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

somasimple

http://highered.mcgraw-hill.com/site...ump_works.html

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

somasimple

What is the relation with this question and the subject?

Pythagorean

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

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

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

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

Link is broken. But your picture is a reduction of Goldman to Nernst... It's valid depending on the question being asked.

somasimple

It comes from the book Cells.

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Pythagorean

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

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

yes....

somasimple

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

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

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

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

Some instuctors' points come from: Nelson, Johnston, Kandel, Nilsson , Sten-Knudsen ...

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

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.