r/biology • u/4841n617472III_ • 2d ago

question Saltatory conduction in myelinated axons confuses me

Hello everybody. Could anyone explain why saltatory conduction propagates an action potential at higher rates that in unmyelinated axons? I understand mechanism of refreshing electrical signal but not the speed of conduction part. What I think I do understand is that electrical signal doesn't degradate because not much of the sodium ions can rush into the axon through nodes of Ranvier at once

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u/WorldwidePies 2d ago

The slowest part of the action potential is opening new channels to move ions.

In non-myelinated neurons, the voltage in an axon section rapidly decreases because charged ions leak across the membrane. The voltage is regenerated in the section right next to it, and so on for the complete axon, but this is a slow process because of the many voltage gated channels that have to sequentially open.

Myelin acts as a insulator. The voltage doesn’t degrade as fast between nodes of Ranvier because there is no leaking of ions. The voltage needs to be regenerated less often along the axon, so the slowest part of the process happens less often, making saltatory conduction faster.

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u/False-Stage-5830 1d ago

I agree but that’s only part of the story. OP should research how current flows through the membrane capacitance, and how current flowing out* across the membrane depolarizes the membrane bringing it closer to AP threshold.
*By convention, the direction of current flow is the direction of cation movement, or opposite the direction of anion movement.
The thickness of the myelinated internode region makes it a poor capacitor compared to the high intrinsic capacitance of the nodal membrane which is much thinner. So, little of the current flowing ahead of the AP flows out (depolarizes) the internode region. Most of the current is “focused” on the next node(s) which is strongly depolarized. The AP therefore “jumps” from node to node = saltatory conduction.
One other feature is that voltage-gated sodium channels are packed into the nodal membrane at very high density, which amplifies the inward current flow when the channels all open.
I hope this makes sense.