The loss of dopamine-producing brain cells which causes muscle tremor and lack of the brain telling the muscles what to do. Without dopamine the two do not communicqte well. Much more complicated really and symptoms vary with individuals and can be a bit weird. More and more younger people are getting it. Usaully occurs after 55. I've had it for 7 years now. There is no cure as of yet but medications are many and other options are available and being researched. It's a nasty slow progressing diease.