Discharges of nucleus interpositus neurones during locomotion in the cat.

Extracellular recordings were made from ninety-five cerebellar nuclear neurones in the cat. All were studied during periods of steady walking at 0.5 m/s and most were also studied in the resting animal. Most neurones were in nucleus interpositus anterior; forty-four cells were shown by antidromic invasion to project to the mid-brain. Most neurones discharged tonically in the absence of overt movements and the mean rate was 42 impulses/s (S.D. +/- 23). During locomotion the mean rate was 68 impulses/s (S.D. +/- 32). In all but seven neurones the discharge during locomotion was frequency modulated but in different neurones the depth of modulation varied from 5 to 161 impulses/s (mean 52 impulses/s; S.D. +/- 30) and the time of peak discharge relative to the step cycle in the ipsilateral forelimb also varied widely. Despite the individual differences the population as a whole was much more active during forelimb swing than during stance, both in numbers of neurones strongly active and in over-all average discharge rate (74 impulses/s as compared with 55). Most neurones had tactile receptive fields on the ipsilateral forelimb while others received input from head and neck or from both ipsilateral limbs. The tendency to discharge preferentially during early swing was greatest for the first group, especially the subpopulations with receptive fields around or proximal to the elbow. Cells encountered in close sequence during a micro-electrode track had similarly located receptive fields and usually showed similar patterns of discharge during locomotion. These findings are discussed in relation to the suggestion by Orlovsky (1972a, b, c) that nucleus interpositus assists in regulating locomotion by evoking rubrospinal discharges which facilitate the flexor muscle activities produced by the spinal mechanisms responsible for generating the swing phase of the step cycle.