According to MedicineNet, RSD involves "irritation and abnormal excitation of nervous tissue, leading to abnormal impulses along nerves that affect blood vessels and skin."
Animal studies indicate that norepinephrine, a catecholamine released from sympathetic nerves, acquires the capacity to activate pain pathways after tissue or nerve injury, resulting in RSD.
http://arthritis.about.com/od/rsd/a/rsd.htm