When a muscle contraction is required, it receives a message from the brain for voluntary contractions and from the spinal cord for reflex contractions. This message, called a nerve impulse or action potential, is carried along motoneurons to the muscle. It would not be practical for all the fibers in a given muscle to contract every time the motoneuron discharged an action potential. Therefore, before the motoneuron reaches the target muscle, it divides into many separate branches that independently activate groups of muscle fibers. These groupings are referred to as motor units. The…