Final answer:
Motor units vary in size and number of fibers within different muscles, with some muscles having as few as 10 fibers controlled by a motor unit and others having up to 1000 fibers. The number of muscle fibers in a motor unit determines the precision of control of that muscle.
Step-by-step explanation:
The size of motor units and number of fibers vary within different muscles. Motor units are composed of a motor neuron and the muscle fibers it controls. The size of motor units can range from as few as 10 muscle fibers in an extraocular muscle to as many as 1000 muscle fibers in the quadriceps. The number of muscle fibers in a motor unit corresponds to the precision of control of that muscle. Muscles with finer motor control have more motor units connecting to them, requiring a larger topographical field in the primary motor cortex.