Responses of identified cells in postcentral cortex of awake monkeys during comparable active and passive joint movements

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Abstract
In awake rhesus macaques [Macaca mulatta] trained to allow passive somatic stimulation without active resistance, the adequate natural stimuli of single neurons in postcentral cortex were studied. Of the neurons sampled in areas 1, 2 and 5, 36% responded to cutaneous stimulation, and 43% responded to manipulation of deep tissue and/or joint rotation; 10% were activated by other, more complex stimuli; and 11% were unaffected by any of the stimuli tested. The deep cells either responded to stimulation of single joints (shoulder, 33%; elbow, 20%; wrist, 6%), or of multiple joints (14%), or they responded to palpation of muscles and rotation of associated joints (27%). Activity of postcentral cortex cells, identified by adequate stimulus and cytoarchitectonic locus, was documented during comparable active and passive elbow movements with the forearm held in a cast. With active elbow movements, the firing rate of many postcentral cortex cells changed well before activation of contralateral arm muscles. Relative to onset of agonist elbow muscles, activity of postcentral neurons changed on the average 61.4 ms earlier; and a third of the changes preceded agonist muscle onset by 100 ms or more. Mean onset times did not differ appreciably for deep and cutaneous cells, nor for cells in areas 1 and 2. Such early changes in neural activity during these self-paced elbow movements suggest a centrally originating input to these postcentral neurons. Responses of certain elbow joint and muscle cells appeared to be reduced before active elbow movement, indicating central inhibition. Other deep cells showed evidence of central excitation well before agonist muscle activity. Cutaneous cells exhibited relatively little evidence of central or peripheral inhibition of phasic responses during active movements. Depending on their projections, postcentral neurons that become active before agonist muscles may participate in generation of active movements.