3

I am aware of the saltatory conduction model, nodes of Ranvier and all that, and that myelin lets electrical signals "jump". What does not add up to me entirely is what the myelin sheath insulates against, why it improves conduction through axon. If the cell membrane is already impermeable to ions, why does a "thicker membrane" improve signal conduction? There are nearly infinite memes on the internet about action potentials through axons, have not really seen any answer this though. Some vaguely mention "less leakage", but isn't the cell membrane preventing that to begin with?

Hodgkin
  • 31
  • 1
  • 1
    Here's an answer I think is fit to answer your question. It gives you a better conception of how myelin does its job of insulation. – S Pr Mar 02 '20 at 15:31
  • Ions have an electrical charge which generates an electrical field that decreases in strength with distance from the ion. The cell membrane of the axon is not thick enough to stop ions on either side from being attracted to one another and interacting. The electric fields of these ions are what myelin 'insulates against'. This answer to a related question provides some good diagrams and in-depth discussion of how increasing the distance between the oppositely charged ions helps nerve function. – timeskull Mar 02 '20 at 16:26
  • @Hodgkin Can you look at the two related questions linked in comments by S Pr and timeskull? I think your question should be closed as a duplicate of one or the other. – Bryan Krause Mar 02 '20 at 16:30
  • So, myelin increases the distance to the positive ions on the outside of the axon membrane, preventing them from repelling the positive sodium ions that travel down the internodal segment by electrotonic conduction, overall decreasing resistance to sodium ion current? – Hodgkin Mar 02 '20 at 22:52
  • If that is the answer, then technically it isn't really insulating (as insulation does not insulate against electric fields?), but, just increasing distance (with non-charged matter, the myelin)? insulation in wires prevents leakage of electrons, doesn't it? isn't the metaphor, if so, misleading? – Hodgkin Mar 02 '20 at 23:01
  • A capacitor involves plates holding charge separated by an insulator - adding more insulator increases the separation. Adding a non-insulator would not. There is also increased resistance overall. So no, I don't think it's misleading nor an analogy, though it's important to realize that biological electrical systems only approximate the perfect/ideal mathematical circuit components (even more so than non-biological versions). – Bryan Krause Mar 03 '20 at 00:44
  • I don't think your statement about positive ions repulsing is correct, either. Just think about it as the amount of charge necessary to change the membrane voltage is less. – Bryan Krause Mar 03 '20 at 00:47
  • The dielectric in a capacitor is insulating, good point. If not by decreased repulsion from the positively charged ions on the other side of the myelin dielectric, then how does the decreased capacitance in the internodal segment improve propagation of the sodium ions? – Hodgkin Mar 03 '20 at 01:05
  • (what I meant with misleading, it seems to me that insulation around a copper wire is used to prevent leakage, not to decrease capacitance, since the space around the wire isn't electrically charged whereas extracellular space is. better to liken it to a capacitor then, like you did. I got that analogy, which was not an analogy because it is what it is, but maybe you get what I mean) – Hodgkin Mar 03 '20 at 01:07

0 Answers0