The loss of pigmentation in the substantia nigra of the midbrain at the left in a patient with Parkinson disease is contrasted with a normal midbrain at the right in which dark pigmentation appears in the region of the substantia nigra above the cerebral peduncles.

The loss of dopaminergic neurons affects movement control circuitry. Parkinson disease is marked clinically by a resting 3 to 5 Hz tremor, rigidity, bradykinesia, mask-like facies, and festinating gait, among other findings. Some cases occur in association with Lewy body dementia, a form of dementia with findings similar to Alzheimer disease.