Action Potential Calculators

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An action potential is a rapid, transient reversal of the electrical potential across a neuron's plasma membrane — the fundamental electrical signal by which neurons communicate over long distances. Triggered when the membrane potential reaches the threshold (~−55 mV in most neurons), the action potential proceeds through stereotyped phases of depolarization, repolarization, and hyperpolarization driven by the sequential opening and closing of voltage-gated Na⁺ and K⁺ channels. Action potentials propagate without decrement along axons and are transmitted to other cells at synapses, enabling all neural computation and muscle contraction.

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Resting Membrane Potential

At rest, the inside of a neuron is ~−70 mV relative to outside (negative inside). Maintained by:

  • K⁺ leak channels — K⁺ diffuses out along its concentration gradient
  • Na⁺-K⁺ ATPase — pumps 3 Na⁺ out and 2 K⁺ in per cycle, maintaining gradients
  • High intracellular K⁺ (~150 mM inside vs. ~5 mM outside)
  • High extracellular Na⁺ (~145 mM outside vs. ~12 mM inside)

Phases of the Action Potential

1. Threshold (−55 mV)

Sufficient depolarizing stimulus opens enough voltage-gated Na⁺ channels to trigger the positive feedback cycle — the action potential fires all-or-nothing.

2. Rising Phase (Depolarization)

Voltage-gated Na⁺ channels open rapidly — Na⁺ rushes in along both electrical and concentration gradients. Membrane potential rises from −70 mV to about +30–40 mV in ~1 ms.

3. Falling Phase (Repolarization)

Na⁺ channels inactivate (fast inactivation gate closes). Voltage-gated K⁺ channels open (slower) — K⁺ rushes out. Membrane potential returns toward resting.

4. Undershoot (Hyperpolarization)

K⁺ channels close slowly — membrane transiently hyperpolarizes below resting potential (~−80 mV). Prevents immediate re-firing (absolute + relative refractory period).

Propagation

Action potentials propagate along unmyelinated axons by local current spread. In myelinated axons, propagation jumps between nodes of Ranvier (saltatory conduction) — dramatically increasing conduction velocity (up to 120 m/s vs. ~1 m/s unmyelinated).

Glossary

Action Potential
A rapid, transient reversal of membrane potential (~−70 mV to +30–40 mV) driven by sequential opening of voltage-gated Na⁺ and K⁺ channels. The all-or-nothing electrical signal neurons use for long-distance communication.
Threshold Potential
The membrane potential (~−55 mV in most neurons) at which enough voltage-gated Na⁺ channels open to trigger the self-reinforcing depolarization of an action potential. Stimuli below threshold cause only graded potentials that fade without propagating.
Saltatory Conduction
The propagation of action potentials by jumping between nodes of Ranvier in myelinated axons. Dramatically faster than continuous propagation in unmyelinated axons (up to 120 m/s vs. ~1–2 m/s) and more energy-efficient.

Frequently Asked Questions

An action potential is a rapid reversal of membrane potential (~−70 mV to +30–40 mV) triggered when a depolarizing stimulus reaches threshold (~−55 mV). Voltage-gated Na⁺ channels open — Na⁺ rushes in (depolarization). Na⁺ channels inactivate; K⁺ channels open — K⁺ rushes out (repolarization). Membrane hyperpolarizes slightly then returns to resting. The entire process takes ~1–3 milliseconds. Action potentials are all-or-nothing — either full amplitude fires, or nothing.

Resting: ~−70 mV. Threshold: depolarization to ~−55 mV triggers Na⁺ channel opening. Depolarization (rising phase): Na⁺ influx drives membrane to +30–40 mV. Repolarization (falling phase): Na⁺ channels inactivate; K⁺ channels open, driving membrane back negative. Hyperpolarization (undershoot): K⁺ channels close slowly, membrane temporarily below resting (~−80 mV). Resting potential restored by K⁺ leak channels and Na⁺-K⁺ ATPase.

The absolute refractory period occurs while Na⁺ channels are inactivated (during and just after the action potential) — no stimulus of any strength can trigger another action potential. The relative refractory period follows, during hyperpolarization — a larger-than-normal stimulus can fire another action potential, but with higher threshold. Refractory periods limit maximum firing rate (~500–1,000 Hz), ensure unidirectional propagation (can't travel backward because channels behind are inactivated), and set the temporal resolution of neural coding.

In myelinated axons, myelin sheaths insulate the axon between nodes of Ranvier — ion channels are concentrated only at the nodes. Action potentials 'jump' from node to node (saltatory conduction from Latin saltare = to jump) rather than propagating continuously along the membrane. This dramatically increases conduction velocity (up to 120 m/s for thick myelinated axons vs. ~0.5–2 m/s for unmyelinated) and reduces the energy cost — fewer Na⁺-K⁺ pumps needed to restore ion gradients.