Cat pontocerebellar network: numerical capacity and axonal collateral branching of neurones in the pontine nuclei projecting to individual parafloccular folia.

We have studied the convergence and divergence in the pontocerebellar pathway. Two or three different fluorescent tracers were injected in separate folia of the parafloccular complex. Retrogradely labelled cells were quantitatively recorded. The estimated total number of labelled neurones in the pontine nuclei contralateral to the injection sites was 18000 (median; range 5000-46000; 14 cell populations, six animals). Using stereological principles, the total number of neurones on one side in the pontine nuclei was estimated to be 490000 (mean; n = 6). Thus, approximately 4% of the total number of neurones in the pontine nuclei would project to a single parafloccular folium. Assuming that the highest estimates of labelled cells are the most representative, the proportion would be 9%. Considering that the volume injected makes up a tiny fraction of the total cerebellar cortical volume, these figures reflect an extreme convergence. After injections in adjacent folia we observed 19-27% double labelling. The double labelling frequency dropped steeply with increasing distance between injections. The strong convergence and limited local axonal branching suggest the existence of extensive branching to widely separated cerebellar regions.