Histological evidence supporting the inferior olive as the major source of cerebellar climbing fibers in the rat

Complete bilateral destruction of the inferior olive was achieved in rats by 3-acetylpyridine intoxication. Active secondary degeneration of olivo-cerebellar fiber terminals was studied in the cerebellar cortex of 35 treated rats by means of a new silver method which selectively impregnated degenerating nerve cell processes and perikarya. Degenerating climbing fibers were observed in both the molecular and the granular layers as early as 12 h after the treatment. Degenerating climbing fibers were no longer detectable beyond 48 h of survival. The following observations were made in this study. (1) All olivo-cerebellar fibers end as climbing fibers in the cerebellum; the inferior olive is not a source of mossy fibers. (2) Climbing fibers branch frequently, mainly in the infra-ganglionic plexus. Hence, it is concluded that a single olivary neuron can provide several Purkinje cells with climbing fibers. (3) As a consequence of complete inferior olive destruction, all climbing fibers were observed to degenerate over large areas of the cerebellar cortex. In the experimental material taken as a whole, massive degeneration of climbing fibers occurred over the entire extent of the molecular layer. Therefore, it is concluded that the inferior olive is the major and probably the sole significant source of cerebellar climbing fibers. © 1974.SCOPUS: ar.j