1. Glass-insulated tungsten micro-electrodes were used to record from single neurones in the intermediate zone of the cerebellar cortex of cats in a state of quiet wakefulness. 2. Two hundred and seventy Purkinje (P) cells were recorded extracellularly, 95% of which displayed an irregular tonic discharge at rates between 19 and 95/s (over-all mean 44/s), including complex spikes (c.s.) which occurred at 1.0--2.5/s )over-all mean 1.5/s). The remaining cells discharged c.s. at the usual rate but only one or two simple spikes (s.s.) per minute. C.s. of spike plus wavelet and of multi-spiked type were present in approximately equal numbers of cells. 3. Presumed climbing fibre-e.p.s.p.s were recorded from fifty-six P cells and occurred both singly and in groups of two to six e.p.s.p.s at an intra-group frequency of about 500/s. The cells giving rise to the c.f.s therefore discharge propagated impulses both singly and in short bursts as previously reported for anesthetized animals. A single e.p.s.p. can give rise to more than one spike in the multi-spiked type of c.s., and probably to a complete c.s. event. 4. Following spontaneous c.s. the interval to the next s.s. varied from 8 to 600 ms. There was an inverse correlation between duration of the post-c.s. interval and the rate at which s.s. were discharged in the preceding 100 ms. The duration exceeded the mean s.s. interval provided s.s. rate was less than 40--50/s, and the post-c.s. interval would then constitute a real interruption of s.s. discharge. 5. When the superficial radial (s.r.) nerves were stimulated with single shocks too weak to produce a behavioural response changes in discharge pattern were detected in eighty-eight of 151 P cells tested. The initial responses were almost always excitatory and consisted in seventy-two cells of a c.s., in eleven of a c.s. preceded by a brief increase in s.s. and in two cases of a s.s. discharge alone. The spino-olivo-cerebellar paths responsible for the c.s. showed transmission characteristics similar to those reported for animals anaesthetized with barbiturates. 6. C.s. were readily evoked by tapping or squeezing the forepaws. 7. Excitatory responses to nerve stimulation were usually followed by a depression of the tonic s.s. discharge. Its duration ranged widely in different cells (from 10 to 500 ms) and it would coincide with equally variable periods of facilitation previously seen in neurones of nucleus interpositus. It is therefore likely that such facilitations of the cerebellar nuclear cells result at least in part from reductions in the tonic inhibitory input from the P cells. 8. Thirty-six units were classes as 'probable cortical interneurones'. They discharged more regularly, at rates between 9 and 28/s. Twenty such units (56%) responded to s.r. stimulation with a brief excitation not usually followed by any pronounced depression.
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