The Facts

• Dopamine is one of two neurotransmitters (the other is acetylcholine) associated with the control of movement. Under normal conditions a balance is maintained between these chemicals.
  
• Degeneration of that part of the upper brain stem known as the substantia nigra, results in reduced levels of dopamine in the brain.
  
• The loss of dopamine and the imbalance with acetylcholine result in the symptoms of PD.
  
• It is not known what triggers this process.

The PD experience

Wherever two or more PWP get together the subject of what causes PD will eventually come up. I suspect it will continue to do so until someone actually finds out. Contenders include:

• Head injury,
• Exposure to toxic substances,
• Genetic factors,
• Exposure to one or other metal,
• 'Enviromental factors' (always a nice one as it includes just about everything),

Current thinking seems to be headed in the direction of an enviromental trigger activating a genetic predisposition. Speaking for myself I am reserving judgement.