The cortico-striato-thalamo-cortical circuit in a healthy individual.
In the normally functioning cortico-striato-thalamo-cortical circuit, glutamatergic signals from the frontal cortex lead to excitation in the striatum, which increases inhibitory GABA signals to the GPi and the SNr. This then decreases the inhibitory output via GABA from the GPi and SNr to the thalamus, resulting in thalamic excitatory glutamatergic output to the frontal cortex. This is a positive feedback loop. An indirect external loop composed of the GPe and subthalamic nucleus (STN) is postulated to contribute to a steady state of excitation/inhibition. The striatum inhibits the GPe, which decreases its inhibition on the STN. The STN is then free to excite the GPi/SNr and therefore inhibit the thalamus (adapted from ref.
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