The hallmark feature of Parkinson's disease is the dopaminergic neurodegenerationprogressive loss of dopamine-producing neurons in the substantia nigra, a region of the brain involved in movement control. Dopamine is a neurotransmitter that plays a critical role in regulating motor function and coordination. Its depletion leads to the characteristic motor symptoms of PD, such as tremors, rigidity, and bradykinesia (slowness of movement).