Basal Ganglia Circuits

The concepts of the direct pathway and indirect pathway take up a day of medical school neuroscience courses. They usually follow a discussion of the four dopamine pathways: the nigrostriatal, mesocortical, mesolimbic, and tuberoinfundibular.

These circuits were recently featured in the journal Neurology.

Benarroch E. Endocannabinoids in basal ganglia circuits. Neurology 2007;69:306-309.

In the diagram featured in that article the only variation in the above schematic is the contribution of the substantia nigra pars compacta. According to the article, "dopaminergic inputs from the SNc modulate the activity of these pathways, exerting a net excitatory effect on the direct and a net inhibitory effect on the indirect pathway."

Dr. Benarrach says it better than I could: in the direct pathway "striatal neurons elicit a phasic inhibition of the GPi/SNr, leading to disinhibition of the thalamus and brainstem motor areas". The indirect pathway is explained thus: "since the GPe tonically inhibits the STN, activation of the indirect pathway disinhibits the STN. The resulting increase in STN activity leads to an increase in inhibitory output from the GPi/SNr, resulting in inhibition or termination of motor programs."