Reciprocal effect of single corticomotoneuronal cells on wrist extensor and flexor muscle activity in the primate

Abstract

Action potentials of single motor cortex cells were recorded extracellularly in awake monkeys trained to make alternating wrist movements between flexion and extension position zones. Spike-triggered averaging of rectified EMG activity was used to test each cell's output effect on both the coactivated (agonist) forearm muscles and their antagonists. Five of 18 adequately tested motor cortex cells, whose activity covaried strongly with alternating wrist movement, not only produced postspike facilitation of agonist muscles but also postspike suppression of antagonist muscles. Five additional cells only facilitated the agonist muscles and had no effect on the recorded antagonists; 8 cells had no effect on either agonist or antagonist muscles. We conclude that the synaptic terminations of some motor cortex cells with flexor and extensor spinal motoneurons are reciprocally organized.