The top axon has no myelin: every patch of membrane must open its own voltage-gated channels to regenerate the impulse, so the glow crawls continuously along the whole length.
The bottom axon is wrapped in insulating myelin sleeves, exposed only at the nodes of Ranvier (the bright rings). Current leaps passively — and far faster — through each insulated internode, so the signal appears to jump node-to-node ("saltatory conduction").
- Lesion extent — how many internodes near the centre are diseased.
- Severity — how much insulation is stripped from those internodes. A stripped internode conducts no faster than bare membrane, and a severely stripped one can fail to relay the signal at all — a real conduction block, exactly what a clinical nerve-conduction study detects as slowed or absent velocity.
Fire the pulse to race both axons over an identical distance and watch the timers and the live velocity readout respond to the damage you dial in.