Parkinson's disease is a chronic progressive neurological disease that affects a small area of nerve cells (neurons) in an area of the brain known as the substantia nigra. These cells normally produce dopamine, a chemical (neurotransmitter) that transmits signals between areas in the brain that, when working normally, coordinate smooth and balanced muscle movement. Parkinson's disease causes these nerve cells to die, and as a result, body movements are affected. WebMD