Loss of dopaminergic neurons in the substantia nigra pars compacta in Parkinson's disease produces its cardinal motor features primarily through which effect on basal ganglia output?

• A Reduced inhibition of the thalamus via decreased activity of the indirect pathway
• B Direct hyperactivity of the pedunculopontine nucleus causing rigidity
• C Excessive excitation of the subthalamic nucleus leading to reduced GPi firing
• D Increased inhibitory output from the globus pallidus interna to the thalamus ✓

Explanation

Dopamine normally facilitates the direct pathway and suppresses the indirect pathway. Its loss tips balance toward the indirect pathway, increasing subthalamic nucleus drive onto the globus pallidus interna. The result is increased inhibitory GPi output to the ventrolateral thalamus, which suppresses thalamocortical motor facilitation and produces bradykinesia and rigidity. This model also explains why subthalamotomy or GPi deep brain stimulation relieves symptoms.

Reference: Ganong's Review of Medical Physiology, 26th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.