Neuron Electrical Potential Question

[gea113]

Hello,

I came across a question that asked what would happen if only the Na+/K+ pumps were allowed to function on the membrane of a neuron. So no diffusion through any other ion channels could occur. The answer is that the membrane potential would continuously drop, which I took to mean that it would approach 0V from -70mV (but maybe I'm interpreting that terminology wrong, and that is my issue). I thought however that the membrane potential would rise, because according to the Nernst Equation, the more the concentration of potassium ions inside and outside the cell differ, the more negative the electrical potential will be, meaning the stronger the potential right? If K+ can't diffuse out of the cell, the pump would create a huge concentration of it inside the cell, and a very low concentration of it outside. So doesn't that mean that the membrane potential increases if only the Na+/K+ pumps are the only things functioning that transport ions?

Thanks!

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[Cawolf]

Drop, as in hyperpolarize, i.e. become more negative.

 

[gea113]

Drop, as in hyperpolarize, i.e. become more negative.

Ok, cool! Thanks!

 

[rabbott1971]

The explanation is that for every "pump" you have 3+ (Na) leaving, and 2+ (K) entering, for a net of -1 per "pump" from the intracellular perspective. This is in my EK materials which I assume is where you also got the question.

I am a little confused on this one too. I thought "membrane potential" referred to the net difference in polarity inside versus outside. Because if it keeps getting more and more negative inside, increasing membrane polarity (hyperpolarize) then I had the same thought as you gea113, that the "net" membrane potential would increase. Maybe Aldol or desperadoflakes can help. Action potentials are not my best topic.

 

[desperadoflakes]

The explanation is that for every "pump" you have 3+ (Na) leaving, and 2+ (K) entering, for a net of -1 per "pump" from the intracellular perspective. This is in my EK materials which I assume is where you also got the question.

I am a little confused on this one too. I thought "membrane potential" referred to the net difference in polarity inside versus outside. Because if it keeps getting more and more negative inside, increasing membrane polarity (hyperpolarize) then I had the same thought as you gea113, that the "net" membrane potential would increase. Maybe Aldol or desperadoflakes can help. Action potentials are not my best topic.

I think that by "drop" they just meant that the value would become more negative. If the only action on the ions was the ATPase than the potential would definitely be negative. Drop was a bad word to use because it's so ambiguous and I doubt you'd see something like that on the mcat

 

[rabbott1971]

So then membrane potential is not the net difference between intra- and extra-, but instead is a measurement of one side's (we are to presume intra- I suppose) electrical state? Because it seems to me one side doesn't really have "potential" unless it is measured against another value. It simply has a quantity of charge. Where am I going wrong with this thought process?

 

[rabbott1971]

I think I have seen in other places that the extra-cellular side is presumed to have a value of zero, so perhaps this is where the comparison is coming from. If that is the case, then that gives us two values to compare and derive potential. But it still seems to me that the potential (absolute value of difference between intra- and extra-) will increase as the intra- side becomes increasingly negative. At least isn't this how we think of redox towers? One side will have a positive reduction potential, the other side a negative one, and you sum the difference? Is this not the same?

 

[desperadoflakes]

I think I see what you're saying, but after thinking about this question, I actually think I was totally wrong. The cell will definitely depolarize, not hyperpolarize. The term 'drop' meant depolarization. I think maybe I'm burning out lol..testing this saturday

Anyway, the most important thing to know about the membrane potential, and the thing the mcat will definitely try to spank us with if it wants to ask about the resting membrane potential, is that the resting membrane potential is not due to the action of the pump. The function of the pump is to create a concentration gradient where K+ will want to flow out of the cell (if channels are available) and Na+ wants to flow in (again).

When we think about the pump we pretty much think about it being basically not permeable to sodium at all. But it is permeable to K+. The membrane potential is due to the fact that K+ is flowing out of the cell, which now isn't happening. The membrane will depolarize.

After looking this up just now, it turns out that the membrane potential due to just the ATPase would be -12mV.

Sorry for the confusion lol

 

[desperadoflakes]

The way that I was thinking about it before that led me down the totally wrong road was making up this pretend system in my head where there was unlimited K+ and Na+ ions and eventually there'll be infinite Na+ outside and K+ inside which is totally wrong because the concentrations of the ions on the inside and the outside are regulated.

The right way to think about this is "what if you have a regular neuron where everything is working fine, and now all of a sudden you take out all the ion channels." Well you already have almost all of your K+ inside and almost all of your Na+ outside, so your pump really isn't going to change anything very much. What will happen is you now no longer have the electromotive force generating that potential

 

[desperadoflakes]

Idk maybe the 'unlimited' interpretation was what this test prep company wanted. As long as we know the net flow of K+ from in to out is the force generating the potential and not the pump

 

[desperadoflakes]

And yeah the membrane potential is the net difference between in and out, it's just that we arbitrarily call "out" 0. Will be helpful to have an idea of the nernst and goldman equations.

The nernst equation tells you what the membrane potential would be if only one ion was permeable in the cell.

V = (RT/zF)*ln([ion outside]/[ion inside]

For body temp 37C this simplifies to V = (61.5)/z*log(out/in) where z is the charge of the ion, make sure for anions like chloride you switch out/in to in/out.

The nernst potential for K+ is about -90, for Na+ its about +60. And without both apparently the ATPase is just -12.

Also helpful to understand the goldman equation, which is basically a modified version of the nernst but if you have multiple permeable ions. You say that your total permeability through the membrane is = 1, and the relative P of each ion is represented by a fraction of that. You basically just replace the (out/in) part of the Nernst: add up the permeability*[ion] for all the outside cations and the inside anions, and divide by the sum of the permeability*[ion] for inside cations/outside anions. This is the same as just doing the nernst potential and pretending that all the permeable ions are actually just one weighted average of an ion. For example, let's say that potassium, sodium and chloride are all permeable at 0.7, 0.2, 0.1 respectively.

Vm = (61.5/z)*log((0.7*K+out + 0.2*Na+out + 0.1*Cl-in)/(0.7*K+in + 0.2*Na+in + 0.1*Cl-out))

I don't think we need to memorize the goldman but I guess it might show up on a passage. Also, it can only calculate ions that have the same z, so you can't include Ca2+ in a calculation with K+ for example. The z for the example is 1.

Also, if we think of the neuron as the wire of a circuit with an internal resistance, and the mcat asks about conductance, it's helpful to know that conductance is 1/resistance, and we can think of the conductance as being exactly the same as permeability. When more channels are open more current can flow. We can think of the cytoplasmic fluid of the cell as being the wire and the transmembrane channels oriented perpendicular to it as being resistors in parallel. The more channels we have, the lower the equivalent resistance, and the higher the conductance

 

[rabbott1971]

Thanks for that, I need to spend some more time on this because we just didn't really hit on it in my biochem class for whatever reason. I think you are in good shape for the test. A lot of the books say to take some time off in the days leading up to the test but I know that will be really hard for me. How are you feeling about it?

 

[desperadoflakes]

lol I'm still memorizing psych terms ;( good luck man

 

[rabbott1971]

Thanks you too.

 

[desperadoflakes]

Thanks you too.

Crazy thing. Today I was taking one of the AAMC sample tests, and that resistor analogy I wrote up there was literally on it. Like an hour after I wrote it lol

 

[rabbott1971]

See all your helping people on SDN was also helping you too. Taper off over the next day or two if you can help it. Good luck hombre.