[0:00]Whenever you're trying to learn a complex concept, as complex as an action potential, you should always try to think of a memory technique. And for this, I want you to think of a salty banana. How is this going to help us? Stick around.
[0:25]All right, guys, welcome to Psych Explained. In this video, we're going to break down an action potential. Now, if you look at different textbooks, you might see different names for it. You might see the phrase a neuro impulse or nerve impulse or spark, but essentially it means the same thing. It is a brief electrical charge that travels down a neuron or more specifically, the axon. And this is going to allow neurons to communicate with each other and get the message across, whatever that message may be. And in this video, we're going to go step by step by step by step how this process works. But let's begin by understanding what are we actually looking at behind me. Let's actually start with this visual right here. This is a neuron. Now, I do have a separate video on the parts of a neuron and their functions. I recommend to watch that first and then maybe come back to this video. We have different parts. We have our soma and cell body, that sits in the middle. We have our dendrites that receive the message from the sending neuron. We have the axon, which takes the action potential, the charge away from the soma, and our axon terminal, which contains the neurotransmitters. This visual right here is essentially a close-up of different parts of the neuron. This is considered the beginning of the neuron. So this is going to represent our soma, our cell body, and the receptor sites on the dendrite. Okay, so what we're looking at right here is a close-up of this area. And this long tube is going to represent our axon, okay? Because this is where the electrical charge, or actual potential, is going to travel down all in the same direction. You'll also notice there's a membrane, this outside. This protects the neuron, right? It keeps things in, it keeps things out. The only way to get things in and out are through these little channels right there. We'll talk about those in a moment. Now, another thing to pay attention to is that a neuron is surrounded by ions. What's an ion? It's a charged particle or molecule, all right? Some of them are negative, and some of them are positive. When we're referring to a neuron, there's two that are really important to us: sodium and potassium. You'll notice there's a high concentration of positively charged sodium ions that sit outside the neuron, right? We have sodium that sits outside. On the inside of the neuron, here's our axon, you have a higher concentration of potassium, positively charged potassium ions, all right? Do you remember our memory technique earlier? We have our salty banana. Let's think about that together. Here's our salty banana, right? Here's our banana. How does this help us? We have potassium on the inside. Potassium on the inside. What do we have on the outside? Sodium, right? Imagine we're pouring salt all over the banana. Salty banana. There is our memory technique. So, when a neuron is at rest, is not doing anything, we have positive, uh, potassium on the inside, positively charged sodium ions on the outside. Okay, so there's where we're at, all right? Now, here's what I want us to know. When a neuron is at rest, right? The neuron is not doing anything. What we have is a negative 70 millivolts. In other words, if you take the voltage inside the neuron, we're at negative 70. It's very negative on the inside. Okay, that's where we're starting. The next question is, how do we then excite the neuron? Right? How do we actually have the neuron do something? Well, let's just imagine that we are reaching for a glass of water, right? There's a lot of things that can stimulate a neuron. It could be thinking about something. It could be smelling something. It could be putting cold water on my, on my arm. But in this case, let's imagine that you're reaching for a glass of water. What's going to spark that? We have these neurotransmitters. We're going to represent these as acetylcholine. What is the acetylcholine? This is a neurotransmitter that controls our muscles to move, right? To move our muscles, pick something up, to bend down. This is acetylcholine. And what's going to actually happen is our first step. We'll write this in together, is there has to be some sort of stimulus that excites the neuron. The stimulus is going to be acetylcholine reaching for something. These acetylcholines are going to bind to the receptors on the dendrite. All right? And what's going to happen is, these are going to open the channel and take with it sodium, our positively charged sodium ions, all right? And these are very positive, okay? So there we go. Once we bind a receptor, this is going to allow sodium to flow into the neuron as well. And then we have another acetylcholine is going to bind to another receptor site. What's going to happen? It's going to allow this channel to open up, this gate to open up and more sodium is going to rush in, okay? Once again, it is very positive, okay? And this is going to happen over and over and over again all on the receptor sites. They're going to bind to those little pockets. We have our sodium that's going to rush in, becoming more positive, positive, and positive. So right now, this area is extremely positive. What does this mean? This graph here is showing the voltage, from negative to positive, right? As we go up, it gets more positive, we're getting close to zero, and then as we go down, we're getting more negative. The neuron is starting to become more positive. So what's going to happen? The inside of the neuron, the charge is going to start going up, up and up and up, okay? It's getting more positive until it reaches negative 55, okay? This is considered the threshold. What is it considered? The threshold. This is the magic number. If a neuron gets positive enough to reach negative 55, something amazing happens. The action potential is likely to fire. We call this the all or none principle because if we do reach that threshold, a neuron's going to fire every time. Now, not every neuron does reach that threshold. There could be some kind of false alarms where it's like, it's almost there, and then nope. It's almost there and then nope, right? There wasn't enough stimulation. But if we reach that threshold, that's the magic number. So what's that going to do? All these sodium, positively, uh, positive sodium ions are then going to trigger these voltage-gated sodium channels that go all along the axon. What's going to happen? This is going to cause these gated channels to open up. These are going to open up the lid, right? Thinking about like a channel that opens up like, right? A little gate that opens up, and this is going to draw more very quickly sodium into the neuron, okay? Once again, very positive, okay? And this is going to go to negative 55. What's going to happen? This is going to trigger then the next gate to open up, the next voltage-gated channel open up, and then this sodium is going to enter, okay? You get the idea here, right? All this sodium is entering. It's going to become more positive, and then finally, it's going to open up the last one, all right? This is going to open up. This is going to open up this, and our sodium is going to rush in as well, okay? We got positive, positive, positive. All right, so what's happened here? We have so much positive sodium ions within the neuron. That this is going to take the charge not just negative 55, but it's going to change it all the way up to positive 30. It is so positive now on the inside, it's reached positive 30, okay? We'll go right there, okay? So what does this mean right now? What we have is what we call depolarization. That is the next phase in our action potential. Depolarization, okay, or depolarized. What this means is when a neuron starts to become more positive, the neuron is depolarized. This is going to start our action potential. So what does this mean? It means our sodium ions are going to start rushing in to our neuron. And it's going to create more positive. So as they rush in, all everything starts to become more positive on the inside. What do you think happens on the outside? It's going to become more negative on the outside. The voltages have switched. This is the beginning of our, this is our action potential, right? This is our depolarization is going to cause or evoke or trigger the action potential. So now everything of electrical electricity traveling throughout the body, essentially what we're just saying is, all these sodium gates opening up and letting sodium into the cell. All right? So that's where we're at right now. Now, what happens once it reaches positive 30? Well, the neuron has to go back to being negative, right? It has to go back to the resting state. So what happens is, these sodium gates are going to close, right? It's almost like we've had a, we have enough sodium inside the cell. We don't need any more. We're going to close the gate. What's going to open up? The potassium voltage-gated ions are going to open up. These channels are going to open up, okay? And this is going to cause these positively charged sodium ions are going to rush outside of the cell, okay? All outside. Potassium rushes out. Why is that important? Because this is going to make the neuron more negative, and this is going to rush back down all the way to negative 90, okay? I'll explain why it's at negative 90 there. The process of closing the sodium, positive, positive sodium ions, and opening up the potassium. We call this phase repolarization, repolarization, okay? Repolarization. And this is when our potassium rushes out of our cell, okay? Because we want to go back to the original, right? We want to go back to the original resting state. So let's actually write this in together. This charge going up, this graph going up is sodium rushing into the cell. And the voltage going back down to negative is our potassium rushing out, okay? This whole thing together, right? This whole process together is our action potential, okay? The rising and the falling. Now you'll notice, the voltage goes even more negative than at rest. Why is that? It's because one, we have so much potassium leaving the cell, and also it takes a long time for these to close. That actually undershoots and goes even more negative and it goes to negative 90, okay? Why is that important? This phase right here is what we call hyperpolarized. What is it called? Hyperpolarized, okay? You can also call this the refractory period. Refractory period. Okay, what happens here? We have the neuron recharging, right? It's like trying to flush a toilet twice in a row. It can't, right? You have to let the toilet fill up, the water has to fill up, and then it could fire again. Well, it's the same thing with the neuron. It can't fire twice in a row. It has to recharge. The charges have to return back to normal, they have to switch, and then we can fire again. And eventually, we're going to go back to our resting potential, and we'll be ready to charge again. So there we go. We have our resting state or resting potential. We have our threshold negative 55, positive 30 goes back down. There's a nice way to think about it as well. And by the way, our last little box right here. We go back to our resting potential, our resting state. We are ready to fire again, our resting potential, okay? And instead of depolarize, instead of repolarized, we are now back at polarized, okay? Polarized. And what happens here? Well, everything changes back. We now are negative on the inside, okay? And we return to being positive on the outside. Everything returns back to normal, okay? And the neuron is ready to be fired again.
