hyperpolarization

[chiddler]

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How is RMP returned to -70 after hyperpolarization?

Book says that K+ leak channels and Na+/K+ ATPase is responsible but what I do not understand is this: if the cell is hyperpolarized after K+ channels open, shouldn't the presence of K+ leak channels cause the cell to remain at -90 mV?

So hyperpolarization occurs because K+ goes down its concentration gradient. But leak channels are present; K+ should still be able to do that, right? Which means it'll remain hyperpolarized.

thanks!

 

[chiddler]

responding to my own question, the answer is because there are sodium leak channels as well. the membrane is very permeable to K+ which is evident by the fact that equilibrium potential for K+ is -90. But that it is at -70 says that some sodium is leaked to make it slightly more positive. Though Na+ channels are present at only 1% of the K+ channel levels.

 

[chiddler]

responding to my own question, the answer is because there are sodium leak channels as well. the membrane is very permeable to K+ which is evident by the fact that equilibrium potential for K+ is -90. But that it is at -70 says that some sodium is leaked to make it slightly more positive. Though Na+ channels are present at only 1% of the K+ channel levels.

wow thanks for the helpful response chiddler you are so smart

 

[chiddler]

wow thanks for the helpful response chiddler you are so smart

i know. now get back to studying!

 

[milski]

I think I need to post something here, just so that you don't feel too lonely. 😛
And no idea what you're talking about. But I'm not done with bio, so need to worry. Yet.

 

[MedPR]

responding to my own question, the answer is because there are sodium leak channels as well. the membrane is very permeable to K+ which is evident by the fact that equilibrium potential for K+ is -90. But that it is at -70 says that some sodium is leaked to make it slightly more positive. Though Na+ channels are present at only 1% of the K+ channel levels.

Not to mention the Na/K ATP-ase that is always pumping 3Na out and 2K in. Thus decreasing the positive charge following an action potential.

 

[Pisiform]

wow did he just spoke to himself ....

 

[FIREitUP]

When a cell starts repolarizing K+ is going down both its electrical AND chemical gradient. This makes the driving force for K+ extremely high and leads to hyperpolarization. So why does it go back to its resting membrane potential rather than getting more and more hyperpolarized? Well, this lies in the fact that, as K+ repolarizes the cell, the driving force for K+ decreases. Why does it decrease? Because whenever a K+ leaves the cell, the inside of cell has proportionally more negative ions. At a certain point, the attraction of K+ to the negative ions inside the cell balances out the driving force of K+ out leading to an equilibrium. This equilibrium, coupled with the very low conductances for Na+ at rest leading to Na+ coming in (and of course the Na+/K+ ATPase) leads to the resting potential going to back to the equilibrium of -70mV (RMP is actually different in different cells, such as SA node cells in the heart, this is due to a decreased conductance of K+ at RMP in the SA node cells). It's important to note that despite the presence of K+ leakage channels making it 100 times more easy for K+ to leave than Na+ to go in at rest, equilibrium is reached at -70mV due in part because of the driving force for Na+ is higher (despite the lower ability for Na+ to go out) due to the concentration gradient being higher for Na+.

I was always angry at how poorly this was taught in review books for the MCAT.

 

[MT Headed]

Not to mention the Na/K ATP-ase that is always pumping 3Na out and 2K in. Thus decreasing the positive charge following an action potential.

No. This a common mistake. Even KhanAcademy got this wrong and he had to publish a second video clarifying and correcting himself.

Experimentally people have modified sodium potassium pumps so that they only pump out two sodiums instead of three. The difference in resting potentials is negligible. Basically all of the -70mV potential comes from leaky potassium channels.

It's worth watching the second khanacademy video if you really want to learn what's going on (although i'm sure it is waaay beyond the scope of the mcat).

 

[NuttyEngDude]

i know. now get back to studying!

i lol'd. 👍

 

[MedPR]

No. This a common mistake. Even KhanAcademy got this wrong and he had to publish a second video clarifying and correcting himself.

Experimentally people have modified sodium potassium pumps so that they only pump out two sodiums instead of three. The difference in resting potentials is negligible. Basically all of the -70mV potential comes from leaky potassium channels.

It's worth watching the second khanacademy video if you really want to learn what's going on (although i'm sure it is waaay beyond the scope of the mcat).

Oh wow I didn't know this. I just learned about this in A&P last semester. What's the purpose of burning all that ATP?

 

[MT Headed]

Truthfully? Most of it is used to keep yourself warm. Bodies spend a lot of energy trying to warm up to normal temperature, and it's a useful sidd effect of Na K pumps.

Any time you see a cell being inefficient, like making RNA just to destroy it, the waste is released as heat, which the cell was going to have to do anyway to keep warm.

 

[MedPR]

Truthfully? Most of it is used to keep yourself warm. Bodies spend a lot of energy trying to warm up to normal temperature, and it's a useful sidd effect of Na K pumps.

Any time you see a cell being inefficient, like making RNA just to destroy it, the waste is released as heat, which the cell was going to have to do anyway to keep warm.

Cool. Sometimes I wish the MCAT and MCAT prep books were more specific because of situations like this. For example, my A&P class taught me that all spinal nerves and cranial nerves are PNS, but it also taught me that the optic nerve is considered CNS because it does not regenerate if it gets damaged.

According to EK, PNS is all 12 cranial nerve pairs and all 31 spinal nerve pairs. So, if a question about CNII were to pop up on the MCAT, I wouldn't know which answer to go with (well, I would since I learned specifics about it in school, but otherwise I wouldn't).

For instance that question in TBR (maybe it was EK) about what myelinates CN II. If you go with "all cranial nerves are PNS" your answer will be schwann cell. But if you know that optic nerve is CNS, and more specifically if you already know that it is myelinated by oligodendrocytes, you would know the real answer (oligodendrocyte).