All of the following statements about the indirect pathway of basal ganglia are true EXCEPT:

Explanation

## Indirect Pathway: Structure and Function The indirect pathway is the second major motor circuit of the basal ganglia, responsible for suppressing unwanted movement through increased inhibition of the thalamus. ### Anatomical Components **Key Point:** The indirect pathway follows this sequence: Striatum → GPe → STN → GPi/SNpr → Thalamus. ### Detailed Circuit Analysis | Component | Neurotransmitter | Target | Function | |-----------|------------------|--------|----------| | Striatum → GPe | GABA (inhibitory) | GPe | Removes tonic inhibition from GPe | | GPe → STN | GABA (inhibitory) | STN | Reduces STN excitatory drive | | STN → GPi/SNpr | Glutamate (excitatory) | GPi/SNpr | Increases inhibitory output | | GPi/SNpr → Thalamus | GABA (inhibitory) | Thalamus | Suppresses motor cortex activation | ### Dopamine's Role in the Indirect Pathway **High-Yield:** D2 dopamine receptors on striatal neurons are **inhibitory** (Gi-coupled). When D2 receptors are activated, they **REDUCE** the firing of indirect pathway neurons, thereby **DECREASING** indirect pathway output and reducing movement suppression. **Warning:** The statement "Dopamine D2 activation enhances indirect pathway output" is **FALSE**. D2 activation actually **suppresses** indirect pathway neurons, leading to disinhibition of the thalamus and facilitation of movement. ### Functional Consequence ```mermaid flowchart TD A[Dopamine released at striatum]:::action --> B{D1 vs D2 receptor?}:::decision B -->|D1 receptor on direct pathway| C[Direct pathway ENHANCED]:::action B -->|D2 receptor on indirect pathway| D[Indirect pathway SUPPRESSED]:::action C --> E[Thalamus disinhibited]:::outcome D --> F[Thalamus disinhibited]:::outcome E --> G[Movement FACILITATED]:::outcome F --> G ``` **Mnemonic:** **D2 = Decreased indirect pathway activity = Disinhibition of movement.** Both D1 and D2 dopamine actions converge on the same net effect: movement facilitation. ### Verification of Other Statements - **STN involvement:** TRUE — the indirect pathway uniquely includes the subthalamic nucleus as an excitatory relay. - **Increased thalamic inhibition:** TRUE — the net effect is more GPi/SNpr output, increasing thalamic suppression. - **GPe→STN GABA projection:** TRUE — this is a well-established inhibitory synapse that reduces STN excitatory drive.