The corticopontocerebellar pathway to crus I in the cat as studied with anterograde and retrograde transport of horseradish peroxidase.

In 13 cats injections of horseradish peroxidase (HRP) in various parts of the cerebral cortex were combined with injections of HRP in the cerebellar crus I in the same animal in order to study the cortical regions which may influence the crus I via the pontine nuclei. In the pons terminal regions anterogradely labeled from the cerebral cortex and cell groups retrogradely labeled from crus I were carefully plotted. Overlap between sites of ending of cortical fibers and sites of origin of fibers to crus I was relatively modest in all experiments. On the other hand, partial overlap was usually found at multiple sites. The largest single input to crus I appears to come from the parietal region, particularly the anterior part of the suprasylvian gyrus, while the sensorimotor region contributes much less. Area 6, the second somatosensory cortex and the orbital gyrus, all seem able to influence pontine cells projecting to crus I. Least overlap is found after injections of the visual cortex. The size and orientation of the dendritic fields of pontine cells were studied in Golgi-impregnated material. The dendritic fields average 187 X 339 microns in the transverse plane and are so small that they will only moderately increase the overlap. It is concluded that small subgroups of neurons projecting to crus I receive somewhat different sets of cortical afferents. The input to crus I must originate in wide areas of the cerebral cortex, and probably exhibits a high degree of spatial order, with an intricate pattern of specific divergence and convergence. The present results are compatible with previous physiological evidence from micromapping studies of a precise and complicated mosaic pattern of connections to the cerebellar hemispheres.