Which neurotransmitter is released by dopaminergic neurons of the substantia nigra pars compacta in the direct pathway of basal ganglia circuits?

Explanation

## Direct Pathway in Basal Ganglia Circuits **Key Point:** The direct pathway (also called the "go" pathway) facilitates voluntary movement through dopaminergic inhibition of the globus pallidus internus (GPi). ### Anatomical Sequence The direct pathway follows this sequence: 1. Substantia nigra pars compacta (SNc) → dopamine release 2. Dopamine acts on D1 receptors in the striatum 3. Striatal neurons project directly to GPi 4. GPi inhibition → reduced thalamic inhibition → movement facilitation ### Dopamine's Role **High-Yield:** Dopamine from SNc is the critical neurotransmitter that: - Activates D1 receptors (excitatory) on direct pathway neurons - Facilitates movement initiation and execution - Is depleted in Parkinson disease, causing hypokinesia (poverty of movement) ### Clinical Correlation **Clinical Pearl:** Loss of dopaminergic neurons in Parkinson disease impairs the direct pathway, resulting in: - Bradykinesia (slow movement) - Rigidity - Tremor at rest - Difficulty initiating voluntary movement ### Comparison with Indirect Pathway | Pathway | Neurotransmitter | Effect | Function | | --- | --- | --- | --- | | Direct | Dopamine (D1 agonist) | Facilitates movement | "Go" | | Indirect | Dopamine (D2 antagonist) | Inhibits movement | "No-go" | [cite:Snell's Clinical Neuroanatomy Ch 9]