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

[sodium.dioxid]

If resting potential is negative, then K+ ions shouldn't be passively exiting the cell. It doesn't make sense for a male that is attracted to females to leave a club full of females and go outside where there are lots of males. There are females to be had!

 

[tal444]

The neuron's firing depends on the K+ not being inside. The sudden rush of K+ is what causes the firing. Neurons wouldn't work otherwise.

 

[sodium.dioxid]

There are two levels to learning anything in biology. One is by saying that "things wouldn't work otherwise". The other level is trying to understand the seeming contradictions. THAT is the one I'm interested in.

 

[aytell]

There are two forces at work here:
1. Electric force--because the inside of the cell is negative, it pulls K+ in.
2. Diffusion force--because the concentration of K+ is much, much higher inside the cell, K+ tends to move out. This is because things move from regions of high concentration to regions of low concentration, like a drop of food coloring spreading out in a glass of water.

In the resting state, the diffusion force (pushing K+ out) is much stronger than the electromotive force (pulling K+ in), so K+ moves out of the cell.

In terms of gradients: the concentration gradient pushing K+ out is stronger than the electrical gradient pulling K+ in.

(And by the way, K+ doesn't create the action potential; Na+ does.)

 

[somasimple]

But if the external side is positive by the presence of positive ions across the membrane, why Na⁺ ions are not stopped/repulsed by them since the electrostatic force that exists in the extra cellular compartment near the membrane is stronger that the ones that may exist on the other side of the membrane?

 

[Neurofreak114]

But if the external side is positive by the presence of positive ions across the membrane, why Na⁺ ions are not stopped/repulsed by them since the electrostatic force that exists in the extra cellular compartment near the membrane is stronger that the ones that may exist on the other side of the membrane?

Na+ IS repelled by the excess of Na+ and other positive charges. Only you have to factor
in permeability.

Neurons are designed to maintain a negative resting membrane potential. Meaning they have
channels that open only at specific threshold voltages. Extracellular signals at the synapse
stimulate the opening of ion channels making the synapse more "permeable" to these
specific ions. If the voltage within the cell reaches a specific threshold, voltage gated Na+
channels open allowing an all or none (non-decrementing) action potential.

So there can't simply be an influx of ions into or out of the cell. You need channels to
open to allow permeability. There are diverse types of channels. One's that respond to
mechanical pressure, light, vibration, voltage, chemical signals etc.

 

[Neurofreak114]

There are two levels to learning anything in biology. One is by saying that "things wouldn't work otherwise". The other level is trying to understand the seeming contradictions. THAT is the one I'm interested in.

There are no contradictions, there is only a balance of polar opposites.
Understand equilibrium and you will understand biology.

yin and yang

 

[somasimple]

Na+ IS repelled by the excess of Na+ and other positive charges. Only you have to factor
in permeability.

If the membrane is permeable, at rest, to K⁺ there is an excess of K⁺ outside the cell. The Na⁺ ions will encounter them firstly when the membrane will change its state?

 

[Neurofreak114]

If the membrane is permeable, at rest, to K⁺ there is an excess of K⁺ outside the cell. The Na⁺ ions will encounter them firstly when the membrane will change its state?

The membrane has negligible/leaky permeability to K+ at rest. K+ permeability increases
AFTER the influx of Na+

Also if the membrane is fully permeable to K+ then it won't be at the resting membrane potential. K+ will move out as it is less concentrated outside the cell, this efflux causes hyperpolarization of the cell (decreasing voltage below resting potential V_r).

as "aytell" stated already there are 2 forces at work.
One caused by the the concentration gradient, the other by the voltage (charge difference).
There are a sequence of events happening.

1. Some stimulus (light/acoustic/mechanic/ligand/etc) mediated voltage increase causes threshold level voltage.
2. At this threshold Na+ channels are frequently open and allow influx from outside (positive
and high in Na+ concentration) to the inside (negative and low in Na+ concentration). This
known as the rising portion of the action potential (also called depolarization)
3. Increasing voltage due to Na+ influx triggers the opening voltage gated K+ channels, resulting in a net efflux of K+ from the cell. Against its electrical gradient (- to +) and towards its concentration gradient (high to low concentration). The concentration gradient dominates over the electrical gradient in this circumstance, resulting in the falling phase of
the action potential and slight hyperpolarization.

The movement of ions is determined by the Goldman equation. It is the net sum
that determines the fate of all ion movements.

 

[somasimple]

The membrane has negligible/leaky permeability to K+ at rest. K+ permeability increases

In a healthy animal cell Na+ permeability is about 5% of the K permeability or even less, whereas the respective reversal potentials are +60 mV for sodium (ENa)and -80 mV for potassium (EK). Thus the membrane potential will not be right at EK, but rather depolarized from EK by an amount of approximately 5% of the 140 mV difference between EK and ENa. Thus, the cell's resting potential will be about −73 mV.

From Resting Potential

 

[somasimple]

And, if Na⁺ channels are closed at rest and permeability to K⁺ at rest is near to 1 then the Na⁺/ K⁺ exchanging pump may not function at all?

If a theoretical model is clear/simple you must/may be able to draw every step between each phase. In that case, I can't draw it.

 

[Neurofreak114]

And, if Na⁺ channels are closed at rest and permeability to K⁺ at rest is near to 1 then the Na⁺/ K⁺ exchanging pump may not function at all?

If a theoretical model is clear/simple you must/may be able to draw every step between each phase. In that case, I can't draw it.

Na+/K+ pump is an ATPase
Phosphate bond hydrolysis ΔG=−30.5 kJ/mol

it pumps 3 Na+ and 2 K+ against their respective electrochemical gradients.
this is a form of active transport and does not depend on the gradients unless the
energy stored in a phosphate bond is comparable to the gradient differential.

 

[somasimple]

In order to maintain the cell membrane potential, cells keep a low concentration of sodium ions and high levels of potassium ions within the cell (intracellular). The sodium-potassium pump moves 3 sodium ions out and moves 2 potassium ions in

From Na/K pump
At rest Na⁺ channels are closed so no Na⁺ is entering the cell.
The pump can not function.

 

[Pythagorean]

From Na/K pump
At rest Na⁺ channels are closed so no Na⁺ is entering the cell.
The pump can not function.

The pump is not the same protein as the receptor that operates the Na flood channels. They're two completely different kinds of gateways in/out of the cell.

 

[somasimple]

The pump is not the same protein as the receptor that operates the Na flood channels. They're two completely different kinds of gateways in/out of the cell.

Yes, did I said something different? Does it change the problem?

 

[Pythagorean]

Why do you think the pump cannot function?

 

[somasimple]

At rest:
Na⁺ channels are closed. Sodium intake may be quite low.
K⁺ channels are open so potassium ions goes out.

Na/K pump is theorized as functioning all the time.

If sodium intake is meant low and potassium outtake is meant high, how a pump that may put outside 3 Na+ (taken from inside) and may put inside 2 K+ (taken from the outside) find sodium ions, inside, when they remain outside because the sodium permeability is low?

 

[somasimple]

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

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

 

[somasimple]

What is a relation of biology and nutrition ?

What is the relation with this question and the subject?

 

[Pythagorean]

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_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]

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]

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?

 

[Pythagorean]

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

 

[somasimple]

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.

 

[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]

Do you agree the text tells us that the potential depends of concentrations?

yes...

 

[somasimple]

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]

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.
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 homework 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]

... which was the instructor's point in assigning us that particular problem.

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

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]

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]

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]

Really?
http://www.neurophysiology.ws/membranepotentials.htm

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.

 

[somasimple]

Not yup!
If the resting potential is not null then the capacitor is charged.
If a membrane capacitor contains a charge, it comes from concentrations across the membrane but a capacitor has one and only one mathematical solution to find this voltage where I'm able to find tons.

Since I'm able to create conditions where I find a lot of solutions where I'm commonly used to find only one, I must reexamine the problem: The theory is false and/or the facts are false.

 

[Pythagorean]

Yes yup! A charged capacitor is only a meaningful concept if it's sometime going to discharge or if it got charged in the first place, which each require state evolution through time; additionally, each are separate states that are separated by time.

Also, you are imagining a single engineered capacitor with a (mostly) constant capacitance?

Here, we have a thing who's capacitance does change slightly at different membrane potentials, but because of the parallel distribution of capacitors and leak channels, the effective capacitance changes very little. Most of the change comes from the charge reshuffling (i.e. leak channels and ion pumps). Because of the parallel nature of the capacitors and the escape routes available, we can get huge fluctuations without changing the capacitance much (so the dQ mostly makes up for the dV, the small fluctuations in the capacitance are considered insignificant) . Think about a population of channels and capacitors with some distribution across the membrane, not just one set.

 

[somasimple]

(Not yup)²,

It does not change anything about the membrane potential computation.
You must explain how it is possible to get a same result with different charges densities that come from concentrations?

It is a violation of Thermodynamics: You may obtain a lesser voltage with less charges, not the same one.

 

[somasimple]

If I remove the limitations about concentrations that exist in the original form of the Nernst equation, I will be able to create/produce batteries with very few quantities of chemical products. They would be cheaper and I do not understand why manufacturers haven't thought about this simple fact.

Perhaps there is some fate in Electro-Chemistry that may not exist in Biology?

 

[Pythagorean]

You must explain how it is possible to get a same result with different charges densities that come from concentrations?

It's already been explained; you just haven't absorbed it yet. Give it some time; think about a sea of coupled capacitors with current pathways of mixed species all over and adjustable resistors changing dynamically. It's not a pretty picture that can be understood by the Nernst equation or a simple capacitor. Capacitance is a property of many different arrangements of matter; engineered capacitors are a very specific case of capacitance. And remembrer the capacitance isn't constant, it's approximately constant.

Charge accumulation and distribution isn't as straightforward with ions in a cell as it is with electrons in an engineered capacitor.

 

[somasimple]

It's already been explained; you just haven't absorbed it yet.

I may be blind or something else since I'm unable to find the sentences where you gave the results.

Charge accumulation and distribution isn't as straightforward with ions in a cell

I agree totally because I know that the cytoplasm is a gel but your talk contradicts the HH model where ions cross the membrane instantaneously and now the model of diffusion becomes... dubious.
Did you say... rubbish?

 

[Pythagorean]

I was calling your constant knit-picking and subject changing rubbish, not your confusing model with reality.

The HH actually models delays well compared to say, the Morris Lecar model, which is still a good approximation for many questions (and was derived empirically from the barnacle giant muscle fiber).

 

[somasimple]

not your confusing model with reality.

Reality: Is cytoplasm a gel?
Reality: Do diffusion cited in the book was meant to function with gel?
Reality: Does a pump function against a gel?
Reality: How Na+ ions enter the cell gel?

Etc...

How may I be confusing a model since I cite only pieces of... books and papers?
Put together, you get effectively a confusing model and the model stop to work.

 

[Pythagorean]

There is no panacea

 

[somasimple]

If the change is 0, then capacitance doesn't matter; see a simple neuron model

The HH actually models delays well compared to say, the Morris Lecar model, which is still a good approximation for many questions (and was derived empirically from the barnacle giant muscle fiber).

Perhaps I'm confused within a model that contains dozen of mistakes, wrong facts and wobbly theories. It seems somehow, perfectly logical.

But if you suppose that I will accept a neuron model that was empirically derived from a barnacle muscle fiber then I'll hypothesize you're lost in yours (models).

 

[somasimple]

There is no panacea

I'm sorry you lost any curiosity.

 

[Pythagorean]

Let me know when you fix everything that's wrong with science. I'll be here in this thread, holding my breath.

 

[somasimple]

A better model is still possible but I'm quite sure you're not interested.

 

[Pythagorean]

Nope; a better skill for my work is being able to use the right model for the right question. You could spend months on a better model and it will likely give the same result for the kinds of questions it's being used for.

There are people that go yet even simpler than Morris-Lecar but their purpose is to study synchronicity in a million neuron network. Time delays you are worried about are not important to them. It's the same for HH.

To take your argument to absurdity, why not just model neurons as an ensemble of quantum particles. Or... Why ignore the system effects of astrocytes or the underlying genetic expression?

You only focus on electrochemistry because that is apparently your area. But choosing that view skews you from other perspectives.