AbstractIt is widely assumed that the spinal stretch reflex (i.e., EMG activity 25-50 ms after a muscle is stretched) only acts to regulate muscle length. Recently, however, we showed that the triceps' spinal stretch reflex was appropriately modulated to help return the hand to its original static position, and not by the amount the triceps was stretched. Here we provide further evidence that the spinal stretch reflex does not just regulate muscle length by showing that this reflex is also modulated to support a reaching action. Participants grasped the handle of a planar 3 degree-of-freedom robot and, starting from a home-location, reached towards a goal-target that required 10 degrees of elbow extension. At movement onset on roughly half of the trials the robot mechanically flexed the participant's elbow – stretching the triceps – and simultaneously flexed or extended the participant's wrist. These perturbations displaced the participant's hand away from the goal-target, but critically, the perturbation that yielded the largest hand displacement relative to the goal-target did so with the least amount of elbow flexion (and vice versa). We found that the triceps' spinal stretch reflex was tuned to the hand's displacement away from the goal-target, and thus the amount of elbow extension needed to complete the reach, and not by the amount the triceps was stretched. These findings show that spinal circuitry can process sensory information from multiple muscles to generate rapid motor responses that support reaching, adding to our understanding of this circuit's functional capacity to control the hand.
Acknowledgments: NSERC, BrainsCAN