The substantia nigra pars compacta projects to the striatum via the nigrostriatal pathway. Loss of these neurons in Parkinson's disease affects which neurotransmitter system and causes what net effect?

• A Increased dopamine → decreased inhibition of globus pallidus
• B Decreased acetylcholine → reduced striatal output
• C Decreased dopamine → increased inhibition of thalamus via STN ✓
• D Decreased GABA → increased thalamic activity

Explanation

Loss of dopaminergic neurons in substantia nigra pars compacta reduces dopamine in the striatum, shifting the balance toward the indirect pathway. The indirect pathway (striatum → GPe → STN → GPi/SNr → thalamus) is disinhibited, increasing GPi/SNr inhibitory output to the thalamus, reducing thalamo-cortical activity and producing the hypokinetic features of Parkinsonism. Acetylcholine in the striatum is relatively increased when dopamine is lost, not decreased.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.