The organization of cerebellar and basal ganglia outputs to primary motor cortex as revealed by retrograde transneuronal transport of herpes simplex virus type 1

We used retrograde transneuronal transport of herpes simplex virus type 1 t o map the origin of cerebellar and basal ganglia "projections" to leg, arm, and face areas of the primary motor cortex (M1). Four to five days after v irus injections into M1, we observed many densely labeled neurons in locali zed regions of the output nuclei of the cerebellum and basal ganglia. The l argest numbers of these neurons were found in portions of the dentate nucle us and the internal segment of the globus pallidus (GPI). Smaller numbers o f labeled neurons were found in portions of the interpositus nucleus and th e substantia nigra pars reticulata. The distribution of neuronal labeling v aried with the cortical injection site. For example, within the dentate, ne urons labeled from leg M1 were located rostrally, those from face M1 caudal ly, and those from arm M1 at intermediate levels. In each instance, labeled neurons were confined to approximately the dorsal third of the nucleus. Wi thin GPI, neurons labeled from leg M1 were located in dorsal and medial reg ions, those from face M1 in ventral and lateral regions, and those from arm M1 in intermediate regions. These results demonstrate that M1 is the targe t of somatotopically organized outputs from both the cerebellum and basal g anglia, Surprisingly, the projections to M1 originate from only 30% of the volume of the dentate and <15% of GPI, Thus, the majority of the outputs fr om the cerebellum and basal ganglia are directed to cortical areas other th an M1.