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Action Potential

For medical students2 min readUpdated 2026-10-10

Action potential (AP) is a rapid transient change in the cell membrane potential during propagating excitation. It occurs when depolarization reaches a critical threshold and is accompanied by an avalanche-like movement of ions.

AP AmplitudeApproximately 100 mV in a nerve fiber. It remains constant for any specific cell.

Resting Membrane PotentialApproximately -70 mV, maintained by the activity of the sodium-potassium pump.

Critical ThresholdDepolarization to -50 mV opens all voltage-gated sodium channels and triggers an impulse.

Stimuli and Local Response

For a cell to become excited, it must be acted upon by a stimulus. Stimuli are classified as subthreshold, threshold, and suprathreshold.

With a weak (subthreshold) stimulus, the resting potential decreases only slightly at the electrode site, producing a local response (LR). Its main feature is a direct proportionality: the stronger the stimulus, the higher the response amplitude.

If the stimulus reaches the threshold value, the local response reaches the critical firing level (threshold of depolarization). At this exact moment, the local membrane change transitions into a full-fledged action potential.

Dynamics and Phases of the Action Potential

The development of excitation involves several consecutive stages displayed on a graph:

Pre-spike (Depolarization prepotential): A slow partial depolarization corresponding to the local response.
Depolarization (Spike, peak): A rapid voltage surge. The potential shifts from the critical threshold (-50 mV) to positive values (e.g., +30 mV). This polarity reversal is called overshoot (the inner surface of the membrane becomes positive).
Repolarization: Voltage drops and returns toward the resting level. At the end of this phase, a negative after-potential (NAP) or after-depolarization may occur.
After-hyperpolarization: The potential drops below the baseline -70 mV. This is designated as the positive after-potential (PAP).

The duration and shape of these phases depend on the tissue type. In neurons and skeletal myocytes, the spike is very brief, and after-potentials are variable.

Ionic Mechanisms and Channel Function

All potential changes are driven by transmembrane tubular proteins known as ion channels.

Resting state: The membrane potential is -70 mV. Potassium channels are closed. Sodium channels are also closed but primed for activation: their activation m-gates are shut, and inactivation h-gates are open. Potassium concentration is higher inside the cell, while sodium is higher outside.
Onset of stimulation (LR): The m-gates begin to slowly open, but the inward sodium current remains negligible.
Depolarization phase: Upon reaching the threshold (-50 mV), all m-gates open. An avalanche-like, passive influx of Na⁺ ions begins down their concentration gradient.
Repolarization: The incoming positive charge triggers the closure of the h-gates (inactivation of sodium channels). Simultaneously, voltage sensors on potassium pores respond by opening. K⁺ ions rush outward, restoring the negative charge inside.
Hyperpolarization and restoration: Because potassium channels close slowly, K⁺ efflux continues (sometimes supplemented by Cl⁻ influx), driving the potential below -70 mV. Finally, the Na⁺/K⁺-ATPase actively pumps ions back to baseline. Sodium channels reset (closing m-gates and opening h-gates), and the cell is ready for a new impulse.

The All-or-None Law

This law reflects the constancy of the action potential amplitude. Its core principle is simple:

A subthreshold stimulus produces no impulse ("none").
A threshold or suprathreshold stimulus generates an AP of the maximum possible amplitude for that specific cell ("all").

Single structures obey this law: neurons, individual nerve and muscle fibers, smooth muscle, and cardiac tissue. However, whole nerve trunks or skeletal muscles do not obey it because they consist of numerous fibers with varying excitation thresholds.

Mnemonic

How to remember sodium channel gates at rest: M (m-gates) Make you stay out (closed), while H (h-gates) are Hanging wide open in the wind.

Frequently asked questions

How does a local response differ from an action potential?

A local response occurs in response to a subthreshold stimulus, whereas an action potential occurs in response to a threshold or suprathreshold stimulus. Unlike an AP, a local response cannot propagate over significant distances (spreads with decrement and rapidly decays), does not obey the all-or-none law (its amplitude varies gradedly with stimulus strength), is capable of summation, and lacks a refractory period.

Feature	Local Response	Action Potential
Stimulus	Subthreshold	Threshold / Suprathreshold
Propagation	Does not propagate / decremental	Propagates without decrement
Summation	Possible	Impossible
Law	Graded / Strength	All-or-none
Refractoriness	Absent	Present

What is the conduction velocity of action potentials in myelinated and unmyelinated nerve fibers?

Conduction velocity depends on myelination and fiber diameter: myelinated fibers conduct faster than unmyelinated ones, and thick fibers faster than thin ones. In myelinated fibers, conduction is saltatory, resulting in a higher propagation velocity than continuous conduction.

Myelinated fibers: Includes type A and B fibers. The fastest are type Aα fibers, with a velocity of approximately 100 m/s; A-δ fibers conduct at 4–30 m/s.
Unmyelinated fibers: Includes type C fibers. They are the slowest: approximately 1 m/s (ranges cited include 0.5–2 m/s).

What is absolute refractoriness, and during which phase of the AP does it occur?

Absolute refractoriness is a period of complete membrane unexcitable state during which generating a new action potential is impossible; the stimulation threshold approaches infinity, and excitability drops to zero. It is caused by the temporary inactivation of the majority of Na⁺ channels.

Absolute refractoriness occurs during the spike: the depolarization phase and, according to data on muscle contraction, the early repolarization phase of the AP.

What is the role of the sodium-potassium pump during spike generation?

The pump itself does not participate in the rapid phases of depolarization and repolarization. Its role is to restore original ion concentrations after the action potential has already completed.

Why doesn't the AP amplitude increase with a stronger stimulus?

In accordance with the all-or-none law, a threshold stimulus already opens all available sodium pores, producing the maximum possible ion current. The cell simply cannot respond any more strongly.

Go deeper

AP characteristics in cardiomyocytes and smooth muscle
Role of chloride ions in forming after-hyperpolarization
Mechanism of voltage-sensing charge receptors in potassium channels
Constitutively open potassium leak channels and maintenance of resting potential