The Hodgkin-Huxley model (1952 Nobel Prize) describes the action potential using four coupled ODEs: membrane voltage V, and gating variables m (Na activation), h (Na inactivation), and n (K activation). The inward sodium current (blue) rapidly depolarizes the membrane, while the delayed potassium current (green) repolarizes it. The total current I = gNa·m³h·(V−ENa) + gK·n⁴·(V−EK) + gL·(V−EL). The threshold arises from a positive feedback loop: depolarization opens Na channels, which further depolarizes — creating an all-or-nothing spike.