The reasoning: a conductance sequence with positive feedback
Hodgkin–Huxley model: voltage‑gated Na⁺ channels have an activation gate (m, fast) and an inactivation gate (h, slower); delayed‑rectifier K⁺ channels have activation (n). Threshold is the voltage at which inward Na⁺ current exceeds outward K⁺/leak current — a regenerative (positive‑feedback) upstroke follows.