PHYS20008 membrane potential question

[ChickenCh0wM1en]

Hi guys,

I just had a few questions about membrane potential..

1) I know that if I add K+ into the ECF, Resting membrane potential and equilibrium increases (becomes less negative) - more likely to fire but how does this occur? If we add K+ to ECF, the concentration gradient is reduced since K+ is higher in the inside of the cell - less negative charge is required to keep K+ inside the cell but how does this change anything? (i don't really see the link)
2) Would the opposite effect occur if I increase ICF K+?
3) If I add Na+ into the ECF, concentration gradient for Na+ is increased and because the inside is more negatively charged, eq potential increase, RMP increases - more likely to fire (I get that I think) - correct me if I'm wrong. Would an increase in ICF Na+ have the opposite effect?
4) What would happen to RMP if I increase # leak channels for Na+ and K+ respectively and why?

Thanks!

[Shenz0r]

1) I think that sort of scenario is just trying to test your understanding of RMP. If we need less negative charge to pull the K+ back into the cell then the equilibrium potential would be more positive. This doesn't actually happen during an action potential.

2) Yeah, so if we increase K+ in our cells, the concentration gradient increases and we need more negative charge so that it will equal the diffusion force. So equilibrium potential would be more negatve.

3) Correct.

4) Remember that the RMP is determined by relative permeability between K+ and Na+. Normally our RMP is at -70mV. If we increase the permeability of K+, we increase the number of K+ leak channels then more K+ is diffusing out the cell, so we need more negative charge to pull those K+ back. So the RMP would become more negative and will move closer to the equilibrium potential for K+, whichs is -90mV.  If we increased permeability to Na+ then we increase the number of Na+ leak channels so more Na+ ions didfuse into the cell. We need a positive charge inside the cell to repel the Na+ and push it out, so we bring the RMP closer to the equilibrium potential for Na+ which is +60mV.

[Starlight]

1)Hyperpolarization (where MP becomes more negative) is basically movement of K+ from the inside of the cell (provided its there at a high concentration) to the outside of the cell. Here, since the concentration gradient is not as strong, MP becomes more +ve because less K+ is moving out of the cell. Shezn0r's explanation of the EP is good so I won't elaborate on that.

2)Yep see the first line I wrote in Q1. High conc K+ inside the cell means more K+ moves to the outside of the cell and MP becomes more -ve (as it is less +ve inside the cell with K+ ions moving out). The charge needed to bring that K+ back to the inside of the cell needs to be greater i.e. more negative because K+ is being driven out the cell with a concentration gradient.

3)Increase in ICF Na+: Concentration gradient to move sodium into the cell from the ECF decreases. Less Na+ moves into the cell. Equilibrium potential/charge needed to bring Na+ ions into the cell needs to be more -ve to attract those Na+ ions.

4)More leak channels for K+: More K+ moves from the ICF to the ECF. This is hyperpolarization so MP decreases. Charge/Equilibrium potential needed to bring K+ Into the cell needs to become more -ve to attract those K+ ions back into the cell.

More leak channels for Na+: More Na+ moves from ECF to ICF. This is depolarization so MP increases. Charge/Equilibrium potential needed to bring Na+ Into the cell becomes more +ve as the concentration gradient for Na+ is already working to bring those Na+ ions into the cell.

You will want to know everything about how different scenarios affect MP and EP & in addition, knowledge of the graph about RMP, EP for K+ and Na+ and labelling the axes for that graph in prep for the exam. Keep practicing this kind of stuff with the human phys past exams.

[Starlight]

Thanks guys!

I'm probably going to get scolded for this but what does it mean by a positive/negative equilibrium potential - it's related to the Nernst potential right?
So would a +ve EP correlate to high concentration on outside v.s inside and vice versa?

Thanks again C:

All you need to know for the equilibrium potential for physiology is that it's the charge needed to attract ions into a compartment (either the ICF or ECF). E.g. if there is a large concentration of ions in compartment 1 then compartment 2 will need to become more negative (I.e. EP becomes -ve) in order to attract those ions to compartment 2.

Again, using the examples above:

"if I add K+ into the ECF". Under resting conditions, the concentration of K+ is higher in the ICF than the ECF. By adding K+ into the ECF, you are decreasing the concentration gradient by which K+ moves from the ICF to the ECF. Therefore the EP becomes more positive as the ICF doesn't need to attract those ions as much.

"if I add K+ into the ICF" Under resting conditions, the concentration of K+ is higher in the ICF than the ECF. By adding K+ into the ICF, you are increasing the concentration gradient by which K+ moves from the ICF to the ECF. Therefore the EP becomes more negative as the ICF needs to have more of a negative charge to attract those ions back into that compartment.

[Starlight]

You're a legend mate, thanks so much.
Ugh I feel so stupid not understanding this before, gotta brush up and fully wrap my head around it

No probs. Don't worry, it's a bit difficult to get your head around at first but by the time you are revising for the MST & the exam you will find that interpreting these membrane potential scenarios isn't too bad at all!

[MelonBar]

Osmotic equilibrium means that osmolarity is the same inside and outside the cell, which is the concentration of both penetrating and non-penetrating solutes ?? If that is true, then the total number of particles on either side of the membrane can vary, because the volume of water/fluid can change the concentration of ions etc whether it is inside the cell or outside the cell - is this correct?

[MelonBar]

ty men. physo is hard as... harder than anatomy and biochem?

[Starlight]

ty men. physo is hard as... harder than anatomy and biochem?

I never once thought physiology was harder than anatomy & biochem :l

[hobbitle]

Hi guys,
in the attached screenshot, I'm kind of confused about one thing. 
If you look at the yellow area (the active area where the Na channels are open) you can see that the polarity across the membrane is reversed (negative outside, positive inside).
Is this really accurate? 
So, I understand that the -90mV charge that holds K+ inside the cell at RMP is due to negatively charged proteins and stuff like that (I actually wish they didn't gloss over it so much, because its pretty confusing to just say "the cell has a negative charge". Like, what?).
But at the peak of a action potential, is it REALLY a negative charge on the outside of the membrane?  Or did they just draw it like that for illustrative purposes?  I don't really understand how the polarity totally swaps - I understand that the concentration gradient across the membrane gets reduced, but to have it switch polarity completely?
Does anyone know what I'm on about? I'm confused.

[Starlight]

Hi guys,
in the attached screenshot, I'm kind of confused about one thing. 
If you look at the yellow area (the active area where the Na channels are open) you can see that the polarity across the membrane is reversed (negative outside, positive inside).
Is this really accurate? 
So, I understand that the -90mV charge that holds K+ inside the cell at RMP is due to negatively charged proteins and stuff like that (I actually wish they didn't gloss over it so much, because its pretty confusing to just say "the cell has a negative charge". Like, what?).
But at the peak of a action potential, is it REALLY a negative charge on the outside of the membrane?  Or did they just draw it like that for illustrative purposes?  I don't really understand how the polarity totally swaps - I understand that the concentration gradient across the membrane gets reduced, but to have it switch polarity completely?
Does anyone know what I'm on about? I'm confused.

So the definition of an AP is that it should go from -50 mV(threshold) to around -70 mV, where after-hyperpolarization is expected to kick in. In the resting state, the RMP of the cell is expected to be at -70 mV (correspond to the inside of the cell membrane having negative charges). As soon as RMP reaches threshold (-50 mv) i.e. when the trigger event occurs, the activation gate of the VG sodium channel rapidly opens. As is shown in the active area of the diagram, sodium ions rush into the cell from the ECF and hence take their positive charges into the cell. The active area in the picture is representing the peak of the action potential, i.e. just before the inactivation gate closes the VG sodium channel and the potassium VG channel is triggered to open and allow K+ ions to exit from the ICF to the ICF.

Basically I think the reason there are + charges lining the inner membrane of the active area is because of the fact that this area of the cell is at a MP ranging anywhere from 0 to +30 mV during the AP event(here the example is at peak potential: +30 mV). The adjacent inactive area still has negative charges lining the inner membrane because although sodium ions are flowing from the active area to that region to stimulate a depolarization, the AP event has not occurred and so it does not have a positive MP in that area yet.

[hobbitle]

I think I must have expressed my question badly because I was talking about the -ve charges on the outside of the membrane (ECF) that are still within the active area... not the -ve charges adjacent.
However I think you still answered my question inadvertently!
I think it's definitely more about thinking of it like a relative charge difference thing rather than absolute.

[Starlight]

I think I must have expressed my question badly because I was talking about the -ve charges on the outside of the membrane (ECF) that are still within the active area... not the -ve charges adjacent.
However I think you still answered my question inadvertently!
I think it's definitely more about thinking of it like a relative charge difference thing rather than absolute.

Haha whoops. Yeah I think i think you are right about it being a case of relative charge difference. The sodium ions are rushing from the ECF to the ICF and you are taking away positive ions from the ECF in the active zone during the AP. The negativity on the outside is further enhanced from 0 to +30 mV because the potassium VG channels are not yet open to allow K+ to go from the ICF into the ECF and reverse that negative charge on the outside that has been established whilst the cell is undergoing an AP

[MelonBar]

Anyone have any tips for the physiology blogs ??

[Starlight]

Anyone have any tips for the physiology blogs ??

Post frequently, i'd aim for about 5 good-quality posts. Try commenting on posts made by other members of the blog.

[MelonBar]

RMP = -70mV, equilibrium potential for K+ = -90mV. Over time then, the concentration gradient would disappear as potassium ions regularly leave the cell through the K+ leak channels. But this is countered by the action of Na+/K+ pumps that drive sodium and potassium ions against their concentration gradient, thereby maintaining the relative ion concentrations and the RMP.

Is this correct?


edit: also when we say the positive charge is pulled along the membrane attracted by the negative charge that lines the cell during an AP (activation gate of VG sodium channel opens) --- what exactly is this negative charge? Why is it there... what is it

Btw thank you El etc for helping out.