SPINAL REFLEX CIRCUITS SUPPORTING SKILLED HAND MOVEMENTS IN PRIMATES

Skillful hand movements are a defining feature of primates and rely on precise coordination between motor planning and execution. While cortical mechanisms underlying dexterous control have been extensively studied, the contribution of spinal circuits to skilled voluntary movements remains less well understood.　In this presentation, we report recent findings from studies in behaving non-human primates that reveal an active role of excitatory spinal reflex circuits during voluntary wrist movements. Using electrophysiological recordings combined with predictive neuromuscular simulations, we identified a population of excitatory spinal interneurons that form a closed-loop, positive feedback circuit with their target muscles. This circuit is characterized by bidirectional interactions between interneuronal spiking and muscle activity, mediated by motoneuronal efferent drive and proprioceptive afferent feedback from the same agonist muscles.
Our results indicate that key features of muscle activation during movement execution, including amplitude and duration, are specified at the level of spinal interneurons during motor planning, effectively acting as a force-feedback gain within the excitatory circuit. These findings suggest that autogenic, Ib-related spinal excitatory pathways play a central role in shaping muscle activity during voluntary movements, provided that reflex gain is appropriately set by higher motor centers.
In addition, we will present preliminary data from ongoing multichannel recordings in the spinal cord of awake, behaving animals, revealing population-level coding by spinal interneurons during skilled hand movements.