A horseradish peroxidase study of projections from the main and external cuneate nuclei to the cerebellum of the North American raccoon.

Neurons of the raccoon main (MCN) and external (ECN) cuneate nuclei having terminal fields in the anterior lobe and paramedian lobule of the cerebellum were studied by means of the retrograde transport of horseradish peroxidase (HRP). In the MCN, neurons of the polymorphic regions, but not of the round cell clusters, were labeled following HRP injections of the anterior lobe. HRP-labeled fusiform, triangular, and stellate cells were observed from 3-4 mm caudal to the obex to 2-3 mm rostral to it. The main body of labeled MCN neurons was located in the 1.25 mm immediately rostral to the obex, there forming a recognizable subnucleus in the ventral MCN. The ECN displayed uniform labelling from the anterior lobe with no particular cell type or nuclear region being labeled preferentially. A minor projection to the anterior lobe appeared to originate in the contralateral MCN and ECN. Injections of the paramedian lobule produced MCN labeling similar to that observed after anterior lobe injections, but the majority of labeled ECN neurons were found in the dorsomedial part of the nucleus. In lobule V of the anterior lobe, sagittal zone c contained the greatest density of cuneocerebellar terminals as judged by the degree of retrograde labeling seen in neurons of both the MCN and ECN. This observation was consistent with evoked potentials recorded in the anterior lobe of the raccoon cerebellum. The position of cuneocerebellar neurons in the ventrolateral MCN suggests that the bulk of the information being transferred to the cerebellum by the raccoon MCN is related to deep, rather than cutaneous, modalities.