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. However, there is no overlap in the innervation of the muscle fibers by different efferent neurons. Each individual muscle fiber is connected to only one efferent neuron.
The figure, however, is quite diagrammatic. There are many more efferent neurons innervating a muscle and many more muscle fibers in one motor unit. For example, a large muscle used for powerful movements, such as the gastrocnemius in the calf, is controlled by hundreds of efferent neurons. Moreover, each efferent neuron innervates hundreds, or even thousands, of individual muscle fibers.