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Let's talk a little more neuroanatomy.

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and neurophysiology of the brain.

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I'd like to start with the Globus Pallidus.

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This is a young individual with tumefactive MS.

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We're not going to pay any attention

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to that right now,

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but I'd like you to focus on the anterior limb of

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internal capsule that bleeds into the region

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of the globus pallidus.

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Now, later on,

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we're going to be able to separate the globus pallidus

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with iron-sensitive sequences

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into a GPI inner part,

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which is right up against the genu of the internal

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capsule and a GPE or external or outer part.

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The putamen is also divided into

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an inner and outer part.

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And we can sort of see a little division

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right here with this thin white line.

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Then beyond the outer aspect of the putamen is

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the external capsule, then the claustrum,

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which is this white little band right here.

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Then the extreme capsule,

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which is the dark band outside the claustrum.

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And then a lighter area,

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the centro-sylvian cortex.

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So let's do it again with a

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little more magnification.

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Internal capsule,

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inner and outer aspect of the globus pallidis.

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Inner and outer aspect of the putamen.

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External capsule, dark, claustrum, white.

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Extreme capsule, dark, centrosylvian cortex,

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white.

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You can't see them as well

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on the accompanying FLAIR image.

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Now, activity in the GPI.

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The inner aspect of the globus pallidus is

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modulated by a dual set of striatal pallidal

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GABAergic projections,

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which have been termed the

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direct and indirect pathways because one is a

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single neuron direct projection to

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the globus pallidis internal portion,

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and the other runs through synaptic

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stations out here in the GPE or external portion

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of the internal capsule,

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and also in the subthalamic nucleus,

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which we see better and described

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in the coronal projection.

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Now, there are diverse types of chorea with different

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causes such as Huntington's disease.

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Another one you probably haven't thought

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about in a while is levodopa-induced chorea.

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Yes, levodopa is used to treat Parkinson's,

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but it can cause a chorea in Parkinson's disease.

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And you have to sort these out.

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Hemichorea can also occur after lesions in the

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subthalamic nucleus, and this is known as ballism.

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And they can all be explained by deficient

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globus pallidus internal region,

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inhibitory input to the motor thalamus,

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resulting in excessive thalamo-motor facilitation.

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So in other words,

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the thalamus is constantly driving the car.

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You want the car to slow down a little bit

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so it's in control.

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The GPI is responsible for that control and

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slowing down. And when it fails,

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now there's no inhibition on the thalamus.

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It steps on the accelerator and it just simply

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speeds up with these movement disorders.