In the basal ganglia motor circuit, which is the most common site of dopaminergic neuronal loss in Parkinson disease, leading to hypokinetic movement disorder?

Explanation

## Dopaminergic Pathology in Parkinson Disease ### Anatomical Basis of Dopamine Loss **Key Point:** The substantia nigra pars compacta (SNpc) is the primary source of dopamine for the striatum in the basal ganglia circuit. Selective degeneration of SNpc dopaminergic neurons is the pathological hallmark of Parkinson disease. ### Circuit Disruption **High-Yield:** The nigrostriatal pathway: 1. SNpc dopaminergic neurons project to dorsolateral putamen (motor striatum) 2. Dopamine normally facilitates the direct (Go) pathway and inhibits the indirect (No-Go) pathway 3. Loss of dopamine → unopposed indirect pathway activity → excessive inhibition of thalamus → hypokinesia (bradykinesia, rigidity, tremor) ### Pathological Features **Clinical Pearl:** - Lewy bodies (α-synuclein inclusions) are found predominantly in SNpc neurons - 50–70% neuronal loss in SNpc is required for motor symptoms to manifest - This selective vulnerability of SNpc is not fully understood but relates to high oxidative stress and mitochondrial dysfunction in these neurons ### Functional Consequence | Pathway | Normal State | With Dopamine Loss | |---------|--------------|-------------------| | Direct (Go) | Facilitated by DA | Reduced → hypokinesia | | Indirect (No-Go) | Inhibited by DA | Disinhibited → excessive inhibition of thalamus | | Net effect | Movement facilitation | Movement suppression | **Mnemonic:** **SNpc = Substantia Nigra pars compacta = Striatal Dopamine source = Selective loss in PD** [cite:Harrison 21e Ch 428]