Membrane Potential and Concentration Gradient

[drXanthine]

In order to have a membrane potential, there must be a concentration gradient for the ion as well as membrane permeability to that ion. The classic example is the neuron and Na-K pump. The membrane is relatively very permeable to K and barely permeable to Na. K tries to flow down its concentration gradient, which develops a "negative" charge on the inside of the cell. The equilibrium potential is reached at the point the concentration gradient is balanced by the developing potential.

Now to where I get confused. My book states that if the Na-K pump was not present, the concentration gradient would dissipate. In my head, it makes sense that the concentration gradient might never get established, but if the gradient was already there I feel like there would still be an equilibrium potential. Is this only the case if the membrane was completely impermeable to one of the ions? I think the reasoning has to do with the fact that the membrane is slightly permeable to both, however, this still doesn't completely make sense to me.

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[mcloaf]

The key is that the cell membrane is leaky because it always has some ions channels open (conveniently called leak channels). Even while the cell is pumping ions out and in to establish and/or maintain these gradients, the ions are continuously leaking back across the membrane. As long as these pumps are active the cell can maintain the resting potential, but if the pumps were turned off or removed the ions would slowly run down their electrochemical gradients and drain the membrane potential away.

A couple other things: remember that because we're talking about ions, we're talking about an electrochemical gradient rather than just one of concentration. Also, it's important to remember the difference between equilibrium potential, which you've already explained above, and resting membrane potential, which is the dynamic balance of influx and efflux of multiple ions across the membrane.

 

[drXanthine]

OK, so I think I have it figured out. The electric potential difference is a byproduct of the concentration gradient. Since the channels are leaky, the concentration gradient would dissipate without the Na+-K+ ATPase. Without a concentration gradient, there can not be an equilibrium potential.

Due to the membrane permeability, the K+ flows out faster than the Na+ flows in. This is the primary reason why the inside of the cell is (-) with respect to the outside (+). Consider the inside of the cell high in K+ and low in Na+ and vice versa for the outside. If the membrane was completely impermeable to Na+, a concentration gradient would still develop in the absence of the Na+-K+ pump, right? It's the leaky channels that cause the Na+-K+ pump requirement.

 

[mcloaf]

While the cell membrane is much more permeable to potassium than sodium, I would argue that the primary reason for the relative negativity is that the Na/K pump moves three sodium ions out for each two potassium ions in, so with each pumping cycle you've moved one net positive charge to the exterior of the cell.

 

[sps27]

I had some confusion regarding this as well so I am just trying to clear it up. I hope you don't mind my butting in. My understanding of action potential goes something like.....

1) AcH is released, binds to the receptor which allows Na+ to get in via facilitated diffusion. So that is fine. Na+ is not leaking in via leakage channels, it is getting in via facilitated diffusion through AcH. When Na+ gets in, that activates the 'voltage gated Na+ channels in the neighboring section of axon hillock' and they open up. It also slowly activates the K+ voltage gated channels for the same section. The potential goes past the threshold and then to 35+ve. So this would be depolarization. At that point K+ voltage gated channels open up in the same section, AcH sterase breaks up AcH and Na+ channels close down. Na+ is not getting in anymore. Now the stage is set for re-polarization Re-polarization takes place primarily due to K+ voltage gated channels which have recently opened up. Meanwhile the Na+ voltage gated channels of the neighboring section are depolarizing.

2) Since the affinity of K+ voltage gated channels is far more than Na+ and they are far more leaky, so they typically allow more K+ to get in the cell thereby causing it to hyper-polarize.

3) As soon as hyper-polarization sets in, the Na/K pump protein kicks into gear and starts throwing 2 K+ ions out and takes 3 Na+ ions in to reset the balance.

This last point 3, is my conjecture. I have not read it per say. So I guess my question is, can we safely assume that Na/K pump protein gets activated only to bring the hyper-polarization back to resting potential for the membrane???

I was told by SDNers that Na/K pump is operating continuously and it is not like they kick in during hyper-polarization. If that be the case then I would assume that although Na/K pump does play a role in action potential but to rule out the possibility of action potential due to absence of Na/K pump would not be the case because really facilitated diffusion with AcH+Na, voltage gated channels are the key players here. Is that correct?

 

[drXanthine]

According to this, the Na+-K+ ATPase pumping 3 Na+ for 2K+ has a negligible effect on the resting potential when compared to the permeability.

sps27: I do not believe point (3) is correct. My understanding is that the closing of the K+ channels is what repolarizes the membrane. I'm also not sure that the Na+-K+ pump ever was turned off.

 

[mcloaf]

Ah, good find drXanthine, guess I was wrong there.

And I agree that there's no reason for the Na/K pump to be turned off during an action potential or leakage channels to be closed for that matter. The voltage-gated ion channels are what dictates the AP as they allow the movement of ions much faster than pumping ever could.