NONRECIPROCAL 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 inve stigate the organization of projections from the perihypoglossal nucle i to the cerebellar paramedian lobule, and possible reciprocal cortico perihypoglossal connections in rabbits. The absence of labeled Purkinj e cells in the paramedian lobule after HRP injections of the perihypog lossal nuclei, compared with labeling in the nodulus, ipsilateral floc culus and adjacent ventral paraflocculus, strongly suggests that corti coperihypoglossal projections do not originate in the paramedian lobul e. Comparison of labeling patterns in the perihypoglossal nuclei follo wing HRP injections into various sublobules of the paramedian lobule s uggests that projections from the perihypoglossal nuclei to the parame dian lobule exist and that some degree of topographic organization is present exclusively in bilateral projections from the caudal part of n ucleus prepositus hypoglossi. All perihypoglossal projections end in d eep parts of sublobules e-b of zones C-2 and D-1 (as defined by inferi or olivary :labeling). It is significant that in the ipsilateral nucle us prepositus hypoglossi, two groups of cells projecting to zone C-2 a re arranged rostrocaudally between three other groups projecting to zo ne D-1.