Which neurotransmitter imbalance in the basal ganglia is responsible for the motor symptoms of Parkinson disease?

Explanation

## Neurochemical Basis of Parkinson Disease Motor Symptoms **Key Point:** Parkinson disease results from loss of dopaminergic neurons in the substantia nigra, leading to dopamine depletion in the striatum. This creates a relative excess of acetylcholine, which normally is balanced by dopamine. The imbalance between dopamine (↓) and acetylcholine (relatively ↑) disrupts motor control. **High-Yield:** The dopamine-acetylcholine imbalance explains why anticholinergic drugs (e.g., benztropine, trihexyphenidyl) provide symptomatic relief in Parkinson disease — they reduce acetylcholine activity and restore the balance. ### Basal Ganglia Circuit Dysfunction ```mermaid flowchart TD A[Loss of SNpc dopamine neurons]:::urgent --> B[Striatal dopamine depletion]:::outcome B --> C[Reduced D1 receptor activation<br/>in direct pathway]:::outcome B --> D[Reduced D2 receptor inhibition<br/>in indirect pathway]:::outcome C --> E[↓ Facilitation of motor output]:::outcome D --> F[↑ Inhibition of motor output]:::outcome E --> G[Bradykinesia, rigidity, tremor]:::outcome F --> G H[Relative acetylcholine excess<br/>unopposed by dopamine]:::outcome --> G ``` **Clinical Pearl:** The relative dopamine-acetylcholine imbalance is why anticholinergic agents were historically the first-line treatment for Parkinson disease (before levodopa became available). Modern dopamine replacement therapy (levodopa, dopamine agonists) is more effective because it directly restores dopamine rather than just blocking acetylcholine. ### Motor Pathway Effects | Pathway | Normal Function | In Parkinson Disease | Result | |---------|-----------------|----------------------|--------| | **Direct** (D1 dopamine-dependent) | Facilitates movement | Reduced facilitation | ↓ Movement initiation | | **Indirect** (D2 dopamine-dependent) | Inhibits unwanted movement | Reduced inhibition → ↑ GPi output | ↑ Inhibition of movement | | **Net effect** | Balanced motor control | Unopposed inhibition + acetylcholine excess | Bradykinesia, rigidity | **Mnemonic:** **DAC imbalance** = **D**opamine ↓, **A**cetylcholine relatively ↑, **C**ontrol lost [cite:Harrison 21e Ch 429]