PD is a neurodegenerative disease. It occurs when certain nerve cells, or neurons, die or become impaired. This degeneration takes place in a small area of the brain called the substantia nigra, located in the brainstem. People living with PD have usually lost 80% or more of dopamine-producing cells in this area by the time symptoms are apparent. In addition, there is also neuronal cell loss and related pathology in other brain regions, responsible for other non-motor symptoms.
The main reason why these cells die or become impaired is still a mystery. Most scientists believe that both genetic and environmental factors are contributing causes. However, how they lead to PD is still unclear.