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Current time:0:00Total duration:10:47

Video transcript

every day we make hundreds of movements from reaching for our first cup of coffee in the morning to waving hello or goodbye to someone that we know - using utensils to eat our food and we actually have this really incredible system in our brains that allows us to make these movements and it prevents unwanted movements from happening and it does this in this really smooth streamlined way that we don't even really notice so this system is called the basal ganglia and the basal ganglia is actually a collection of nuclei and here when we say nuclei 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 clusters of 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 in the back of the brain and now we're looking inside one of these sections and we call this a coronal section so if we look here this is the putamen and over here is the body of 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 form what we call the striatum and if we look back over here we can see what we call the Globus pallidus and this is the external part the external Globus pallidus and this is the internal part and if we look down here this black structure that we have on our diagram this is the substantia [ __ ] and we call it this we call it the substantia [ __ ] which means black substance because the neurons here they have this pigment in them this coloring that makes them actually look black in the brain we can actually see this darkness of the substantia [ __ ] quite nicely if we look at an MRI so in this MRI of the brain you can see these little black areas here on both sides of the brain and this is the substantial [ __ ] and if we head back over to our diagram of the brain we have over here the subthalamic nucleus and we call it this because it actually sits below sub meaning below the thalamus so except for the thalamus these are the components of the basal ganglia that we need to know about to talk about movement so all of these structures including the thalamus they work together to control our movements and the way that they do that the way that they communicate with each other is through these pathways and we're going to talk about these pathways but this communication in these pathways is controlled by neurons neurons talking to each other so before we dive into these details of these pathways I'm going to throw some terminology at you just so that things make a little bit more sense as we're going along so when a neuron goes from one part of the brain to another it actually communicates with 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 and they talk by sending chemicals that we call neurotransmitters so the presynaptic neuron sends a bunch of chemicals to the postsynaptic neuron and depending on what kind of chemicals they send the postsynaptic neuron may have different things happen to it so one important neurotransmitter that the presynaptic neuron can send is gaba and gaba we call our main inhibitory neurotransmitter and we call it this this inhibitory neurotransmitter because it has this inhibitory effect on the postsynaptic neuron so it kinda 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 and this increases activity in the postsynaptic neuron when we excite it so all of this will become important as we go through these pathways so there are two big things that we need help with when it comes to movements the first is that we actually need help making a movement so we need help getting from saying to our bodies hey I want to move my arm I want to grab that cup of coffee to the point where we actually are moving our arm so everything in between we need help with and the second thing we need help with is not moving making sure our muscles are not moving when we're at rest or when we just don't want them to so the pathway that takes care of this first one here we call the direct pathway and the pathway that takes care of this 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 go through the direct pathway so these are the components of our pathway and before we begin it's important that we recognize that the thalamus here the thalamus it's normally under what we call inhibition so this means that unless things change the thalamus is its activity is being suppressed it's not allowed to be as active as it wants to be so the aim of the direct pathway is to take away this inhibition to allow the thalamus to be more active and that's because the thalamus is what talks to the motor cortex which then talks to our muscles telling them to move so if we want to get movement going if we want to move our arm we need the thalamus to be able to be active so that's the aim of the direct pathway so the first thing that happens 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 cortex goes to the striatum so this this is something that's already there but the motor cortex sends an excitatory message to the striatum and this excitatory neuron here it actually synapses with an inhibitory neuron in the striatum that's heading to the Globus pallidus internal so when this excitatory message comes down this excitatory neuron and synapse is on this inhibitory neuron in the striatum heading for the Globus pallidus internal what this does is it excites the striatum and these inhibitory neurons in the striatum they become more active because the striatum is excited so these inhibitory neurons they're more active and so they actually inhibit the Globus pallidus internal more than it would have been before we sent this excitatory message from the motor cortex so this excitation that's happening here is happening because of glutamate being released and this inhibition on the Globus pallidus internal is happening over here because of gaba being released so the Globus pallidus internal normally is what's actually holding the thalamus down keeping its activity down so when it's inhibited by these striatal neurons its activity is turned down so when the activity of the Globus pallidus internal is turned down it can't inhibit the thalamus as much as it normally would so the thalamus is now no longer as inhibited as it was so it's able to get a bit more excited a bit more active and it's able to send excitatory messages to the motor cortex because it has these excitatory neurons that go there so it sends more and more messages 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 those movements that we want to make so while all of this is going on the substantia [ __ ] and the subthalamic nucleus they're actually kind of working in the background to fine tune things so the substantia [ __ ] has these neurons that are dopamine neurons and they actually go from the substantia [ __ ] to the striatum where they synapse with inhibitory neurons in the striatum that are going to the Globus pallidus internal so kind of those ones that we talked about before so when the substantia [ __ ] is more active it sends more and more dopamine to these inhibitory neurons in the striatum that are heading for the Globus pallidus internal and these inhibitory neurons in the striatum they have these dopamine receptors that we call d1 receptors and when dopamine the substantial Niagra binds to these d1 receptors on these inhibitory neurons in the striatum they get excited and so the dopamine coming from the substantia [ __ ] further excites these inhibitory neurons heading for the Globus pallidus internal and this results in even more reduction in activity even more inhibition of the Globus pallidus internal and this allows the thalamus to be even more active because we further blocked that signal and back over here the subthalamic nucleus is actually what's exciting the substantia [ __ ] so it sends excitatory messages through excitatory neurons from the subthalamic nucleus to the substantia [ __ ] and this is what excites is substantia [ __ ] and allows it to send more dopamine to the striatum and the substantia [ __ ] can actually talk back to the subthalamic nucleus and it does this through inhibitory neurons and this allows it to say hey stop exciting me I've had enough excitement so it actually inhibits the subthalamic nucleus which then stops the subthalamic nucleus from being able to excite the substantia [ __ ] so when this happens when the substantia [ __ ] isn't being as excited by the subthalamic nucleus then it's not adding to that extra activity in the thalamus it's not allowing the striatum to further inhibit the Globus pallidus internal and so we don't get as much movement from muscles as we would if the substantia [ __ ] was excited so together these structures in the direct pathway they work together to ultimately increase excitation of the motor cortex so to make it more active and allow us to make more muscle movements