I suppose that anyone with a hobbie/obsession/career which involves the use of very fine movements of the hands and fingers finds that its a bit of a bugger when you develop a motor control disorder. With musicians there is also such a strong emotional connection with playing your instrument(s) that I feel as though we are dealt a double whammy there. We can't carry out the physical activity at all or as well as we used to, and we can't satisfy a fundamental emotion need. It's like losing a loved one: you go through a process of grieving in fact.
So its absolute heaven when you start the dopamine treatment! Ropinirole, then a couple of years later, Madopar, each became my very best friends when they consecutively returned to me the ability to play the guitar after months of pain, tremor and immobility had made it impossible for me even to hold up my left arm never mind whip through a bit of Classical Gas or Layla.
One of the interesting developments, though, is that PD seems to make you more creative. Before my symptoms began I did not, as a rule, do much in the way of composing or improvising. And I most certainly did not write song lyrics. It was after my diagnosis in particular that I began to - sometimes quite obsessively - write poetry, songs and compositionsimprovisations. I go for months and months and don't write a thing, then - sometimes for no apparent reason, or sometimes because something has upset me - I go into overdrive and write and write.
The research into music and the brain is absolutely fascinating. Rhythm cues for walking, extra release of dopamine in the brain when you listen to your favourite music. There is a link between music and intelligence. Something to do with building neural networks in the brain.
Anyway, if there are any musicians out there who also happen to be of the shakier persuasion, and who perhaps don't want to play in public, how about a few jam sessions?
There's some really interesting comments here and food for thought. Do check out the new thread I have just posted, "Has Parkinson's affected your creatity?", which was prompted by your message.