Just as the symptoms caused by a concussion vary greatly in presentation and duration, so do the theories behind the pathophysiology of mild TBI. The transient loss of cerebral function after a head injury was formally differentiated from severe head injury for the first time by the Persian physician Rhazes in 900 AD and has since caused much speculation and varying clinical descriptions throughout history. It took another 500 years before a "commotion" or shaking of the brain was theorized to be responsible for clinical signs. Presently, the exact cause of concussive symptoms continue to remain an enigma; a major question however, is whether a concussion is due to a lesser degree of diffuse structural change seen in severe traumatic brain injury, or if it is in fact a mechanism caused by reversible functional changes of the neurons and axons.