The University of Washington describes a motor unit as consisting of a somatic motor or efferent neuron and all the muscle fibers or cells it stimulates or innervates. An efferent axon reaches the muscle, where it forms branches and synapses with several muscle cells.
Each efferent neuron, however, innervates or stimulates the muscle fiber to which it’s connected. The branching of individual motor axons within a muscle is essential because there are more muscle fibers than efferent neurons, according to "Neuroscience." The mechanism ensures muscle action is not compromised in the event that one or a few motor neurons are damaged.