When a motor unit is stimulated, what happens to all of its muscle fibers?

• A. Some contract
• B. All contract
• C. Only slow-twitch contract
• D. Only fast-twitch contract

Answer: (B)

When a motor unit is stimulated, all of the muscle fibers it innervates contract. A motor unit consists of a single motor neuron and every muscle fiber it connects to; when that neuron fires, it triggers an action potential in all those fibers, causing cross-bridge cycling and contraction in each one. This coordinated response happens because the neuron’s signal is delivered to every fiber in the unit, so the unit acts as a single functional unit. The overall force you feel from the unit depends on how many motor units are recruited and how rapidly they fire, but within a single motor unit, all of its fibers contract when stimulated.