Medical Marijuana's effect on Parkinson's

With medical marijuana now legalized in 25 states and Washington, D.C., it is obvious that there is strong interest in its therapeutic properties. Researchers are testing marijuana, which they call cannabis, as a treatment for many illnesses and diseases, including neurological conditions, with Parkinson's disease (PD) high on the list. But despite several clinical studies, it has not been demonstrated that cannabis can directly benefit people with Parkinson's. What is the science and pharmacology behind marijuana, and can it be used to treat Parkinson's symptoms?  
(Reference: http://www.parkinson.org)