We plan to investigate the neural mechanisms controlling voluntary hand and arm movement in primates. The functional roles of premotor (PreM) cells in motor cortex and spinal cord will be directly compared. PreM cells with a correlational linkage to forelimb motoneurons will be identified by post-spike effects in spike-triggered averages of EMG activity. The activity of PreM cells and multiple muscles will be documented during multidirectional wrist movements. Monkeys will operate a multi-jointed manipulandum that will allow wrist movements in three directions: flexion-extension, radial-ulnar deviation and pronation-supination. In addition a grip handle will transduce force during a power grip. This repertoire of movements will activate muscles in different synergistic combinations and resolve whether PreM cells and non-PreM cells are organized primarily in terms of muscles or movement parameters. The directional tuning of forearm muscles will be compared with the tuning curves of PreM cells and non-PreM cells. We anticipate finding functionally significant differences between motor cortex cells and spinal interneurons with regard to their relation to muscles and movements. Spinal cord interneurons have been studied largely in immobilized animals; our study will provide new information about the involvement of interneurons in preparation and execution of voluntary movements. These interneurons will be identified by their synaptic inputs from different forelimb muscles and from functionally identified cortical sites. We will also systematically map the movements of arm and hand evoked by electrical stimulation of spinal cord sites; the modulations of these responses during an instructed delay task will reveal the interaction of intraspinally evoked responses with preparation and execution of voluntary movements. Activity of dorsal root afferent fibers also will be recorded during an instructed delay task to document the afferent input to the central nervous system during movement. The axonal excitability of afferent fibers will be tested to investigate task-related modulation of presynaptic inhibition. These studies of the primate motor system will provide unique information essential to understanding and effectively treating clinical motor disorders, like cerebral palsy, stroke and spinal cord injury.
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