How a Neuron Fires — the Action Potential

Concept — the all-or-none action potential

A neuron signals by briefly flipping the voltage across its membrane. At rest the inside sits at about −70 mV. A stimulus that pushes the membrane to the threshold (−55 mV) triggers a self-reinforcing spike: voltage-gated Na⁺ channels snap open and Na⁺ floods in, driving Vm up toward +40 mV (depolarization). Those Na⁺ channels then inactivate while slower K⁺ channels open and K⁺ flows out, pulling Vm back down (repolarization) and briefly past rest to ≈ −80 mV (hyperpolarization / refractory) before settling. The spike is all-or-none: below threshold nothing fires; at or above threshold every spike has the same size.

What the animation shows

Left: a membrane cross-section with one voltage-gated Na⁺ channel and one K⁺ channel. Ion motion is tied to the gate state — Na⁺ always moves in, K⁺ always out; only how open each gate is (g_Na, g_K) changes with phase, and the membrane charge (+/−) flips as Vm crosses 0. Right: a chart-recorder plotting Vm against time, with a marker on the same clock. Move the stimulus slider below +15 mV to see a sub-threshold bump that fails to fire.

Quantities

• V_m — membrane potential (mV): −70 rest, −55 threshold, +40 peak, −80 undershoot.
• g_Na, g_K — relative conductances (0–1) = how open each voltage-gated channel is.
• t — time since the stimulus (ms); a full spike + afterhyperpolarization lasts ≈ 5 ms.
• E_Na = +61 mV, E_K = −89 mV — Nernst equilibrium potentials; E_K recomputes live as you change [K⁺]ₒ.
• F = 96485 C/mol — Faraday constant; RT/F = 26.7 mV at 37 °C.

Sources

Hodgkin & Huxley, J. Physiol. 117 (1952); Kandel et al., Principles of Neural Science 5th ed.; Purves et al., Neuroscience; Nernst equation; CODATA 2018 (F, R). Vm(t) is an idealised teaching waveform, not a recorded trace.