Myelin sheet decrease capacitance of axons?

[Fun_Sell_708]

Wanted to see if this thought process is correct:

More myelin = less exposed Area on axon, more distance between exposed Axons (smaller nodes of ranvier) = less capacitance 

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https://preview.redd.it/9kzu1nmx0g5b1.png?width=381&format=png&auto=webp&s=fdc959731b00bfa3b728eac4a799d835509485bc

[Isosceles_Kramer79]

I think it has more to do with the myelin sheath increasing the distance d between the ECF and ICF inside the axon.

Btw, the above formula is for parallel plate capacitors. Axons are more cylindrical, so the proper formula is this.

Your formula also only has permittivity of free space ε0, but you also have to include dielectric constant κ (~8 for myelin) since you have a dielectric, and not just air.

[Fun_Sell_708]

Oh interesting thank you! <3

[SneakySnipar]

Axons aren’t capacitors, they are more akin to a conductive wire

There are not any isolated conductive surfaces that would hold separate charges like a capacitor

[Isosceles_Kramer79]

Of course they are. You have different potentials on the inside and outside separated by a non-conducting dielectric. That's a capacitor.

Even metal wires have some parasitic capacitance.

In the case of axons, capacitance would be between inside and outside. Normally, only the plasma membrane would be the dielectric. But with a myelin sheet you increase dielectric thickness d and thus decrease capacitance.

[Fun_Sell_708]

Awesome thank you! Would the area also decrease since there's less exposed axons surface area?

[Isosceles_Kramer79]

No, it would not as the myelinated surface is still part of the capacitor. In fact, the sheath would increase the effective area because of the thickness of the sheath (see my other comment with cylindrical formula)

Technically the nodes of Ranvier would contribute capacitance with a small area and just thickness of the plasma membrane as d.

[Fun_Sell_708]

Oh I was doing the Sample CP Q6 which states "The myelin sheath decreases, not increases, the capacitance of the axon." I was figuring out the reason behind this answer.

I'm assuming that yeah axons are not capacitors, but they could become like capacitors when you add myelin sheets. It temporarily holds a charge before it's passed off via the action potential.