January 24, 2011

022 Re: Akbar – Inactivation of V-gated Sodium Channels

This is a video answering Akbar’s question regarding the difference between the inactive and closed states of voltage-gated sodium channels.

Watch as Leslie explains this difference using a box.


Transcript of Today’s Episode

Welcome to another episode of Interactive Biology TV, where we’re making biology fun! My name is Leslie Samuel. In today’s episode, I’m going to be answering a question that was left on my blog. On Episode 12 where I spoke about the absolute and relative refractory periods, a question was left by Akbar, and his question was, “Can you please elaborate a bit more on the inactive state of the sodium channel? How does it differ from the closed state?” That’s a very good question and I’m glad he asked it.

In order to illustrate the difference, I’m going to use this box here. The type of box doesn’t matter too much. Let’s say that this box is a voltage-gated sodium channel. There are 3 states that this box can be in. It can be closed, which you can see right now, and it can be open, and it can be inactivated. The way this works is we have 2 gates. These are the 2 gates. I’m going to call this the activation gate, and I’m going to call this the inactivation gate.

So we have a stimulus that comes along, and the stimulus causes the membrane potential to become more positive, and it reaches the threshold. Now, the cool thing about this activation gate is that it’s positively charged. Imagine that you have a bunch of positive charges inside here, and let’s say it’s inside the cell, what is that going to do to the positive charge of the gate? Now, you probably know that like charges repel, and opposite charges attract. Well, we have like charges here, and as that charge builds up, that positive charge is going to cause the positively charged activation gates to open.

When that happens, within 0.5-1.0 millisecond, the inactivation gate automatically closes. That has to do with the structure of the gate, the structure of the channel, and a few different properties that I’m not going to go into. But basically, the idea is once it opens, this just closes automatically after a certain period of time. That period of time would be 0.5-1.0 millisecond. Once it’s inactive, you can’t re-stimulate it. The only way for you to re-stimulate the channel is that it needs to be reset to the closed state. As we said, the activation gate is positively charged, so when the second phase of the action potential happens and potassium rushes out of the cell, the cell becomes more negative, and as it becomes more negative, this gate now gets pulled in so that the channel closes.

So the difference between inactive and closed is: inactive means that the inactivation gate is closed; when it’s fully closed, it’s when both gates are closed, so the activation gate now is closed, and it’s ready to start another action potential.

I hope that clarifies it for you, Akbar, or anyone else that’s listening. If you have any more questions about any of the topics, please feel free to leave them in the comments below, and like I said, like you’re seeing here, I’d be more than happy to make a follow-up video or answer your question in the comments. That’s it for this episode, and I’ll see you in the next one.

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