The striatum serves as the input center for the basal ganglia. The primary input comes from the cerebral cortex; there is a secondary input from the thalamus. Specific areas of the cortex map to specific parts of the striatum. These associations are maintained in projections throughout the basal ganglia.
The globus pallidus is broken into internal and external segments. The internal section is one of two main output nuclei of the basal ganglia, along with the substantia nigra pars reticulata . The pallidal output is passed by the thalamus primarily to the supplementary motor area, and possibly also to the premotor cortex . The supplementary motor area is also a major input to the basal ganglia; therefore there is a loop between the supplementary motor area and the basal ganglia.
As mentioned earlier, it appears that the supplementary motor area is involved with planning sequences of movements. The loop between the supplementary motor area and the basal ganglia therefore provides a clue to the function of the basal ganglia.
The external section of the globulus pallidus passes information within the basal ganglia, in particular to the subthalamic nucleus.
The subthalamic nucleus takes the output from the motor and premotor cortices, and from the the external segment of the globus pallidus. The subthalamic nucleus projects to both segments of the globus pallidus and to the substantia nigra pars reticulata.
The substantia nigra, along with the internal globus pallidus, is one of the two major output sections of the basal ganglia.
There are two major pathways through the basal ganglia, arising from different cellular populations of the putamen. The inhibitory direct pathway projects monosynaptically from the putamen to the motor regions of the internal globus pallidus and the substantia nigra pars reniculata .
The indirect pathway is inhibitory from the putamen to the external globus pallidus, inhibitory from there to the subthalamic nucleus, and excitatory to the external globus pallidus or substantia nigra pars reticulata.
It appears that the net affect of the direct pathway is to facilitate motion by exciting the supplementary motor area; the indirect pathway has the opposite effect .
In addition (and crucially for Parkinson's disease), there is a loop from the striatum to the substantia nigra pars compacta and back to the striatum. The return route is mediated by dopamine. Dopamine excites the direct pathway and inhibits the indirect pathway: since these two pathways have opposing effects, dopamine tends to facilitate motion. In effect, ``the dopamine terminals are in a position to `gate' the influence from cortex'' .
There are also reciprocal inhibitory connections between the internal and external globus pallidus.
The major output of the basal ganglia, from the internal globus pallidus and the substantia nigra pars reticlata, project to three nuclei in the thalamus: the ventral lateral, ventral anterior, and mediodorsal. These thalamic nuclei in turn project to the prefrontal cortex, the premotor cortex, the supplementary motor area, and the motor cortex .
From an input-output analysis, therefore, the basal ganglia do not appear to generate motions directly; they take input from the cortex, modify it in some way, and pass it back to the cortex (via the thalamus).