Zonal organization of cortico-nuclear and nucleo-cortical projections of the paramedian lobule of the cat cerebellum. 1. The C1 zone

 The cortico-nuclear (C–N) and nucleo-cortical (N–C) projections of the C₁ cortical zone in pars anterior (pa) and pars copularis (pc) of the paramedian lobule (PML) in the posterior lobe of the cat cerebellum were investigated with a combined electrophysiological and neuroanatomical technique. In each experiment the medio-lateral boundaries of the zone were located on the cortical surface by recording field potentials mediated via climbing fibres and evoked in the zone by activity elicited in spino-olivocerebellar paths through percutaneous stimulation of fore- and hindlimbs; a small (15–30 nl) injection of WGA-HRP was then made into the zone. The distributions in the deep cerebellar nuclei were determined (with light microscopy) both for terminal labelling due to anterograde axonal transport by Purkinje cells and for cell bodies labelled due to retrograde transport in N–C axons. The extent to which injection sites were confined to the C₁ zone was assessed both by comparing injection site and zone widths and by determining the distributions of retrogradely labelled neurones within the contralateral inferior olive. The C–N projection from the part of the zone in PML pa (a forelimb part) terminates almost exclusively (perhaps exclusively) in nucleus interpositus anterior (NIA), primarily in caudal and dorsal parts, where it overlaps heavily with the C–N projections from the lobule V parts (also forelimb parts) of the C₁ and C₃ zones as previously defined. The C–N projection from the part of the zone in PML pc (a hindlimb part) also terminates virtually exclusively in NIA but primarily in almost all parts of the medial third of the nucleus. There is, nevertheless, sufficient overlap between the PML pa and PML pc projections that approximately one third of the termination territory of each projection overlaps that of the other. The PML pc part of the zone is almost entirely lacking in a N–C projection, as previously found for the lobule V part of the C₁ zone (and C₃ zone). However, the PML pa part of the zone receives N–C projections that arise, in descending order of size, from nucleus interpositus posterior (NIP), from NIA, from the NIA/nucleus lateralis (NL) fusion area and (perhaps) NL. The projection from NIP is similar in size to that provided by the nucleus to the C₂ zone in lobule V of the anterior lobe. The findings are discussed, with particular emphasis on their implications for the hypothesis that the cerebellum is divisible into a number of olivo-cortico-nuclear complexes or compartments. Received: 3 April 1997 / Acccepted: 22 July 1997