Figure 1.

SIMPLIFIED FRONTAL-SUBCORTICAL CIRCUIT STRUCTURE. This figure depicts the general organizational structure of the frontal-subcortical circuits. This is a simplified illustration showing only the direct pathway. All FSC circuits share a general structure consisting of an information loop connecting the cortex, basal ganglia, and thalamus. Information originates from many areas in the cerebral cortex, travels first to the basal ganglia, then on to the thalamus, and finally returns to the cortex. The caudate, putamen, and ventral striatum make up the input nuclei and receive excitatory glutamate projections from multiple areas of the cortex. The input nuclei then connect by way of inhibitory GABA fibers to the major output nuclei, which consist of the internal segment of the globus pallidus and the pars reticulata of the substantia nigra. The output nuclei send inhibitory GABA efferents to the thalamus. Finally, the circuit is closed by way of excitatory glutamate fibers that project back to the cortex.