Which statement describes the all-or-none principle?

Which statement describes the all-or-none principle?

• A. Action potentials vary in amplitude with stimulus strength.
• B. An action potential occurs only if the threshold is reached; the amplitude is fixed.
• C. Once the threshold is reached, an action potential of fixed amplitude occurs.
• D. The frequency of firing is independent of stimulus strength.

Answer: (C)

The all-or-none principle means that a neuron fires a spike of essentially the same size every time the membrane potential reaches the threshold. If the stimulus is too weak, no spike occurs at all; if it is strong enough, a spike is produced, and its peak amplitude is fixed, not graded with stimulus strength. Stronger stimuli don’t enlarge the spike; they can increase how often spikes occur (firing rate) but not the size of each individual spike. That’s why the statement describing this idea is that, once threshold is reached, the action potential has a fixed amplitude.

The all-or-none principle means that a neuron fires a spike of essentially the same size every time the membrane potential reaches the threshold. If the stimulus is too weak, no spike occurs at all; if it is strong enough, a spike is produced, and its peak amplitude is fixed, not graded with stimulus strength. Stronger stimuli don’t enlarge the spike; they can increase how often spikes occur (firing rate) but not the size of each individual spike. That’s why the statement describing this idea is that, once threshold is reached, the action potential has a fixed amplitude.