Myelin increases resistance across the cell membrane

In summary, myelin significantly increases resistance across the cell membrane and decreases capacitance, resulting in a 5,000-fold increase in resistance and a 50-fold decrease in capacitance. This decrease in capacitance can be explained by the myelin sheath wrapping around the neuron multiple times, acting as a series of capacitors. This also explains why myelin speeds up the action potential, as it only occurs at the nodes of Ranvier where the myelin sheath is absent and the full depolarization can take place. De-myelination can significantly decrease the speed of the action potential and can only be observed in pathological conditions.
  • #36
http://sfbay.craigslist.org/forums/?act=Q&ID=102572102
 
Biology news on Phys.org
  • #37


somasimple said:
50 turns of myelin (50*d) and a length that is 2000 time longer (2000*A). :zzz:

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.
 
  • #38


somasimple said:
DaleSpam,
Give the results in our example.
50 turns of myelin (50*d) and a length that is 2000 time longer (2000*A). :zzz:

Your question is rather ambiguous the way it is worded. Length of what? What "result" are you asking to have explained?
 
  • #39


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

Attachments

  • node_internode_cap.png
    node_internode_cap.png
    17.8 KB · Views: 465
Last edited:
  • #40
granpa said:
http://sfbay.craigslist.org/forums/?act=Q&ID=102572102
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 [Broken]
It does not change the length segment problem.
 

Attachments

  • node_internode_cap_02.png
    node_internode_cap_02.png
    21.1 KB · Views: 454
Last edited by a moderator:
  • #41


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

somasimple said:
It does not change the length segment problem.

Time constant~RC, so if you include R (membrane resistance)?
 
  • #42
Last edited:
  • #44


atyy said:
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.
Are you serious?
I do not contest... numbers.
 
  • #45


somasimple said:
Are you serious?
I do not contest... numbers.

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
 
  • #46


atyy said:
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.
Someone is wrong: Is it Mathematics or Pr C Koch?:redface:
Edit: 250 turns does not change anything since 250 < 1000
 
  • #47


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

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 :redface:
 
Last edited:
  • #48


atyy said:
Order of magnitude means correct to within a factor of <10 (I usually think ~3-4)
But Rm varies inversely to Cm and they are linked...
You may test a simple linear function:
Rm=f(C)=-a(C)+b
 
  • #49


somasimple said:
But Rm varies inversely to Cm and they are linked...
You may test a simple linear function:
Rm=f(C)=-a(C)+b
Yes, that's why I said:
atyy said:
Edit: But there is something very fishy with this explanation. Time constant Tm~RmCm. What's the point of decreasing Cm, but increasing Rm by the same amount?

Edit: Another part of the puzzle. Space constant Lm~Rm/Ra

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.
 
  • #50


Atyy said:
That's why I was thinking about the length constant (frequency-independent component) ~Rm/Ra, where Ra is the axial resistance.
If you reject Cm then you reject the whole theory...
Atyy said:
So I don't understand what's going on.
I do... but I can't say anything on this site without being thunder lightened by modos.
 
  • #51


somasimple said:
If you reject Cm then you reject the whole theory...

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.

somasimple said:
I do... but I can't say anything on this site without being thunder lightened by modos.

Maybe to be careful, you should say "at least one modo (singular)". :rofl:
 
  • #52


atyy said:
Maybe to be careful, you should say "at least one modo (singular)". :rofl:
I can't. It is out of my capacitance and resistance.:rofl:
 
  • #53


somasimple said:
I can't. It is out of my capacitance and resistance.:rofl:
:rofl::rofl::rofl:
 
  • #54


atyy said:
Order of magnitude means correct to within a factor of <10 (I usually think ~3-4)
with the function:
The minimal value found with 250 turns is a Time Constant that is multiplied by a factor 20.
the worst is... 1250
 

Attachments

  • tau_variation_1024.png
    tau_variation_1024.png
    9.8 KB · Views: 545
Last edited:

Similar threads

  • Biology and Medical
Replies
3
Views
1K
  • Electrical Engineering
Replies
6
Views
6K
  • Electromagnetism
Replies
2
Views
2K
Replies
7
Views
9K
Replies
2
Views
3K
  • Advanced Physics Homework Help
Replies
1
Views
2K
  • Introductory Physics Homework Help
Replies
4
Views
2K
  • Biology and Medical
Replies
2
Views
20K
Replies
4
Views
44K
Back
Top