EK Bio In-Class Lecture 4 #90

[dougkaye]

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This one was a bit confounding.

90. According to Figure I. at 2 msec after the action potential
begins, Na+ voltage gated channels are most likely:
A. open
B. closed
C. inactivated
D. activated

In the studies I've done at the basic level, there was very little mention of whether Na gates were activated or inactivated - profs simply used open or closed. I am assuming this has to do with the fact that Na voltage-gated channels have activation and inactivation gates. The answer is logical but the information is not in the EK book! Upon reading my bio book, it says that the "activation gate" opens FAST in response to action potential. When the neuron is repolarizing, evidently the "deactivation gate" closes SLOWLY. It make sense - during repolarization, Na channels close but not instantly, and at the same time K+ channels open fast. It is a good detail to know, I suppose, but seems perhaps not worth a flash card because of the level of detail.

-- Have any of you seen real MCAT questions dealing with activation/inactivation versus open/closed? Seems really cryptic in my opinion.

 

[shffl]

That is a bit weird for B and C could be the right answer. However in this case, C is clearly a stronger answer because we know that during that phase of AP, it is going through absolute refractory period where the Na channels CANNOT open no matter what and hence inactivated.

 

[NYR56]

I don't remember if I've seen more questions about this topic, but it was discussed in my physio class and it was pretty significant. Honestly, I don't think it is overkill for the MCAT, nor is it really hard to remember.

 

[dougkaye]

yeah i agree... I've also noticed that the pattern with EK is to ask pretty tough questions that might go a bit deeper than the MCAT requires. But I think you're right about the importance of absolute and relative refractory periods being important.

 

[BestDoctorEver]

The EK 30 min in-class questions are extremely hard...If the mcat questions are like that, I am toast.

 

[NYR56]

The EK 30 min in-class questions are extremely hard...If the mcat questions are like that, I am toast.

Not to be a downer, but they are of similar style to the MCAT, maybe slightly harder. Thing is, you likely just reviewed the content so the difficulty should balance out.

 

[Swagster]

The EK 30 min in-class questions are extremely hard...If the mcat questions are like that, I am toast.

From what I've read around here, you shouldn't worry too much. The EK mini test sets are overkill and not indicative of the MCAT. They're good for exposure to the material, but if you are missing most of the questions don't lose sleep over it.

 

[dougkaye]

cool. thx for your input folks! that's one more random fact i know, now.

 

[jphwki82]

This one was a bit confounding.

90. According to Figure I. at 2 msec after the action potential
begins, Na+ voltage gated channels are most likely:
A. open
B. closed
C. inactivated
D. activated

In the studies I've done at the basic level, there was very little mention of whether Na gates were activated or inactivated - profs simply used open or closed. I am assuming this has to do with the fact that Na voltage-gated channels have activation and inactivation gates. The answer is logical but the information is not in the EK book! Upon reading my bio book, it says that the "activation gate" opens FAST in response to action potential. When the neuron is repolarizing, evidently the "deactivation gate" closes SLOWLY. It make sense - during repolarization, Na channels close but not instantly, and at the same time K+ channels open fast. It is a good detail to know, I suppose, but seems perhaps not worth a flash card because of the level of detail.

-- Have any of you seen real MCAT questions dealing with activation/inactivation versus open/closed? Seems really cryptic in my opinion.

Here is a brief synopsis of what is going on with Na+ channels during an action potential. (Fast Na+ Channels)

1. Cell at resting membrane potential - Activation Gate is Closed & Inactivation Gate is Open

2. Membrane depolarized to threshold - Activation Gate Opens & Inactivation Gate stays Open (Na+ can enter the neuron)

3. AP just after the peak membrane potential - Activation Gate stays Open & Inactivation Gate Closes (No more Na+ can enter the neuron)

4. Repolarization prior to hyperpolarization - Activation gate Closes & Inactivation Gate Opens (Just as in #1)

Think of Na+ channels as a two way door. For Na+ to get in, it needs both to be open. Once threshold is reached, both doors are open and Na+ can enter to depolarize. After peak membrane potential, the inactivation gate closes so the cell can repolarize.

Voltage Gated K+ Channels (Delayed Rectifier Channels) - these have only one door

1. Resting membrane potential - K+ gate is closed
2. Depolarized to threshold - K+ gate stays closed
3. Just after peak membrane potential - K+ opens (repolarize neuron)
4. End of AP & back to resting membrane potential - K+ gate closed

Depolarization to threshold causes a voltage-gated K+ channel to open after a delay, hence the Delayed Rectifier nomenclature.

This shouldn't be too difficult to remember for the MCAT. It is still getting at the point of the physiology of an AP. Hope this better explains the lack of material in EK.