The cerebellar paramedian corticonuclear and nucleocortical connections in the cat were studied by means of anterograde and retrograde transport of HRP. Previous experimental studies have given evidence that the paramedian cortex in a lateromedial direction can be subdivided, into six longitudinal zones. These are recognized as zones D2, D1, C3, C2, C1 and B. An analysis of our material suggests that each cortical zone has its own field of termination in the cerebellar nuclei and that the Purkinje fibres from one zone have only one terminal region. The nuclear terminal areas for the fibres from the described cortical zones are the ventral nucleus lateralis (the D2 zone), the transition area between the nucleus lateralis and nucleus interpositus anterior (the D1 zone), the dorsolateral nucleus interpositus posterior (the C2 zone), the ventromedial nucleus interpositus posterior (the C1 zone), and the dorsomedial nucleus interpositus anterior (the B zone). A separate nuclear terminal region for the fibres from a cortical C3 zone could not be positively demonstrated, but a comparison of cases makes it likeky that it is located in the transition area of nucleus lateralis and nucleus interpositus anterior, medially to the D1 zone. The rostral folia of the paramedian lobule project more laterally in the cerebellar nuclei than do the caudal folia. Furthermore, our findings indicate that the axons of the Purkinje cells in one folium from medial to lateral terminate along a mediolateral nuclear band which loops from the dorsomedial nucleus interpositus anterior down into the ventral nucleus interpositus posterior, and from a bend in this part to the dorsal nucleus interpositus posterior, and hence into the transition zone of nucleus interpositus anterior and nucleus lateralis, from here to proceed caudally to its end. The nucleocortical projection shows with some exceptions the same zonal arrangement as the corticonuclear, but a few labelled nuclear neurons were in some cases found in the fastigial nucleus. This nucleus does not receive Purkinje axons from the paramedian lobule. This shows that although retrogradely labelled nuclear cells usually were located among or just adjacent to anterogradely filled terminal fibres, there is not a complete reciprocity in the corticonuclear and nucleocortical projections. The observations furthermore indicate that the cortical afferents terminate as mossy fibres. The advantages and problems encountered with the use of HRP as an anterograde tracer are discussed and the observations are related to previous observations on the corticonuclear and nucleocortical cerebellar projections. © 1979 Springer-Verlag.
