The FitzHugh-Nagumo model (FHN) is a simplified version of the Hodgkin-Huxley for the action potential in giant squid axon.
$$\dot v = v - v^3/3 - w + I(t)$$ $$\dot w = 0.08 (v + 0.7 - 0.8w)$$The variable v is the membrane potential, w is a recovery variable, and I(t) is the intensity of a stimulus current.