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- [Voiceover] Imagine it's the World Cup in soccer, and you are actually playing, and it's the final game, the score is 1 - 1, right at the end of the game, and you are standing on the field, you're about to take the last shot, a penalty shot, to decide who wins the game. So this is a big deal, right? Your body needs to work together to make sure that you can kick that ball, and score that point. There are a few things that could go wrong through, right? Imagine if you were standing there, you were about to kick the ball with your left foot, and all of a sudden your right foot came up instead. Or imagine if you were about to kick the ball and both of your feet came up at the same time. Well that would't be very good, right? Because you can't very well kick a ball very well and stay on your feet if both of your feet come up. So we need something in our bodies, something in our brains, to help us make sure that when we want to make a certain movement, we can. And when there's certain parts of our bodies that we don't want to move, they don't move, right? And we actually have this system in the brain. This system is called the basal ganglia. And the basal ganglia is just a term for a bunch of different nuclei that we have in our brains. And these nuclei, when we say nuclei, that just means that these structures are these little specialized collections of neurons, they're specialized because they have their own little function. So the basal ganglia is made up of a bunch of these nuclei, and they work together to make sure that we can make the movements that we want to make. So kicking the soccer ball with our left foot, and also to make sure that we don't make the movements that we don't want to make, that would interfere with the movement that we want to do. So in this case, having both of our feet come up when we're trying to kick the ball with just our left foot. So some of the structures in the basal ganglia, they work together to make sure the we can make the movements that we want to make. And they work together in this way that we call the direct pathway. And by pathway, we just mean that's the way that they talk to each other. And parts of the basal ganglia work together to make sure that we aren't making any movements that we don't want to make. And we call the way that the structures talk together to do this, the indirect pathway. And that's the one that we're going to focus on now. So let's start off by bringing up the brain here. So let's pop these nuclei, these structures of the basal ganglia, let's pop them on the brain and just kind of see where they're at. And I'm going to put them kind of roughly where they are on the brain, but I might move them around a little bit, just so that we can see how they chat with each other, a bit more clearly. So over here we have one of these nuclei which is called the striatum. And down here we have something called the globus pallidus. And it actually has two parts to it, an internal part, and we say that because it's closer to the inside of the brain, and then up here it has the external part. so the globus pallidus external. and that's just because it's closer to more of the outside of the brain. And down here we have the substantia nigra, and over here we have the subthalamic nucleus. And the basal ganglia, they actually have to chat with some other structures in our brain to make sure that this gets done. So one really important structure in the brain is this structure called the thalamus. And the thalamus is the major relay station of the brain. So pretty much anything that's coming from one part of the brain to another, from one structure to another, has to go through the thalamus. So it's kind of like Grand Central Station in New York. You know how if you want to get from one part of the city to another, you pretty much have to go through Grand Central Station? Well that's kind of what the thalamus is like, it's sort of this relay hub in the brain where all information is sent to before it's sent on its way to other parts of the brain. So the basal ganglia have to talk to the thalamus in order to get their messages to our muscles to control how much they move. And there's one other really important structure in this pathway, that we need in order to control our muscles, and that's the section over here called the motor cortex. And the motor cortex is kind of that final stop, before talking to our muscles because it has these neurons that head back down through the brain here and down the spinal cord, and out to our muscles. So the motor cortex is what actually chats with our muscles, right? So we need the motor cortex in our little pathway in order to control our muscles, because that's the final stop before muscle control. So the thalamus has this really interesting relationship with the motor cortex. It always wants to excite the motor cortex, that's all it wants to do in life, it just wants to excite, excite, excite the motor cortex. And if it does that, that means that the motor cortex will just get really really active and it'll start pumping out all these messages to our muscles and make them move all out of control. So imagine you're out on the field, and you're about to kick that ball, and if the thalamus starts exciting your motor cortex too much, it's going to excite the motor cortex, and the motor cortex is going to tell all your muscles to move. So, all of sudden, your left leg will move but your right leg will move too, and your hands and your wrists and your head, and there's no way that you're going to be able to get that ball into the goal, right? It's just not going to happen if your whole body's moving around. So that's what the thalamus wants to do all the time, and so it's kind of like a dog, it's like that cute puppy that looks so innocent until you take it off its leash and then it's gone, it's running around the park, it's exciting all the other dogs, it's digging everything up, it's getting wet, It's just chaos. So what do you do with your dog if you can't trust it? Well, you put it on a leash, right? And that means that you get to control what your dog is doing. So it can only get up to so much mischief if it's on a leash. And that's kind of what it's like with the thalamus. The thalamus is that dog that you can't trust. Because if you leave it alone, it's going to run over, well, not exactly run over, you know it has these neurons that go to the motor cortex and when it wants to chat with it it sends these messages, but you know what I mean, so it runs over to the motor cortex and it's going to go nuts on it. It's just going to send lots and lots of messages, and it's just going to overexcite the motor cortex. That's what the thalamus is like. You can't trust it, you need to put a leash on it. And we actually have that in the brain. One of these nuclei of the basal ganglia, that is kind of the leash of the thalamus. And so that's the globus pallidus internal. So it has this permanent leash on the thalamus. It's keeping the activity in the thalamus down by sending these neurons to it, that send these little inhibitory messages that turn its activity down. And that means that the thalamus can't just send whatever it wants to the motor cortex, and telling it to get too excited, and so we don't have too much muscle movement. So that's great. We have this leash on our thalamus that makes sure that our thalamus doesn't just overexcite our motor cortex which would overexcite our muscles. But it's not that simple because we need a bit more flexibility than that, because we can't just have the thalamus on this constant leash, that's kind of at a fixed length, right? So we need something in the brain to allow us to adjust this leash on our thalamus. Because sometimes we need to move a little bit more. Imagine you're holding an ice cream cone and you dropped it, and you had to reach down to pick it up. It's still lick-able, it was only on the floor for about three seconds, so you need to move a little bit more to get it from the ground, to your mouth, right? And sometimes we need to move a little bit less. Maybe you're already holding that ice cream cone, and it's quite close to your face. Well you don't want to move the same amount that you just moved before when it was on the ground because that ice cream it's going to hit you in the face if you try to move that much, and then you're going to have ice cream all over your face. And in the moment, really quickly, we may need to change how much we need to move. So if you're on the soccer field, and you want to pass the ball to one of your teammates, what happens if your opponent suddenly comes in front of the teammate that you're going to pass it to, and you need to decide to pass it to someone who's a little bit further away. So we need to be able to change how much we let the thalamus talk to our motor cortex so that we can change how much our muscles get excited. So that's really all the indirect pathway is. It's the basal ganglia chatting with each other to control our leash on the thalamus so that it can control our muscles, it can turn off messages going to our muscles when we don't want them to move.