Collateralized projection of visual climbing fibers to the flocculus and nodulus of the rabbit.

Electrical stimulation of the rabbit cerebellar flocculus evoked responses in Purkinje cells of the nodulus by an axon reflex through collateralized climbing fibers (latency 4.2 ms). These responses interfered with those evoked by stimulation of the optic nerve in the manner of occlusion. Stimulation of the nodulus induced similar axon reflex responses in the flocculus. To identify the cells of origin of this collateralized climbing fiber projection, different fluorescent retrograde tracers, propidium iodide and 4',6-diamidino-2-phenylindole, were injected into the nodulus and flocculus. In the contralateral inferior olive, the majority (85%) of double-labeled cells were found in the dorsal cap and the rest (15%) in the ventrolateral outgrowth. These results lead to a conclusion that some inferior olive neurons convey common visual signals with collateralized axons to both the flocculus and the nodulus.