NextStep FL 4 B/BC #21

[jeep1010]

I wanted to clarify something about the deactivation and inactivation gates that was discussed in this question. Please let me know if I have this correct.

From the answer explanation I found that deactivation means that the ion channel has its inactivation gate open, but its activation gate closed. This means that a very strong stimulus is required to cause the Action Potential to occur. Therefore, the deactivated channels would be present during relative refractory periods?

The other type of ion channel discussed was the inactivated type, which has its inactivated gate closed and activation gate closed, therefore no action potential can occur. So this would be present during absolute refractory period.

If you could please let me know if I have this right, especially in terms of the question that would be very helpful.

Thanks!

@NextStepTutor_2 @NextStepTutor_3

---

Members don't see this ad.

 

[NextStepTutor_3]

Hi there, great question!

For the most part, you're correct. But just to make sure you have this mastered, I wanted to respond directly to what you wrote and clarify some points.

From the answer explanation I found that deactivation means that the ion channel has its inactivation gate open, but its activation gate closed. This means that a very strong stimulus is required to cause the Action Potential to occur. Therefore, the deactivated channels would be present during relative refractory periods?

Yes, when a sodium channel is deactivated, it has its inactivation gate open, but its activation gate closed. Let's think of this really simply - "deactivated" means "not totally open, but not inactivated." Since it's "not totally open," we know that one of the two gates must be closed. But since it's "not inactivated," it must not be the inactivation gate that's closed - it must be the activation gate.

The activation gate is the gate that opens at the beginning of an action potential, in response to depolarization. So if we depolarize this "deactivated" neuron, its activation gate will open, meaning the channel as a whole will be open and sodium can flow through. In other words, an action potential can occur. Since an action potential can occur during the relative refractory period (it's just more difficult than usual, since the neuron is hyperpolarized), it makes sense that channels during this period are in the deactivated state.

Note that the correct answer choice uses the term "de-inactivated" instead of simply "deactivated," but it doesn't really matter to us here. De-inactivation is just the term that refers to the "removal of inactivation" - in other words, the transition a sodium channel makes when it stops being inactivated and starts just being deactivated. Here, we've already reasoned out that "deactivated = good" for this part of the answer, so this is fine.

The other type of ion channel discussed was the inactivated type, which has its inactivated gate closed and activation gate closed, therefore no action potential can occur. So this would be present during absolute refractory period.

You're correct here as well - I just wanted to point out that they technically aren't different types of ion channels - just different conformations that the sodium voltage-gated ion channel can exist in. But yes, inactivation is like a "plug" that blocks the sodium channel entirely. Depolarization doesn't open the inactivation gate, so even a really strong stimulus won't cause an action potential to occur. This is exactly what we think of when we imagine the absolute refractory period - no matter how strong the stimulus, no action potential can be generated.

NOTE: We can sum this up really simply by returning to the question and using process of elimination.

Choices B and D: Wrong, because sodium channels can't be inactivated during the relative refractory period.

Choice C: Wrong, because potassium channels are still open during the relative refractory period - remember, they were open during the latter part of the action potential, to allow potassium to flow outward and repolarize the cell.