A motor unit is a group of cells under the control of a single motor neuron; groups of motor units work together, as a single muscle. The number of cells within each unit can vary: thigh muscles can have a thousand cells 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