The Action Potential? (NextStep FL #4, Q21)

[marcosma]

The question is as follows:

When an odorous molecule binds to olfactory receptors, the cell transduces the information into an electrical signal that travels to the brain for processing. Which of the following accurately describes the state of the voltage-gated channels on this cell during the relative refractory period?

a) Na+ channels are de-inactivated, and K+ channels are activated. (correct)
b) Na+ channels are inactivated, and K+ channels are activated.
c) Na+ channels are de-inactivated, and K+ channels are inactivated. (my choice)
d) Na+ channels are inactivated, and K+ channels are inactivated.


I'm a neuroscience major, so getting this wrong irked me! My understanding of the situation is that the K+ flow during the relative refractory period is due to K+ leak channels, and NOT due to K+ voltage-gated channels (which is what the question is asking about). So, I said choice C, that Na+ channels are de-inactivated (or, back to normal RMP state) and that K+ channels are inactivated, assuming we are working with inactivating K+ channels like the Shaker channel.

NextStep (@Next Step Tutor) claims that the answer is choice A - because the K+ ions are still flowing, the v-gated K+ channels must be activated.

Am I correct or just ill-informed? Can anyone clear up what is really happening during the relative refractory period with regards to K+ channels?


Thanks!

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[Nugester]

So resting potential ~ -70 mV, maintained by sodium potassium pump (with leaky K+ channels, which help the ~ - mV). Depolarization (open Na+ gates) to reach + mV (forget amount). At that point, sodium channels are deactivated and potassium channels activated (slow). There is a brief hyperpolarization (~-90 mV?) before settling back to resting potential.

Absolute refractory period - no stimulus can elicit another action potential, basically anywhere between depolarization until ending of Na+ channel inactivation
Relative refractory period - from beginning of repolarization ; a much STRONGER stimulus is needed to start an action potential; K+ channels should be activated/open

 

[deleted647690]

Yeah, I remember this question. I just knew there was a delay in the closing of the voltage K+ channels, so they would still be in the process of closing, which contributes to the increased K+ current. When the membrane potential rises above threshold, the conductance of Na and K rises above the leak channel conductance

And I think the fact that K+ channels are delayed in their closing is what contributes to the hyperpolarized state of the membrane

 

[rabbott1971]

They didn't really write "de-inactivated" did they? I'm hoping you meant to say "deactivated" possibly? Or "inactivated?"

 

[deleted647690]

They didn't really write "de-inactivated" did they? I'm hoping you meant to say "deactivated" possibly? Or "inactivated?"

It refers to the structure of the Na+ gated ion channel. This channel has both an activation gate and a deactivation gate. In the natural state, the activation gate is closed while the inactivation gate is open. When the membrane depolarizes, this pops open the activation gate, and Na+ ion can flux into the cell. Once the peak is reached, the inactivation gate closes, stopping inward Na current. At this point, the channel is inactivated. During hyperpolarization, the channel returns to its natural state (open activation gate and closed inactivation gate), thus it is de-inactivated.
Please correct me if this is wrong

 

[rabbott1971]

An activation gate and deactivation gate? I don't think that's right man! They talk about activation and deactivation because some gates are naturally closed, whereas others are naturally open. For example, the normal cAMP-gated ion channel is closed (i.e. olfactory, gustatory signal transduction), but in the vision signaling pathway, the cGMP-gated ion channel is open normally, and closes when activated. But I don't think there are "separate" gates for activation and deactivation. Man I hope I have got this right because else this is going to suck. @aldol16

 

[deleted647690]

An activation gate and deactivation gate? I don't think that's right man! They talk about activation and deactivation because some gates are naturally closed, whereas others are naturally open. For example, the normal cAMP-gated ion channel is closed (i.e. olfactory, gustatory signal transduction), but in the vision signaling pathway, the cGMP-gated ion channel is open normally, and closes when activated. But I don't think there are "separate" gates for activation and deactivation. Man I hope I have got this right because else this is going to suck. @aldol16

Check out this animation

Animation: Voltage-Gated Channels and the Action Potential (Quiz 2)

 

[rabbott1971]

Wow. Well OK thanks man, I honestly have not seen it presented that way before. I better dig back in on this stuff.

 

[deleted647690]

Wow. Well OK thanks man, I honestly have not seen it presented that way before. I better dig back in on this stuff.

Don't worry; I didn't really know about it until I took some upper level bio courses. I have a feeling it's not important for the actual MCAT, even though it was on this NS exam. Maybe someone else could chime in as to whether it's important to understand or not.

 

[rabbott1971]

That probably has to do with why the sodium gate cuts off fast, but the potassium gate cuts off slower causing hyperpolarization. Hopefully for MCAT its enough to know sodium closes quickly when the action potential has maxed out, but potassium has a little bit of lag to it. Is that how you understand it?

 

[deleted647690]

That probably has to do with why the sodium gate cuts off fast, but the potassium gate cuts off slower causing hyperpolarization. Hopefully for MCAT its enough to know sodium closes quickly when the action potential has maxed out, but potassium has a little bit of lag to it. Is that how you understand it?

Yeah, I actually doubt many would know about the timing of the channels. That's not something I learned in my intro level classes, and I think they'd provide that kind of information in the passage (not what they did here, but I think they would on an AAMC exam)