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Action Potential Simulations

An Exploration of Neuron Membrane Physiology

Introduction Phases Summation All-or-None Refractory Periods

Absolute and Relative Refractory Periods

Introduction

The all-or-none principle tells us that an action potential is a binary event. It either happens completely or it does not happen at all. Think of it like a light switch: it is either on or off, never halfway.

An action potential begins when a stimulus depolarizes the axon membrane to approximately −55 mV, the threshold potential.

Feedback Cycle

Reaching threshold opens the activation gates of nearby voltage-gated Na⁺ channels. Na⁺ then flows into the axon along its concentration gradient. This influx of positive charge further depolarizes the membrane, causing still more Na⁺ channels to open. This rapid, self-amplifying process is called the Na⁺ positive feedback cycle.

Na+ channel positive feedback cycle diagram

As the Na⁺ positive feedback cycle continues, nearby Na⁺ channels open rapidly. As a result, the membrane potential rises from about −55 mV to a peak of approximately +30 to +40 mV in less than a millisecond.

All-or-None

This is where the all-or-none principle becomes clear. A subthreshold stimulus, one that does not quite reach −55 mV, does not open enough voltage-gated Na⁺ channels to trigger the positive feedback cycle.

But once threshold is reached, a full action potential is triggered. A stronger stimulus does not produce a larger action potential. Whether the stimulus is just barely at threshold or far above it, the action potential has approximately the same amplitude and duration.

The neuron’s response is preset: all or none, every time.

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Analysis / Quiz
Strong (28 µA).
The action potential was triggered at approximately −55 mV, which is the threshold potential.
When the membrane reached threshold at the axon hillock, most voltage-gated Na⁺ channels in that region opened their activation gates, allowing Na⁺ ions to rush into the axon down their concentration and electrical gradients.
Reaching threshold triggered a positive feedback cycle. Opening Na⁺ channels allowed Na⁺ to flow in, which depolarized the membrane further. This opened more Na⁺ channels, allowing even more Na⁺ to enter. This self-amplifying cascade produced the sharp rising phase of the action potential.
The membrane potential peaked at approximately +40 mV.
Yes. Both stimuli produced action potentials with approximately the same peak amplitude.
Once threshold is reached, the neuron produces an action potential with approximately the same amplitude and duration, regardless of whether the stimulus is just above threshold or much stronger. This is the all-or-none principle. The response is either complete or absent. Increasing stimulus strength above threshold does not make the action potential larger.

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Reference and Attributions

BioPhysiology – “The Action Potential“.

Cellular Neurophysiology – “Synaptic Transmission in the Central Nervous System“.

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Wikipedia – “Summation (neurophysiology)“.