Regarding the direct and indirect pathways of the basal ganglia, all of the following statements are correct EXCEPT:

Explanation

## Basal Ganglia Circuitry: Direct vs Indirect Pathways ### Anatomical Origins and Terminations **Key Point:** Both the direct and indirect pathways originate from the same structure (putamen) but have fundamentally different downstream projections and functional outcomes. | Pathway | Origin | Intermediate Targets | Final Output | Functional Effect | |---------|--------|----------------------|--------------|------------------| | Direct | Putamen | GPi directly | Thalamus (disinhibition) | Facilitates movement | | Indirect | Putamen | GPe → STN → GPi | Thalamus (increased inhibition) | Suppresses movement | ### Why Option 2 Is Incorrect **High-Yield:** The indirect pathway does NOT terminate directly in the globus pallidus internus. Instead, it follows a polysynaptic route: 1. Putamen (D2 receptor neurons) → GPe (external globus pallidus) 2. GPe → STN (subthalamic nucleus) 3. STN → GPi (internal globus pallidus) This multi-step pathway creates a net **increase** in inhibition of the thalamus, thereby suppressing movement. ### Dopaminergic Modulation **Key Point:** Dopamine from substantia nigra pars compacta (SNpc) acts on two receptor populations: - **D1 receptors** (on direct pathway neurons): dopamine **enhances** direct pathway activity → facilitates movement - **D2 receptors** (on indirect pathway neurons): dopamine **inhibits** indirect pathway activity → reduces suppression of movement Both effects converge to **promote voluntary movement**. ### Clinical Pearl In Parkinson's disease, loss of SNpc dopamine neurons causes: - Reduced facilitation of the direct pathway (hypokinesia) - Unopposed indirect pathway activity (rigidity, tremor) This explains why L-DOPA restores movement — it replenishes dopamine and rebalances both circuits.