[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Nerve and muscle](https://durgical.com/en/physiology/#s-nerve_muscle) › The nerve action potential

[Physiology](https://durgical.com/en/physiology/) · [Nerve and muscle](https://durgical.com/en/physiology/#s-nerve_muscle)

# The nerve action potential

- Mechanism
- Exam priority: high

A nerve signal is a brief, all-or-none flip of the voltage across the cell membrane, driven by sodium rushing in and potassium leaving.

Updated 2026-11-01

Also called: Nerve impulse, Spike

**See it in 3D:** [Sciatic nerve](https://durgical.com/en/anatomy/sciatic-nerve/) · [Median nerve](https://durgical.com/en/anatomy/median-nerve/)

## Key points

- All-or-none: once threshold (≈ −55 mV) is crossed, the spike has the same amplitude; stronger stimuli raise the firing rate.
- Upstroke = Na⁺ influx through voltage-gated Na⁺ channels (positive feedback); repolarization = Na⁺ channel inactivation plus K⁺ efflux.
- The Na⁺/K⁺-ATPase does not make the spike; it restores the gradients over many spikes. Very few ions actually cross per spike.
- After-hyperpolarization: slow K⁺ channels stay open, so the membrane briefly sits closer to E_K (≈ −90 mV).
- Local anesthetics block voltage-gated Na⁺ channels from inside the cell and work best on fibers that fire often (use-dependent block).

## What it is

An action potential is a fast, stereotyped reversal of membrane potential that a neuron uses to send information along its axon. Voltage-gated Na⁺ channels open first and make the inside positive; slower K⁺ channels then bring the voltage back down. Because the size never changes, the nervous system codes stimulus strength as firing frequency, not as spike height.

## How it works, step by step

- At rest the membrane sits near −70 mV because it is far more permeable to K⁺ than to Na⁺.
- A stimulus (synaptic current, receptor potential) depolarizes the axon hillock toward threshold.
- At threshold enough voltage-gated Na⁺ channels open that Na⁺ entry outruns K⁺ exit: depolarization feeds itself.
- The membrane overshoots to about +30 mV, approaching but never reaching E_Na (≈ +60 mV).
- Within about 1 ms the Na⁺ channels inactivate (the inactivation gate closes), stopping further Na⁺ entry.
- Voltage-gated K⁺ channels, which open more slowly, let K⁺ leave and drive repolarization.
- K⁺ channels close late, giving a short after-hyperpolarization before the membrane returns to rest.

### Action potential

**Rest** −70 mV **Threshold** −55 mV **Peak** +35 mV **Duration** ≈ 1–2 ms

Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.

- Mainly Na⁺
- Mainly K⁺
- Rest
- Threshold
- ARP: absolute refractory period
- RRP: relative refractory period

Tap each phase to see which channel is driving the voltage.

## Equations

**Nernst potential (37 °C)**Eion = (61.5 / z) × log10([ion]out / [ion]in) mV

**z**
ion valence (+1 for Na⁺ and K⁺)

**[ion] out, [ion] in**
extracellular and intracellular concentrations

With K⁺ ≈ 4 mM outside and ≈ 140 mM inside, E_K ≈ −94 mV; Na⁺ ≈ 142 mM outside and ≈ 14 mM inside gives E_Na ≈ +61 mV.

## Typical values (large mammalian neuron)

| Parameter | Value | Note |
| --- | --- | --- |
| Resting membrane potential | ≈ −70 mV | −70 to −90 mV depending on the cell |
| Threshold | ≈ −55 mV | about 15–20 mV above rest |
| Peak (overshoot) | ≈ +30 to +40 mV | |
| Spike duration | ≈ 1–2 ms | |
| E_K / E_Na | ≈ −90 mV / ≈ +60 mV | |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Clinical notes

- Local anesthetics block Na⁺ channels: impulses stop in the blocked segment, so pain does not reach the spinal cord.
- Tetrodotoxin (puffer fish) and saxitoxin (shellfish) block Na⁺ channels from outside and can cause fatal paralysis.

## Memory aid

Sodium starts, potassium parks: Na⁺ opens the door to the spike, K⁺ closes the show.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does a stronger painful stimulus not produce a bigger action potential in a nociceptor axon?

The spike is all-or-none: once threshold is crossed, Na⁺ channel positive feedback runs to completion. Intensity is coded as higher firing frequency and recruitment of more fibers.

## What ends the upstroke of the action potential, given that E_Na is never reached?

Na⁺ channel inactivation (the inactivation gate closes within about 1 ms) together with the delayed opening of voltage-gated K⁺ channels, which push the voltage back toward E_K.

## If the Na⁺/K⁺-ATPase is poisoned, why can a neuron still fire many action potentials?

Each spike moves only a tiny fraction of the ions present, so the gradients run down slowly. The pump restores them over time; it does not generate the spike itself.

## Why do local anesthetics block an actively firing pain fiber faster than a quiet one?

They reach their binding site from inside the channel and bind best to open or inactivated Na⁺ channels. Frequent firing keeps channels in those states longer (use-dependent block).

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/the-nerve-action-potential/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Sciatic nerve](https://durgical.com/en/anatomy/sciatic-nerve/)
- [Median nerve](https://durgical.com/en/anatomy/median-nerve/)

## Related topics

- [The resting membrane potential](https://durgical.com/en/physiology/the-resting-membrane-potential/)
- [The Nernst and Goldman equations](https://durgical.com/en/physiology/the-nernst-and-goldman-equations/)
- [Refractory periods and excitability](https://durgical.com/en/physiology/refractory-periods-and-excitability/)
- [Impulse conduction and myelin](https://durgical.com/en/physiology/impulse-conduction-and-myelin/)

## In Biochemistry

- [Na⁺/K⁺-ATPase (sodium pump)](https://durgical.com/en/biochemistry/enzymes/na-k-atpase/)

## In Pharmacology

- [Local anesthetics](https://durgical.com/en/pharmacology/classes/local-anesthetics/)
- [Lidocaine](https://durgical.com/en/pharmacology/lidocaine/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [12.4 The Action Potential](https://openstax.org/books/anatomy-and-physiology-2e/pages/12-4-the-action-potential) (OpenStax (Rice University), 2022); [3.1 The Cell Membrane](https://openstax.org/books/anatomy-and-physiology-2e/pages/3-1-the-cell-membrane) (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 5). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/the-nerve-action-potential/