NUMERICAL ASPECTS OF PONTINE, LATERAL RETICULAR, AND INFERIOR OLIVARYPROJECTIONS TO 2 PARAVERMAL CORTICAL ZONES OF THE CAT CEREBELLUM

Two different olivo-cortico-nuclear zones in the cat cerebellum have b een compared quantitatively as regards the numbers of cells projecting to them from within several sources of mossy fibres (MFs), namely the basal pontine nuclei (BPN), nucleus reticularis tegmenti pontis (NRTP ), and the ipsilateral lateral reticular nucleus (LRN). The zones chos en were the C-3 zone in lobule V of the anterior lobe and the C-1 zone in pars copularis of the paramedian lobule (PMLpc), localised by reco rding climbing fibre-mediated potentials evoked on their surface as a result of volleys set up in their spino-olivocerebellar paths. The zon es were injected with fluorescent-labelled latex microspheres and cell bodies, retrogradely labelled in the MF source nuclei and in the cont ralateral inferior olive, were counted and mapped. Evidence was obtain ed that tracer efficiency was very high in both the MF projections and the olive-cerebellar projection and that each olivocerebellar axon ma y provide only one climbing fibre to the upper part of a lobule V foli um but an average of nearly two to the same part of a PML folium. When the numbers of labelled cells in each MF source nucleus were expresse d as a percentage of the total number of labelled pontine cells, the b iggest source for lobule V was the contralateral BPN, followed by LRN, contralateral NRTP, ipsilateral BPN, and ipsilateral NRTP. For PMLpc, the order was similar except that ipsilateral BPNs exceeded contralat eral NRTPs, but the dominance of contralateral BPN as a source was muc h greater. Cell totals were converted into projection densities (i.e., numbers of cells labelled per square millimetre of cortical sheet inv olved in the injection site); densities for PMLpc were found to be alm ost three times greater than those for lobule V for contralateral BPN but the two densities were not significantly different for ipsilateral BPN. The three other MF sources projected at higher densities to lobu le V than to PML. These findings indicate that two cortical zones, bot h of which receive climbing fibres from the rostral part of the dorsal accessory olive and project to nucleus interpositus anterior, neverth eless differ markedly in regard to both the relative and the absolute sizes of the projections they receive from several of their most impor tant sources of MFs. (C) 1998 Wiley-Liss, Inc.