This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.Spinal neurons play a crucial role in generating voluntary limb movements. We investigated the way that spinal interneurons are engaged in controlling muscle synergies by documenting their responses in monkeys performing a repertoire of movements about the wrist (flexion-extension, radial-ulnar deviation, pronation-supination and grip). Most interneurons in cervical segments were modulated during one or more of these hand movements, showing preferential relationships to particular muscle patterns. We have also been documenting the response patterns of motor cortical cells during the same response repertoire to compare the tuning properties of cortical and spinal neurons.We also documented the output effects on muscles evoked by intraspinal stimulation at different cervical sites. In behaving monkeys the excitatory and inhibitory post-stimulus effects evoked by single intraspinal stimuli in upper and lower cervical segments were investigated by stimulus-triggered averages. In anesthetized monkeys the movements and muscle responses were mapped systematically with brief trains of stimulation. At most spinal sites the stimuli evoked responses in multiple muscles, even at threshold for movement. This means that spinal cord is a better target for stimulation by a recurrent brain-computer interface than muscles, because synergistic responses can be evoked and motor units are recruited in a more natural order.Other experiments evaluated the modulation of synaptic transmission from descending corticospinal neurons. We previously reported evidence that descending commands that generate active wrist movements also evoke presynaptic inhibition in afferent fibers and thereby decreases the effect of peripheral input to first-order relay INs. In contrast we found no evidence that descending corticospinal fibers could be influenced by peripheral input. The functional consequence of both patterns of presynaptic inhibition is to prevent re-afferent activity produced by movements from interfering with the accurate control of movement by descending commands.
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