What is the difference between a motor unit and a single muscle fiber?

• A. A motor unit is a single muscle fiber innervated by multiple motor neurons.
• B. A motor unit consists of a single motor neuron and all the muscle fibers it innervates.
• C. A single muscle fiber is controlled by a network of neurons.
• D. A motor unit is the contractile element of a muscle fiber.

Answer: (B)

A motor unit is the functional unit of muscle contraction, defined as a single alpha motor neuron and all the muscle fibers it innervates. When that neuron fires, all the fibers in its motor unit contract together, producing force. A single muscle fiber is the basic contractile element that shortens in response to its motor neuron's signal, but it does so as part of a motor unit rather than by itself. In a muscle, one neuron can branch to control many fibers, forming multiple motor units that are recruited as needed to generate varying amounts of force. This arrangement allows precise, graded control of movement: activating more motor units or increasing firing frequency increases total contraction. The motor unit is not just a single neuron or a single fiber, and it isn’t merely the contractile element itself; it includes the neuron and all the fibers it controls.