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 · Median nerve
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 mVThreshold −55 mVPeak +35 mVDuration ≈ 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
Equations
- 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.
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).
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- The resting membrane potential
- The Nernst and Goldman equations
- Refractory periods and excitability
- Impulse conduction and myelin
In Biochemistry
In Pharmacology
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 (OpenStax (Rice University), 2022); 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.