Parkinson’s disease is a neurodegenerative disease associated with degeneration of dopaminergic neurons on the substantia nigra region of the midbrain. The specific causes of the disease are not currently known, and diagnoses are based on clinical criteria. Symptoms are related to the loss of motor abilities, and examples are tremors, rigid movement, loss of postural reflexes, impaired speech abilities and reduced facial expressions.
Jankovic J (April 2008). “Parkinson’s disease: clinical features and diagnosis”. Journal of Neurology, Neurosurgery, and Psychiatry. 79 (4): 368–76.