Myelin increases resistance across the cell membrane

DaleSpam

What result and computation are you talking about?

Your previous post where you said:
It is very difficult for me to communicate with you. I know that a large part of that is a language barrier, so I am trying to be patient.

granpa

http://sfbay.craigslist.org/forums/?act=Q&ID=102572102

atyy

Yes, myelin allows the total capacitance of an internode and a node to be roughly the same even though the internode is ~1000 longer than the node.

Moonbear

Your question is rather ambiguous the way it is worded. Length of what? What "result" are you asking to have explained?

somasimple

A visual perhaps?
Time constant for internode is, at least, 120 longer in that case.

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somasimple

You're right. Normally a cylindrical capacitor must be computed that way but biologists do not.
http://butler.cc.tut.fi/~malmivuo/bem/bembook/21/21.htm
It does not change the length segment problem.

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atyy

Time constant~RC, so if you include R (membrane resistance)?

somasimple

http://www.physicsforums.com/showpos...66&postcount=1
If C and R are increased then...
Of course the internal axonal resistance remains unchanged but since its length did.

atyy

So your point is that the numbers in wikipedia are wrong?

somasimple

Are you serious?
I do not contest... numbers.

atyy

Yes - but only "same order of magnitude" - Koch: Even though the length of the interaxial node is typically 1000 times larger than the node, its total capacitance has the same order of magnitude.

But Koch is talking about the frog axon: made up of 250 myelin layers

somasimple

Someone is wrong: Is it Mathematics or Pr C Koch?
Edit: 250 turns does not change anything since 250 < 1000

atyy

Order of magnitude means correct to within a factor of <10 (I usually think ~3-4)

So I think we need better numbers, and from the same species - not some squid, some frog, and some rabbit ...

Edit: not squid - that's not myelinated

somasimple

But Rm varies inversely to Cm and they are linked...
You may test a simple linear function:
Rm=f(C)=-a(C)+b

atyy

Yes, that's why I said:
There are other equations in Koch's book where Cm enters, for example in the frequency-dependent length constant, but it always enters in the combination RmCm, so if the primary job of myelin is to change capacitance, I don't see how it affects anything.

That's why I was thinking about the length constant (frequency-independent component) ~Rm/Ra, where Ra is the axial resistance. The length constant determines how signals decay over distance, so to conduct in the internode where sodium channels are low, and signals cannot be actively boosted, the length constant has to be increased, perhaps by increasing Rm with myelination. Unfortunately, this increases the time constant ~RmCm - unless you decrease the capacitance by the same amount, which I think myelin does. However, most expositions do not feature the length constant, and they also say that the job of myelin is to increase the time constant, not to keep it the same. So I don't understand what's going on.

somasimple

If you reject Cm then you reject the whole theory...
I do... but I can't say anything on this site without being thunder lightened by modos.

atyy

Yes, and no. I think the decrease in Cm is required to offset the increase in Rm, so that the time constant remains the same. There are no outright contradictions between what I'm saying and the standard explanations. BUT there are enough differences in emphasis that I should look at the equations carefully and see whether the apparent lack of contradiction between the two explanations is due to a real similarity in the underlying mathematics, or just due to chance. But those details are not in Koch's book.

Maybe to be careful, you should say "at least one modo (singular)".