membrane potential question (EK bio #350)

[astronaut135]

Hyperkalemia is a condition where the extracellular concentration of K+ is significantly increased. Which of the following is an effect of hyperkalemia?

A) resting potential will become more negative
B) increased difficulty in action potential generated
C) decreased membrane threshold
D) decreased urine production

The correct answer here is C, and the book explains that it's because the K+ outside will prevent the K+ inside from leaking out. This leads to an overall more positive membrane potential, so it's easier to depolarize the cell and reach threshold. However, I'm wondering why the K+ outside doesn't influence the membrane potential? Won't it make the potential more negative, since there are more positive ions on the outside of the cell? That's why I initially chose A, but that's the wrong answer.

Thanks for your help!

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[jtran7]

the condition hyperkalemia doesn't necessarily mean that the excess K+ outside the cell came from the inside of the cell so you logic that the inside of the cell will become more negative isn't correct. the excess K+ could have been a result of excess dietary intake or some other disorder leading to the extracellular concentration of K+ increasing.

 

[GomerPyle]

How does it exactly decrease the membrane threshold to begin an action potential?

And what is it that exactly sets the membrane potential around -90 mv?

 

[jtran7]

you have the Na/K pump that pumps 3 na out and 2 K in resulting in a negative charge within the cell

eventually a chemical gradient is formed and so K+ leak channels will allow K that are pumped in to leak out of the cell and Na leak channels to allow Na to leak back in. At that point the Na/K pump works to restore the potential and maintain it at -70mv.

However in the question above what you have is a situation where the K leak channel isn't working and there is an accumulation of K inside cell resulting in a more positive Resting Potential and a decreased membrane threshold

 

[jtran7]

the decrease threshold is a result of the higher resting potential. So basically the AP can result with a smaller change in membrane potential

 

[FeinMS]

How does it exactly decrease the membrane threshold to begin an action potential?

And what is it that exactly sets the membrane potential around -90 mv?

I know right? It's the increase in resting potential that makes it easier to fire AP. The threshold shouldn't change!! Can someone clarify?

the decrease threshold is a result of the higher resting potential. So basically the AP can result with a smaller change in membrane potential

Threshold is a membrane potential value that fires AP when reached. By higher resting potential, it's easier to reach the threshold, but the actual threshold value shouldn't change.

 

[jtran7]

I know right? It's the increase in resting potential that makes it easier to fire AP. The threshold shouldn't change!! Can someone clarify?



Threshold is a membrane potential value that fires AP when reached. By higher resting potential, it's easier to reach the threshold, but the actual threshold value shouldn't change.

yes sorry for the confusion that was the point i was trying to get accross

my logic is its a bad answer but the best of the 4 so its the right one

 

[image187]

you have the Na/K pump that pumps 3 na out and 2 K in resulting in a negative charge within the cell

eventually a chemical gradient is formed and so K+ leak channels will allow K that are pumped in to leak out of the cell and Na leak channels to allow Na to leak back in. At that point the Na/K pump works to restore the potential and maintain it at -70mv.

However in the question above what you have is a situation where the K leak channel isn't working and there is an accumulation of K inside cell resulting in a more positive Resting Potential and a decreased membrane threshold

more K inside the cell wouldn't lead to a more positive resting potential. Things favoring a more positive resting potential are less K inside the cell and more K outside the cell:

Normally, the cell has a huge amount of K inside and a little bit outside, and there is a constant net diffusing pf K outside the cell down it's gradient through leak channels creating a negative charge on the inner cell membrane. Sodium is normally high outside the cell and the same thing but it diffuses in, making the charge on the inner membrane more positive. Normally, these two actions would cancel out but the cell membrane is much much more permeable to K at a resting state which causes the resting potential to lie closer to that which happen of only potassium were considered.

In hyperkalemia, high K OUTSIDE ("-emia" means "of the blood") the cell disrupts this concentration gradient and creates less of a drive for K to leave the cell to make things even. This makes the potential of the inner cell membrane LESS negative, and the resting potential is increased to a voltage closer to the threshold.

This is a bad question though, because the threshold isn't really being changed, it just takes less stimulation to fire an action potential.

 

[DrBTS]

Neurobiologist here. This question must be thrown out, it has absolutely no right answer. Image187, an increase in the RMP definitely does NOT change the threshhold which is dependent on the Na+ voltage-gating properties. Higher RMP would be the right answer here. Where was this q taken from?

 

[jtran7]

thanks for correcting my error I did some research and it seems other proteins are responsible for creating the K+/Na+ chemical gradients and the Na/K pump just serves to maintain the gradients. And also you helped me realize less negative isn't the same as being more positive I was thinking in relative terms

 

[Pediateix]

yea i don't get this question. if the inside of the cells i more positive, doesn't that mean the membrane is more negative??? @examracker

the solution says... a cell has a resting membrane potential of -70 with -30 threshold for AP. if the cell interior has a buildup of positive charge, the resign membrane potential moves from -70 to -55 and closer to the threshold.

it could also be a mistake, the cell interior does not have a build up, its the extracellular

 

[Professional Student]

yea i don't get this question. if the inside of the cells i more positive, doesn't that mean the membrane is more negative??? @examracker

the solution says... a cell has a resting membrane potential of -70 with -30 threshold for AP. if the cell interior has a buildup of positive charge, the resign membrane potential moves from -70 to -55 and closer to the threshold.

it could also be a mistake, the cell interior does not have a build up, its the extracellular

Yes, the word "interior" in the solution should be changed to "exterior." This word change now matches the solution with the question and also makes C the right answer.

 

[Pediateix]

Yes, the word "interior" in the solution should be changed to "exterior." This word change now matches the solution with the question and also makes C the right answer.

thanks. wasted some time wondering why the outside of the cell would become positive if the inside does too

 

[Professional Student]

thanks. wasted some time wondering why the outside of the cell would become positive if the inside does too

Well, the outside of the cell is becoming more positive because that is where the K atoms are in excess compared to their normal levels. A more correct way to say it is that the inside is becoming less positive, and the outside is becoming less negative.

 

[liquidsodium]

I'll contribute a tip to figuring this question out, even if you are unsure or thrown by the wording:

If you simply know that a more negative resting potential = harder to reach AP, you'll notice that answers A and B are describing the same scenario and are equivalent answers. Since the MCAT is a single answer multiple choice test, neither A nor B can be the correct answer.

This is a common trend in MCAT answer choices and will grant you a 50/50 shot at the right answer if you can spot them


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[desperadoflakes]

-Natural flow of K+ is down its concentration gradient, in to out
-Increasing [out] means less of a gradient
-Less of a gradient means less flows out
-Less flowing out means more cation inside the cell means the potential is more positive or less negative
-If it's less negative than its depolarized, and closer to the threshold

Can't be A or B because it's more positive, not negative. If the question was the same but for sodium, A or B would be true.

Can't be D because more K+ in the interstitial fluid means that some of it won't be reabsorbed in the nephron and more urination needs to happen to excrete it I think?

 

[Pediateix]

so when -70 is given, its the potential on the membrane? so the more positive inside the cel the more negative on the outside?

 

[desperadoflakes]

so when -70 is given, its the potential on the membrane? so the more positive inside the cel the more negative on the outside?

No, -70 means the cell is -70 with respect to the outside, which is 0

 

[Pediateix]

No, -70 means the cell is -70 with respect to the outside, which is 0

but a depolarization happens with respect to the inside of the cell right?

 

[aldol16]

but a depolarization happens with respect to the inside of the cell right?

First, it's easier to think of the membrane potential as a relative concept rather than an absolute one. In fact, you can think of it as a delta G, as it's directly related to a delta G of ion flow from outside to inside. Since the inside of the cell is negative relative to the outside by 70 mV, that means that there's a relative potential of -70 mV (again, inside relative to outside). Second, depolarization, by its very definition, means to get less polar. Going from -70 to anything more positive than -70 all the way up to zero is making the membrane less polar because there's less of a potential difference. Similarly, hyperpolarization means to get more polar. Therefore, the -70 mV should increase in magnitude.