In the basal ganglia direct pathway, which neurotransmitter is released by the substantia nigra pars compacta onto the striatum?

Explanation

## Direct Pathway Neurotransmission **Key Point:** The substantia nigra pars compacta (SNpc) is the primary source of dopamine to the striatum and is essential for normal motor control. ### Dopamine's Role in the Direct Pathway Dopamine released from SNpc neurons acts on **D1 receptors** (dopamine-1) located on medium spiny neurons (MSNs) in the striatum that form the direct pathway. This D1-mediated signaling: 1. Facilitates excitation of direct pathway neurons 2. Reduces inhibitory GABAergic output to the globus pallidus interna (GPi) 3. Decreases tonic inhibition of thalamic neurons 4. Promotes motor execution and movement initiation **High-Yield:** Loss of dopamine in Parkinson's disease leads to hypokinetic movement disorders because the direct pathway becomes underactive, while the indirect pathway becomes relatively overactive. ### Basal Ganglia Neurotransmitter Summary | Structure | Primary Neurotransmitter | Target | Effect | |-----------|-------------------------|--------|--------| | SNpc | Dopamine | Striatum (D1 & D2 receptors) | Modulatory | | Striatum (direct) | GABA | GPi/SNpr | Inhibitory | | Striatum (indirect) | GABA | GPe | Inhibitory | | GPe | GABA | STN | Inhibitory | | STN | Glutamate | GPi/SNpr | Excitatory | | GPi/SNpr | GABA | Thalamus | Inhibitory | **Clinical Pearl:** Deep brain stimulation (DBS) of the GPi in Parkinson's disease works by reducing the excessive inhibitory output caused by dopamine loss, effectively bypassing the defective dopamine system.