RMP of neurons

[549]

How exactly does the Na+/K+ pump restore the resting membrane potential after a hyperpolarization? Doesn't moving 3 (+) charges out and only 2 (+) charges in add to the negative charge inside the cell? I get that it moves the ions back to establish the concentration gradients, but how does it bring the membrane voltage up?

Also, how do the K+ leak channels help to bring the membrane potential back to the RMP? At that point, the inside of the cell is close to the equilibrium potential for K+, and I don't get how more K+ moving out of the leak channels would help bring the membrane potential back up.

Thanks a lot!

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[Pons Asinorum]

Quicker than typing it all up...
http://www.khanacademy.org/video/electrotonic-and-action-potentials

How exactly does the Na+/K+ pump restore the resting membrane potential after a hyperpolarization? Doesn't moving 3 (+) charges out and only 2 (+) charges in add to the negative charge inside the cell? I get that it moves the ions back to establish the concentration gradients, but how does it bring the membrane voltage up?

Also, how do the K+ leak channels help to bring the membrane potential back to the RMP? At that point, the inside of the cell is close to the equilibrium potential for K+, and I don't get how more K+ moving out of the leak channels would help bring the membrane potential back up.

Thanks a lot!

 

[549]

Thanks for the link- so I watched the video and his explanation for how exactly the Na+/K+ pump restores RMP was just, "the Na+/K+ pump might get us back to about -70 mV." Unfortunately, that doesn't answer my question of how moving two positive charges in and three out will help to make the membrane potential more positive after the hyperpolarization. Also, he never addressed the face that K+ leak channels are involved in restoration of the RMP. Can someone please explain this? Thanks!

 

[jsfkt7]

The answer is simple, seems like you're over thinking it and maybe assuming that it must be harder than it actually seems.

You have to consider that the membranes are dynamic and not static. There are periods where the Pk(permeability of K) is much much much greater than that of Pna(permeability of Na), and vice versa.

What does that have to do in terms of your question ---> A lot!

The purpose of the Na/K pump is simply to restore the asymmetrical balance of ions. Some people may argue that it has more functions but in the essence of this explanation, just concede that is all there is to it.

Now, since you already know that, what does that mean?

Remember that after the cell hyperpolarizes, its relatively impermeable to Na. Prior to hyperpolarization, the Na that burst into the cell during depolarizing it, thus becoming stuck inside. During hyperpolarization the ATPase pumps it out. Okay, but thats counter intuitive because wouldn't that add more negative charge inside? Yes in no, depends on how short-sighted you are.

Why?

Because the membrane is very permeable to K. And as the membrane (inside) become more negative K rushes down its electrochemical gradient UNTIL (maybe a fraction more) it is effectively repelled by the charge build up inside. That charge build up can be relatively calculated using the nernst equation but a more pragmatic assumption is that K goes down its electrochemical gradient till the inside is roughly -80mV (numbers vary slightly in different texts, but this is the one that I committed to memory a long time ago).

Another side note, there's not that big of a difference between -87mV and -80mV. So the way I'm looking at it, it doesn't seem illogical to assume that K goes inside the cell due to:
1.) electrochemical gradient set up by the ATPase Pump (remember that it effectively restores the asymmetrical distribution of ions)
2.) & because the membrane is permeable to K.

That's my $0.02 anyways

 

[549]

"So the way I'm looking at it, it doesn't seem illogical to assume that K goes inside the cell due to:
1.) electrochemical gradient set up by the ATPase Pump (remember that it effectively restores the asymmetrical distribution of ions)
2.) & because the membrane is permeable to K."

Oh, okay- so you're saying that what brings the neuron back to a slightly more positive RMP after the hyperpolarization is that K+ actually moves back into the cell because of the electrostatic force from the negative potential inside the cell, and it's therefore "pulled" back in (I hope I understood your explanation right!). But K+ rushed out of the cell to create that large negative potential inside the cell because it was moving towards its equilibrium potential, where it's "happy." And since the K+ equilibrium potential is lower than the RMP (because it's not the only ion contributing to the RMP), I don't get why K+ would suddenly be pulled into the cell after it hyperpolarized it in the first place.

I'm probably overthinking this, like you said, but I'd still like to try and understand this fully- thanks for your help!

 

[fizzgig]

ok this is the best i can do. look up 'resting membrane potential' and read a bit. every ion involved has an electric gradient and a chemical gradient. given, at rest, which ions leak and how much, and which ions have pumps and what those ratios are, rmp is -70mV (or whatever, depending on the cell). ok. so basically, no matter WHAT YOU DO, if all ion channels go back to functioning the way they did originally, and pumps too, you will end up getting back to rmp.

yes after the AP the inside is more negative than rmp, and 3Na are going out for every K that goes in... but that is happening all the time, right? and the cell does not just continually get more and more negative.

remember the current membrane potential changes the electrochemical gradient for an ion.

once again, CVphysiology comes through for the win. look at the potassium potential paragraph.
http://www.cvphysiology.com/Arrhythmias/A007.htm
you mention that if K were the only ion it would want to leave the cell and make it more negative. the equation on this site reflects that. this is true GIVEN the resting K concentration gradient. GIVEN that gradient, you'd need -96mV inside the cell attracting the positive K to keep equilibrium. IF the potassium gradient changes, so does the amount of negative charge inside the cell required to keep that gradient. make sense, ish? assuming the site's values, if the concentration of K outside the cell goes up 10x, only -35mV would be required to keep that gradient, and you're hyperpolarized right now, at -80, right? so at that moment in time K actually does want INTO the cell.

i wouldn't try to get super step by step in how you think this through, because it's a differential equation that changes with EVERY change in concentration for each component, so it's kind of a pain.

hope that's useful.