A 58-year-old man from Delhi presents with a 2-year history of progressive tremor at rest in his right hand, which disappears with intentional movement. He also reports difficulty initiating gait and a stooped posture. Neurological examination reveals bradykinesia, cogwheel rigidity, and a resting tremor of 4–5 Hz. MRI brain is normal. Which of the following best explains the pathophysiology of his motor symptoms?

Explanation

## Pathophysiology of Parkinson's Disease in Basal Ganglia Circuits ### Normal Basal Ganglia Motor Control The basal ganglia regulate motor output through two opposing pathways: - **Direct pathway (facilitatory):** Striatum → internal globus pallidus (GPi) → thalamus → cortex. This pathway *facilitates* movement. - **Indirect pathway (inhibitory):** Striatum → external globus pallidus (GPe) → subthalamic nucleus (STN) → GPi → thalamus → cortex. This pathway *inhibits* movement. Dopamine from substantia nigra pars compacta (SNpc) **facilitates the direct pathway** (D1 receptors on striatal neurons) and **inhibits the indirect pathway** (D2 receptors on striatal neurons). This balance allows selective, coordinated movement. ### In Parkinson's Disease **Key Point:** Loss of dopaminergic neurons in the SNpc removes both facilitation of the direct pathway AND inhibition of the indirect pathway. **High-Yield:** The net result is **unopposed inhibitory output from the indirect pathway**, leading to: 1. Excessive inhibition of the thalamus 2. Reduced thalamic activation of motor cortex 3. **Hypokinesia** (difficulty initiating movement, bradykinesia) 4. **Rigidity** (co-contraction of agonist–antagonist muscles) 5. **Resting tremor** (oscillatory imbalance in basal ganglia circuits) ### Circuit Diagram ```mermaid flowchart TD A["SNpc dopamine loss"]:::urgent --> B["↓ Direct pathway facilitation"]:::outcome A --> C["↑ Indirect pathway activity"]:::outcome B --> D["↓ GPi inhibition"]:::outcome C --> E["↑ GPi inhibition"]:::outcome D --> F["↑ Thalamic inhibition"]:::urgent E --> F F --> G["Hypokinesia, rigidity, tremor"]:::outcome ``` **Clinical Pearl:** The resting tremor in Parkinson's disease is thought to arise from oscillatory activity in the thalamus due to unopposed inhibitory basal ganglia output. This tremor **disappears with intentional movement** because cortical activation can override the abnormal basal ganglia signal. **Mnemonic:** **DIRECT = Dopamine-dependent, Facilitates movement** | **INDIRECT = Inhibits movement, needs Dopamine removal to activate** [cite:Guyton & Hall 14e Ch 56]