An innervation ratio is defined as:

• A. Number of motor neurons in a motor unit
• B. Number of muscle fibers stimulated by one motor neuron
• C. Volume of muscle tissue affected by a nerve
• D. Rate of impulse conduction along a neuron

Answer: (B)

The main idea tested is what innervation ratio means in motor units. A motor unit is one motor neuron and all the muscle fibers it innervates, and the innervation ratio is simply how many muscle fibers are stimulated by that single motor neuron. This ratio helps determine how precisely a muscle can be controlled: a low innervation ratio (fewer fibers per neuron) supports fine, delicate movements, while a high ratio (many fibers per neuron) supports larger, less precise actions.

So the correct definition is the number of muscle fibers stimulated by one motor neuron. The other descriptions describe different concepts: the count of motor neurons in a motor unit (a motor unit contains one neuron), the volume of muscle tissue served by a nerve, or the rate at which impulses travel along a neuron (conduction velocity).