The activity of cerebellar nuclear neurones in relation to stimuli which evoke a pectoral fin reflex in dogfish.

Extracellular single-unit recordings from the cerebellar nucleus were classified into type I and type II units on the basis of their spontaneous discharges. Type I units discharged at a very regular frequency, giving interspike interval histograms with narrow distributions. Type II units had irregular discharges. Type I units were identified as cerebellar nuclear units by their antidromic responses to stimulation of the contralateral brachium conjunctivum (b.c.) in the mid-brain and by their inhibitory responses to stimulation of the cerebellar cortex. Type II units were not driven antidromically by b.c. stimulation but were inhibited by stimulating the cerebellar cortex. Activity of the nuclear neurones was monitored following subcutaneous electrical stimulation of a fin that elicits a reflex elevation. 67% of units responded, the majority with an increased discharge frequency (excitation, 59%) but some with a decreased discharge frequency ('inhibition', 8%). Latencies of both excitatory and inhibitory responses were greater than 50-400 msec. Most excitatory responses lasted for at least 500 msec; several lasted for more than 10 sec. Inhibitory responses lasted for about 500 msec. With units tested by bilateral fin stimulation, the same qualitative response was obtained whichever fin was stimulated. These results are discussed in relation to the known responses of cerebellar Purkinje cells recorded under similar experimental conditions.