Motor units and innervation ratio (A) (B) Motor neuron in spinal cord Innervation ratio Fibers per motor neuron Extraocular muscle 3: 1 Gastrocnemius 2000: 1 Muscle fibers innervated by a single motor neuron PUe织g164 o 2001 Sinauer Associates, Inc.Motor units and innervation ratio Purves Fig. 16.4 Innervation ratio Fibers per motor neuron Extraocular muscle 3:1 Gastrocnemius 2000:1