Origin of cerebellar climbing fibers in the rhesus monkey

The inferior olivary nuclear complex is generally considered to be the major source of climbing fibers to the cerebellum (Bell and Dow, '67). To test the validity of this statement, experiments using monkeys were designed in which the olivocerebellar fibers were transected in their course through the medulla to the contralateral hemicerebellum. After allowing for anterograde degeneration of these transected fibers, the cerebellum of each monkey was stained and examined for the presence and condition of climbing fibers. In each monkey olivocerebellar fibers were transected at one of three sites: (1) within the right inferior olivary nucleus in one rhesus monkey, (2) at the decussation of the olivocerebellar fibers, within the region of the raphe, in five monkeys and (3) in the left parasagittal plane, just lateral to the inferior olivary nucleus, in one monkey. No noticeable reduction was found in the number of silver stained climbing fibers associated with the Purkinje cells of each hemicerebellum after these lesions. None of the monkeys exhibited durable, specific cerebellar signs. On the basis of the histological evidence obtained from the above experiments, it is concluded that the inferior olivary complex is not the major source of climbing fibers to the cerebellum. Climbing fibers were readily identified in each of the sections examined. However, silver-stained climbing fibers were not associated with many Purkinje cells in every one of these sections. There were 1.7 times more Purkinje cells without climbing fibers than those with climbing fibers. This ratio of 1.7 to 1 characterized normal cerebella as well as both hemispheres of the cerebella of the cerebella of the experimental monkeys.