What defines a motor unit?

• A. A motor neuron and all of the muscle fibers it innervates.
• B. A motor neuron
• C. A group of motor neurons controlling the same muscle
• D. A motor neuron firing without innervation of fibers

Answer: (A)

A motor unit is the functional unit formed by a single motor neuron and all the muscle fibers it innervates. When that neuron fires, all of its connected fibers contract together, producing a coordinated twitch in that unit. The overall force of a muscle arises from the sum of the contractions of many motor units recruited and varying firing rates.

This definition is best because it captures both the neural element (the motor neuron) and the effector elements (the specific fibers it controls). A motor neuron alone doesn’t constitute a unit without its fibers, and a group of motor neurons controlling the same muscle describes a larger pool rather than one unit. A neuron firing without any innervation wouldn’t produce contraction, so it isn’t a functional motor unit.