Discharges of nucleus interpositus neurones during locomotion in the cat.

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Abstract
Extracellular recordings were made from 95 cerebellar nuclear neurons 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 neurons were in nucleus interpositus anterior; 44 cells were shown by antidromic invasion to project to the mid-brain. Most neurons discharged tonically in the absence of overt movements and the mean rate was 42 impulses/s (SD .+-. 23). During locomotion the mean rate was 68 impulses/s (SD .+-. 32). In all but 7 neurons the discharge during locomotion was frequency modulated but in different neurons the depth of modulation varied from 5-161 impulses/s (mean 52 impulses/s; SD .+-. 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 neurons strongly active and in overall average discharge rate (74 impulses/s as compared with 55). Most neurons 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 1st group, specially 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. The hypothesis that the 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 is discussed.