# Autogenic spinal excitatory circuit ensures skilled hand movements in primates

## TL;DR

This study shows that spinal circuits in primates actively support skilled hand movements, not just passively relaying signals from the brain.

## Contribution

The discovery of an autogenic spinal excitatory circuit that contributes to voluntary hand movements in primates.

## Key findings

- Spinal interneurons form a positive feedback loop with hand muscles during voluntary movements.
- The spinal circuit's activity is tightly coupled to muscle output and is preset during motor planning.
- Spinal reflex loops operate in parallel with cortical mechanisms to enable skilled hand movements.

## Abstract

Skilled hand movements are a hallmark of primate behavior and are usually attributed to cortical mechanisms. Yet the degree to which spinal circuits contribute directly to voluntary dexterity has remained unclear. By combining in vivo recordings, peripheral stimulation, and computational modeling, we identify an excitatory spinal circuit in macaques that forms an autogenic positive feedback loop: spinal interneurons receive proprioceptive input from hand muscles and in turn provide excitatory drive back to those muscles. This loop generates task-related activity tightly coupled to muscle output, contributing to cortical control of skilled hand movements via spinal reflex pathways. These findings preclude any view of the spinal cord as a passive relay and highlight its role in volitional motor control and rehabilitation strategies.

Skillful hand movements are a hallmark of primates, including humans, requiring sophisticated motor planning and execution. Building on the well-established cortical basis of dexterous control, our findings show that spinal excitatory reflex circuits form a critical complementary pathway that contributes substantially to the planning and execution of skillful hand movements. Using a combination of experimental approaches with behaving nonhuman primates and predictive simulation, we identified a group of excitatory spinal interneurons that orchestrate a closed-loop, positive feedback mechanism during voluntary wrist movements. This mechanism is characterized by a bidirectional interaction between interneuronal spiking and muscle activity, mediated by motoneuronal efferent signals and proprioceptive afferent signals from the same agonistic muscles. Furthermore, we demonstrate that the temporal profile of muscle activity during movement execution, including amplitude and duration, is predetermined during motor planning at the spinal interneurons, functioning as a force-feedback gain within the excitatory circuit. These findings suggest that autogenic, Ib spinal excitatory circuits play a predominant role in shaping overall muscle activation during motor execution, provided the proper reflex gain is preset by higher neural systems during motor planning. Together, our findings provide cellular-level evidence that spinal reflex loops operate in parallel with cortical mechanisms to support skilled voluntary movements in primates.