READ MORE: "PD develops over time as neurons in certain areas of the brain that control muscle movement slowly waste away, according to a study posted online today that will appear in the April 2006 print issue of the Archives of Neurology, one of the JAMA/Archives journals. The degeneration occurs in areas that produce a neurotransmitter known as dopamine. Typical symptoms of PD-including severe tremors, rigidity and slow movements-begin when the brain loses more than half of its dopamine production ability."