FitzHugh–Nagumo–Rinzel — dx/dt=x−x³/3−y−z+I, dy/dt=ε(x+a−b·y), dz/dt=μ(x−γ−z). Classic a=0.7 b=0.8 ε=0.08 I=0.5 μ=0.1 γ=−0.775. Dual I+ε sliders; a/b/μ/γ fixed. Display embed (x,y,z). Distinct from lotka-volterra.html, brusselator.html, operon.html, hindmarsh-rose.html, morris-lecar.html, arneodo.html. Bulk centroid ≈ (−0.5236, 0.2212, 0.2519). RK4 neuron fold. Gold low-alpha trail, cap 650. RM = settled silhouette, no animate-on.