Membrane Potential

[TheSeanieB]

Advertisement - Members don't see this ad

If you decrease the extracellular concentration of Na+, does that cause the membrane potential to become more positive or negative?

 

[kaleerkalut]

If you decrease the extracellular concentration of Na+, does that cause the membrane potential to become more positive or negative?

Completely guessing, but I think it would make it more negative (i.e. it would be harder to get an action potential going) because the concentration gradient wouldn't be as high and that is one of the components. If I'm wrong, I hope somebody would correct this.

 

[Jorje286]

Completely guessing, but I think it would make it more negative (i.e. it would be harder to get an action potential going) because the concentration gradient wouldn't be as high and that is one of the components. If I'm wrong, I hope somebody would correct this.

No I think you're wrong. The membrane potential is measured with the reference electrode (V = 0) outside of the cell, so normally, the inside of the cell is a lot more nagative than the outside (hence why the potential is negative). If you remove positive ions from the outside, the inside will become more positive (i.e less negative) in comparison to the outside, and the membrane potential will increase.

 

[Ty Webb]

No I think you're wrong. The membrane potential is measured with the reference electrode (V = 0) outside of the cell, so normally, the inside of the cell is a lot more nagative than the outside (hence why the potential is negative). If you remove positive ions from the outside, the inside will become more positive (i.e less negative) in comparison to the outside, and the membrane potential will increase.

This makes no sense. I think you're confusing the amount of charge on either side of the membrane with the concept of electrochemical potential, which is what dictates the membrane potential. If you add more positive sodium ions to the extracellular space, that means there is a larger driving force of positive ions wanting to cross into the membrane, making the membrane potential more positive (just like what happens when sodium ions enter the cell during an action potential). The reverse then of course will be true if you reduce the concentration of extracellular sodium ions: you'll have less driving force, and thus a more negative resting membrane potential, since potassium's contribution to RMP will further outweight that of sodium leak channels.

A little more explanation:

The transmembrane potential of sodium is ~+65mV, hence why when sodium channels are opened during an action potential, there is a massive influx of sodium, making the cell more positive relative to the extracellular space.

The cell membrane also permits some degree of Na+ to leak through during rest, which also makes the resting potential more positive than it would be otherwise, as potassium, which has a high membrane permeability due to a relatively large number of potassium leak channels, has a potential of ~-85mV is somewhat counteracted by a small influx of sodium, causing the resting potential (~-65-70mV) to be less negative than it would be if the membrane were permeable to potassium alone. Thus, if sodium were reduced extracellularly, there would be less influx of sodium during rest, and a more negative RMP would result.

As far as the clinical importance of hyponatremia goes, I'm pretty sure the osmotic effects on neurons are much more significant than the change in resting potential. I don't think a modest change in sodium concentration would change the resting potential all that much, but feel free to plug it into the Nernst equation to check for yourself.

 

[TheSeanieB]

This makes no sense. I think you're confusing the amount of charge on either side of the membrane with the concept of electrochemical potential, which is what dictates the membrane potential. If you add more positive sodium ions to the extracellular space, that means there is a larger driving force of positive ions wanting to cross into the membrane, making the membrane potential more positive (just like what happens when sodium ions enter the cell during an action potential). The reverse then of course will be true if you reduce the concentration of extracellular sodium ions: you'll have less driving force, and thus a more negative resting membrane potential, since potassium's contribution to RMP will further outweight that of sodium leak channels.

A little more explanation:

The transmembrane potential of sodium is ~+65mV, hence why when sodium channels are opened during an action potential, there is a massive influx of sodium, making the cell more positive relative to the extracellular space.

The cell membrane also permits some degree of Na+ to leak through during rest, which also makes the resting potential more positive than it would be otherwise, as potassium, which has a high membrane permeability due to a relatively large number of potassium leak channels, has a potential of ~-85mV is somewhat counteracted by a small influx of sodium, causing the resting potential (~-65-70mV) to be less negative than it would be if the membrane were permeable to potassium alone. Thus, if sodium were reduced extracellularly, there would be less influx of sodium during rest, and a more negative RMP would result.

As far as the clinical importance of hyponatremia goes, I'm pretty sure the osmotic effects on neurons are much more significant than the change in resting potential. I don't think a modest change in sodium concentration would change the resting potential all that much, but feel free to plug it into the Nernst equation to check for yourself.

Thanks. Your explanation makes sense.

 

[richse]

If you decrease the extracellular concentration of Na+, does that cause the membrane potential to become more positive or negative?

I disagree with the post above this.

The resting membrane potential (RMP) for a cell is negative. This means the inside is more negative than the outside. You can also think of this as the inside having less positive charges than the outside. Thus, if you decrease the positive charges on the outside of the cell then there will be less of a difference between the outside and the inside. The RMP will move closer to 0 (ie. become more positive).

 

[Zenfudge]

I disagree with the post above this.

The resting membrane potential (RMP) for a cell is negative. This means the inside is more negative than the outside. You can also think of this as the inside having less positive charges than the outside. Thus, if you decrease the positive charges on the outside of the cell then there will be less of a difference between the outside and the inside. The RMP will move closer to 0 (ie. become more positive).

I tutor physio and this is a common error - re: Caddieshack is correct.

Prove it to yourself: http://www.physiologyweb.com/calculators/ghk_equation_calculator.html

Fill the cells with sample data and then decrease [Na]o ----> hyperpolarization. The easiest way to think about it is if you decrease Na outside, the Na is not going to want to go in the cell as much. It's that simple.

This is an important concept for electrolyte disturbances in DKA, SIADH, hyperkalemia, etc. It takes a while for the shifts to start making sense intuitively.

 

[richse]

I tutor physio and this is a common error - re: Caddieshack is correct.

Prove it to yourself: http://www.physiologyweb.com/calculators/ghk_equation_calculator.html

Fill the cells with sample data and then decrease [Na]o ----> hyperpolarization. The easiest way to think about it is if you decrease Na outside, the Na is not going to want to go in the cell as much. It's that simple.

This is an important concept for electrolyte disturbances in DKA, SIADH, hyperkalemia, etc. It takes a while for the shifts to start making sense intuitively.

Edit: I used the calculator at the link you provided so I see that the Vm gets more negative but I'm just not conceptualizing it very well at the moment.

 

[just one]

Edit: I used the calculator at the link you provided so I see that the Vm gets more negative but I'm just not conceptualizing it very well at the moment.

Sodium normally wants to go inside the cell. When this happens the membrane potential depolarizes (becomes more positive). If you decrease the outside concentration of sodium there is less of a gradient so the cell gets less sodium coming in (hence the cell becomes more polarized).

Remember that sodium does enter the cell in small quantities at resting membrane potential. This helps account for the fact that resting membrane potential is slightly more positive than the potential for potassium alone