Skip to main content
Neuroscience · Process

Action Potential Diagram

How a neuron fires: from rest at −70 mV to threshold, sodium rushing in, potassium flowing out, the undershoot and back, the spike itself over in 1 to 2 ms.

An action potential is the brief electrical signal a neuron sends along its axon. This graph follows the voltage across one patch of membrane as the signal passes, phase by phase, with the ion channels that cause each phase. The numbers are those of a typical neuron; they vary from cell to cell.

Also called Nerve impulse or Spike.

A graph of membrane potential in millivolts against time in milliseconds for an action potential. The potential rests at minus 70 millivolts, rises to the threshold of minus 55, shoots up to a peak of about plus 35 as sodium channels open, falls back as potassium channels open, dips below rest to about minus 82, and returns to rest by about 4.5 milliseconds. Brackets under the time axis mark the absolute and relative refractory periods.+40+200−20−40−60−80012345Membrane potential (mV)Time (ms)Threshold, −55 mVResting potential, −70 mVheld by K⁺ leak and the Na⁺/K⁺ pumpDepolarisationNa⁺ channels openPeak, about +35 mVRepolarisationK⁺ channels openHyperpolarisationAbsolute refractory periodRelative refractory period
View

Step 1 of 9

The action potential at a glance

The graph shows the voltage across a neuron's membrane as an action potential passes: a fast rise and fall that take only 1 to 2 milliseconds, then a brief dip below rest that lasts a few milliseconds more. Sodium flowing in causes the rise; potassium flowing out causes the fall.

Use ← and → to move between steps.

Step 1 of 9: The action potential at a glance

Download the diagramChecked against its sources on 25 October 2026.

Step by step

The steps of the Action Potential

  1. 01The action potential at a glance

    The graph shows the voltage across a neuron's membrane as an action potential passes: a fast rise and fall that take only 1 to 2 milliseconds, then a brief dip below rest that lasts a few milliseconds more. Sodium flowing in causes the rise; potassium flowing out causes the fall.

  2. 02Resting potential

    At rest the inside of the neuron is about −70 mV relative to the outside. Potassium leaks out through channels that are always open, and the sodium-potassium pump keeps the gradients, moving 3 Na⁺ out for every 2 K⁺ in.

  3. 03A stimulus reaches threshold

    Input from other neurons makes the inside less negative. If it reaches threshold, about −55 mV, enough voltage-gated sodium channels open to start an action potential. Below threshold nothing follows, so the signal is all or none.

  4. 04Depolarisation: sodium rushes in

    Voltage-gated sodium channels open, and Na⁺ rushes in down its gradient. Each opening depolarises the membrane further and opens more channels, so the potential shoots up to about +35 mV in under a millisecond.

  5. 05The peak

    Within about a millisecond the sodium channels close by inactivation, and voltage-gated potassium channels, which respond more slowly, open. The rise stops at about +30 to +40 mV.

  6. 06Repolarisation: potassium flows out

    With the potassium channels open, K⁺ flows out of the cell, and the potential falls back towards rest.

  7. 07Hyperpolarisation: the undershoot

    The potassium channels close slowly, so for a moment more K⁺ leaves than at rest and the membrane dips below −70 mV, to about −80 mV.

  8. 08The refractory periods

    While the sodium channels are inactivated, no stimulus can fire another action potential: the absolute refractory period. During the undershoot only a stronger stimulus than usual can: the relative refractory period. This is why the signal travels one way along the axon.

  9. 09Back to rest

    As the potassium channels close, the leak channels bring the membrane back to rest, ready to fire again; the pump works over the long run, not spike by spike. The signal has moved on to the next patch of membrane along the axon.

Parts

What each part of the action potential does

Resting potential
About −70 mV inside compared with outside. Potassium leaks out through channels that are always open, leaving the inside negative.
Sodium-potassium pump
Moves 3 Na⁺ out for every 2 K⁺ it brings in, using ATP. It keeps the concentration gradients that every action potential runs on.
Threshold
About −55 mV. A stimulus that brings the membrane this far opens enough sodium channels to set off an action potential; a smaller one dies away.
Depolarisation
The fast rise, as sodium ions rush in through voltage-gated sodium channels and the inside turns positive.
Voltage-gated sodium channels
Open quickly when the membrane reaches threshold, then shut by an inactivation gate within about a millisecond, and cannot open again until the membrane is back near rest.
Peak
About +30 to +40 mV, as the sodium channels close and the potassium channels open.
Repolarisation
The fall, as potassium ions flow out through voltage-gated potassium channels and the inside turns negative again.
Voltage-gated potassium channels
Open more slowly than the sodium channels, at about the time of the peak, and close slowly, which causes the undershoot.
Hyperpolarisation
The undershoot below rest, to about −80 mV, while the slow potassium channels are still open.
Refractory periods
In the absolute refractory period no stimulus can fire the cell again; in the relative one only a stronger stimulus than usual can. This keeps the signal moving one way.

Dictionary

The terms to know

Practise it

The quizzes and cards that fit

In 3D

The molecules, to turn and explore

Next to it

More diagrams

Sources

Where the facts come from

Drawn and written by biofideX from these sources.

Cookie Notice

We use cookies

biofideX uses cookies and similar technologies to keep you signed in, remember your preferences, and (with your consent) measure how the platform is used. Change your choices any time from the Cookie Policy.