Which statement reflects the all-or-none principle of action potentials?

Which statement reflects the all-or-none principle of action potentials?

• A. The amplitude of an action potential varies with stimulus strength.
• B. Action potentials occur in bursts with repeated stimuli.
• C. Once the threshold is reached, an action potential of fixed amplitude occurs.
• D. The number of action potentials depends on stimulus duration.

Answer: (C)

The all-or-none principle states that once the membrane potential crosses threshold, an action potential is generated with a fixed, maximum amplitude. This happens because voltage-gated sodium channels open in a rapid, concerted wave to drive depolarization to a characteristic peak, and then those channels inactivate while potassium channels repolarize the membrane. The result is a spike that reaches about the same height each time, regardless of whether the stimulus that triggered it was just above threshold or much stronger. If the stimulus is too weak to reach threshold, no spike occurs at all. Stronger or longer stimuli don’t make the spike taller; they can influence whether a spike occurs or the firing rate, but not the amplitude of the individual action potential.

The all-or-none principle states that once the membrane potential crosses threshold, an action potential is generated with a fixed, maximum amplitude. This happens because voltage-gated sodium channels open in a rapid, concerted wave to drive depolarization to a characteristic peak, and then those channels inactivate while potassium channels repolarize the membrane. The result is a spike that reaches about the same height each time, regardless of whether the stimulus that triggered it was just above threshold or much stronger. If the stimulus is too weak to reach threshold, no spike occurs at all. Stronger or longer stimuli don’t make the spike taller; they can influence whether a spike occurs or the firing rate, but not the amplitude of the individual action potential.