Myelin G-Ratio & Saltatory Conduction Velocity

Fiber Geometry

Axon diameter 8.0 μm

Myelin g-ratio (axon / fiber Ø) 0.60

thick myelinoptimum ≈0.61thin myelin

Demyelination severity 0%

Live Readouts

Conduction velocity— m/s

Internode length— μm

Segment conduction time— ms

How it works

Myelin wraps the axon between nodes of Ranvier, forcing the action potential's inward Na⁺ current to regenerate only at the exposed nodes. Because the myelinated internode has high resistance and low capacitance, the depolarization spreads passively — and fast — from node to node: saltatory conduction.

Fiber diameter rule (Hursh, 1939):
  v_myelinated ≈ 6 · d[μm]  m/s   (at the optimal g-ratio)

g-ratio dependence (Rushton, 1951):
  v(g) ∝ g·√(−ln g),  maximised at g* = e^(−1/2) ≈ 0.6065

Internode spacing:
  L ≈ 100 · d[μm]

Unmyelinated (demyelinated) segment (Hodgkin-type d^0.5 law):
  v_bare ≈ 1.7 · √(d[μm])  m/s

Diameter slider — thicker axons conduct faster; velocity scales linearly with d for a fixed g-ratio.
g-ratio slider — the ratio of axon diameter to total fiber (axon+myelin) diameter. Too little myelin (g→0.9) or too much (g→0.3) both slow conduction; real myelinated axons cluster near the g≈0.6–0.7 optimum, matching the marker on the bar.
Demyelination severity — models a demyelinating plaque (as in multiple sclerosis): the fraction of internodes shown loses its sheath (red bands) and falls back to slow, continuous conduction, dragging down the fiber's average velocity — exactly the mechanism behind conduction slowing/block in demyelinating disease.
Fire action potential — launches a glowing impulse that hops node-to-node at the computed velocity; each node flashes as the impulse regenerates there.

Action potential Healthy myelin Demyelinated segment

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