Schematic and Cross Section of Basal Ganglia

A biconvex structure, the subthalamic nuclei (STN) receives glutamatergic inputs from the cerebral cortex, GABA inhibition from the GPe, and provides glutamatergic innervations to the GPe, GPi, SN, and PPN. The STN has become a structure of interest because of its pivotal role in our understanding of basal ganglia function.

The postsynaptic dopamine receptors are divided into two major broad categories, D1/D5 and D2, D3, D4 family of receptors, segregated into two main pathways. The direct pathway, subserved by D1 dopamine receptors, sends its projections to the subthalamic nuclei via the GPi, and the indirect pathway, via the D2 family of receptors, influences the STN via the GPe.

Recently, the excitatory-inhibitory interplay between the direct and indirect pathways has been conceptualized as focused selection and tonic inhibition (surround inhibition hypothesis). By suppressing excitability in an area that is surrounding an activated neural network, neuronal activity focuses to select desired responses. Simultaneously, other pallidal neurons projecting to the thalamus, act to permit desired movements. By decreasing their discharge, through focused striatal output chiefly via the direct pathway, tonic inhibition to the thalamus is removed, releasing the cortical generators for normal or desired movement to occur. Therefore the presence of abnormal involuntary movements results from either failure of inhibition or excessive excitation of the surrounding structures.

Based on the models discussed above, it is important to recognize the pallidum as the major outflow structure of the basal ganglia. Most fugal pathways pass throught the fields of Forel. Presently, the STN is the preferred target for the surgical treatment of idiopathic Parkinson disease (iPD), the ventral intermediate (VIM) thalamus for the treatment of essential and certain other types of tremor, and the GPi for dystonia, with deep brain stimulation (DBS) being the favored surgical procedure.

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