Which of the following correctly distinguishes the DIRECT and INDIRECT pathways of the basal ganglia and their roles in movement?

• A Direct pathway: striatum → GPe → STN → GPi/SNr → inhibits thalamus → suppresses movement. Indirect: striatum → GPi → thalamus → facilitates movement
• B Direct pathway: striatum (D1, GABA) → GPi/SNr → disinhibits thalamus → facilitates desired movement. Indirect pathway: striatum (D2, GABA) → GPe → STN (glutamate) → GPi/SNr → inhibits thalamus → suppresses competing movements ✓
• C Direct pathway uses D2 receptors; indirect uses D1 receptors; both converge on GPi
• D Both pathways ultimately excite the thalamus; they differ only in the speed of signal transmission

Explanation

The basal ganglia has two opposing pathways through the striatum. Direct pathway: striatum (D1 receptors, co-expressing substance P/dynorphin) → GPi/SNr (GABA — inhibit) → thalamus is disinhibited → facilitates desired movement. Indirect pathway: striatum (D2 receptors, co-expressing enkephalin) → GPe (GABA — inhibit) → STN is disinhibited → GPi/SNr (via glutamate from STN, excitatory) → thalamus is strongly inhibited → suppresses competing movements. Dopamine from SNc: activates D1 (direct) → facilitates movement; inhibits D2 (indirect) → also facilitates movement. In Parkinson's (dopamine loss): direct pathway underactive + indirect pathway overactive → net thalamic inhibition → bradykinesia/rigidity.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.