In the indirect pathway of basal ganglia circuits, which structure receives inhibitory (GABAergic) input from the striatum and projects to the globus pallidus interna?

Explanation

## The Indirect Pathway: Striatum → GPe → STN → GPi **Key Point:** The globus pallidus externa (GPe) is the **first relay** in the indirect pathway. It receives inhibitory GABAergic input from striatal medium spiny neurons (MSNs) expressing D2 dopamine receptors. ### Indirect Pathway Sequence | Step | Structure | Neurotransmitter | Effect | |------|-----------|------------------|--------| | 1 | Striatum (D2+ MSNs) | GABA | Inhibits GPe | | 2 | GPe | GABA | Inhibits STN | | 3 | STN | Glutamate | Excites GPi | | 4 | GPi | GABA | Inhibits thalamus | | 5 | Thalamus | Glutamate | Excites motor cortex | ### Mechanism of Action - **Dopamine effect on D2 receptors:** Decreases excitability of indirect pathway MSNs - **Loss of dopamine (Parkinson disease):** Indirect pathway becomes overactive → excessive inhibition of thalamus → hypokinesia **High-Yield:** The indirect pathway is the **"stop" or "brake" pathway**. Dopamine normally inhibits it (via D2 receptors); when dopamine is lost, the brake is released, causing rigidity and bradykinesia. **Mnemonic:** **DI-RECT** = **D1 receptors** facilitate movement (direct pathway); **IN-DIRECT** = **D2 receptors** inhibit movement (indirect pathway). **Clinical Pearl:** Levodopa and dopamine agonists restore balance by reducing indirect pathway activity, improving motor symptoms in Parkinson disease. [cite:Kandel Principles of Neural Science 6e Ch 42]