Parkinson's disease is a neurodegenerative disorder, more specifically a movement disorder, that affects the cells of the brain that control muscle movement. These cells, located deep in a section of the brain called the substantia nigra, produce an important neurotransmitter called dopamine. Dopamine is important because it is the chemical messenger used to send signals from the brain to the muscles to complete smooth, coordinated movements. When 80% of these dopamine-producing cells have died, symptoms of Parkinson's disease appear. The cause of death of these dopamine-producing cells is still unknown.