 So I went and found a graph of an action potential on Wikipedia, so this is all open and you can totally go check it out. But it shows you not only a clean view. Now, this is just the change in membrane potential at one point on an axon. So just because, let's just do this, you've got your entire neuron, and then you've got your axon coming all the way down. And then you get to your axon terminal, but what we're looking at is the events like Ruther. And right there, the membrane potential changes. But visualize this for a second. Can you imagine that the membrane potential changes right here, okay, all the way across here? We have a new membrane potential. What's that going to do to the voltage-gated sodium channels right next to this place? They're going to open. And now you have an action potential right here. What's that going to do to the sodium, voltage-gated sodium channels that are right here? They're going to open. And so on and so on, et cetera, et cetera, et cetera. And so you literally get this wave of a charge once you reach threshold, once the stimulus is big enough to take you from here and depolarization. That, like, done. This is going to cause all of the voltage-gated sodium channels nearby to open and potassium channels, which they just don't open quite as fast. And then when the potassium does finally get open, the sodium snaps shut. Here's the deal. Within this space right here, this is considered the absolute refractory period. And during the absolute refractory period, it doesn't matter how big the bear is. It will not fire another action potential. No serene. Now, these are milliseconds, my friends. So you basically have, you know, two milliseconds where you will not fire another action potential. That probably does limit our reaction time, but it doesn't feel like it limits our reaction time. I mean, yeah, you might want to react faster, but this absolute zone, this is physiologically incapable of firing another action potential, and it actually has to do with the anatomy of the sodium channels. They actually have this little inactivation gate that is time-sensitive and refuses to open up until this point. And that actually prevents the action potential from moving two directions at once. Now, that's a little bit... I would love to have some kind of visual activity that we could do where we acted that out so that I could try and visualize. Like, why is that the case that it won't go backwards? I trust that it has to do with the fact that these guys close and they won't reopen. And if they don't reopen, then I guess you just can't change the voltage behind you. You can only change the voltage in front of you. It's not going to go there because I can't explain that. But once you have reached, like, the end of repolarization in this process, the potassiums are rushing out, and we're repolarizing the cell, then you have a relative refractory period. This is relative. And during that period of time, you can, if the stimulus is big enough, you can regenerate or redo another action potential. And then ultimately, you reach the point where, yeah, now it doesn't matter what size the stimulus is, if it's a big enough stimulus to help me reach threshold, the action potential will fire. The other thing that I wanted to talk about in this section is I want to make sure... I want you to think about the concept of a myelin sheath. Now, look at this. Myelin, the oligodendrocytes, the Schwann cells are responsible for producing myelin, which is basically just little fat blobs. And I want you to think of the myelin as insulation. And here's the deal. The myelin sheath is like a little fatty structure that surrounds the axon. And the only way for an action potential to pass through here is for the, basically, for the action potential to be generated in the spaces between the sheaths. So there are only sodium and potassium channels. I'm just going to draw the little sodium ones in here. And these little nodes where the myelin sheaths connect. And you can think of this as if you had an unmyelinated neuron that, basically, in order to propagate the action potential, you have to open up every single sodium and potassium channel all the way down the axon. If you have a myelinated neuron, you do not. I mean, dude, go ahead and open sodium channels in here, but they're surrounded by this fatty insulation marshmallow bubble, and so there's no, you're not going to have any ions moving through there. And in fact, you're going to maintain the charge till you get here, because it's insulated. And then you're going to keep it going. It's called saltatory conduction. And the analogy that helped me with this is if you have a Word document and you want to get your cursor from one side all the way to the other side, you can push space and open every single potassium channel and every single sodium channel and space your way all the way across. And how long is it going to take you? I don't know, a minute. Or you can push tab and only open up the sodium and potassium channels in the nodes of wrong VA. And then tab, tab, tab, tab, to take you a fraction of the time, because myelin sheets are that cool. All right, how do you feel? Awesome? Okay, I will see you for the next one next time. And I hope that you bless your little sodium and potassium channels. See you in class.