Sensory and motor responses of precentral cortex cells during comparable passive and active joint movements

Abstract

In awake macaques trained to permit passive stimulation, the adequate natural stimuli that reliably evoked responses in precentral motor cortex neurons were documented. Of the cells in leg and arm areas 75% responded only to passive joint movement, 8% were activated by cutaneous stimulation, and 17% did not respond to the somatic stimuli tested. Half of the units with cutaneous input could also be activated by passive joint rotation. A small number of precentral neurons responded to complex visual stimuli, such as approaching objects or appearance of novel objects. Of the precentral cells responsive to passive joint movement, over 3/4 responded only phasically during joint rotation and exhibited no tonic discharge related to joint angle. Two-thirds responded to movement of only a single joint. For most arm and leg joints, the numbers of neurons responding to flexion and to extension were almost equal; some cells were activated by both flexion and extension of the same joint. In exploring specific precentral regions, cells with input from different parts of a limb were found to be extensively intermingled. The response patterns of precentral neurons, identified by anatomical location, adequate natural stimulus and pyramidal tract projection, were documented during comparable active and passive elbow movements, with the forearm held in a cast. Responses to controlled passive elbow movements with the arm restrained were usually consistent with the cells'' adequate stimulus. Some precentral neurons responded to active and passive elbow movements in both directions. Most of these had adequate stimuli restricted to elbow rotation; such neurons appear to be related to joint movement per se, independent of direction. The results, in conjunction with lesion and stimulation studies, are consistent with a sensory and a motor role for precentral cortex neurons. Under passive conditions, the responses evoked by joint rotation and cutaneous stimulation may be utilized in perception of such stimuli, particularly in the absence of the more sensitive postcentral cells.