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The basal ganglia - Concepts of the indirect pathway

Video transcript

imagine it's the World Cup in soccer and you are actually playing and it's the final game the score is 1:1 we're 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 there are a few things that could go wrong though right imagine if you were standing there and you're 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 and both of your feet came up at the same time well that wouldn't be very good right because you can't really kick a ball very well and stay on your feet if both of your feet come up so we need something in our body 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 well 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 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 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 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 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 that 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 bringing up the brain here so let's pop these nuclei these 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 gonna I'm going to put them kind of roughly where they are in the brain but I might move them around a little bit just 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 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 [ __ ] 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 then we need in order to control our muscles and that's this 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 would actually chats with our muscles right so we need the motor cortex in kind of our little pathway in order to control our muscles because that's the final the final stop before muscle control so the thalamus the thalamus has this really interesting relationship with the motor cortex it always wants to excite the motor cortex that's all 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 on the field right and you're about to kick that ball and if the 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 a sudden you know your left leg will move but your right leg will move too and your hands and your 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 is moving around so that's what the thalamus wants to do all the time and so it's kind of it's kind of like a dog you know it's like that that cute puppy that looks so innocent until you take it off its leash and then it just is 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 it's just chaos so what do you do with your dog if you can't trust it well you 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 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 over excite the motor cortex that's 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 is keeping the activity in the thalamus down by sending these neurons to it that 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 over excite our motor cortex which would over excite our muscles but it's not that simple because we we need a bit more flexibility than that because we can't just have the thalamus on this this constant leash that's that's kind of at a fixed length right so we need something in the brain to kind of allow us to adjust this leash on our thalamus because sometimes we we need to move a little bit more imagine you were holding the ice cream cone and you you dropped it and you had to reach down to pick it up it still it's still lickable you know 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 gonna hit you in the face if you try to move that much and then you're gonna have ice cream all over your face and in the moment you know really quickly we may need to change how much we need to move so you know if you're on the soccer field and you want to pass the ball to one of your one of your teammates well 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