Pathophysiology of Parkinson’s Disease

Core Pathological Mechanisms

Parkinson’s disease is a synucleinopathy characterized by progressive loss of dopaminergic neurons in the substantia nigra pars compacta, with misfolded α‑synuclein forming Lewy bodies that spread from the brainstem to cortical regions. 1, 2
α‑Synuclein misfolding and aggregation constitute the central pathological hallmark, producing insoluble Lewy bodies composed mainly of α‑synuclein and ubiquitin. 1, 2
The pathological spread follows a stereotyped sequence: initial involvement of the medulla oblongata, pontine tegmentum, and olfactory system; subsequent deposition in the substantia nigra and other deep gray nuclei (coinciding with symptom onset); and final accumulation in the cerebral cortex. This pattern underlies the progression of clinical features. 1, 2, 3

Degeneration specifically targets dopaminergic neurons that project from the substantia nigra to the striatum. 1, 2, 4
Clinical motor symptoms appear only after approximately 40 %–50 % of these substantia nigra dopaminergic neurons have been lost, corresponding to an estimated five‑year interval between the onset of neuronal loss and the emergence of overt symptoms. 1, 2, 4
The substantial pre‑clinical phase explains why patients already exhibit significant functional impairment at the time of diagnosis. 4

Resting tremor, bradykinesia, and rigidity arise directly from loss of dopaminergic neurons in the substantia nigra‑striatal pathway. 1, 2, 4

Autonomic dysfunction, behavioral changes, and dementia develop as α‑synuclein pathology extends beyond the substantia nigra into extra‑striatal regions. 1, 2, 4
Early involvement of the olfactory system and brainstem accounts for prodromal non‑motor symptoms that precede the classic motor manifestations of Parkinson’s disease. 1, 2