The axon is modelled as a 1-D excitable cable: a chain of membrane patches, each obeying FitzHugh-Nagumo kinetics (a simplified Hodgkin-Huxley reduction), diffusively coupled to its neighbours exactly like current spreading passively through the axoplasm.
dV/dt = V - V^3/3 - W + I_ext + D*(V_i-1 + V_i+1 - 2V_i)
dW/dt = eps*(V + a - b*W)
a=0.7 b=0.8 eps=0.08
V is membrane voltage (mapped to mV for the readout), W is a slow recovery variable standing in for K+ channel inactivation, D is the internal-resistance coupling between patches, and I_ext is the injected stimulus. When I_ext pushes V past the cubic nullcline's unstable branch, a fast spike is regenerated, and diffusive coupling drags neighbouring patches over threshold in turn - this is what makes the pulse travel rather than just decay.
- Stimulus current - sets I_ext injected at the axon hillock; below ~0.35 nA no spike is triggered (subthreshold).
- Axon type - toggles the coupling constant D between unmyelinated (slow, continuous) and myelinated (large D at nodes, fast saltatory jumps) conduction.
- Fire pulse - injects one stimulus at t=0; Auto-fire repeats it once the refractory period ends.
- Orbit-drag rotates the view; scroll zooms along the axon.
Myelin sheaths (formed by oligodendrocytes/Schwann cells) insulate the membrane between nodes of Ranvier, forcing the regenerative current to "jump" node-to-node - this saltatory conduction is why myelinated axons carry signals 10-100x faster than bare ones, and why demyelinating diseases like MS slow or block transmission.