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

Monkeys were trained to perform a maintained isometric grip of the thumb and forefinger that elicited a simultaneous cocontraction of the antagonist muscles of the forearm. The same monkeys were also trained to flex and extend the wrist against a stop with the fingers extended and to maintain an isometric wrist position for 1.0-1.5 s. During wrist movement, some of the synergist forearm muscles contracted during both flexion and extension. However, during the maintained isometric wrist position, the prime mover and synergist muscles were reciprocally active or silent. In the culmen-simplex region of the cerebellar cortex bordering on the primary fissure, 62% of the Purkinje cells that were identified by the climbing fiber discharge and that changed firing frequency decreased activity during maintained prehension. Almost all of these same Purkinje cells were reciprocally active during isometric wrist flexion and extension, although three neurons had similar discharge patterns during movements in both directions. In contrast, 79% of the unidentified neurons recorded from the same region of the cerebellar cortex increased discharge frequency during prehension. In general, most of these same neurons had reciprocal patterns of discharge during wrist movement even though a few cells were active during the dynamic phase in both directions. Together, the Purkinje cells and the unidentified neurons with bidirectional response patterns were thought to be related to muscles active during both flexion and extension wrist movements. No cells were found that increased discharge with the static isometric wrist torque exerted in both directions. The discharge frequency of some Purkinje and some unidentified neurons could be shown to be related to prehensile force as well as wrist movement velocity and isometric wrist torque. These data suggest that the discharge of about two-thirds of the Purkinje cells related to forearm muscles located along the borders of the primary fissure may depend on whether antagonist muscles are activated reciprocally or coactively. As a consequence, these cells may play a role in the selection or alternation between either of these two modes of muscular contraction. The increased discharge of the remaining one-third of the Purkinje cells excited during antagonist coactivation may provide inhibition of nuclear cells to stabilize the posture at joints other than the wrist and fingers or, alternatively, they may act to reduce nuclear cell discharge in proportion to the intensity of cutaneous stimulation.