Multiple cerebellar zones are involved in the control of individual muscles: a retrograde transneuronal tracing study with rabies virus in the rat.

To identify cerebellar regions that are involved in the control of limb muscles, rabies virus was injected into the tibialis anterior (TA), the gastrocnemius (GC) or, for comparison, into the flexor digitorum (FD) muscles of the rat. Progression of retrograde transneuronal infection at supraspinal levels was assessed after variable time spans and was divided into three groups. Initially, infected neurons were observed in the reticular formation, lateral vestibular nucleus, red nucleus and motor cortex (group 1). Group 2 was characterized by labelling within the cerebellar nuclei as well as of two vermal strips of Purkinje cells (PCs). Double-labelling with zebrin enabled identification of these strips as the lateral part of the A1- and B-zone. For TA both zones were ipsilateral, whereas for GC the A1 strip predominated contralaterally. Group 3 infections showed additional labelling of multiple, in part bilateral, identifiable strips of PCs in vermis, paravermis and hemisphere. FD injections resulted in less robust labelling of vermal strips and more pronounced labelling within paravermal and hemispheral zonal regions. Only sporadic labelling in corresponding regions of the inferior olive and no labelling of cortical interneurons or granule cells was observed. Prolonged infection was seen to result in degeneration of PCs and possibly of motoneurons. We conclude that vermal, paravermal as well as hemispheral zones of the cerebellar cortex converge upon motoneurons that innervate a particular muscle. In addition, individual zones may control motorpools of different muscles and thus contribute to muscle synergies.