Zonal organization of cortico-nuclear and nucleo-cortical projections of the paramedian lobule of the cat cerebellum. 2. the C2 zone.

The cortico-nuclear (C-N) and nucleo-cortical (N-C) projections of the C2 cortical zone in pars anterior (pa) and pars posterior (pp) 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 mediolateral boundaries of the zone were localized 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 the fore- and hindlimbs; a small (15-30 nl) injection of 1-2% WGA-HRP was then made into the zone. Distributions in the deep cerebellar nuclei were determined with light microscopy both for C-N 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 retrogradely labelled olivary neurones were confined to the part of the rostral medial accessory olive that innervates the C2 zone was estimated to provide an indication of the degree to which the injected tracer might have spread beyond the boundaries of the zone. The C-N projection from the part of the C2 zone in PML pa terminates almost exclusively (probably exclusively) in nucleus interpositus posterior (NIP) at all medio-lateral levels of the nucleus but most extensively at middle and lateral levels. At most levels the C-N termination territory forms a crescent with its outer curve following the caudal, dorsal and rostral borders of the nucleus and as a result it is mainly in the dorsal half of the nucleus. There is heavy overlap with the projection from the lobule V part of the C2 zone previously studied by us. The projection from the C2 zone in PML pp terminates entirely in NIP, but although at middle medio-lateral levels in the nucleus there is substantial overlap with the PML pa and lobule V projections, the projection territory is confined to the medial half of the nucleus. Evidence was obtained compatible with the view that throughout the C2 zone its lateral and medial parts project to different parts of NIP. In both PML pa and pp the C2 zone receives N-C projections from NIP. Most of the N-C cells concerned are in the dorsal half of NIP and the great majority lie within the corresponding C-N projection territory. However, the N-C projection to PML pa appears c. 6 times heavier than that to PML pp and the PML pa part of the zone also receives a minor additional projection from nucleus lateralis (NL). The findings are discussed in relation to the hypothesis of olivo-cortico-nuclear complexes or compartments, with particular reference to the internal organization of the C2 complex.