Knockout blows cause brain damage:

A blow to the head that knocks a person unconscious can result in widespread loss of brain tissue, Canadian researchers said recently, explaining why some people who suffer head injuries are never quite the same. The more severe the injury, the more brain tissue is lost, they said. "There is more damage and it is more widespread than we had expected," said Dr Brian Levine of the Rotman Research Institute and the University of Toranto, whose study appears in the journal Neurology. "When you have a blow to the head, it causes neurochemical reaction in the brain cells that lead to cell death," Levine said. "The more cells that die, the less tissue you have."