A spinal origin for the obligate flexor synergy in the non-human primate: Implications for control of reaching

Stroke survivors frequently develop the flexor synergy, an obligate co-contraction of shoulder abductors and elbow flexors; the neural substrate has proven elusive. Here we trained healthy monkeys to generate isometric elbow and shoulder torques to move an on-screen cursor, and recorded neuron firing from motor cortical areas and the reticular formation. In all regions we found cells correlated with activity around a single joint. Neurons coding co-contractions showed a bias towards combinations orthogonal to the post-stroke flexor synergy, e.g. shoulder abduction with elbow extension. Threshold microstimulation in the spinal cord but not in either motor cortex or the reticular formation generated coactivation aligned to the flexor synergy. We suggest the evolution of prehension required descending systems either to control or bypass locomotion-dedicated spinal circuits. Loss of descending input after stroke constrains the upper limb to spinal synergies best suited to primitive locomotor functions.