Principles of organization of the corticopontocerebellar projection to crus II in the cat with particular reference to the parietal cortical areas.

In 13 cats injections of horseradish peroxidase-wheat germ agglutinin in various parts of the cerebral cortex were combined with injections in the cerebellar crus II in the same animal in order to study the cortical regions that may influence the crus II via the pontine nuclei. In 2 cats lesions in the cerebral cortex were combined with horseradish peroxidase injections in the crus II. In the pons terminal regions (anterogradely labelled from the cerebral cortex or containing terminal degeneration) and cell groups retrogradely labelled from crus II were carefully plotted. The pontocerebellar projection to crus II is mainly crossed, on the average 26% of the labelled cells were found in the ipsilateral pons. Some overlap between sites of ending of cortical fibres and sites of origin of fibres to crus II was present in all cases, but the degree of overlap varied considerably, depending on which cortical region was injected. Typically, partial overlap between terminal patches and groups of labelled cells occurred at multiple sites in the pontine nuclei. A major input to crus II appears to come from the parietal region. Experiments with bilateral cortical injections showed that the pontine projection from the parietal region is topographically organized in a precise mosaic pattern of adjacent but apparently non-overlapping patches of termination. Area 6 also has strong connections with crus II, while only very few of the corticopontine fibres from the sensorimotor region overlap with cell groups labelled from crus II. The second somatosensory area and the visual cortex both seem able to influence a small but significant proportion of cells projecting to crus II. In contrast to other cortical regions, the auditory cortex appears to send fibres mainly to cell groups projecting to the ipsilateral crus II. It is concluded that the input to crus II originates in wide areas of the cerebral cortex. Small subgroups of neurons projecting to crus II can be differentiated on the basis of their cortical afferents. It appears likely that each subgroup receives fibres mainly or in some instances only from one cortical site. The corticopontocerebellar projection to crus II probably exhibits a high degree of spatial order providing a specific pattern of convergence and divergence in the cerebellar cortex, in agreement with recent physiological evidence from micromapping studies.