The basal ganglia - Details of the indirect pathway

so in this video we're going to take a look at the indirect pathway that circuit between some of the nuclei of the basal ganglia that helps to make sure that we don't make any unwanted muscle movements so maybe an unwanted movement would be throwing a hot cup of coffee at your crush on your first date that that would definitely be an unwanted movement but before we get into all the all the details of this pathway and see how we can avoid this unwanted muscle movement that would definitely ruin our first date let's just take a look at this special structure here the thalamus now the thalamus it really really likes chatting with the motor cortex and and this is the motor cortex here this structure that chats with our muscles and helps to control them now if the thalamus could have its way it would just send tons and tons of excitatory messages to the motor cortex and just turn its activity up turn the activity up in the motor cortex and when the activity in the motor cortex is turned up the motor cortex chats more with our muscles and this turns their activity up and this makes us move more so when I think of all of this I like to picture the thalamus as a puppy a really excited puppy that that just cannot be trusted and it needs to be kept on a leash because that's what you do with puppies that can't be trusted right and we keep it on a leash to control how much can run around and cause trouble and in this case the trouble is the thalamus over exciting the motor cortex making it too active because that makes our muscles too active and that makes us move too much so we actually have this leash in the brain we have a leash for the thalamus and it's the Globus pallidus internal one of these nuclei of the basal ganglia so this is the Globus pallidus internal here and it controls the thalamus it keeps it on a leash by sending it these inhibitory messages and this keeps the activity in the thalamus down so this is what the Globus pallidus internal does in the brain it keeps the thalamus on a leash and when the basal ganglia work together to keep us from making an unwanted movement what they do is they tighten the leash on the thalamus they make it even less active right because you pull that leash in can't run around as much so to do this all they do is they they chat with each other and ultimately they send a message to the Globus pallidus internal and they tell it to tighten that leash to send more inhibitory messages to the thalamus and this turns the activity in the thalamus down so it can't can't chat with the motor cortex as much so that's the aim of the indirect pathway to tighten the leash on the thalamus by chatting with the Globus pallidus internal so let's go through this pathway and have a look at how it all happens so the first thing that happens is the motor cortex it chats with the striatum and it turns up activity in the striatum so to do that it sends this excitatory message along this excitatory neuron here that heads over to the striatum and so the striatum gets really excited right it gets excited and its activity it turns up and so the striatum has these neurons here these red neurons let's trolleys in red that are inhibitory neurons that head over to the Globus pallidus external that that other part of the Globus pallidus so when the striatum gets excited these red inhibitory neurons get excited and they turn down activity in the Globus pallidus external so we have these neurons that go from the Globus pallidus external to the subthalamic nucleus so when the Globus pallidus external activity is turned down it can't chat with the subthalamic nucleus as much as it used to and and what it usually does is it normally kind of holds down the activity in the subthalamic nucleus so when it's not so active it can't do that so the subthalamic nucleus it gets more excited right and this is this is the really important part so this is that this is the part that matters is the subthalamic nucleus it gets excited and it's what sends excitatory messages to the Globus pallidus internal our leash on the thalamus so what happens if we excite our leash well that's going to tighten our leash right because we have our leash our inhibitory neurons that go from the Globus pallidus internal to the thalamus and so if we excite those neurons we're gonna we're gonna tighten our leash because those neurons are inhibiting the thalamus so all that kind of all that you need to kind of get from this is that when we don't want to move our muscles our motor cortex sends this little signal it goes through this little pathway here and the end result is that we excite the Globus pallidus internal and that tightens our leash on the thalamus and so you know from before that if we tighten that leash if we inhibit the activity of the thalamus even more we're going to have less chatting between the thalamus and the motor cortex and less chatting with our muscles between the motor cortex and our muscles right now there's a little bit more to the pathway remember what we said before about how we want to be able to adjust our leash well we have something kind of in the background that does just that it adjusts our leash based on what we need to do so if we head back over here to the subthalamic nucleus it actually chats with the substantia [ __ ] and the substantia [ __ ] what it does in this pathway is it kind of does the opposite of what we just did it is our adjustment of our leash it lets our leash out a little bit so allow our muscles to move a bit more so we'll just go over how that happens but that's that's the important part that's the goal of the substantia [ __ ] in the indirect pathway so the subthalamic nucleus it turns up activity in the substantia [ __ ] so does that buy these excitatory neurons by sending these little excitatory messages and the substantia [ __ ] it has these other types of neurons these dopamine neurons and these dopamine neurons head over to the striatum and they chat with these excitatory neurons these neurons that turn up activity in these inhibitory neurons here that go to the Globus pallidus external so when the substantia [ __ ] gets excited when it's deciding that it needs to allow the thalamus to be out a little bit more on its leash to allow it to have a bit more bit more freedom and allowed to play a bit more its sends dopamine to the striatum and the dopamine binds to these little receptors here on these excitatory neurons and these receptors are called d2 receptors and when dopamine binds to these it turns down activity in the neurons in these excitatory neurons and so the they can't they can't chat with these inhibitory neurons as much and turn their activity up so what happens is these inhibitory neurons that would be inhibiting the Globus pallidus external they're not able to do their job as much as they were before when we returning their activity up so they can't inhibit the Globus pallidus external as much as before so that's kind of the opposite of what we had before right so before we were inhibiting the Globus pallidus external and now the substantia [ __ ] is kind of taking away some of that it's allowing the Globus pallidus external to be a little bit more active so that means that the Globus pallidus internal is going to be a little less active and so if the Globus pallidus internal our leash our leash on our thalamus is less active then that means that it can't hold the leash so tight it's going to have to let it go a little bit and the thalamus is going to be allowed to play a bit more it's going to be allowed to be more active so it's going to talk to the motor cortex more and the motor cortex is going to talk to our muscles more and we're gonna have more muscle movement in the substantia [ __ ] it's smart right it knows that our dog cannot be trusted so we need to control how much we're letting it out we can't just let everything go so the substantia [ __ ] knows when enough is enough and it sends messages back to the subthalamic nucleus telling the subthalamic nucleus to stop telling it to be more active and the substantia [ __ ] stops allowing this little pathway to let our leash out right so that that is the indirect pathway that's how these structures talk to each other to adjust the leash on the thalamus and that's how they adjust how much movement we make