Action Potential
The Hodgkin-Huxley model (Nobel Prize 1963) describes how neurons fire. A brief depolarizing stimulus triggers a cascade: voltage-gated Na⁺ channels open, driving V toward +40 mV; they then inactivate as K⁺ channels open, repolarizing the membrane. Click the axon to stimulate.
Hodgkin-Huxley equations
The membrane capacitance Cm·dV/dt = I_stim − gNa·m³·h·(V−ENa) − gK·n⁴·(V−EK) − gL·(V−EL). Each gate variable (m, h, n) follows dX/dt = α_X(V)(1−X) − β_X(V)·X, where α and β are voltage-dependent rate constants measured in squid giant axons by Hodgkin and Huxley in 1952.
Key reversal potentials: ENa ≈ +55 mV (Na⁺ rushes in during upstroke), EK ≈ −77 mV (K⁺ rushes out during repolarization), EL ≈ −54.4 mV (leak). The absolute refractory period (h gate inactivated) prevents re-stimulation for ~2 ms.
Temperature scales the rates by Q₁₀ = 3 per 10°C above 6.3°C. Higher temperature → faster, narrower spikes and higher maximum firing rate.