Which ion channels are responsible for maintaining the resting membrane potential?

[m25]

So I came across this question, but I am confused about their explanation to why the answer is not B, because I thought it was the voltage gated sodium-potassium pump that maintains the resting membrane potential? I feel like I'm missing some sort of big picture here but I don't know what.

Which ion channels are responsible for maintaining the resting membrane potential?

A. Ungated channels
B. Voltage-gated channels
C. Ligand-gated channels
D. No ion channels are involved in maintenance of the resting membrane potential.

Correct answer is A. The resting membrane potential is displayed by cells that are not actively involved in signal transduction. Ungated or “leak” channels permit limited free flow of ions, while the sodium–potassium pump is also active and corrects for this leakage. Ligand-gated and voltage-gated channels are involved in cell signaling and in the pacemaker potentials of certain cells, but cause deviation from—not maintenance of—the resting membrane potential.

EDIT: Actually, am I confusing the Na+/K+ pump (which I am guessing is not even a gated channel?) with Na+ voltage gated channel and K+ voltage gated channel...?

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[amy_k]

Yes, the sodium-potassium pump requires ATP but it is not gated, it is constantly working to maintain the resting membrane potential. The voltage gated Na+ and K+ channels are separate.

 

[m25]

Actually, am I confusing the Na+/K+ pump (which I am guessing is not even a gated channel?) with Na+ voltage gated channel and K+ voltage gated channel...?

Yes, the sodium-potassium pump requires ATP but it is not gated, it is constantly working to maintain the resting membrane potential. The voltage gated Na+ and K+ channels are separate.

Ok, so do all gated channels not require ATP?

 

[NextStepTutor_3]

The main "gated" channels on the MCAT are voltage-gated and ligand-gated channels. Both of these allow facilitated diffusion (which is a form of passive, not active, transport). In other word, as far as I know the answer to your question is yes. Neither of these types of gated channels require ATP, because neither function to move ions against their gradient. Think of the word "gated" as the key. The presence of certain conditions, such as a specific ligand or an increased voltage, allows the gate to open, but ions will still only flow in the direction that is made favorable by their gradient.

As a side note, both the Na+ / K+ pump and leak channels are involved in maintaining the resting potential, and they are completely independent of each other. The pump, to put it simply, is what causes the cell to be negative in comparison to the external fluid, because 3 Na+ ions get pumped out for every 2 K+ ions that get brought in. Leak channels, on the other hand, allow small amounts of ions to continuously travel down their gradients. You know the period of time right after an action potential, where a neuron is slightly hyperpolarized (too negative)? Leak channels allow some ion flow to bring the voltage back to -70 mV (again, talking about an average neuron).

Hope this helps, and I'd love to hear what other people have to say about this! The nervous system is so complex, but this is the gist of what the MCAT tests.

 

[m25]

You know the period of time right after an action potential, where a neuron is slightly hyperpolarized (too negative)

So this is due to both the leak K+ channel and the slow voltage gated K+ channel?

 

[sazerac]

The mismatch between sodium and potassium in the Na/K pump contributes almost nothing to the net charge of a cell. Sure at first glance everybody sees the pump as pumping 3 positive sodiums out and 2 positive potassiums in, and thinks "hey, that's a loss of one positive charge per pump cycle, that's why cells are negatively charged". And they would be wrong.

As an experiment, scientists have modeled what would happen if the sodium potassium pump only pumped out two sodiums and pumped in two potassiums. Result? Cells are still massively negative, and just slightly more positive than the usual -70mV.

The negative charge on cells comes from the fact that cell membranes are really good at keeping the sodium out, but they are horrible at keeping potassium in. They leak out potassium like a barrel full of holes.

Don't worry if this wasn't obvious to you or you never had it explained to you before... even the almighty Kahn of Kahn Academy got this wrong and he had to post a rare followup correction video, sheepishly admitting he was wrong about how much the three sodiums in the sodium potassium pump contribute to cell resting potential.

The answer to the original question is that the resting membrane potential mostly comes from the ungated, leaky potassium ion channels.

 

[amy_k]

The mismatch between sodium and potassium in the Na/K pump contributes almost nothing to the net charge of a cell. Sure at first glance everybody sees the pump as pumping 3 positive sodiums out and 2 positive potassiums in, and thinks "hey, that's a loss of one positive charge per pump cycle, that's why cells are negatively charged". And they would be wrong.

As an experiment, scientists have modeled what would happen if the sodium potassium pump only pumped out two sodiums and pumped in two potassiums. Result? Cells are still massively negative, and just slightly more positive than the usual -70mV.

The negative charge on cells comes from the fact that cell membranes are really good at keeping the sodium out, but they are horrible at keeping potassium in. They leak out potassium like a barrel full of holes.

Don't worry if this wasn't obvious to you or you never had it explained to you before... even the almighty Kahn of Kahn Academy got this wrong and he had to post a rare followup correction video, sheepishly admitting he was wrong about how much the three sodiums in the sodium potassium pump contribute to cell resting potential.

The answer to the original question is that the resting membrane potential mostly comes from the ungated, leaky potassium ion channels.

This is true that alone the Na+ / K+ pump would not be sufficient to generate the resting membrane potential, BUT it is important in helping maintain it, otherwise we would expect the concentration of potassium outside the cell to become so high that no more k+ would diffuse out since it is going against the concentration gradient. The sodium potassium pump is therefore crucial for maintaining the potential by pumping some K+ back in.

 

[amy_k]

What would actually happen if the sodium potassium pump stopped working and charge distribution was determined by the leaky channels (both sodium AND potassium leak through the membrane) is that, over time there would be equilibrium in the concentration of both charges and the membrane potential would be zero.

 

[NextStepTutor_3]

Thanks @sazerac and @amy_k , those are great explanations! I'll correct my original wording - the Na+ / K+ pump has a role in MAINTAINING the net negative charge. (As an old neuroscience major, I should know how important these distinctions are, haha.) And yes, potassium leak channels definitely massively overwhelm sodium ones, though both exist. That was definitely what this particular question was going for! I get so used to explaining "here's how we can remember that it's 3 Na+ out, but only 2 K+ in" that it's easy to oversimplify the pump's role.

 

[amy_k]

I think it would be difficult to ask an either/or question, as both are responsible for creating and maintaining the resting membrane potential, just that the sodium-potassium pump plays more of a significant role in maintenance than it does establishing, if you agree

 

[NextStepTutor_3]

Yes, I absolutely agree! It's fun to discuss, but I can't see the MCAT ever asking which of the two has a greater role. They're much more likely to ask a question similar to the one originally posted, where the Na+/K+ pump was never mentioned - the question just asked which of the types of channels had the greatest effect.

 

[bearwoot]

The main "gated" channels on the MCAT are voltage-gated and ligand-gated channels. Both of these allow facilitated diffusion (which is a form of passive, not active, transport). In other word, as far as I know the answer to your question is yes. Neither of these types of gated channels require ATP, because neither function to move ions against their gradient. Think of the word "gated" as the key. The presence of certain conditions, such as a specific ligand or an increased voltage, allows the gate to open, but ions will still only flow in the direction that is made favorable by their gradient.

As a side note, both the Na+ / K+ pump and leak channels are involved in maintaining the resting potential, and they are completely independent of each other. The pump, to put it simply, is what causes the cell to be negative in comparison to the external fluid, because 3 Na+ ions get pumped out for every 2 K+ ions that get brought in. Leak channels, on the other hand, allow small amounts of ions to continuously travel down their gradients. You know the period of time right after an action potential, where a neuron is slightly hyperpolarized (too negative)? Leak channels allow some ion flow to bring the voltage back to -70 mV (again, talking about an average neuron).

Hope this helps, and I'd love to hear what other people have to say about this! The nervous system is so complex, but this is the gist of what the MCAT tests.

Over two years later and this still helped me,
thank you

 

[NextStepTutor_3]

Glad to hear it! Good luck with your prep