How don't neurons get enough K+ to balance out their inner negative charge?


by sodium.dioxid
Tags: balance, charge, negative, neurons
somasimple
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Apr13-12, 05:22 AM
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What is a relation of biology and nutrition ?
What is the relation with this question and the subject?
Pythagorean
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Apr13-12, 09:52 AM
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Quote Quote by somasimple View Post
http://highered.mcgraw-hill.com/site...ump_works.html

This animation does not provide any clue how Na+ ions entered in the cell.

Sodium is still there even if "low" so it can still be pumped out and there are also always leak currents that ignore channel gating.
somasimple
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Apr13-12, 12:09 PM
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Quote Quote by Pythagorean View Post
Sodium is still there even if "low" so it can still be pumped out and there are also always leak currents that ignore channel gating.
That's a wise response but the contribution of Na/K pump was known as already low in a resting potential.
If its contribution is low at rest, it may not change when the neurons fires since it depends of ATP. Thus, you get another problem: The recovery phase becomes too long since Na+ (voltage gated) are still closed since the end of the rising phase...
Pythagorean
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Apr13-12, 12:24 PM
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I still don't understand what you're trying to say, the pumps is always operating (yes, through ATP); it doesn't need to "change".

Perhaps you are imagining much more Na floods in then actually does? It doesn't require a lot to depolarize the cell. The pumps are able to keep up with the help of leak currents which are always permeable and will go whichever way goldman-hodgkin-katz (the force balance) tells them to in the moment of the neuron's state.
somasimple
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Apr14-12, 01:13 AM
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Perhaps it is time to get a simpler example?
Here is a link to a simple explanation.
Do you agree with this concept?
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Pythagorean
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Apr14-12, 03:27 AM
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Link is broken. But your picture is a reduction of Goldman to Nernst... It's valid depending on the question being asked.
somasimple
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Apr15-12, 11:48 PM
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Quote Quote by Pythagorean View Post
Link is broken. But your picture is a reduction of Goldman to Nernst... It's valid depending on the question being asked.
It comes from the book Cells.
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Pythagorean
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Apr15-12, 11:58 PM
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The text is demonstrating the concept of the Nernst potential. In a real neuron, there are several channels with different Nernst potentials; together they make the resting potential, so you can have a constant membrane potential maintained while Na leaks in and K leaks out through their respective channels; you would use the goldman-hodgkin-katz equation instead of Nernst in most dynamical cases.
somasimple
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Apr16-12, 12:43 AM
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I would yet stay with this simple example because the original Nernst equation belongs to Chemistry while the second belongs to Biology.
There is some major differences between the two equations :

In the original:
1/ There is a redox equation.
2/ The notion of concentrations is limited.

In Biology:
1/ It introduces a semi permeable membrane.
2/ The notion of concentrations is not limited.
3/ It introduces charges that stick across the membrane: It belongs to Electrostatics.
4/ It introduces the notion of capacitor that belongs to Electricity.
5/ It introduces the notion of violation of Electroneutrality near the membrane.

Do you agree the text tells us that the potential depends of concentrations?
Pythagorean
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Apr16-12, 02:11 PM
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Quote Quote by somasimple View Post
Do you agree the text tells us that the potential depends of concentrations?
yes....
somasimple
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Apr17-12, 12:35 AM
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Quote Quote by Pythagorean View Post
yes....
Before I may answer a "yes", I will carefully examine if the hypothesis satisfy each scientific domain and its limitations:

Electrostatics:
1/ Each compartment contains negative and positive charges.
2/ The hypothesis creates an attraction from a charge contained in a compartment to the opposite:
The distance that exists between these two charges must be fewer than the distance that exists between opposite charges in a single compartment.
The membrane thickness must be thinner than the distance that exists between opposite charges in a single compartment.
These two conditions must be valid for each ion specie (Na+, K+, Cl- ...).
These conditions are false because the point #1.
These conditions are false because the thickness is larger than the distance that exists between charges on each side.

Electricity:
since Electrostatics is not made possible then there is no capacitor effect.
It is also possible to discard this scientific field with the concentrations ratio.

Since these two first points are not validated then there is no reason that the electroneutrality rule may be violated.
Pythagorean
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Apr17-12, 01:04 AM
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You're not making much sense; is English your first language? I'm not sure if you don't know what you're talking about or you're just not communicating effectively, but it sounds like a lot of rubbish.
somasimple
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Apr17-12, 01:37 AM
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Hey man. It is not because English is not my first or native language that you have the ability to say it's rubbish.
As a scientist, you must examine sentences. They contain facts or arguments. You must reply with scientific arguments that contradict the previous without any "ad hominem" allegation.

Make a simple drawing with some charges and try to compute the needed forces that may answer the hypothesis.
Pythagorean
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Apr17-12, 10:37 AM
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You should take your advice and try the calculations out. I happen to have done it already as a hw assignment in molecular neuroscience. As I've already suggested, you may be overestimating how much Na leaves the cell, which was the instructor's point in assigning us that particular problem.
somasimple
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Apr18-12, 01:22 AM
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Quote Quote by Pythagorean View Post
... which was the instructor's point in assigning us that particular problem.
Some instuctors' points come from: Nelson, Johnston, Kandel, Nilsson , Sten-Knudsen ...

Quote Quote by A. E.
The important thing is not to stop questioning. Curiosity has its own reason for existing. One cannot help but be in awe when he contemplates the mysteries of eternity, of life, of the marvelous structure of reality. It is enough if one tries merely to comprehend a little of this mystery every day. Never lose a holy curiosity.
We are still in the example of the book with a single K+ problem.
As it is expected, as the concentration (of the most concentrated side) grows, the voltage across the membrane must grows because a capacitor will get its voltage growing as the charge density grows.

In the above example the concentrations numbers are 155/4 and it gives 93 mV.
Try with 15.5/0.4 or 310/8 or 75.25/2 and you'll find 93 mV. In fact, there is a lot/infinity responses.
Thus you have a voltage that remains constant where the charge density may vary at will.
A capacitor dos not allow such a thing.
Pythagorean
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Apr18-12, 02:19 AM
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Quote Quote by somasimple View Post
In the above example the concentrations numbers are 155/4 and it gives 93 mV.
Try with 15.5/0.4 or 310/8 or 75.25/2 and you'll find 93 mV. In fact, there is a lot/infinity responses.
Thus you have a voltage that remains constant where the charge density may vary at will.
A capacitor dos not allow such a thing.
In the resting state (the polarized state) there needn't be capacitance. The capacitance is a term in the differential equation pertaining to the change in membrane potential. If the change is 0, then capacitance doesn't matter; see a simple neuron model

But I don't see how that has anything to do with your previous claim about the pumps (which rely on ATP) not functioning.
somasimple
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Apr18-12, 02:27 AM
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Quote Quote by Pythagorean View Post
In the resting state (the polarized state) there needn't be capacitance. The capacitance is a term in the differential equation pertaining to the change in membrane potential. If the change is 0, then capacitance doesn't matter; see a simple neuron model
Really?
http://www.neurophysiology.ws/membranepotentials.htm
Because the membrane behaves as if it were in part composed of parallel capacitors, we are interested in the rules governing parallel capacitors
Pythagorean
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Apr18-12, 02:38 AM
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Yup!

Capacitance is only important to the dynamics. Once you are sitting still, it's just a charge distribution. If the charges must flow an indirect route, the system experiences a delay (characterized by the time constant) and your system can "jiggle" or propagate waves (since it's parallel capacitors, it's a spatial extension).

Parallel capacitors are like a mattress of springs. Springs only really function as springs if you perturb the mattress (like knocking over a wine glass at one end by jiggling the other end). If nothing ever changes, it's no different than a rigid body.


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