Olivary branching projections to the flocculus, nodulus and uvula in the rabbit. I. An electrophysiological study.

Olivocerebellar branching projections to the flocculus, nodulus and uvula were studied electrophysiologically in pigmented rabbits anesthetized with pentobarbital and halothane. Neurons in the dorsal cap of the inferior olive were antidromically activated by stimulation of the contralateral flocculus, nodulus and uvula. The antidromic responses in the dorsal cap from the flocculus, nodulus and uvula partially occluded with each other. Electrical stimulation of the nodulus (or uvula) evoked early and late climbing fiber responses in the flocculus with latencies of 4.0 ms and 10.0 ms, respectively. The early response was resistant to repetitive stimulation and not affected by interruption of the olivocerebellar fibers at the lower medulla, while the late response was abolished by these two procedures. This indicated that the early response was evoked through climbing fiber branches to the flocculus and nodulus (or uvula) as an axon reflex, while the late response was evoked via the inferior olive. Of 76 Purkinje cells in the flocculus, 35.6% showed climbing fiber activation through branches to the nodulus and/or uvula, 70% of which were orthodromically activated through climbing fibers from the contralateral optic tract. Electrical stimulation of the flocculus or uvula evoked the early and late climbing fiber responses in the nodulus. Of 60 Purkinje cells in the nodulus, 63.4% showed axon reflex activation from the flocculus and/or uvula, 42% of which were activated through climbing fibers from the contralateral optic tract. These findings demonstrated that the flocculus, nodulus and uvula receive branching climbing fiber projection from the inferior olive, a part of which conveys visual information.