Medial segment of globus pallidus

The medial segment of globus pallidus, also called the globus pallidus internus (GPi), is the medial subdivision of the globus pallidus. It lies medial to the medial medullary lamina, which separates it from the globus pallidus externus (GPe). It lies lateral to the internal capsule.

Role of medial segment of globus pallidus (GPi) in direct and indirect neuronal circuitries

The medial segment of globus pallidus (GPi) is one of the principal output nuclei of the basal ganglia. It receives inhibitory GABAergic input from the striatum and excitatory glutamatergic input from the subthalamic nucleus. Its neurons provide tonic inhibitory GABAergic output mainly to the ventral anterior and ventral lateral nuclei of the thalamus, thereby regulating thalamocortical motor activity.

• In the direct pathway, inhibition of the GPi reduces its inhibitory influence on the thalamus, facilitating desired movement.

• In the indirect pathway, GPi activity increases, producing greater thalamic inhibition and helping suppress excessive or unwanted movement.

Clinical correlate

Abnormal GPi activity contributes to movement disorders such as Parkinson disease and dystonia.