Non-reciprocal connections between the perihypoglossal nuclei and the cerebellar paramedian lobule in rabbits and their relationship to the climbing fiber zones.

Retrogradely transported horseradish peroxidase (HRP) was used to investigate the organization of projections from the perihypoglossal nuclei to the cerebellar paramedian lobule, and possible reciprocal corticoperihypoglossal connections in rabbits. The absence of labeled Purkinje cells in the paramedian lobule after HRP injections of the perihypoglossal nuclei, compared with labeling in the nodulus, ipsilateral flocculus and adjacent ventral paraflocculus, strongly suggests that corticoperihypoglossal projections do not originate in the paramedian lobule. Comparison of labeling patterns in the perihypoglossal nuclei following HRP injections into various sublobules of the paramedian lobule suggests that projections from the perihypoglossal nuclei to the paramedian lobule exist and that some degree of topographic organization is present exclusively in bilateral projections from the caudal part of nucleus prepositus hypoglossi. All perihypoglossal projections end in deep parts of sublobules e-b of zones C2 and D1 (as defined by inferior olivary labeling). It is significant that in the ipsilateral nucleus prepositus hypoglossi, two groups of cells projecting to zone C2 are arranged rostrocaudally between three other groups projecting to zone D1.