Movement Synergies Elicited by Intraspinal Microstimulation Compared to Stimulation of Muscles, Nerves and Roots in the Cat

This study tested the idea that there are sets on neurons in the spinal cord responsible for generating elementary synergies (movement primitives) that could have implications for FES. Intraspinal, intramuscular, epineural and spinal root stimulation were compared in the anesthetized cat. Ventral root stimulation produced downward and backward movements of the leg while dorsal root stimulation produced upward and forward movements. Muscle, nerve and spinal cord stimulation generated selective movements in all directions. Muscle stimulation produced the most smoothly graded recruitment curves, but recruitment depended heavily on the location of electrodes relative to the motor point. Intraspinal stimulation within intermediate “movement primitive” regions of the cord activated synergistic groups of muscles, and produced relatively graded recruitment curves. However, the direction of movement often changed as stimulus strength increased. Nerve stimulation produced the largest movements, but the recruitment curves were quite steep, which might make fine control difficult.