Responses of single units in cervical spinal cord of alert monkeys

Abstract

Activity of single units was recorded extracellularly in the cervical spinal cord of five unanesthetized monkeys (Macaca mulatta), three of which had been trained to flex and extend the right wrist. Units were recorded with tungsten microelectrodes advanced through a chamber mounted over two cervial segments between C6 and T1. Unit responses to electrical stimulation of the medullary pyramidal tract and three peripheral nerves of the arm at the brachial plexus were tested with chronically indwelling electrodes. The adequate natural stimuli for each unit also were characterized. Many spontaneously active units were not influenced by natural or electrical stimulation, or during active wrist movements. Units activated by cutaneous stimulation were isolated in the dorsal portion of the cord and most were not spontaneously active. Isolated units with well demarcated cutaneous receptive fields responded to brushing hairs, touching skin, gentle pinching, or some combination of these. Units responding to passive joint movements of the fingers, wrist, elbow, and/or shoulder were isolated at all depths of the cord and most exhibited some spontaneous activity. Many units responded during the phasic components of active wrist movements, and some were driven by comparable passive wrist movements.