## Dopamine and the Direct Pathway **Key Point:** Dopamine released from substantia nigra pars compacta (SNpc) neurons acts on D1 receptors in the striatum to facilitate the direct pathway, which promotes movement initiation. ### Direct Pathway Mechanism The direct pathway operates as follows: 1. Dopamine binds D1 receptors on striatal medium spiny neurons (MSNs) 2. D1 activation **disinhibits** the thalamus by reducing GABAergic output from globus pallidus internus (GPi) 3. Reduced inhibition of thalamus → increased thalamocortical drive → **facilitation of movement** ### Basal Ganglia Circuit Overview | Pathway | Neurotransmitter | Effect on Movement | |---------|------------------|--------------------| | **Direct** | Dopamine (D1 agonism) | Facilitates movement | | **Indirect** | Dopamine (D1 antagonism) | Inhibits movement | | **Nigrostriatal** | Dopamine (SNpc → Striatum) | Modulatory | **High-Yield:** In Parkinson's disease, loss of SNpc dopamine neurons disrupts this balance, causing the indirect pathway to dominate, resulting in bradykinesia and rigidity. **Clinical Pearl:** Dopamine replacement therapy (L-DOPA) restores direct pathway facilitation, improving movement initiation in Parkinson's patients. [cite:Kandel & Schwartz Principles of Neural Science Ch 43] 
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