Basal ganglia and associated circuitry. Several brain regions and numerous neurotransmitters are involved with the function and regulation of neuronal activity within the basal ganglia. In Parkinson's disease, the loss of dopamine neurons projecting from the SNpr to the neostriatum (1) leads to neuronal imbalance in this brain region and alterations in striatal outflow pathways. The direct pathway (2) the striatum to SNpr) becomes less active, whereas the indirect pathway (3) striatum to globus pallidus to STN to SNpr) increases in activity. The consequence is an enhanced activation of SNpr outflow neurons (4), which exert an overall reduction of the thalamic-cortical pathway (5). DA, dopamine; Glu, glutamate, excitatory transmitter; GABA, a-amino butyric acid, inhibitory transmitter; ACh, acetylcholine, excitatory intrastriatal transmitter. GP globus pallidus; SNpc, substantia nigra pars compacta; SNpr, substantia nigra, pars reticulate; STN, subthalamic nucleus.
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