Talk:Membrane potential

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Article Grading: The following comments were left by the quality and importance raters: (edit · refresh) rated top as high school/SAT biology content - tameeria 14:50, 17 February 2007 (UTC) I'm not an expert on membrane potential, so please adjust the rating if B doesn't seem right. - tameeria 18:50, 18 February 2007 (UTC)

Contents 1 Requested move 2 dt=0 3 Merging 4 prior texts still to be edited and incorperated 5 Dissipation of ion gradiets by transmembrane ion flux 5.1 voltage difference or potential difference? 6 Introduction 7 Electroneutrality!! 8 Merge? 9 expert tag off

[edit] Requested move

• I have merged the contents from the article "Transmembrane potential" into this article and transformed the former in a redirect. I suggest that this article - "transmembrane potential difference" - is moved to membrane potential, currently existing as a redirect, as "membrane potential" is the most often used term and as far as I know, it denotes the same. --Eleassar777 13:22, 14 May 2005 (UTC)

• Support Eleassar777 22:46, 14 May 2005 (UTC)
• Support. Eleassar777 is quite correct on all counts. --TenOfAllTrades (talk/contrib) 18:25, 15 May 2005 (UTC)
• Support ~~~~ ashleyisachild 19:18, 19 May 2005.
• Support—membrane potential is definitely the more commonly used of these terms. JeremyA 02:07, 20 May 2005 (UTC)
• Support absolutely correct Synaptidude 7 July 2005 00:38 (UTC)

This article has been renamed as the result of a move request. violet/riga (t) 21:28, 21 May 2005 (UTC)

[edit] dt=0

If you set dt=0 in the equation for capacitive current, then it doesn't eliminate the time dependance. Rather it gives an infinite capacitve current. You are dividing by 0. Maybe you mean to approximate the derivative by the differential dV/dt ~= deltaV=0.1V.

Which still makes no sense. There should clearly be an estimate of the time involved.Peiter 13:59, 21 December 2006 (UTC)

[edit] Merging

If there is a merge, this article should redirect to bioelectromagnetism; not the other way around. - Omegatron 14:55, Jun 19, 2005 (UTC)

• I strongly object to this proposal: I don't care what happens to bioelectromagnetism (or bioelectricity) but membrane potential is a widely used term in the fields of biophysics and electrophysiology (see, for example, the 461,000 google hits). It is also used in many textbooks—for example Hille (ISBN 0-87893-321-2), which is considered by many to be the bible of biophysics. JeremyA 15:42, 19 Jun 2005 (UTC)
• I'm afraid I have to strongly disagree on that one. Membrane potential–as JeremyA notes–is a term used very heavily used in cell biology. Bioelectromagnetism is–I think–a related topic but I don't think there's enough overlap to warrant a merge. That article could use some cleanup, though. The scope of and definitions in that article aren't as clear as they could be. --TenOfAllTrades(talk) 16:20, 19 Jun 2005 (UTC)

ITA with Jeremy . There is no way that this article should be merged with bioelectricity or biomagnetism. Membrane potential is a specific biophysical property of cells, while biomagnatism and bioelectricity, at least as discussed in the referenced wikipedia articles, are epiphenomona at best, and utter nonsense at worst. Those two articles are pretty much incomprehensible in any case. Furthermore, students, scientists of every ilk, including cell biologists, are going to look for the term "membrane potential" if they want to know about it. It is from membrane potential that all of these other phenomena (e.g. EEG) arise. I've deleted the merge request, because

• it's ridiculous on the face of it, and,
• I'm being bold

Synaptidude 7 July 2005 00:37 (UTC)

[edit] prior texts still to be edited and incorperated

I cut all of the following text from the original article. Basically there were several, well not factual errors exactly, but enough factual ambiguities, that I found it easier to re-write it from scratch. I kept the text below in case anyone objects and wants to talk about reinstating some of it.

Cells are surrounded by a plasma membrane, which defines their extent and acts as a barrier between the cells and their external environment, for example interstitial fluid or blood plasma. The membrane, as a result of its lipid bilayer structure and specific membrane proteins, is selectively permeable (the hydrophobic interior prevents the passage of both large polar molecules and ions) and therefore will only allow certain species through. This selective permeability allows asymmetric concentrations of ions to exist between the intra- and extracellular fluids. These differences can be chemical or electrical (i.e. the difference in charge between the inside and outside). Most cells maintain a “membrane potential” of around –80mV relative to the surrounding fluid. The membrane potential is negative because usually cells have a net negative charge due to leakiness of potassium channels and the large size of negatively charged macromolecules such as proteins and RNA.

In animal cells, passive ion movement accounts for the majority of the electrical potential across the plasma membrane. This passive ion movement mostly consists of K+ ions. A helps maintain an osmotic balance by keeping the concentration of intracellular Na+ low. Because the concentration of Na+ is so low inside the cell, other cations must be present to balance the negative charge carried by the cell's fixed protein anions. This balancing act is largely performed by K+ which is pumped in through the Na+/K+ pump and is also free to leave or enter the cell through the K+ leak channels. There is an electrostatic attraction for K+ due to the protein anions. This attraction balances against the tendency of K+ to diffuse out of the cell, down its concentration gradient, and it is these combined actions that create the membrane potential.

This can also be explained in the following way. Suppose that a cell initially has a membrane potential of zero – i.e. has no voltage gradient across the plasma membrane. However, the concentration of K+ inside the cell is higher than outside and so K+ will tend to leave the cell, driven by the concentration gradient. As it leaves the cell, the K+ leaves an unbalanced electrical charge. This creates a negative electrical charge, which is the membrane potential. The electrical field also opposes any further K+ leaving the cell. The membrane potential also tends to keep anions like Cl- out of the cell because their charge is also negative.

The cytosol, or interior, of a cell possesses a uniform electric potential or voltage compared to the extracellular solution. This voltage is the resting cell potential, also sometimes called the transmembrane potential of the resting cell. As an example, retinal ganglion cells have a resting cell potential of about -60 mV. Cells whose voltage is more negative than typical are said to be hyperpolarized, and those more positive are said to be depolarized. Healthy cells do not naturally hyperpolarize or depolarize except for brief intervals, for example during a nerve impulse or action potential. Among other roles, the cell potential acts as a reservoir for metabolic energy, which cells use to drive the transport of solute molecules across the membrane, to communicate with other cells and to trigger intracellular events.

Between the inside and outside of the cell (which is typically uniform electrically like the cytosol) the voltage rises very steeply just at the boundary created by the membrane. This create an electric field across the membrane, which exerts a force on ions and controls voltage-gated ion channels. Integral membrane proteins such as channels, pumps, and exchangers establish the membrane potential by transporting specific ions in or out. In essence, resting cells are negative because positively charged potassium ions, which are more concentrated inside than outside, are allowed to leak out. The resulting negative voltage difference between inside and out is therefore approximately equal to the reversal potential for potassium. Sodium-potassium exchangers maintain intracellular potassium at a high concentration while pumping sodium into the extracellular solution, where the concentration of sodium typically is high.

The Goldman equation can be used to calculate the membrane potential given the concentration of ions on either side of the membrane and their permeability.

Synaptidude 8 July 2005 22:17 (UTC)

[edit] Dissipation of ion gradiets by transmembrane ion flux

I removed the statement that action potentials run down the membrane ion concentration gradients. As I explained and calculated earlier in the article, the number of ions that cross the membrane during a large swing in membrane potential is infitessimally small compared to the ion concentrations supporting the gradients. Indeed it has been know since early days of studying action potentials, that upon disabling the sodium potassium pump, it takes many 10's of thousands of action potentials to run the gradient down appreciably. So while it is theoretically possible to run down gradients, it can really only happen under two conditions: 1) A highly unnatural experimental manipulation and 2) death.

[edit] voltage difference or potential difference?

I've struggled a little with this one myself. It has been changed to "voltage difference", but I'm not sure that is right. The voltage IS the difference between one side and the other of a resistor. One side of the resistor can't have a voltage, it is by definition a comparison between one side and the other. Still, I'm not sure "potential" is any better. In trying to think this through I'm considering the analogy to altitude. If you hold a ball 10 feet off the floor, the potential energy is the difference between the altitude of the ball and the altitude of the floor (just as voltage is the difference between one side and the other of a resistor). But the question is, what is the equivalent voltage term for the altitude of each thing (the floor and the ball)? I think "potential difference" comes closest, but what does anyone else think? Synaptidude 00:02, 25 August 2005 (UTC)

See Electric potential. And for your analogy with gravity, see Gravitational potential. I think the technical term from physics is "potential difference" between the inside and the outside of a cell. This, from the fact that only differences in potential are important, not absolute values. Another point, maybe knit-picky, to keep in mind is that potential is NOT "potential energy", its potential energy per unit charge (for electrical potential) or per unit mass (for gravitation). I agree that we shouldn't use the term "voltage difference". One other point, people do sometimes talk about the voltage measured on one side of a resistor, but this is always in reference to an arbitrary 0 point (ground). In cell biology, often the fluid outside the cell used as the ground reference. I think this leads to somewhat sloppy terminology like "at rest, the cell is at -60mV".

I'd agree, but add the nit-picky point that even in the case you mention (one side of a resistor), even if recording to an indifferent ground, you still won't get any voltage if there is no resistance between your measuring points. Good point on the /per unit comment. Thanks!Synaptidude 21:55, 25 August 2005 (UTC)

Actually, this is kind of moot. I just went to the article to change it back to "electrical potential difference" from "voltage", but I realized that I'd misread it. It says "the electrical potential difference (voltage) across a cell's membrane...". This is a correct definition of voltage ("potential difference" = voltage). NEVERMIND! Synaptidude 21:59, 25 August 2005 (UTC)

[edit] Introduction

We need to tighten the introduction . It seems to be about three times longer than desirable. What do others think? David D. (Talk) 21:30, 25 January 2006 (UTC)

[edit] Electroneutrality!!

I am no expert in biophysics at all, but during my chemistry degree "physical chemistry of biological processes" It was CLEARLY stated that both compartments (inside and outside) are electrically neutral, that means that there are many wrong sentences in the article, specially in the generation of the resting potential section. The electrical fields needed in order to generate a tiny deviation (10E-6 mol/liter from neutrality are millions of volts, clearly too much) The potential comes from the thermodynamic definition of electrochemical potential (chemical potential + electrical potential) and so and so.

I hope we can go through this but for the momento i'm putting on the expert tab, i think the article really requires rewriting by an expert

"Such a movement of one ion across the membrane would result in a net imbalance of charge across the membrane and a membrane potential"

"As potassium leaves the cell, it is leaving behind the anions" Those are clearly wrong, since cations must be accompanied by anions. An increase in permeability of a cation forces an increase of permeability in all anions, as the are forced to go with it through the membrane.

05:42, 5 November 2006 (UTC)Knights who say ni I'll post my references in a couple of hours

(as a matter of fact there can be a slight local deviation of electroneutrality on the membranes due to their own electric load but that is not the cause of the potential ...)Knights who say ni 18:07, 17 September 2006 (UTC)

What do you mean by "since cations must be accompanied by anions. An increase in permeability of a cation forces an increase of permeability in all anions" You don't seem to be considering that these ions have specific channels (more for some than others) and for some, whether they are open or not can be regulated. Or am i misunderstanding your point?

Also this artical seem to be discussing the electrical potential ONLY. This is not synonymous with the electrochemical potential. What do you mean by "both compartments (inside and outside) are electrically neutral". If there is an electrical charge across the membrane how can both be electrically neutral? David D. (Talk) 04:46, 18 September 2006 (UTC)

The opening of channels can be interpreted as an increase in average permeability of that ion. And that forces other ions with the opposite charge to circulate more, because there can't be a difference in charge across the membrane. The electrical charge across the membrane (which obviously exists) is very small compared to the causes of the membrane potential and is not taked into account when studying the system theoretically (for example in the Goldman equation Knights who say ni 07:30, 18 September 2006 (UTC)

I found this article confusing for this exact reason, and I have to say I support the expert tab for the moment. I'm worried that there is some confusion regarding terminology (which I can't clear up, because I'm not an expert), but here is what I do know: just two days ago I had a lecture (I'm doing a masters in neuroscience) wherein the teacher was careful to dispel the notion that the membrane potential is due to a difference in the charges inside and outside the cell. In fact he was very careful to point out that the charges are neutral. Like Knights who say ni says, the potential difference is due to the thermodynamic properties of any cell with ion concentration differences inside and outside the cell (see Goldman eqn which Knights linked, and also Nernst equation). Now, this may seem totally counterintuitive if we imagine that the membrane potential is an "electrical charge across the membrane" but it is not. Charge is measured in coulombs, and a difference in charges would also be measured in coulombs. This is distinct from voltage (though related). Voltage is a difference in electric potential, which indicates a non-zero electric field, i.e. there is a non-zero amount of force per coloumb in the area. I need to learn a lot more before I'm comfortable rewriting some of this article, but I think that it suffers from some very common misconceptions about what voltage is. (In my second year bio-psych course I was told the difference in charge was the cause, but I was told that by a psychologist who was no expert on cellular biophysics.) Tyrell turing 20:40, 2 November 2006 (UTC)

I went and picked up a good text to figure out why this all seemed so weird to me (Principles of Neural Science, Kandel, Schwartz, and Jessell). I've discovered that I was confused in some ways, but right that the charges are neutral. I think the article does little to address my confusion though, or the confusion of people who think that the charges aren't neutral. Here's what I'd say about it: the article doesn't say anywhere that the charges aren't neutral, but it also doesn't spell out how the charge can be neutral when there is a seperation of charges across the membrane. From what I gather in the book, the article is right in that it is the movement of K+ ions across the membrane (as a result of the chemical gradient), that creates a separation of charge across the membrane, which creates a balancing voltage gradient (because voltage = charge/capacitance). This balancing voltage gradient eventually causes K+ ions to flow in, such that the overall net flow of K+ ions across the membrane is nil, thus reaching electrochemical equilibrium. Of course, although the numbers going in and out balance, there are still a few more positive ions outside, i.e. we still have a separation of charge across the membrane. However, what the article should spell out is that this small excess of positive and negative ions attract each other and line up along the membrane, such that the charges are 100% neutral everywhere except right across the membrane. I didn't find that clear reading it.Tyrell turing 20:48, 3 November 2006 (UTC)

The key here is that the membrane is only permeable to some ions AND is a closed system. Clearly from above you do understand this. Try this reference. http://advan.physiology.org/cgi/content/full/28/4/139 it may help you more. If you can not access it, send me an e-mail and I'll respond with the pdf. David D. (Talk) 23:52, 3 November 2006 (UTC)

It is vital to understand that all you say is obviously correct; BUT the thing is that whenever K+ ions are drawn in one direction, some Na+ or Cl- ions have to cross the membrane in order to balance the charge. So there can be an imbalane in the concentration of ONE ion because the other ones compensate for itKnights who say ni 05:14, 4 November 2006 (UTC)

These ions (Cl^- and Na⁺) do not necessarily move in response to movement of potassium since they cannot move if there are no open channels. Plus, it also depends on their concentration gradients across the membrane too. Are you really saying that for every potassium that leaves the cell a choride HAS to leave or a sodium HAS to enter the cell? Where have you read such a thing? David D. (Talk) 03:47, 5 November 2006 (UTC)

I'm clearly no expert, otherwise i would have just corrected everything i could prove wrong, The thing is that i don't understand all this well and the article really doesn't help.

There are statements about electroneutrality here [1] and here [2] and here [3] and here [4] Just type electroneutrality and membrane potential in google... The thing is the article needs an expert who can explain what goes on and cite a basic level source (a good old thick book), and hopefully have it sent to us in pdf... The article you cited before states that

These values are meant to represent only hypothetical values for a mammalian cell. Veq, calculated Nernstian equilibrium potentials for each ion gradient; Pi, “typical” permeability values for mammalian neurons. The listed concentrations of Cl� in the cytoplasm and extracellular fluid do not add up to the total concentration of K� and Na�. However, macroscopic electroneutrality is maintained in each compartment through the combined contribution of a diverse array of additional charged solutes (both anionic and cationic).

Knights who say ni 05:42, 5 November 2006 (UTC)

[edit] Merge?

There is a distinct article on resting potential. Maybe the bit about resting potential here can be moved to there? I'd do it, if I wasn't very busy with dutch wikipedia. Methoxyroxy 12:37, 2 November 2006 (UTC)

[edit] expert tag off

I'm finally more or less conviced and the article is mainly right (although the writing could be improved but that's not uncommon in wikipedian topics about chemistry). Maybe it should be noted that (i find that a bit esoteric) although there is an electric potential across the membrane there is no actual measurable difference in the global concentration of positive and negative ions across the membrane, that is, there is no actual charge excess in either size. That's because the effect of charge is hugely greater that the effect of concentration so an undetectable change in concentration creates a great change on electric potential. Knights who say ni 01:45, 24 November 2006 (UTC)