Placebo effect does not belong on the list
Well, maybe the Parkinson's part.
Pain exists to demand that the organism pay attention to a problem. Once the appropriate action has been taken, pain is no longer needed, so the system shuts it down. How it goes about doing that is a great subject for further study. The observation that morphine blockers block the placebo effect suggests that the internal chemistry involved is similar to that involved in how morphine shuts down pain. Or maybe there is operant conditioning happening on a very low level. But there is no mystery in the big picture. Alarm acknowledged, corrective action taken, so alarm turned off or at least down.