Latencies of climbing fiber inputs to turtle cerebellar cortex.

Responses of separate regions of rat cerebellar cortex (Cb) to inferior olive (IO) stimulation occur with the same latency despite large differences in climbing fiber (CF) lengths. Here, the olivocerebellar path of turtle was studied because its Cb is an unfoliated sheet on which measurements of latency and CF length can be made directly across its entire surface in vitro. During extracellular DC recordings at a given Cb position below the molecular layer, IO stimulation evoked a large negative field potential with a half-width duration of approximately 6.5 ms. On this response were smaller oscillations similar to complex spikes. The stimulating electrode was moved to map the IO and the CF path from the brain stem to the Cb. The contralateral brain stem region that evoked these responses was tightly circumscribed within the medulla, lateral and deep to the obex. This response remained when the brain stem was bathed in solutions that blocked synaptic transmission. The Cb response to IO stimulation had a peak latency of approximately 10 ms that was not dependent on the position of the recording electrode across the entire 8-mm rostrocaudal length of the Cb. However, for a constant Cb recording position, moving the stimulation across the midline to the ipsilateral brain stem and along the lateral wall of the fourth ventricle toward the peduncle did shorten the response latency. Therefore a synchronous Cb response to CF stimulation seems to be caused by changes in its conduction velocity within the entire cerebellar cortex but not within the brain stem.