Functional architecture of basal ganglia circuits: neural substrates of parallel processing

Authors: Garret E. Alexander, Michael D. Crutcher
Summary: This paper discusses the evidence for parallelization of circuitry within the individual circuits of the basal ganglia.

• Previous Views:
• The basal ganglia served to integrate converging influences from cortical association and sensorimotor areas during their passage through basal ganglia to common thalamic target zones.
• Same BG recipient zones in thalamus received ascending, convergent inputs from the cerebellum and returned their own projections exclusively to primary motor cortex.
• More Recent Evidence:
• BG and cerebellar projections are directed to completely separate target zones with the thalamus.
• Association and sensorimotor cortex form closed circuits between cortex, BG, and thalamus.
• Output projects not only to primary motor cortex, but to virtually the entire frontal lobe.
• Main Question: 
• Is a parallel functional architecture evident within individual basal ganglia circuits?
• Motor Circuit:
• Features Intrinsic to Parallel Organization
• Cortex $\rightarrow$ glutamatergic $\rightarrow$ striatum $==$ input
• Basal Ganglia $\rightarrow$ tonic, GABA-mediated inhibitory effect on nuclei in thalamus
• modulated by 2 opposing but parallel pathways that pass from striatum $\rightarrow$ basal ganglia output nuclei
• Direct pathway $\rightarrow$ output nuclei
• comes from inhibitory striatal efferents that contain GABA and substance P $\rightarrow \not$ thalamic stage
• Indirect pathway $\rightarrow$ GPe $\rightarrow$ subthalamic nucleus via GABAergic pathway $\rightarrow$ excitatory (Glu)
• GPe neurons exert a tonic inhibitory influence on the subthalamic nucleus.
• GABA/enk projection from striatum suppress activity of GPe neurons $\rightarrow$ disinhibits the subthalamic nucleus $\rightarrow$ excitatory drive on the output nuclei $\rightarrow$ increases inhibition of their efferent targets within the thalamus.