Aphasia's and Basal ganglia

[aspiringmd1015]

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for CNS, my other sources have a billion of the aphasias, and FA only has around 7, so are the ones in FA the only ones i need to know? also im a little confused on d1 and d2 concept of how the substantia nigra stimulates direct and indirect pathway, mostly bc i dont know where the d1 and d2 receptors are located.

 

[Mr. Mojo Risin]

The aphasias in FA are the most HY ones to know.

As for the BG, sit down for a few minutes and follow the excitatory and inhibitory effects of neurotransmission through the BG. It will make sense. The overall effect of dopaminergic input from the SNpc on the putamen is pro-movement. D1 stimulates, while D2 inhibits. Be careful, though, because despite the stimulation/inhibition, what matters is what they are stimulating/inhibiting because that will determine the next step in the cascade of neurotransmission. In short, dopamine stimulates the direct pathway (via D1: Gs) and inhibits the indirect pathway (via D2: Gi).

 

[aspiringmd1015]

i understand that part, but im assuming the d1 receptors are located in the striatum(caudate and putamen) and this leads to increase in firing of these gabanergic neurons to inhibit the inhibitory neurons of the internal segment of the globus pallidus, disinhibition effect, and boom thalamas is firing away. I dont know where the D2 neurons are located though, and how the D2 pathway prevents the indirect pathway from being activated.

 

[Mr. Mojo Risin]

i understand that part, but im assuming the d1 receptors are located in the striatum(caudate and putamen) and this leads to increase in firing of these gabanergic neurons to inhibit the inhibitory neurons of the internal segment of the globus pallidus, disinhibition effect, and boom thalamas is firing away. I dont know where the D2 neurons are located though, and how the D2 pathway prevents the indirect pathway from being activated.

Look at the figure in FA. It's very clear what's going on, you just have to work through the neurotransmission one step at a time. The D1 and D2 receptors are in the striatum, hence the nigrostriatal pathway (substantia nigra to the striatum). It's dopamine effect on the striatal dopamine receptors that is dictating the stimulation of the direct pathway and the inhibition of the indirect pathway. The end goal of dopamine's action on the direct and indirect pathways is disinhibition of the thalamus by making sure the GPi doesn't inhibit the thalamus. The direct pathway doesn't involve the STN, whereas the indirect pathway does involve the STN, which also needs to be inhibited because it stimulates the GPi.

Direct pathway: Dopamine (SNpc)--> D1 on striatum--> direct inhibition of the GPi via striatum, which then disinhibits the thalamus.

Indirect pathway: Dopamine (SNpc)--> D2 on striatum--> inhibition of the striatum leads to disinhibition of the GPe--> inhibition of the STN--> decreased stimulation of the GPi--> disinhibition of the thalamus.