Differential effects of reciprocal wrist torques on responses of somatotopically identified neurons of precentral cortex in awake primates.

The present study was undertaken to investigate the functional significance of previous observations that both reciprocal and bidirectional responses are found in the forelimb area of precentral cortex following oppositely directed torques delivered about a single forelimb joint. Extracellular unit recordings were made in the contralateral precentral cortex of awake monkeys. These units were classified, according to their responses to passive somatosensory stimulation and to the effects of local intracortical microstimulation (ICMS), as being functionally coupled to single joints. Both joint- and cutaneous-sensitive neurons were included. The responses of such classified neurons to oppositely directed torques were then studied. All of the joint-sensitive wrist (flexion-extension) units responded reciprocally to the oppositely directed torques. In contrast reciprocally and bidirectionally responsive neurons were found in the cutaneous-sensitive wrist population and in both the joint- and cutaneous-sensitive non-wrist (F-E) populations. These results are discussed in terms of the expected receptor actions of the applied torques, and are consistent with other evidence that a correlation exists between activities of precentral neurons which respond to sensory events at or about a single forelimb joint, and voluntary movement about the same joint.