 So we need to do the steps of the action potential. Like how does this happen? How do we change the membrane potential? And then how do we use that to create this wave? So are you ready? First of all, what you have to know is that embedded in the cell membrane of a neuron, you have channels. And tell me, what kind of channels are these? Well, if sodium is pink, this is probably a sodium channel. So tell me, is it open or closed? See my little gate? This is a gated sodium channel. And guess what kind of gated channel this is? A gated potassium channel. These things are closed. If they stay closed, can sodium potassium move in and out of the neuron? Heck no, they cannot. Which is why if we have the sodium potassium pump doing its stuff, the sodium potassium pump is going to establish this pretty strong concentration gradient where sodium is going to want to go in or out. Dude, sodium is going to want to go in. The party's happening inside the cell and the sodium's been being kicked out. The potassium is going to want to go out. It wants to go out exploring to see what else is happening in this world. Thank you, sodium potassium pump, for setting up that gradient. Now, the first thing that happens, if you want to fire an action potential, the first thing that has to happen is you have to have some sort of stimulus. And I am going to draw you a graph. And I'm going to draw the graph down here because I'm going to try and make my words right here. And if I planned a little bit more, I would have moved those guys up. So I have a little more room for my graph. But my graph is going to be millivolts on one side. And it's going to be time on the other. And you tell me what is the first measurement of millivolts that we should have on our graph. This is my membrane potential. I'm just going to measure it. I'm going to measure changes in it throughout this whole process. Well, remember, we start at negative 70. I probably should say that negative 70 is like a number that I read in some textbook. And some textbooks report something different and every cell is different. But I'm comfortable with negative 70, that number thereabouts. Your membrane potential will stay at negative 70. I mean, I could make this whole graph go on forever. In fact, I'm going to make this a lot longer so that you know that, dude, this could go on forever. And multiple things can happen in this space. But let's just pretend like along comes a nice purple stimulus. And at that point, and I'm going to draw it up here, I'm going to label my point, this is the stimulus. So that's moment number one up there, the stimulus. Okay, what kind of stimulus? Dude, somebody punches you in the face. How about a light goes on? You hear a fantastic noise. You eat something. You smell something. A spider crawls up your arm. Some kind of stimulus happens. This is crazy. The stimulus will cause stimulus-gated sodium channels to open. Okay? The stimulus causes stimulus-gated sodium channels to open. Okay, seriously? So here's the sodium channel. And what did we say sodium was going to do? Dog-pounds of a big old feather. Sodium is going to rush in. Because sodium potassium pump was kicking it out. Sodium is going to rush in. Now, positive charges just rushed in to this negative 70-ish charged membrane potential. What happens to your membrane potential when positive charges come in? There's only one thing that can happen. The membrane potential increases. Can you visualize that? Positive charges come in to a negative 70-charged place and the charge is going to increase. It's going to become more positive because positive charges came in. And guess what? If the stimulus was big enough to cause a membrane potential change up to about negative 55, that, I need a new color, that's threshold. If threshold is reached, then guess what happens? Then voltage-gated sodium channels open. Okay, the first ones were stimulus-gated. They opened if the stimulus came into the scene. If the stimulus was big enough, enough of them would open and allow enough sodium ions in and change the membrane potential to negative 55 or threshold. And if you reach threshold, voltage-gated sodium channels open. All of them go. And so everything opens. What's going to happen to my membrane potential? Holy madness of opening sodium ions. Seriously, it's like you got to be joking me. Sodium rushes in. I got to make that blue. Sodium rushes in. If the sodium ions rush in, the membrane potential changes and it goes all the way up to positive 30. That's really high. Positive 30 millivolts. And then this is a timing issue. Sodium channels shut. They were open and the stimulus and the voltage caused them to open and they're open for like a millisecond and snap. They snap shut. And can sodium move now? No. And there's actually a little movement of like, oh, really? Nothing's happening here. Because the sodium can't come in anymore. So we're not going to continue to get positive. They literally just shut. Now, guess what happens next? Oh, let's make it, sure. We'll make it this color because next, this is, oh, that was number three. This is number four. I'm not going to organize here. I got to make my colors right. There's three. Number four, potassium voltage-gated potassium channels open. Now, in all actuality, those voltage-gated potassium channels open. They start to open at the same moment that the sodium channels opened. The same reaching threshold causes them to open too. But they're slow. And so that it takes them longer to get open. But now they're open. What? So what's going to happen? Potassium is going to go where? Potassium goes out. Oh, show. True story. Bye-bye. What happens to your membrane potential as the potassium ions rush out? You've got to be kidding me. True story, potassium beats cheeks. I'm out of here. Membrane potential goes down, often drops below the resting potential. But then you end up returning to somewhat of a normal resting potential. Why? What helps reestablish that? Let's make this five. Close. Potassium gates close. Gates close. And then who goes to work? I'm sorry, I just changed my mind because I want it this color. Why? Who's going to work? Six sodium-potassium pump reestablishes. Wow, membrane potential. Extinct membrane potential. Did we catch anything? I mean, did we miss anything? Oh my gosh. That is so incredible. There are some details in there that some details that we are going to just, it's more complicated than this, but doesn't this kind of blow your mind? Okay, I'm going to talk about some other key parts of this action potential process. The thing, I'll do that in a second. Okay, you need a break. Go eat some chocolate.