what is a motor unit
Definition of Motor Unit. A motor unit consists of one somatic efferent (motor) neuron and all of the muscle fibers (cells) that it innervates. In the figure to the right, two motor units are illustrated diagrammatically.Notice that after an efferent axon enters the muscle, it branches and forms synapses with a number of muscle fibers.hus, each motor unit is either relaxed or each presynaptic action potential causes an action potential in all of the muscle fibers of the motor unit, causing them all to contract at once. In the whole muscle, contractions of different strengths necessarily are created by activating different numbers of motor units.