Explain the all-or-none principle in muscle fibers.

• A. A motor unit's fibers respond partially to a single action potential.
• B. A motor unit's fibers respond independently to a neuron firing.
• C. A motor unit's muscle fibers respond as a unit to a neuron action potential; individual fibers do not partially contract; force graded by recruitment.
• D. Force varies with the rate of neuronal firing in a fiber.

Answer: (C)

The key idea is that a motor unit acts as a single functional unit. When the motor neuron sends an action potential, all the muscle fibers it innervates respond together and contract to their fullest extent for that impulse—there isn’t a partial contraction from that single signal. This all-or-none response at the level of each motor unit means the strength of a muscle’s contraction is not built by partial fiber responses, but by how many motor units are activated and how rapidly their neurons fire. So, force is graded by recruitment (how many units are engaged) and, to some extent, by the firing rate of those active units. This explains why small, precise movements use only a few small motor units with low firing rates, while stronger contractions recruit more units and can increase firing frequency.