When you develop PTSD (post traumatic stress disorder) after experiencing a trauma, the memory of the trauma is subsequently stored in a part of the brain called the hippocampus. So whenever you have a trigger or a reminder of the trauma, it will bring up the memory of the trauma stored in the hippocampus, as if the trauma were still occurring. This activated recall of the traumatic memory stored in the hippocampus then activates the amygdala, which then sets off the fight or flight response, where your body is prepared to either run or fight the threat. However, in PTSD, this threat is from the internal memories stored in the hippocampus, rather than from some external threat.
