Ion Channel Biophysics

Hodgkin-Huxley Action Potential

The full ionic model: voltage-gated Na+ and K+ channels with gating variables (m, h, n), pharmacological blockade (TTX, TEA), and clinical correlates from pufferfish poisoning to demyelination.

Presets

Stimulus

Injected Current (µA/cm²)Pulse Onset (ms)Pulse Duration (ms)Duration (ms)

Conductances

Max gNa (mS/cm²)Max gK (mS/cm²)Leak gL (mS/cm²)

Reversal Potentials

E_Na (mV)E_K (mV)E_L (mV)

Pharmacology

TTX Block (%)TEA Block (%)

Membrane voltage

Action Potential Waveform

Summary

Spike metrics

Spikes

Peak voltage

39.4 mV

Na+ block

K+ block

Pharmacology

No pharmacological blockade applied.

Action potential phases

Resting

Membrane sits near -65 mV, set by K+ leak and the Na+/K+ ATPase. Gates: m low, h high, n moderate.

Depolarization

Stimulus raises V past threshold → m gates open fast → Na+ rushes in → positive feedback drives the upstroke to ~+40 mV.

Repolarization

h gates close (Na+ inactivation) while n gates open (delayed K+ rectifier) → outward K+ current pulls V back down.

Undershoot

K+ channels are still open after Na+ channels close → V dips below resting (afterhyperpolarization) before n gates relax.

Recovery

h gates de-inactivate and n gates close → membrane returns to resting, ready for the next spike.

What to notice

The model produces 1 spike with a peak of 39.4 mV.
Watch the gating variables: m (fast activation) leads, h (inactivation) follows, n (K+ activation) is slowest.
The Na+ current (blue) is brief and large; the K+ current (red) is slower and sustains repolarization.
The afterhyperpolarization sets the relative refractory period: another spike requires stronger stimulus during this window.

Biological analogies

gNa

Density of voltage-gated Na+ channels. Reduced in demyelination or by TTX/local anesthetics.

Density of delayed-rectifier K+ channels. Blocked by TEA and 4-aminopyridine.

Background leak conductance. Increases with membrane damage or demyelination.

m gate

Na+ activation — opens within ~0.1 ms of depolarization. The fastest gate.

h gate

Na+ inactivation — closes with ~1 ms delay, creating the absolute refractory period.

n gate

K+ activation — opens with ~1 ms delay, drives repolarization and the afterhyperpolarization.

TTX

Tetrodotoxin (pufferfish toxin) — plugs Na+ channel pore from outside. Also models lidocaine mechanism.

TEA

Tetraethylammonium — blocks K+ channels from outside. Reveals the Na+ component in isolation.