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- [Voiceover] Every day wemake hundreds of movements, from reaching for our firstcup of coffee in the morning to waving hello or good-byeto someone that we know to using utensils to eat our food. And we actually have thisreally incredible system in our brains that allowsus to make these movements, and it prevents unwantedmovements from happening, and it does this in thisreally smooth, streamlined way that we don’t even really notice. So this system is calledthe basal ganglia, and the basal ganglia isactually a collection of nuclei, and here when we saynuclei what we mean are structures that are just kind of made up of a bunch of neurons. So these little clusters of neurons. So the basal ganglia is made up of a few of these clustersof neurons, these nuclei, and before we actually go through how they together control our movements,
- let’s just first have a look at where these structures are in the brain. So here we have a diagram of the brain, and the way that we’re looking at it is as though we sliced it, and we kind of split up the front andthe back of the brain, and now we’re looking insideone of these sections. And we call this a coronal section. So if we look here, this is the putamen, and over here is the bodyof the caudate nucleus. And down here we can actually see the tail of the caudate nucleus, and the putamen and the caudate nucleus together actually formwhat we call the striatum. And if we look back over here, we can see what we callthe globus pallidus, and this is the external part, the external globus pallidus, and this is the internal part. And if we look down hereat this black structure that we have on our diagram, this is the substantianigra, and we call it this, we call it the substantia nigra, which means black substance,because the neurons here they have this pigmentin them, this coloring,
- that makes them actuallylook black in the brain. And we can actually see this darkness of the substantia nigra quitenicely if we look at an MRI. So in this MRI of the brain, you can see these little black areas hereon both sides of the brain, and this is the substantia nigra. And if we head back over toour diagram of the brain, we have over here the subthalamic nucleus, and we call it this becauseit actually sits below, sub meaning below, the thalamus. So except for the thalamus,these are the components of the basal ganglia thatwe need to know about to talk about movement. So all of these structures,including the thalamus, they work together tocontrol our movements, and the way that they do that, the way that theycommunicate with each other, is through these pathways. And we’re going to talkabout these pathways, but this communication in these pathways is controlled by neurons,neurons talking to each other. So before we dive into thesedetails of these pathways, I’m going to throw someterminology at you, just so that things makea little bit more sense as we’re going along. So when a neuron goes from onepart of the brain to another, it actually communicateswith another neuron at its destination, and it does this at what we call a synapse. And it’s here that the first neuron, which we call the presynaptic neuron, that came from the first location, and
- the postsynaptic neuron, which is in the arriving destination, this is where they talk, andthey talk by sending chemicals that we call neurotransmitters. So the presynaptic neuronsends a bunch of chemicals to the postsynaptic neuron, and depending on what kindof chemicals they send, the postsynaptic neuron may have different things happen to it. So one important neurotransmitter that the presynapticneuron can send is GABA, and GABA we call our maininhibitory neurotransmitter, and we call it this, thisinhibitory neurotransmitter, because it has this inhibitory effect on the postsynaptic neuron. So it kind of turns it off. It turns its activity off. It inhibits it. So another neurotransmitter that the presynaptic neuron could send is one that excites the second neuron, excites the postsynaptic neuron and turns its activity up. And the main excitatory neurotransmitter is called glutamate, andthis increases activity in the postsynapticneuron when we excite it. So all of this will become important as we go through these pathways. So there are two big thingsthat we need help with, when it comes to movements. The first is that we actuallyneed help making a movement. So we need help gettingfrom saying to our bodies, hey, I want to move my arm, I want to grab that cup of coffee, to the point where weactually are moving our arm. So everything in betweenwe need help with. And the second thing we needhelp with is not moving, making sure our muscles arenot moving when we’re at rest or when we just don’t want them to. So the pathway that takescare of this first one here, we call the direct pathway, and the pathway that takescare of the second one here, we call the indirect pathway, and both of these pathways, we call these the pathways of the basal ganglia. So they’re involving those structures that we looked at before when we were looking at the brain.
- And we’re going to gothrough the direct pathway. So these are thecomponents of our pathway, and before we begin, it’simportant that we recognize that the thalamus here, the thalamus, it’s normally underwhat we call inhibition. So this means that unless things change, the thalamus is, itsactivity is being suppressed. It’s not allowed to be asactive as it wants to be. So the aim of the direct pathway is to take away its inhibition, to allow the thalamus to be more active, and that’s because thethalamus is what talks to the motor cortex, whichthen talks to our muscles, telling them to move. So if we want to get movement going, if we want to move ourarm, we need the thalamus to be able to be active. So that’s the aim of the direct pathway. So the first thing thathappens is up here, in the motor cortex,and that’s when we say, hey, I want to move. So when we say that, an excitatory neuron from the motor cortexgoes to the striatum. So this is something that’s already there, but the motor cortex sendsan excitatory message to the striatum, and thisexcitatory neuron here, it actually synapsiswith an inhibitory neuron in the striatum that’s heading to the globus pallidus internal. So when this exitatory message comes down this excitatory neuron and synapsis on this inhibitory neuron in the striatum, heading for the globus pallidus internal, what this does is it excites the striatum, and these inhibitoryneurons in the striatum, they become more active,because the striatum is excited. So these inhibitory neurons,they’re more active, and so they actually inhibitthe globus pallidus internal more than it would havebeen before we sent this excitatory messagefrom the motor cortex. So this excitation that’s happening here is happening because ofglutamate being released, and this inhibition on theglobus pallidus internal is happening over here becauseof GABA being released. So the globus pallidusinternal normally is what’s actually holding the thalamus down, keeping its activity down. So when its inhibited bythese striatal neurons, its activity is turned down. So when the activity of theglobus pallidus internal is turned down, it can’tinhibit the thalamus as much as it normally would. So the thalamus is nowno longer as inhibited as it was, so it’s ableto get a bit more excited, a bit more active, and it’sable to send excitatory messages to the motor cortex, because it has these excitatory neurons that go there. So it sends more and moremessages to the motor cortex, and the motor cortex gets more active, and it then sends excitatory messages to the muscles that we want to move. So that’s how we make thosemovements that we want to make. So while all of this is going on, the substantia nigra andthe subthalamic nucleus, they’re actually kind ofworking in the background to fine tune things. So the substantia nigra has these neurons that are dopamine neurons,and they actually go from the substantia nigra to the striatum, where they synapse withinhibitory neurons in the striatum that are going to theglobus pallidus internal. So kind of those ones thatwe talked about before. So when the substantianigra is more active, it sends more and more dopamine to these inhibitoryneurons in the striatum that are heading for theglobus pallidus internal. And these inhibitoryneurons in the striatum, they have these dopamine receptors that we call D1 receptors. And when dopamine fromthe substantia nigra binds to these D1 receptors on these inhibitoryneurons in the striatum, they get excited. And so, the dopamine comingfrom the substantia nigra further excites these inhibitory neurons heading for the globus pallidus internal, and this results in evenmore reduction in activity,
- even more inhibition of theglobus pallidus internal, and this allows the thalamusto be even more active, because we’ve further blocked that signal. And back over here,the subthalamic nucleus is actually what’s excitingthe substantia nigra. So it sends excitatory messages through excitatory neuronsfrom the subthalamic nucleus to the substantia nigra,and this is what excites the substantia nigra and allows it to send more dopamine to the striatum. And the substantia nigracan actually talk back to the subthalamic nucleus,and it does this through inhibitory neurons, andthis allows it to say, hey, stop exciting me,I’ve had enough excitement. So it actually inhibitsthe subthalamic nucleus, which then stops the subthalamic nucleus from being able to excitethe substantia nigra. So when this happens,when the substantia nigra isn’t being as excited bythe subthalamic nucleus, then it’s not adding to thatextra activity in the thalamus. It’s not allowing thestriatum to further inhibit the globus pallidus internal. And so, we don’t get asmuch movement from muscles as we would if thesubstantia nigra was excited. So together these structuresin the direct pathway, they work together toultimately increase excitation of the motor cortex, soto make it more active and allow us to makemore muscle movements.
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