Membrane potential

[EmmaNemma]

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I cannot seem to put together the following facts about membrane potential. They seem to contradict each other.

A large intracellular potassium concentration maintains a negative membrane potential. If the conductance of potassium were increased the cell would become hyperpolarized.

Increasing chloride ion conductance hyperpolarizes a cell.

I don't see how both of these things could be true. Would someone straighten me out? Thanks.

 

[SeekerOfTheTree]

I think the simplest reason is that they are ions with two different charges. When you bring chloride into the cell it has a negative charge so it further makes the interior more negative. With Potassium (K) it is still a positive Ion, the only reason you have a membrane potential that is negative is due to the fact that there is more K outside the cell than there is inside the cell. When K leaves the cell the intrinsice negative charge of the cell increases and without a positive ion the cell becomes hyperpolarized.

So in short

K is + and when it leaves cell becomes more negative
Cl is - and when it comes in the cell becomes more negative

I believe this is the reason. Please feel free to correct me if I stated anything incorrectly.

 

[EmmaNemma]

I think the simplest reason is that they are ions with two different charges. When you bring chloride into the cell it has a negative charge so it further makes the interior more negative. With Potassium (K) it is still a positive Ion, the only reason you have a membrane potential that is negative is due to the fact that there is more K outside the cell than there is inside the cell. When K leaves the cell the intrinsice negative charge of the cell increases and without a positive ion the cell becomes hyperpolarized.

So in short

K is + and when it leaves cell becomes more negative
Cl is - and when it comes in the cell becomes more negative

I believe this is the reason. Please feel free to correct me if I stated anything incorrectly.

Thank for your help Seeker. However, what is throwing me is that the Kaplan USMLE Notes For Physiology state that the resting membrane postential for K+ has a much higher concentration of K+ in the cell that out.

 

[SeekerOfTheTree]

OMG I'm an idiot. I meant to say there is more K inside the cell than outside. That's what makes it negative. Not the other way around. There is more K in the cell than out so it wants to go outside creating the potential.

 

[EmmaNemma]

OMG I'm an idiot. I meant to say there is more K inside the cell than outside. That's what makes it negative. Not the other way around. There is more K in the cell than out so it wants to go outside creating the potential.

OK. So then why does opening Cl- channels make the cell more negative. See my delima? Sorry to be a pain.

 

[medking]

OK. So then why does opening Cl- channels make the cell more negative. See my delima? Sorry to be a pain.

I'm not 100% sure so correct me if I'm wrong:

1. Potassium flows out of cell when channels open--->positive ion flowing out of cell--->hyperpolarization (cell gets more negative)

2. Chloride flows into the cell when channels open--->negative ion flowing into cell--->hyperpolarization (cell gets more negative)

I hope I'm not making things up and that this answers your question. If it doesn't, then I guess I'm not understanding your question.

 

[SeekerOfTheTree]

Exactly what medking said. Couldn't have put it better myself.

 

[EmmaNemma]

I'm not 100% sure so correct me if I'm wrong:

1. Potassium flows out of cell when channels open--->positive ion flowing out of cell--->hyperpolarization (cell gets more negative)

2. Chloride flows into the cell when channels open--->negative ion flowing into cell--->hyperpolarization (cell gets more negative)

I hope I'm not making things up and that this answers your question. If it doesn't, then I guess I'm not understanding your question.

OK that makes sense. So why is the resting membrane potential negative when the potassium concentration inside the cell is so much higher than the concenrtation outside the cell?

 

[medking]

OK that makes sense. So why is the resting membrane potential negative when the potassium concentration inside the cell is so much higher than the concenrtation outside the cell?

This is one of those "that's just how it is" type of things. Membrane potential is mainly determined by a few ions and their permeability and what not. It's just a balance of different factors and it is how things are. The inside of the cell happens to be more negative than the outside.

To remember that the inside is more negative than the outside, I just remember that proteins are inside of cells and that they are usually negatively charged.

If you really want to know HOW they determined all these things, google membrane potential and I'm sure you'll find more than enough information on the topic.

 

[EmmaNemma]

This is one of those "that's just how it is" type of things. Membrane potential is mainly determined by a few ions and their permeability and what not. It's just a balance of different factors and it is how things are. The inside of the cell happens to be more negative than the outside.

To remember that the inside is more negative than the outside, I just remember that proteins are inside of cells and that they are usually negatively charged.

If you really want to know HOW they determined all these things, google membrane potential and I'm sure you'll find more than enough information on the topic.

OK thanks for your help.

 

[CuteTrekkieGirl]

Thank for your help Seeker. However, what is throwing me is that the Kaplan USMLE Notes For Physiology state that the resting membrane postential for K+ has a much higher concentration of K+ in the cell that out.

Are you a premed that's studying for Step1 in anticipation of medical school?

EDIT: So you're gunning for derm... nice

 

[SeekerOfTheTree]

If you want to get really deep you can look at it like this.

1) A cell membrane by itself will have an intrinsic negative charge because the proteins inside have negative charges. This attracts positive charges to the cell.

2) Na/K pump pumps 3 Na out for 2 K in. So if you look at that there is a charge imbalance. You are pumping three out for every two you are pumping in. Right there you are creating an exterior that is more positive and an interior that is more negative.

3) The potasium does stabilize the negative charges in the membrane but with an imbalance between forces which still ultimately gives the cell membrane a negative potential.

4) The negative cell membrane wants to keep the K in the cell; however, the chemical gradient wants to pull it out. That's why when the action potential happens the K rushes out and the Na rushes in.

Ultimately action potetial is current and current is defined as which way the electrons want to go which in this case are represented by the ions because they are nothing more than atoms with orbital spaces that can be occupied or are occupied by less/more electrons than they need.

 

[EmmaNemma]

If you want to get really deep you can look at it like this.

1) A cell membrane by itself will have an intrinsic negative charge because the proteins inside have negative charges. This attracts positive charges to the cell.

2) Na/K pump pumps 3 Na out for 2 K in. So if you look at that there is a charge imbalance. You are pumping three out for every two you are pumping in. Right there you are creating an exterior that is more positive and an interior that is more negative.

3) The potasium does stabilize the negative charges in the membrane but with an imbalance between forces which still ultimately gives the cell membrane a negative potential.

4) The negative cell membrane wants to keep the K in the cell; however, the chemical gradient wants to pull it out. That's why when the action potential happens the K rushes out and the Na rushes in.

Ultimately action potetial is current and current is defined as which way the electrons want to go which in this case are represented by the ions because they are nothing more than atoms with orbital spaces that can be occupied or are occupied by less/more electrons than they need.

Thanks Seeker. That makes perfect sense.

E