## Dopamine and Basal Ganglia Pathways **Key Point:** Dopamine released from substantia nigra pars compacta (SNpc) neurons acts on D1 and D2 receptors on striatal medium spiny neurons, differentially modulating the direct and indirect pathways. ### Mechanism of Action **High-Yield:** Dopamine has **dual effects** on basal ganglia circuits: | Pathway | Receptor | Effect | Result | | --- | --- | --- | --- | | **Direct pathway** | D1 | Excitatory (facilitates) | Promotes movement initiation | | **Indirect pathway** | D2 | Inhibitory (suppresses) | Reduces movement inhibition | ### Functional Significance 1. **Direct pathway activation** → ↓ GPi/SNr inhibition → ↑ thalamic output → Movement facilitation 2. **Indirect pathway inhibition** → ↓ GPe inhibition of STN → ↑ STN activity → ↑ GPi/SNr inhibition → Movement suppression is reduced **Clinical Pearl:** Loss of dopamine in Parkinson's disease leads to: - Reduced D1 activation (weakened direct pathway) - Unopposed D2 activity (overactive indirect pathway) - Net result: **hyperinhibition of thalamus** → bradykinesia, rigidity, tremor **Mnemonic:** **D1-Direct-Dopamine-Do it** (facilitates movement); **D2-Indirect-Dopamine-Don't** (inhibits the inhibitor) [cite:Guyton & Hall Ch 55]
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