The climbing fibers of the cerebellar cortex, their origin and pathways in cat

The sources and pathways of the climbing fibers to the cerebellar posterior vermis were studied with comibined electrophysiological and anatomical methods in cats. Recording from identified cerebellar Purkinje cells, monosynaptic climbing fiber (CF) responses have been obtained both for stimulation of the inferior olive (IO) and various parts of the brain stem (BS). CF responses were found to of three types, IO only, BS only or both IO and BS. However the responses to BS stimulation were very few in number in comparison with IO or IO and BS types of responses. The latencies of the responses were shorter for the BS cases consistent with their distance from the cerebellum. A comparison of latencies and the relative responsiveness of the different area of the brain stem which were studied, indicate that part of the CF ascend through the pontine region and enter the cerebellum by way of the medium and superior penduncles. This finding is confirmed by the results of anatomical studies in which degenerating fibers were found in the molecular layer (using the Nauta technique) after lesion of the brachium pontis but not after lesions of the medial portion of the pons. Similarly, injection of radioactive leucine into the pontine nuclei failed to show any labeled fibers in the molecular layer. Horseradish peroxidase (HRP) was injected into localized regions of the posterior vermis after total bilateral destruction of the inferior peduncles. Large numbers of positive, marked cells were still found in the inferior olive. It is concluded that nearly all, if not all, the climbing fibers originate in the inferior olive and that they ascend to the cerebellum by way of all the peduncles.