Impulse conduction and myelin !
- Mechanism
- Exam priority: high
Myelin wraps the axon like insulation so the signal jumps from gap to gap; that makes it many times faster and cheaper.
Updated 2026-11-01
Also called: Saltatory conduction, Conduction velocity
See it in 3D: Sciatic nerve · Spinal cord · Optic nerve
What it is
An action potential spreads because the current it generates depolarizes the next patch of membrane to threshold. In unmyelinated axons this happens continuously and slowly. Myelin (Schwann cells in nerves, oligodendrocytes in the brain and spinal cord) insulates the axon so current flows to the next node of Ranvier, where Na⁺ channels are concentrated: the impulse jumps node to node (saltatory conduction).
How it works, step by step
- Na⁺ entering at an active site spreads as local current inside the axon.
- In an unmyelinated axon, much of that current leaks out nearby, so only the adjacent membrane reaches threshold: slow, continuous spread.
- Myelin adds many membrane layers in series: capacitance falls and leak resistance rises.
- Current therefore travels farther along the axon and reaches the next node of Ranvier still strong enough to fire it.
- Spikes are regenerated only at nodes, so the impulse appears to jump (saltatory conduction) and fewer ions must be pumped back.
- If myelin is lost, current leaks out before the next node: conduction slows or fails (conduction block).
Action potential
Rest −70 mVThreshold −55 mVPeak +35 mVDuration ≈ 1–2 ms
Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.
- Mainly Na⁺
- Mainly K⁺
- Rest
- Threshold
- ARP: absolute refractory period
- RRP: relative refractory period
Approximate conduction velocities
| Parameter | Value |
|---|---|
| Large myelinated motor axon (Aα) | ≈ 70–120 m/s |
| Thin myelinated pain fiber (Aδ) | ≈ 5–30 m/s |
| Unmyelinated C fiber | ≈ 0.5–2 m/s |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Multiple sclerosis: immune attack on CNS myelin; heat or exercise can transiently worsen symptoms because conduction in demyelinated axons fails more easily at higher temperature.
- Guillain-Barré syndrome: acute immune-mediated polyneuropathy, often after an infection; nerve studies show slowed velocities and conduction block, and reflexes are lost.
- Colombia saw a surge of Guillain-Barré cases during the 2015–2016 Zika outbreak, a reminder that infections can trigger peripheral demyelination.
Test yourself
Think of the answer, then tap the question to see it.
Why does demyelination cause conduction block rather than only slowing, even though the axon itself is intact?
Internodal membrane has few Na⁺ channels. Without myelin, current leaks out before reaching the next node, which may not reach threshold, so the impulse fails.
Why are both a larger diameter and myelin needed for the fastest motor axons?
Larger diameter lowers internal resistance so current spreads farther; myelin lowers capacitance and leak so less charge is wasted. Together they push velocity toward about 100 m/s.
A patient with multiple sclerosis gets worse visual blurring after a hot shower. What is the physiological explanation?
Higher temperature shortens the action potential and Na⁺ channel open time, so less current reaches the next node. Marginally conducting demyelinated axons then block (Uhthoff phenomenon).
Why do Guillain-Barré and multiple sclerosis differ in where demyelination shows up on examination?
Guillain-Barré targets Schwann-cell myelin in peripheral nerves: weakness with lost reflexes. MS targets oligodendrocyte myelin in the CNS: upper motor neuron signs, optic neuritis, sensory tracts.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 12.4 The Action Potential (OpenStax (Rice University), 2022); Multiple Sclerosis (StatPearls (NCBI Bookshelf), 2024); Guillain-Barre Syndrome (StatPearls (NCBI Bookshelf), 2024); Zika virus disease, microcephaly and Guillain-Barré syndrome in Colombia: epidemiological situation during 21 months of the Zika virus outbreak, 2015-2017 (Archives of public health, 2017); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 5). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.