An action potential regenerates itself patch by patch along bare membrane, but a myelin sheath insulates most of the axon and lets current flow passively (almost instantly) beneath it. Regeneration only happens at the exposed Nodes of Ranvier, so the spike appears to "jump" node-to-node — saltatory conduction — which is far faster than continuous conduction along an unmyelinated fibre of the same diameter.
v_myelinated ≈ L_internode / τ_node (jump time set by nodal delay, not distance)
v_unmyelinated ∝ √(d_axon) (every patch must actively regenerate)
- Myelination — raises conduction velocity and concentrates the recordable signal at the Nodes of Ranvier (red rings); under thick myelin the sheath itself hides the depolarization from an external electrode.
- Detection threshold — the minimum voltage the electrode must see to register a spike. A weak, sub-threshold pickup still shows on the trace but isn't counted.
- Electrode position — drag the cyan probe along the axon. Right on a node gives the strongest reading; parked mid-internode under heavy myelin gives almost none.
- Click the axon to inject a stimulus at that point — it fires two action potentials that propagate outward in both directions.
Real-world relevance: this is exactly what a multi-electrode array / neural implant records from a peripheral nerve or cortical surface — signal strength depends critically on electrode placement relative to the active membrane, which is why real arrays use many contacts and threshold-based spike sorting.