FitzHugh-Nagumo equations: ∂u/∂t = Du∇²u + u - u³/3 - v, ε·∂v/∂t = Dv∇²v + u + a - bv
The variable u is the fast activator (membrane voltage analogue), v is the slow inhibitor (recovery). Together they produce excitable medium dynamics. Spiral waves self-sustain indefinitely; Turing patterns arise when the inhibitor diffuses much faster than the activator (large Dv/Du ratio). Adjust parameters to explore different regimes.