Which option correctly defines a motor unit?

• A. A group of muscle fibers innervated by multiple motor neurons.
• B. A single motor neuron and all the muscle fibers it innervates.
• C. The electrical impulse traveling along a motor neuron.
• D. The connective tissue surrounding a muscle fiber.

Answer: (B)

A motor unit is the functional element of contraction: a single motor neuron and all the muscle fibers it innervates. When that neuron fires, its axon branches to multiple fibers, so those fibers contract together as a unit. This arrangement lets the nervous system control force by recruiting more motor units or by selecting smaller versus larger units for finer control or more power. Each muscle fiber follows an all-or-none response when it receives enough signal from its motor neuron, but the overall force of a muscle comes from how many fibers and how many units are activated. The idea that a motor unit could be a group of fibers innervated by multiple motor neurons isn’t correct because a motor unit is defined by one neuron and its specific set of fibers. The electrical impulse along a motor neuron is the action potential, not the motor unit itself. And the connective tissue around a muscle fiber is the endomysium, not related to the motor unit definition.