Post-traumatic stress disorder involves the brain and body’s fight-or-flight response. Those with PTSD may experience extreme mental and emotional responses to danger. When experiencing a traumatic event, the brain prepares the body to take action quickly, and you become more vigilant and alert. However, the stress response diminishes once the dangerous situation ends.PTSD causes you to relive the fight-or-flight response to the traumatic event whenever something triggers you. Researchers think that PTSD affects the hippocampus, which is the part of the brain that converts short-term memory into long-term memory. The hippocampus also helps you to remember to escape dangerous events when you encounter them again.Triggers, memories, and emotions that remind you of the traumatic experience can cause the hippocampus to believe the event was happening again. This causes flashbacks, which are dissociative episodes that make you believe you are experiencing the traumatic event again. You may also experience severe anxiety and nightmares due to your traumatic memories.