 Hi everybody, Dr. O here. In this video we're going to talk about the action potential and how nerves actually fire, how they're turned on and how they're turned off. I've been teaching a long time, this is one of those concepts that students struggle with but I'm going to try to make as much sense of it as I can. We're going to start here with what's called the resting potential or the resting membrane potential and you will be asked to know these voltage numbers. The resting membrane potential of a typical neuron is negative 70 millivolts. So what you see happening here is we do have these leakage channels where because of diffusion, sodium will slowly diffuse and leak into cells and potassium will slowly leak out. So this is a problem because we have to maintain this resting membrane potential of negative 70 millivolts and diffusion can kind of mess all that up. So it takes active forces to actually maintain this voltage at negative 70 millivolts. The key player you see right there in the middle is the sodium potassium exchange pump for every ATP that's used. Three sodiums are going to be pumped back out of this neuron cell membrane and two potassiums are going to be pulled back in. So just remember that resting membrane potential is negative 70 millivolts and it is the sodium potassium exchange pump which we covered way back earlier that maintains it. All right, that is the resting membrane potential. This is the potential energy. This is a locked and loaded gun with the trigger cocked. The energy to depolarize and turn this neuron on is here because of this resting potential. Now we have the actual action potential. We're going to fire this nerve. We're going to depolarize it. So to get to trigger this electronic, this electrical signal, we're going to need this potential, this negative 70 millivolts, this number to move. But we don't need it to move. As you see here, it's going to jump from negative 70 to positive 30. We don't need all that to happen right away. What we need is what's called a graded depolarization, which you see that dashed line there. So the stimulus that actually causes depolarization doesn't need to be that whole 100 millivolts. It needs to be 10 to 15 millivolts. This image is showing it being 15 millivolts because that's the threshold that's actually going to cause the sodium channels to pop open. They're called voltage regulated sodium channels. That's what's going to cause them to pop open and lead to a full depolarization. So we need this small stimulus that's enough to trigger this big explosion. So the best example I can think of is the gun analogy like the trigger. I don't know how fast bullets go. I don't shoot guns. Let's say that I don't even have to go a thousand feet a second or whatever it is. But if I don't need to throw a bullet of a thousand feet a second, and I'm sorry if that number's way off, to get the bullet to go that fast, I need the pressure to pull the trigger on the gun. So we need this graded depolarization, this small stimulus that's enough to lead to the explosion, which would be the depolarization in this case. And then once that graded depolarization has been met, it doesn't matter how much more powerful the stimulus is. You could be hooking this nerve up to a car batter if you want to. You need the 15 millivolts to get the graded depolarization. So that's what that also brings us to the all or none principle like we covered with muscles. A nerve is either either there's a stimulus that's large enough to fire the nerve or it's not. So a nerve is either firing or it's recovering or it's resting and waiting. That's the all or none principle. I cannot make a nerve fire faster or harder. All I can do is fire it. They're on or they're off. That's called the all or none principle. All right, so we had our resting memory potential. Now we have the triggers been pulled, the graded depolarization. Now we're ready for a full on depolarization. And what happens there is those sodium channels are now open. They fly open and sodium rushes into this cell. That's what leads to depolarization and that's what turns the nerve on. The simplest way to look at it. Positive sodium rushing into a neuron fires it with depolarization. So the charge has now gone from negative 70 millivolts all the way up to positive 30 millivolts because of all this sodium rushing in. The nerve is now firing. But you see here when we get to positive 30 millivolts, the sodium channels, very important word, the sodium channels quickly slam closed at positive 30 millivolts. So you don't see the charge go higher than positive 30 millivolts. They slam closed very quickly. At the same time, now potassium channels are going to open, which is going to lead to repolarization. So here on this image, you see what's happening. The sodium channels have closed. No more sodiums coming in. The potassium channels have now flown open and potassium is going to be leaving the cell. So that's going to lead to repolarization. So we're going to go from that positive 30 to positive 20, positive 10, zero all the way down to negative 70 millivolts because we're repolarizing the neuron. But a really important thing happens here. At negative 70 millivolts, the potassium channels start to close, but they close really slowly. There's a delay. So even though we've already reached that negative 70 millivolts, I think of like all the movies where they're closing the castle gate and the last farmer pulling some wheat or a cow or something sneaks in at the end. So whereas the sodium channel slammed closed, as the potassium channels slowly close, more potassium is going to exit the cell than quote unquote needed. And that's why we don't just repolarize this nerve. We hyperpolarize it. So the charges have gone from arresting potential of negative 70 millivolts all the way to positive 30, but now we're at negative 90 millivolts. So we have hyperpolarized the nerve. But this is great. And I'll explain why in the next video because this leads what's called to what's what's what we call the refractory period was actually a short window of time where a section of neuron won't fire. And I'll explain why that's super, super important. What I find amazing about this is this this stimulated neuron can produce 1000 action potentials per second. That's how quickly this is happening. And then so now that we're hyperpolarized, we have to get back to arresting member potential. It is the sodium potassium exchange pump and then just some basic diffusion that gets us back to negative 70 millivolts. So that means in a millisecond or two, the nerve went from resting to depolarized firing turned on to repolarized turned off back to the resting potential and it's ready to do it again. So we're able to think about how quickly this happens and how often this has to happen in your body. Okay, that is an action potential in the next video. I'll explain why these refractory periods are so important. I hope this helps. Have a wonderful day. Be blessed.