Abstract
It is commonly believed that spinal circuitry is only capable of simple sensory-to-motor transformations because the spinal reflex (i.e., EMG activity 25-50 ms after a muscle is stretched) is modulated by only a few factors (e.g., rate of stretch; motor neuron excitability). Here we show modulation of the spinal reflex in the triceps muscle that opposes this belief. Participants grasped the handle of a planar 3 degree-of-freedom robot and placed their hand on a home location. The robot then mechanically flexed the participant's elbow, stretching the triceps. At the same time, the robot flexed, extended, or did not mechanically perturb the participant's wrist from an initial neutral position. Importantly, perturbing the wrist changes how elbow movement effects hand position relative to the home location. Participants were required to bring their hand back to the home location following the perturbation, but did so while grasping the robot's handle in different arm orientations. This manipulation diametrically altered how wrist motion moved the hand with respect to the home location. We report three interesting findings. First, the spinal reflex evoked in the triceps was modulated by the different wrist perturbations. Second, this modulation was dependent on the orientation of the arm. And third, the magnitude of the triceps' spinal reflex was appropriate to bring the hand back to the home location. Our findings indicate that spinal circuitry can integrate and process sensory information from multiple muscles to generate rapid motor responses to help return the hand to a desired location.Acknowledgments: NSERC, CIHR