In mammalian skeletal muscle, each muscle fiber is innervated by only one motor neuron. The combination of a single motor neuron and all the muscle fibers it innervates is called a motor unit. Although the muscle fibers of a given motor unit tend to be located near one another, motor units have overlapping territories.

The strength of muscle contraction depends on the number of muscle fibers active at the same time. However, the central nervous system cannot control each individual muscle fiber. It can only activate the motor neurons and therefore the motor units. The degree of control that can be exerted on the strength of contraction depends on the number of muscle fibers in a motor unit. Motor units of large muscles such as the gastrocnemius, which exert a great deal of power, may contain more than 2,000 muscle fibers. Motor units of small muscles such as the extraocular muscles, which exert very fine control but not much power, may contain as few as six muscle fibers.

Even within a given muscle, the motor units are not equal in size. In general, small motor neurons innervate fewer muscle fibers (they have smaller motor units). Small motor neurons are also more easily activated by synaptic inputs than are large motor neurons. Therefore, when signals from the brain initiate a movement, the smallest motor neurons and motor units are usually activated first. If only a small fine movement is required, the smallest motor units alone can be activated. As more power and less fine control is needed, the larger motor units are progressively recruited. This process is called the size principle of motor control.

The mechanical properties of the muscle fibers are also matched to the size of their motor unit. Because the muscle fibers of the smallest motor units are those most often activated, they must be relatively resistant to fatigue (see Plate 2-15).