ZONAL ORGANIZATION OF CORTICO-NUCLEAR AND NUCLEO-CORTICAL PROJECTIONSOF THE PARAMEDIAN LOBULE OF THE CAT CEREBELLUM - 2 - THE C-2 ZONE

The cortico-nuclear (C-N) and nucleo-cortical (N-C) projections of the C-2 cortical zone in pars anterior (pa) and pars posterior (pp) of th e paramedian lobule (PML) in the posterior lobe of the cat cerebellum were investigated with a combined electrophysiological and neuroanatom ical technique. In each experiment the mediolateral boundaries of the zone were localized on the cortical surface by recording field potenti als mediated via climbing fibres and evoked in the zone by activity el icited in spino-olivocerebellar paths through percutaneous stimulation of the fort-and hindlimbs; a small (15-30 nl) injection of 1-2% WGA-H RP was then made into the zone. Distributions in the deep cerebellar n uclei were determined with light microscopy both for C-N terminal labe lling due to anterograde axonal transport by Purkinje cells and for ce ll bodies labelled due to retrograde transport in N-C axons. The exten t to which retrogradely labelled olivary neurones were confined to the part of the rostral medial accessory olive that innervates the C-2 zo ne was estimated to provide an indication of the degree to which the i njected tracer might have spread beyond the boundaries of the zone. Th e C-N projection from the part of the C-2 zone in PML pa terminates al most exclusively (probably exclusively) in nucleus interpositus poster ior (NIP) at ail medio-lateral levels of the nucleus but most extensiv ely at middle and lateral levels. At most levels the C-N termination t erritory forms a crescent with its outer curve following the caudal, d orsal 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 pro jection from the lobule V part of the C-2 zone previously studied by u s. The projection from the C-2 zone in PML pp terminates entirely in N IP, but although at middle medio-lateral levels in the nucleus there i s substantial overlap with the PML pa and lobule V projections, the pr ojection territory is confined to the medial half of the nucleus. Evid ence was obtained compatible with the view that throughout the CZ zone its lateral and medial parts project to different parts of NIP. In bo th PML pa and pp the C-2 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. Howev er, 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 additi onal projection from nucleus lateralis (NL). The findings are discusse d in relation to the hypothesis of olivo-cortico-nuclear complexes or compartments, with particular reference to the internal organization o f the C-2 complex.