The pathological hallmark of Parkinson's disease is the presence of Lewy bodies, which are intracellular inclusions composed primarily of alpha-synuclein protein. These abnormal aggregates disrupt normal neuronal function and are predominantly found in the substantia nigra, leading to the characteristic loss of dopaminergic neurons and the resulting motor symptoms of Parkinson's disease. Lewy bodies are key in distinguishing Parkinson's disease from other neurodegenerative disorders.

Reference: Adams and Victor’s Principles of Neurology, Raymond Adams, 11th Edition.