The Action Potential Explained Step by Step

Where does the cell start? The resting membrane potential

At rest, a typical neuron sits at about -70 mV, negative inside compared with outside; the exact value varies between cell types. This resting membrane potential exists mainly because the membrane is more permeable to K+ at rest, and K+ tends to leak out down its concentration gradient.

The Na+/K+ ATPase keeps the gradients in place by pumping 3 Na+ out and 2 K+ in for each ATP used. Remember: high Na+ outside, high K+ inside.

The five phases step by step

1. Stimulus to threshold: a stimulus depolarises the membrane. If it reaches threshold (around -55 mV in a typical neuron), an action potential fires.
2. Depolarisation: voltage-gated Na+ channels open quickly. Na+ enters the cell and the inside becomes positive, peaking at roughly +30 mV.
3. Repolarisation: Na+ channels inactivate and voltage-gated K+ channels open. K+ leaves the cell and the potential falls back toward negative.
4. Hyperpolarisation: K+ channels close slowly, so extra K+ leaves and the membrane briefly dips below the resting value.
5. Return to rest: the K+ channels close and leak channels plus the pump restore the resting potential.

The key idea, as described in Guyton and Hall, is timing: sodium channels are fast to open and then inactivate, while potassium channels are slower to open and slower to close.

What are the refractory periods?

During the absolute refractory period, no second action potential can fire, however strong the stimulus, because the Na+ channels are inactivated. During the relative refractory period, a stronger than normal stimulus is needed, because some Na+ channels have recovered but K+ is still leaving the cell.

Refractory periods make the signal travel in one direction and limit how fast a neuron can fire.

Why is it called all-or-none, and how does it travel?

Once threshold is reached, the action potential has the same size every time. A stronger stimulus does not make a bigger action potential; it makes action potentials fire more often. This is the all-or-none principle.

The signal spreads along the axon as each depolarised segment brings the next segment to threshold. In myelinated axons the action potential jumps between nodes of Ranvier, which is called saltatory conduction and is much faster. You will see this again when comparing gray and white matter conduction.

How is this tested in exams?

Questions usually change one thing and ask you to predict the effect: blocking Na+ channels, changing extracellular K+, or applying a stimulus during the refractory period. Answer by asking which channel or gradient is affected. For lessons that walk through physiology topics for your programme and year, explore the Mojaz course catalogue.

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References

1. Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. 15th ed. Elsevier; 2026. Source
2. Costanzo LS. Costanzo Physiology. 8th ed. Elsevier; 2027. Source

Medically reviewed by: Nael Murad, MBBS, MSc in Human Anatomy · Anatomy & Physiology Instructor · Updated 27 Oct 2026

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