A concussion is generally a mild brain injury that is caused by a blow to the head. When this happens, it can cause bleeding, tearing, or bruising of the nerve fibers in the brain. There are three grades of concussions, including:nGrade 1: No loss of consciousness, but you may experience temporary amnesia for up to 30 minutes.nnGrade 2: You lose consciousness and experience amnesia for between 30 minutes and 24 hours.nnGrade 3: You lose consciousness for more than five minutes or suffer amnesia for more than 24 hours.