Which is the most common excitatory neurotransmitter in the direct pathway of basal ganglia circuits?

Explanation

## Basal Ganglia Direct Pathway Neurotransmission ### Overview of Direct Pathway The direct pathway (also called the 'go' pathway) is one of two major output circuits from the basal ganglia. It is responsible for facilitating voluntary movement and motor planning. ### Neurotransmitter Identity **Key Point:** Glutamate is the primary excitatory neurotransmitter in the direct pathway of basal ganglia circuits. The direct pathway consists of: 1. **Cortex → Striatum (putamen/caudate)**: Glutamatergic excitation 2. **Striatum → Globus Pallidus Interna (GPi) / Substantia Nigra Pars Reticulata (SNr)**: GABAergic inhibition (medium spiny neurons) 3. **GPi/SNr → Thalamus**: GABAergic inhibition 4. **Thalamus → Motor Cortex**: Glutamatergic excitation ### Functional Significance **High-Yield:** The direct pathway is tonically inhibited by GPi/SNr. When activated by cortical glutamate, striatal medium spiny neurons release GABA to inhibit GPi/SNr, which *disinhibits* the thalamus, allowing thalamic glutamate to excite motor cortex and facilitate movement. ### Distinction from Indirect Pathway The indirect pathway (putamen → globus pallidus externa → subthalamic nucleus → GPi/SNr) uses the same neurotransmitters but has opposite functional effects (movement inhibition). **Clinical Pearl:** Dopamine from substantia nigra pars compacta (SNc) enhances direct pathway activity (D1 receptors) and inhibits indirect pathway activity (D2 receptors), creating a net facilitatory effect on movement. Loss of dopamine in Parkinson's disease disrupts this balance, leading to hypokinesia.