V -65 mV INa 0 IK 0 µA/cm² Na open 0 % K open 0 % t 0 ms FPS 0
Orange channels pass sodium (Na+ flows in), blue channels pass potassium (K+ flows out). A plug sits on a closed channel; each open channel flickers at random, and the average open fraction is m³h for sodium and n⁴ for potassium. Ion count and speed follow the driving force. Schematic: not to scale. Drag to orbit, wheel or pinch to zoom.
Squid Axon Voltage Clamp: Hodgkin-Huxley Gating, Ionic Currents and the I-V Curve
Hold the membrane at a fixed voltage, step it, and watch sodium and potassium channels open and close. Then switch to current clamp and see the same channels make an action potential.
Voltage clamp Current clamp
Pause Restart protocol Loop: on Quality: High
Protocol
Drugs and temperature
Apply TTX (block Na) Apply TEA (block K) Wash out
Membrane potential Ionic currents (inward is negative) Gating variables Current-voltage curve (voltage-clamp steps) Peak inward INa (blue) and steady IK (orange) at the end of an 8 ms step from the holding voltage. The dip of the Na curve crosses zero near ENa .
Live readout
Experiment: threshold and firing rate
Current-clamp sweep of a steady injected current for 150 ms with the present drug and temperature settings (same equations, a fresh run per row). The bisection estimates the smallest current that fires at least one spike.
Run current sweep
Independent numerical checks
Controls Voltage clamp: choose the holding voltage, the step voltage and the step length. Currents are what the clamp amplifier would have to supply. Current clamp: inject a current step and let the voltage run free. TTX removes the sodium channels, TEA the potassium channels, just as in the original experiments. Playback speed is in milliseconds of membrane time per second of real time. The science
Membrane equation. Cm dV/dt = Iinj − gNa m³h (V − ENa ) − gK n⁴ (V − EK ) − gL (V − EL ) with Cm = 1 µF/cm², gNa = 120, gK = 36, gL = 0.3 mS/cm², ENa = +50, EK = −77, EL = −54.4 mV (the standard parameter set of the squid giant axon at 6.3 °C, in the modern convention where rest is near −65 mV).
Gates. Each gate x ∈ {m, h, n} obeys dx/dt = αx (V)(1 − x) − βx (V) x = (x∞ − x)/τx . At a fixed voltage this has the exact solution x(t) = x∞ + (x0 − x∞ ) e−t/τx , which is why a voltage clamp is such a clean experiment: the current is a product of known exponentials. Sodium has fast activation (m) and slower inactivation (h); potassium has slow activation (n), which is why the potassium current lags the sodium current and ends the spike.
Drugs and temperature. TTX scales gNa , TEA scales gK . Temperature multiplies every rate by φ = Q10 (T − 6.3)/10 with Q10 = 3 (the value used in the original paper).
Channel picture. The 3D panel draws individual channels as two-state switches whose average open fraction is exactly m³h (Na) or n⁴ (K). That is a visualisation choice: real channels are not literally built from 3 + 1 independent gates, and the numbers of channels and ions drawn are for show.
Simplifications. Space-clamped (no propagation along the axon), constant ion concentrations, one membrane compartment, no pumps, no capacitance current during the clamp step (an ideal clamp). The Nernst check uses typical textbook squid concentrations; the model's reversal potentials are the round standard values, not a fit to those numbers.
Educational model of the classic Hodgkin-Huxley equations; not a medical tool.