The Action Potential in Myelinated Axons

Traditionally, physiology and neuroscience courses have taught that the mechanisms underlying the action potential are the same in both unmyelinated axons and myelinated axons. The original work by Hogdkin and Huxley in the 1950s, using the squid giant axon, has held up well over the subsequent 75 years. However, mounting evidence has accumulated which suggests that the mechanisms underlying the characteristic action potential waveform are different in unmyelinated and myelinated axons.

[action potential]

Recall from Objective 8 that the rising phase of the action potential occurs due to the opening of voltage-gated sodium channels. In unmyelinated axons, like the squid giant axon studied by Hodgkin and Huxley, the falling phase of the action potential is due to the action of slightly slower voltage-gated potassium channels.

But in myelinated axons, more recent work has shown conclusively that while the rising phase is still due to voltage-gated sodium channels, as described in Objective 8, that the falling phase occurs because of a high density of leakage potassium channels (K2P channels), as we described in Objective 5.

The genes that encode for these channels are KCNK2, which is also called TREK-1, and KCNK4, which is also called TRAAK.

https://journals.physiology.org/doi/full/10.1152/advan.00171.2021

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