A patch of nerve membrane with its sodium and potassium channels, running the 1952 Hodgkin–Huxley equations in your browser, about a thousand times slower than life. Push it, poison it, block it, and watch the channels open, inactivate and close.
×1,000 slowerOUTSIDEINSIDE
Na⁺K⁺
This browser could not start the 3D view. The model and the voltage trace below still run.
Voltage trace computed from the Hodgkin–Huxley model; ion particles illustrate the direction of flow.
Membrane now−70 mV
Spike?no
EK (Nernst)−82 mV
How hard each zap pushes (0.3 ms long). Below threshold: no spike.
Tetrodotoxin (TTX)
Pufferfish toxin: it plugs voltage-gated Na⁺ channels at their outer mouth.
Plugs voltage-gated K⁺ channels: spikes end slowly. Block about half or more and this model can no longer hold its rest: it fires on its own.
Changes EK by the Nernst equation: watch the resting potential.
Drag the membrane to turn it. Each zap is a 0.3 ms current pulse; the model's threshold for it is about 25 µA/cm².
Four things to try
Na⁺ in, then K⁺ out
Zap at 40. Gold Na⁺ channels open first (activation gate), then their inactivation "ball" swings shut while violet K⁺ channels open and K⁺ flows out.
Try: switch to ×5,000.
TTX: no Na⁺, no spike
With every Na⁺ channel plugged, a zap only nudges the voltage. Even the strongest zap cannot start an action potential.
Try: TTX on, zap at 120.
Block K⁺: a slow fall
Na⁺ still drives the spike up, but with most K⁺ channels plugged the membrane repolarizes slowly: the spike lasts two to three times longer. With too little K⁺ current the model cannot hold its rest and starts firing on its own.
Try: 80% block, then zap.
More K⁺ outside
Raising extracellular K⁺ shrinks the K⁺ gradient: EK and the resting potential move up, closer to threshold. Raise it a lot and Na⁺ channels inactivate.
In a neuron at 37 °C, [K+] is 140 mM inside and 4 mM outside, so its K+ equilibrium potential is E_K = 61.5 mV × log10(4/140) ≈ −95 mV. If extracellular [K+] rises to 8 mM while intracellular [K+] stays at 140 mM, E_K becomes about:
Pick one. The red pen does the rest.
The deciding lineThe Nernst equation for K+ at 37 °C is E_K = 61.5 mV × log10([K+]out/[K+]in); E_K depends on the logarithm of the concentration ratio, so doubling [K+]out shifts it by 61.5 × log10 2 ≈ 18.5 mV toward zero.
Why C: 61.5 × log10(8/140) = 61.5 × (−1.243) = −76 mV. Equivalently, doubling [K+]out adds 61.5 × log10 2 = 18.5 mV to −95 mV: the smaller K+ gradient makes E_K less negative.
Why each choice fails
AScale slip: doubling [K+]out does not double E_K. The potential tracks the logarithm of the ratio, and a smaller K+ gradient makes it less negative: −95 + 18.5 ≈ −76 mV, not 2 × (−95) = −190 mV.
BDirection reversed: the step size, 18.5 mV, is right, but raising [K+]out shrinks the K+ gradient, so E_K moves toward zero (−76 mV), not away from it (−113 mV).
CHolds: 61.5 × log10(8/140) = −76 mV, 18.5 mV less negative than −95 mV.
DScale slip: the K+ gradient does halve (140/4 = 35 to 140/8 = 17.5), but E_K depends on the logarithm of that ratio, not the ratio itself, so it shifts by only 61.5 × log10 2 = 18.5 mV, to −76 mV, not to −95/2 ≈ −47 mV.