## Indirect Pathway Architecture **Key Point:** The indirect pathway is the **inhibitory** "brake" circuit for movement. MSNs of the striatum send GABAergic projections to the **subthalamic nucleus (STN)**, which then sends glutamatergic excitatory projections to the globus pallidus internus (GPi) and substantia nigra pars reticulata (SNpr). **High-Yield:** Indirect Pathway Sequence: 1. Cortex → Striatum (glutamate) 2. **Striatum → STN (GABA)** ← This is the key step 3. STN → GPi/SNpr (glutamate) 4. GPi/SNpr → Thalamus (GABA) 5. Thalamus → Cortex (glutamate) **Mnemonic:** **"Striatum Stops STN"** — The striatum sends inhibitory (GABA) signals to the subthalamic nucleus, which is the only excitatory (glutamate) nucleus in the basal ganglia. ## Why the Subthalamic Nucleus Is Correct The STN is the unique excitatory nucleus in the basal ganglia circuit. It receives inhibitory GABAergic input from indirect pathway MSNs of the striatum. When dopamine is depleted (as in Parkinson disease), indirect pathway activity increases, leading to excessive STN glutamatergic drive on GPi/SNpr, resulting in increased thalamic inhibition and hypokinesia. ## Indirect Pathway Functional Flow ```mermaid flowchart TD A[Cortex]:::outcome --> B[Striatum MSN<br/>Indirect Pathway]:::action B -->|GABA inhibits| C[Subthalamic Nucleus]:::action C -->|Glutamate excites| D[GPi/SNpr]:::action D -->|GABA inhibits| E[Thalamus]:::action E -->|Glutamate| F[Cortex]:::outcome G[Dopamine D2 activation<br/>facilitates MSN firing]:::action -.->|enhances| B ``` **Clinical Pearl:** In Parkinson disease, loss of dopamine reduces D2-mediated facilitation of indirect pathway MSNs. This leads to **reduced inhibition of STN**, causing **excessive STN excitation of GPi**, resulting in increased thalamic inhibition and movement difficulty. This is why STN deep brain stimulation (DBS) is effective — it silences overactive STN neurons. 
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