Talk:Reversal potential
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Questions
What unit of measure is applied to driving force?
After subtracting Membrane potential from ion equlibrium potential and getting the driving force, what unit of measure is used for this numerical value? — Preceding unsigned comment added by Spiked415 (talk • contribs) 19:35, 8 June 2012 (UTC)
- Strictly speaking, it isn't force, but potential, so it's measured in voltage, typically millivolts (mV). --Tryptofish (talk) 21:43, 1 February 2013 (UTC)
Also isn't the reversal potential a synonymous/equivalent term with Resting Potential? Already an article here on Wiki? — Preceding unsigned comment added by Spiked415 (talk • contribs) 19:38, 8 June 2012 (UTC)
- No, they are two different things. That's because there is a reversal potential for each individual kind of ion, whereas the resting potential is for the entire cell. The relationship between them is given by the Goldman equation. --Tryptofish (talk) 21:43, 1 February 2013 (UTC)
Reversal versus equilibrium
An article clarifying the intended difference between reversal potential and equilibrium potential is nice. It offers a good explanation on the origins of the word choice "reversal" in "reversal potential". However, regardless of the number of ions involved, the "reversal potential" described here and in textbooks (such as From Neuron to Brain, 5e, J Nicholls at el) IS the "equilibrium potential" for the whole system.
"Equilibrium potential" is defined as the membrane potential when ion exchange is at equilibrium. And at equilibrium, the entropy change with respect to number of particles in a given microstate reaches zero, and therefore reverses signs when changing from either direction. In our context, entropy change directly relates to change in ion concentration and/or change in membrane potential.(Refer to any statistical mechanics textbook.) Therefore, "equilibrium" and "reversal" refer to exactly the same thing. It should be stressed that equilibrium is ALWAYS about some (fixed) system. Making an unnecessary distinction between "reversal potential" and "equilibrium potential" by statements such as "their numerical values are identical" and "the relationship [of the two terms] holds true only for single-ion system" sends off a very wrong message.
I am not involved with the WikiNeuroscience project. Would an editor fix the writing please?冷雾 (talk) 02:42, 1 February 2013 (UTC)
- I'm not really convinced that something needs to be revised. You say that "regardless of the number of ions involved," they are the same. The problem is that one does not normally consider a reversal potential to apply to more than one ion at a time (although I suppose that in principle one could, by generating, in effect, a weighted average). Equilibrium applies to however many ions are present, as you correctly describe. However, if we are, for example, at the Na+ reversal potential, we are not at equilibrium for K+. --Tryptofish (talk) 21:49, 1 February 2013 (UTC)
- Convention is convention. Concept is concept. I have stated the definitions of both reversal and equilibrium potentials. It seems that you do not dispute the definitions. So the concept should be clear. Then, an additional factor is, by convention, when we say equilibrium, we usually refers to equilibrium of any system, and when we say reversal, we refer to a single-ion system. I think we could afford such a line instead of making the incorrect statement "the relationship [of the two terms] holds true only for single-ion system". As for the example you raised, replace reversal with equilibrium. What is changed? If you do not dispute the definition that equilibrium potential of Na+ is the ionic potential of Na+ (calculated via concentration ratio) when the system is in equilibrium. Nothing is changed. As I have explained, it is also when the rate of change of entropy reverses signs, and hence reversal potential. 冷雾 (talk) 16:12, 13 May 2013 (UTC)
- Well, I guess it depends on whether one is referring to the equilibrium potential of the membrane of a biological cell, or to the equilibrium potential of a specific ion channel. I've see the term used both ways. Reversal potential is used only in reference to single ion species. --Tryptofish (talk) 01:42, 14 May 2013 (UTC)
- Convention is convention. Concept is concept. I have stated the definitions of both reversal and equilibrium potentials. It seems that you do not dispute the definitions. So the concept should be clear. Then, an additional factor is, by convention, when we say equilibrium, we usually refers to equilibrium of any system, and when we say reversal, we refer to a single-ion system. I think we could afford such a line instead of making the incorrect statement "the relationship [of the two terms] holds true only for single-ion system". As for the example you raised, replace reversal with equilibrium. What is changed? If you do not dispute the definition that equilibrium potential of Na+ is the ionic potential of Na+ (calculated via concentration ratio) when the system is in equilibrium. Nothing is changed. As I have explained, it is also when the rate of change of entropy reverses signs, and hence reversal potential. 冷雾 (talk) 16:12, 13 May 2013 (UTC)