Relation of pyramidal tract activity to force exerted during voluntary movement.

Abstract

Monkeys were trained to carry out alternate flexion-extension movements of the wrist for a fruit juice reward. Reward was delivered only when the durations of 2 successive displacements (flexion followed by extension or vice versa) fell between 400 and 700 msec. At certain times a load opposed flexion and, as a result, both the flexor and the extensor displacements of the required duration involved activity of the flexor musculature and exertion of flexor force. At other times the load opposed extension, requiring that both flexor and extensor displacements be associated with extensor force. Force was thus dissociated from direction of displacement, and it was possible to determine whether pyramidal tract neuron (PTN) discharges recorded from the precentral motor cortex in association with this movement were related to direction of force or to direction of displacement. For the majority of PTNs, discharge frequency was related primarily to the force (F) and dF/dt, and was only secondarily related to the direction of displacement. Some PTNs which were unrelated to force were related to the direction of displacement, but not to the fine details of the displacement in the way that numerous other PTNs were related to the fine details of applied force.