For reasons scientists still don't understand, cells that produce a brain chemical called dopamine begin to die in a region of tissue that facilitates muscle movements.

When dopamine levels drop, neurons start to fire abnormally, causing the classic Parkinson's tremors.

One percent of the U.S. population over 60 has Parkinson's disease. Only stroke and Alzheimer's cause more deaths as a result of brain dysfunction. The disease is still incurable.