A motor unit is a single motor neuron and the corresponding muscle fibers it innervates; groups of motor units work together, as a single muscle. The number of muscle fibers within each unit can vary: thigh muscles can have a thousand fibers in each unit, eye muscles might have ten. In general, the number of muscle fibers innervated by a motor unit is a function of a muscle's need for refined motion. Muscles requiring more refined motion are innervated by motor units that synapse with fewer muscle fibers.

See also: motor unit recruitment , contractile summation , tetanic contraction, zootomy