Excitable media (cardiac tissue, chemical BZ reaction) support spiral waves — rotating patterns that can lead to cardiac arrhythmia (reentry). The FitzHugh-Nagumo model: ∂u/∂t = D·∇²u + u(u-a)(1-u) - v, ∂v/∂t = (u - bv)/τ. Click to stimulate. A broken wave front curls into a self-sustaining spiral.