Cerebellar nuclear cell activity during antagonist cocontraction and reciprocal inhibition of forearm muscles.

Monkeys were trained to exert a maintained isometric pinch with the thumb and forefinger. This task reliably elicited a simultaneous cocontraction of the forearm muscles. The same monkeys were also taught to insert the open hand into a manipulandum, flex and extend the wrist 35 and 15 degrees, respectively, and maintain an isometric wrist position against a mechanical stop for 1 s. This second task comprised two conditions: a dynamic or movement phase and a static or isometric phase. Movement always involved a wrist displacement of 50 degrees. Although some forearm muscles demonstrated bidirectional activity during the wrist displacement phase, all the wrist and finger muscles were alternatively active in isometric flexion or extension. Of the neurons in the dentate and interposed nuclei that consistently changed discharge during repeated isometric prehension, over 90% (61/67) of the neurons increased activity during this cocontraction of forearm muscles. About 70% (47/67) of these same nuclear cells discharged with a reciprocal pattern of firing during alternating wrist flexion-extension movements. Forty-six neurons had sustained and reciprocal discharge during the maintained isometric wrist postures. No differences were seen between the activity patterns of dentate and interposed cells with respect to either the prehension task or the reciprocal wrist-movement task. The discharge frequency of some dentate and interpositus neurons could be correlated with prehensile force as well as velocity of wrist movement and torque developed by wrist muscles. Correlation coefficients were calculated between nuclear cell discharge and the amplitude of the surface EMGs of the flexors and extensors of the wrist and fingers during the wrist-movement task. Sixteen nuclear cells showed low-order, but reliably positive, correlations with one of the two forearm muscle groups (mean r = 0.33). In contrast, a sample of seven Purkinje cells recorded during the same task demonstrated low-order correlations that were negative in sign (mean r = -0.30) between discharge frequency and one of the two forearm EMGs.