The activity of cerebellar neurones of an elasmobranch fish (Scyliorhinus canicula) during a reflex movement of a fin.

1. Response of neurones in the corpus cerebelli of Scyliorhinus canicula (Elasmobranchii) have been recorded in decerebrate unanaesthetized fish during the performance of pectoral fin reflexes (p.f.r.) evoked by electrical stimulation of the fins.2. Of 421 single units recorded in the posterior dorsal quadrant of the corpus, 111 (26%) had their discharges modulated when the reflex was evoked; fifty nine were Purkinje (P) cells discharging single spikes, thirteen were P cells discharging complex responses, thirty-six were stellate (S) cells and three were not positively identified.3. The responses of fifty-one units (thirty-seven P cells and fourteen S cells) were analysed in detail. In thirty-one of the P cells and all the S cells the initial response was excitatory, six P cells gave an inhibitory response only and a further three were inhibited after the initial excitation. Only one S cell had an inhibitory component in its response.4. The discharges of the S cells completely overlapped the inhibitory responses of the P cells.5. Only one cell (a P cell) was evoked at a latency shorter than that of the p.f.r. In one other P cell discharging single spikes spontaneously, the evoked response was a long latency (146 msec) complex response. Four other P cells discharged only complex responses but the pattern of their response to the p.f.r. was similar to that of P cells discharging single spikes only.6. No cerebellar responses were evoked if the fin stimulus was below the threshold necessary to evoke a p.f.r. In curarized fish, unit responses were recorded that were qualitatively similar to those recorded in unparalysed fish.7. These results suggest that (i) because of the long latency of the responses, the cerebellum is unlikely to have a role in the initiation of the p.f.r.; (ii) the responses during the p.f.r. were evoked via a mossy fibre-parallel fibre pathway; (iii) the responses were correlated with motor activity rather than sensory input.