Here is an interesting addition to the article published in Nature this month by the UK scientists at University of Sheffield who see PD as a combo of a psychological behavior impairment that is caused by faulty neurotransmission. I must say this is a rather refreshing take on things
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Link to the discussion at PDOnline here

Therefore, our suggestion is that Parkinson’s disease may be understood in terms of a catastrophic impairment of automatisms/habits, which traps patients with Parkinson’s disease forever in goal-directed control of their behaviour. A classic example is that when walking on the street the patient with Parkinson’s disease can suddenly freeze, and find it difficult to start again, when recognizing and saluting an approaching person. The suggested reason is that when the patient starts thinking about the action of saluting, there is a shift of attention from walking to the gesture and articulation of the salute, which because the goal-directed system can only do one thing at a time, the walking becomes unassisted