[0:00]All right guys, welcome to Psych Explained. In this video, we're going to take a deep dive into synaptic transmission. That is to say, how one individual cell or neuron communicates and passes its information to another cell or neuron. Now, the way that you and I communicate and pass information is through spoken words, but the way cells communicate is through chemical messages and electrical signals. And those messages can be excitatory, we're going to increase the likelihood that the cell's going to fire, think about a green light, or these messages can be inhibitory. In which we want to decrease the likelihood that the next cell is going to fire, think about a red light. Now, before we go over the process of uh synaptic transmission, let's first actually understand what parts we are looking at. Now, up here, I have two neurons and neurons are typically made up of the same parts. We have our dendrites, think about our little tree branches, our soma, our cell body, and then the axon that's going to take the signal away from the soma. And you'll notice with these two neurons is there's a tiny space in between them. That tiny space is represented right here. This is where the message is going to travel over, jump over, and then communicate with the next neuron. So, what is this part going to be? This part, we're going to label as the pre the pre what? Presynaptic neuron. Okay, presynaptic neuron, pre because it is before, it's going to initiate that process. And if you think about what part we're talking about, we're talking about the end of the first neuron called the axon terminal. Axon terminal or you might call it the synaptic ball, right? There's so many names for it. And if this is the presynaptic neuron, what are we going to call this one? This is going to be the postsynaptic neuron, right? It's kind of coming afterwards, it's going to receive the information, right? So instead of pre, we have the postsynaptic neuron. All right, and what part of a neuron is this going to be? Well, this is going to be these tiny little tree branches, these are the dendrites, these receive the message. So, we go from the axon terminal, we jump over the synaptic cleft onto the dendrites. Okay, so there's some parts that we are actually looking at. Now, another thing to pay attention to is I have all these little negative lines here. What do those refer to? This is the idea that before a neuron fires, it is negatively charged on the inside. Okay, so we're negatively charged on the inside and positively charged on the outside and we can actually visualize this in a graph, right? We can objectively measure how negative it is and right now, and we'll kind of come back to this graph, a neuron is at -70 millivolts. What is it? -70 millivolts. This is the resting potential, or we'll say the resting membrane potential. Okay, so that's kind of where we're at right now. Now, how does a neuron actually fire? We have to start with an electrical charge, right? We need some charge to go down the neuron and reach the end. Do we know what that charge is called? Well, if you're thinking, and I'll write in here, an action potential, action potential, then you're right. Right, an act, right, in other words, every neuron has the potential to fire and this is the charge that's going to travel down the length of the neuron. Now, how do we actually get an action potential to fire? Well, there has to be some sort of stimulation, right? It could be air in my face, it could be light hitting the cones and rods and starting a signal. Um, it could be internal like my brain having a thought, right? We need some sort of stimulation. And when that stimulation occurs, what's going to happen is the cell is going to start to become a little more positive. Okay, just a little more positive, okay? And here's the magic number. If this cell has enough stimulation, okay? And it reaches this magic number, -55. This is called the threshold potential, so this is the threshold. I like to call this the point of no return, okay, very important number. Is that if this neuron reaches this number, okay, what's going to happen? What's going to happen is something with these gated channels is going to change. Okay, and all these are color coded so we can see them together. Up here, we have voltage-gated sodium channels, channels, because they things go through them. Um sodium, because we have sodium ions on the outside, gated because they open and close, and it mostly responds or opens based on the voltage, right? The membrane potential. If it reaches -5, what's going to happen is these sodium gated channels are going to open up. And what's going to happen? All the sodium is going to start to come inside. Okay, and why does that matter? They are so positive that they're actually going to change the voltage inside the neuron. And it's going to become extra positive, and this area right here, where you can say is become depolarized. Right, it's become so positive, it's going to up to positive 40. Right, it's become so positive, it's going to up to positive 40 and once again, what is this? We can say the neuron or at least this part of it has become depolarized. Okay, now, I do have an extra video, another video on the entire action potential process. I'll put the link above if you want to take a look at it, but this is kind of just a nice brief overview. So, what's going to happen? Well, the next thing that's going to happen is that this is going to talk to the next voltage-gated sodium channel, and this one's going to open up. And what's going to happen? We have more sodium going to enter the cell. Okay, we have more sodium. What's that's going to do? It's going to be more positive, okay? And you get the idea. And then we talk to the next one, and this one's going to open up, and we have more sodium entering the cell. Okay, we have more sodium. Okay, and now we have all this positive, um sodium and positively charged, um cell. Right, so this is basically what an action potential is. It is this kind of depolarizing wave that travels the length of the neuron. Okay, we'll talk about kind of what happens after a cell has become depolarized. Now, what's going to happen next? Okay? Once this reaches, right, we have -55, -55, -55. It's next going to talk to a different voltage-gated channel. What is this one? It is calcium. Okay, so we go from sodium to calcium, and this is where kind of the magic happens. Okay, so once again, I'll put it here, we're at -55, we reach our threshold. This is going to open up our calcium channel our calcium channels, and the calcium is going to enter the cell. And what's the calcium going to do? Well, what it's going to do, it's going to bind to the proteins, the membrane of these little circles. Do we know those circles are called? These are called the synaptic vesicles. Okay, and why are these so special? These contain, think about these like bags, they contain all the chemical messages called neurotransmitters tightly packed together. So, here we have acetylcholine. Acetylcholine affects our muscles, it affects learning and memory, it affects a lot of things. Okay, so we have all the calcium coming in. It's going to dock on these uh vesicles right here. Just like this, I'm going to keep going. We have more vesicles over here. I got to keep make sure I get all of them. There we go. We got another one here. Okay, so what is this going to do? Well, the calcium is going to cause a chemical change in the membrane and the protein of these vesicles. Okay, what's that going to do? It's going to make all these vesicles drop down. Okay, it's going to push them all down towards the membrane of our presynaptic neuron. Okay, so they're all going to come this way. Okay, the second thing it's going to do, and the kind of this is where the magic happens. Is that it's going to cause these vesicles to fuse with the membrane of the synaptic neuron, the presynaptic neuron, it's almost going to become one. And this is going to cross out and it's going to look something like this, right? This is kind of cool, right? It's fusing together. The proteins are fusing together. Okay, there we go. And why is that so important? Because now, all of these neurotransmitters can be released into the synaptic cleft, right? The gap between two neurons. We have all the acetylcholine, maybe I'm lifting weights. I know, my muscles aren't that big. But, we're lifting weights and all these are coming out here. Okay, so there we go. We have our acetylcholine. And by the way, the process of releasing all these neurotransmitters into the synapse, do we know what that's called? It's called exo. Exo what? Exocytosis. I know science science, man, the everything is so complex, right? All these big names, but we have exocytosis, the release of the chemical message from the pre into the gap. Okay, so what happens next? Well, all of these neurotransmitters have to go somewhere, okay? And some of them, if we're very lucky and these are excitatory, they are going to bind or dock themselves on the receptors of the next postsynaptic neuron. And these are chemical voltage, we're going to call these ligand-gated ion channels. And the difference between chemical and voltage-gated challenge uh channels is these open based off of the voltage. These open based on the binding of chemicals, okay? So what happens when it docks? It's going to open, okay? The ligand is going to open, the receptors are going to open. And what's going to happen? The same thing up here. We're going to have our sodium enter the cell. It's the same process. Okay, what's going to happen when that enters? It's going to become super positive. You get the idea. It's the same idea. We have a docking here, we have a docking here. All the ligand receptors are going to open and all the sodium is going to travel down. Okay, we're going to make this super positive again. Okay, down here, down here. I got one more just kind of floating around. Okay, I got there. And so it's going to start the process all over again. We're now depolarizing the cell, and guess what we just caused? We just caused another action potential. Isn't that cool? We just cause another depolarization, if this is excitatory, and we're going to continue that process again and again and again. Okay? Now, it is important to know, not every neurotransmitter is going to dock itself, right on the receptor. Some of them are going to, well, you know, some of them are going to go away, some of them are going to break apart, but some of them, which is kind of cool, are going to recycle themselves through these transporter proteins. And what I mean by that is some of them are going to get reabsorbed, reabsorbed by the original neuron and get repackaged into a vesicle. How cool is that? Right? It's like, um, we have such efficient cells, it's recycling. It's like, well, I'm not going to use you. I'll use you again for a later date. And by the way, do we know what this process is called when we reabsorb? This process is called reuptake. Reuptake. Okay? So we've reabsorbed, we're all reabsorbing, and we're going to be repackaged into a vesicle to be reused for another day. So, let's go over this process again. We'll number it so we know exactly what we're talking about. First, in order for synaptic transmission to occur, we need an action potential. Okay, so that's number one. Oh, I'll write right here. We number one. Action potential, right? The kind of influx of sodium, um turning the cell very positive. So, after the voltage-gated sodium channels open, we now have number two, we activate the voltage-gated calcium channels, and those can run in as well. Those are going to dock or bind themselves to kind of these uh vesicles, right, the proteins. It's going to open up. And we're going to have exocytosis. We're going to release the neurotransmitters. They're going to diffuse across the cleft, and then they're going to bind, number four, to the receptors on the postsynaptic neuron. So we go from action potential, all those gates are opening, to the release of calcium, diffusion. Um, all those neurotransmitters are being released. We're docking and we start another action potential. Now, after that, let's return here. Everything has to return to normal. We can't just always be excited, so then what's going to happen is, and I don't have it necessarily labeled here, but potassium is going to exit the cell, which is going to cause repolarization. And we go back down. Okay, we go repolarizing. And I'll write that here. We have repolarization. Guess where we're back to negative on the inside and positive on the outside. And we actually become so negative, we we go past it and we turn to our resting membrane potential.
[14:48]All right guys, thanks for watching. I really hope you learned something. Don't forget to like the video, subscribe, leave a comment below. I'll see you next time.
