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 number of fibers per motor unit depends on the specific muscle. A small muscle used for intricate movements will have less than 100 fibers per motor unit and less than 100 motor units in total. A large muscle used for weight-bearing and propulsion will have several thousand fibers per motor unit and several hundred motor units.