Precentral unit activity following torque pulse injections into elbow movements.

(1) Precentral neural activity was studied in relation to transient load changes on self-paced elbow movements. Four Cebus monkeys were trained to turn a freely moving handle back and forth between two targets by alternating self-paced elbow flexions and extensions. Torque pulses (of 10 or 100 msec) injected randomly to load or unload the movements stretched or slackened the appropiate prime movers: biceps or triceps. Perturbed movements oscillated at about 5.5 Hz but were completed successfully in about the same time as unperturbed movements. (2) Torque pulses evoked distinct "early" responses with latencies of 20-40 msec in 134 out of 153 precenteral neurons. Oppositely directed torque pulses evoked reciprocal (i.e. increased or decreased) early responses in 61 neurons, and uniform responses in 27 neurons. (3) Early responses were followed by "late" responses with peaks succeeding one another at about 5.5 Hz in 111 neurons, but another 16 exhibited late responses only. (4) Timing of both early and late cortical responses was tightly coupled to peripheral changes. Early responses were timed by the initial torque-induced passive elbow jerk. Timing of late responses was best related to subsequent peak decelerations of accelerations. Intensity of the early but not of all late precentral responses was tightly coupled to peripheral events. (5) Torque pulses that impeded flexions or extensions evoked spinal stretch reflexes in biceps or triceps with EMG latencies of about 15 msec, leading to an acceleration peak about 25 msec later. A second EMG burst followed the first in about 30 msec. The second burst occurred about 20 msec after onset of the early precentral response, which is thought to have caused it, as well as a second acceleration peak that was seen about 60 msec after precentral response (for flexion load pulses). Peaks of late precentral responses were followed by acceleration peaks within about 70 msec. (6) An interaction akin to the spinal stretch reflex is thus revealed between elbow perturbations, early responses of precentral neurons and subsequent elbow movements: discharges of neurons that usually fire in relation to an intended movement can be altered by sudden load changes so that the neurons tend to reduce mismatch between intended and actual movements (cortical load compensation), created by the perturbation. An analagous interaction may also occur with late cortical responses.