion of the medial accessory olive, either (1) involving no portion of the dorsal accessory olive or (2) involving in addition the caudal tip of the dorsal accessory olive. The tissue was processed with tetramethyl benzidine. The locations of all relay neurons were compared with those of dorsal accessory olive projection neurons, as described in a previous study (Molinari, '84a). Localized populations of neurons gave rise to most of the projection to the medial accessory olive. These neurons were found in the peripheral portions of the dorsal column nuclei caudal to the obex and in the ventromedial ventral horn of the entire lumbosacral enlargement. Few projection neurons were found in the lateral cervical nucleus and none in the nucleus z. Neurons in the peripheral dorsal column nuclei and ventromedial ventral horn were labeled by injections in either the medial or dorsal accessory olives. Following medial accessory olive injections, however, they constituted the only labeled somatosensory neurons, while following dorsal accessory olive injections they represented only a small fraction of the labeled neurons. Based on their locations, it is proposed that these neurons might be the source for both the medial and dorsal accessory olives of information signalling movement of the proximal limb. Such a proposal is consistent with functional descriptions of the medial and dorsal accessory olives and the cerebellar anterior lobe.