What does the all-or-none principle imply about action potential amplitude and stimulus intensity?

What does the all-or-none principle imply about action potential amplitude and stimulus intensity?

• A. AP amplitude increases with stimulus intensity
• B. AP amplitude decreases with stimulus intensity
• C. AP amplitude plateaus while frequency remains fixed
• D. AP amplitude remains constant; frequency encodes stimulus intensity

Answer: (D)

The all-or-none principle says that once a neuron’s membrane potential reaches threshold, it fires an action potential with a fixed, standard amplitude and duration. This fixed size happens because the wave of depolarization activates a set, finite number of voltage-gated channels in a consistent way; after firing, those channels go through inactivation and recovery in roughly the same pattern, so the peak voltage reached is roughly the same each time. If the stimulus is below threshold, no spike is produced at all. Because the spike size is fixed, increasing stimulus strength does not make each spike bigger. Instead, stronger inputs make the neuron fire more often, up to a limit imposed by the refractory periods. This means stimulus intensity is encoded by the frequency of action potentials—the rate coding of firing—rather than by changing the amplitude of each spike. Subthreshold stimuli don’t trigger spikes, while supra-threshold stimuli produce spikes of the same height but at a higher rate. So, the amplitude remains constant; frequency encodes stimulus intensity.

The all-or-none principle says that once a neuron’s membrane potential reaches threshold, it fires an action potential with a fixed, standard amplitude and duration. This fixed size happens because the wave of depolarization activates a set, finite number of voltage-gated channels in a consistent way; after firing, those channels go through inactivation and recovery in roughly the same pattern, so the peak voltage reached is roughly the same each time. If the stimulus is below threshold, no spike is produced at all.

Because the spike size is fixed, increasing stimulus strength does not make each spike bigger. Instead, stronger inputs make the neuron fire more often, up to a limit imposed by the refractory periods. This means stimulus intensity is encoded by the frequency of action potentials—the rate coding of firing—rather than by changing the amplitude of each spike. Subthreshold stimuli don’t trigger spikes, while supra-threshold stimuli produce spikes of the same height but at a higher rate.

So, the amplitude remains constant; frequency encodes stimulus intensity.