Neural circuit in the primate spinal cord transform motor command from the brain according to the initial position of body

Proper execution of voluntary movement requires a sensorimotor transformation based on the initial limb state. For example, successfully reaching to a stable target requires the recruitment of different muscle groups depending on limb position at movement initiation. To test if this transformation could occur at the spinal level, we stimulated the cervical spinal cord of anesthetized monkeys while systematically changing initial posture and examined the modulation of the twitch response induced in the upper limb muscles.Results suggest that excitability in the cervical spinal cord is affected by initial arm posture through spinal reflex pathways. This posture-dependency of spinal motor output could affect voluntary arm movement by adjusting descending motor commands relative to the initial arm posture.