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arc5005

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I'm having conflicting information regarding which gates are open/opening vs. closed/closing during different membrane states:

Which of the following voltage gated positions best represents the repolarization phase?
A) m gate open; h gate closed, n gate open.
B) m gate close; h gate closed, n gate open.
C) m gate open; h gate open, n gate open.
D) m gate closed; h gate open, n gate closed.


Here's what information I have, but I really don't feel like this gives me a clear enough picture, and I feel like different sources state different things, so I need clarification please!!!

m-gate:

Resting: Na+ m-gates (activation gates) are closed.
Depolarization: Na+ m-gates are opening, causes others to open.
Repolarization: Na+ m-gates are open at the beginning, but are closing, and closed at the end.
Hyperpolarization: Na+ m-gates are closed.

h-gate:
Resting: open
Depolarization: open, but closes at the end?
Repolarization: closed.
Hyperpolarization: closed.

n-gate:
Resting: K+ n-gates (activation gates) are closed.
Depolarization: K+ n-gates are still closed.
Repolarization: K+ n-gates are opened.
Hyperpolarization: K+ n-gates remain open? slow-to-close, which causes hyperpolarization.

*There are K+ leak-channels that are always open.


Answer: A) m gate open; h gate closed, n gate open.

TBR reasoning:
"During repolarization phase, K+ is flowing out of the cell and down its concentration gradient. If K+ is flowing out of the cell, the n gate must be open. Looking through our choices, we see that the n gate is closed in choice D (eliminate choice D).

We know that during repolarization, we do not want Na+ entering the cell. If Na+ enters the cell as K+ leaves, we will not be able to obtain the resting membrane potential of the cell as quickly as we would like. Which one of the Na+ gates is closed, or are they both closed? We know that during depolarization the h gate begins to slowly close. Let's assume that the h gate has closed as repolarization begins. If this is the case then we can eliminate choice C.

However, all through depolarization Na+ is entering the cell, which means that the m gate is remaining open. By the time the peak of the action potential has occurred, the h gate has closed, the m gate is still open but beginning to close, and the n gate is open."
 

Lawpy

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I don't know whether this question is based on a passage, since a discrete question asking about the Hodgkin-Huxley model is very rare. I don't think the Hodgkin-Huxley model is mentioned in the AAMC content guidelines, since the details are technical and the resulting formulas are differential equations that require knowing calculus, something the MCAT doesn't require nor test.

Basically, the Hodgkin-Huxley model assigns variables to specific phases of sodium and potassium channels in neuronal action potential. From Wikipedia, the variables n, m, and h are dimensionless quantities between 0 and 1 that are associated with potassium channel activation, sodium channel activation, and sodium channel inactivation, respectively (Hodgkin–Huxley model - Wikipedia).

n-gate = potassium channel activation gate
m-gate = sodium channel activation gate
h-gate = sodium channel inactivation gate

Some additional references to follow: The Hodgkin-Huxley Model and Hodgkin and Huxley: Superheroes

In repolarization, sodium channels begin to close more quickly than when potassium channels close. So potassium outflow is greater than sodium inflow, which means net positive charge is leaving the neuron, thereby decreasing the membrane potential. Near the end of depolarization, sodium channel inactivation gates close and potassium channel activation gates open, which is why we see a peak in the action potential. This means at the start of repolarization, the n-gates are open and the h-gates are closed. The sodium channel activation gates close faster than potassium channel activation gates, but at the start of repolarization (so right at the peak of action potential), sodium channel activation gates are open.

So at the start of repolarization, m-gates are open, h-gates are closed and n-gates are open. As repolarization progresses, the m-gates close first. Since n-gates close more slowly, the membrane potential decreases below resting potential, thus hyperpolarizing the neuron. The n-gates close at or just before hyperpolarization, and the Na+/K+ ATPases restore sodium and potassium levels back to initial concentrations. And some small depolarization and occasional sodium channel opening restores the membrane potential back to resting potential levels.
 

arc5005

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Thank you. clarification really helps.

I don't know whether this question is based on a passage, since a discrete question asking about the Hodgkin-Huxley model is very rare. I don't think the Hodgkin-Huxley model is mentioned in the AAMC content guidelines, since the details are technical and the resulting formulas are differential equations that require knowing calculus, something the MCAT doesn't require nor test.

Basically, the Hodgkin-Huxley model assigns variables to specific phases of sodium and potassium channels in neuronal action potential. From Wikipedia, the variables n, m, and h are dimensionless quantities between 0 and 1 that are associated with potassium channel activation, sodium channel activation, and sodium channel inactivation, respectively (Hodgkin–Huxley model - Wikipedia).

n-gate = potassium channel activation gate
m-gate = sodium channel activation gate
h-gate = sodium channel inactivation gate

Some additional references to follow: The Hodgkin-Huxley Model and Hodgkin and Huxley: Superheroes

In repolarization, sodium channels begin to close more quickly than when potassium channels close. So potassium outflow is greater than sodium inflow, which means net positive charge is leaving the neuron, thereby decreasing the membrane potential. Near the end of depolarization, sodium channel inactivation gates close and potassium channel activation gates open, which is why we see a peak in the action potential. This means at the start of repolarization, the n-gates are open and the h-gates are closed. The sodium channel activation gates close faster than potassium channel activation gates, but at the start of repolarization (so right at the peak of action potential), sodium channel activation gates are open.

So at the start of repolarization, m-gates are open, h-gates are closed and n-gates are open. As repolarization progresses, the m-gates close first. Since n-gates close more slowly, the membrane potential decreases below resting potential, thus hyperpolarizing the neuron. The n-gates close at or just before hyperpolarization, and the Na+/K+ ATPases restore sodium and potassium levels back to initial concentrations. And some small depolarization and occasional sodium channel opening restores the membrane potential back to resting potential levels.
 
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