Post-traumatic stress disorder (PTSD) is a particular condition that follows experience of a severe trauma. There are three clusters of symptoms: persistent avoidance of reminders of the event (numbing symptoms), persistent re-experiencing of the event (intrusive symptoms), and autonomic hyperarousal. With numbing symptoms, there is a blunting of emotional reactivity. There is flat affect, avoidance of various activities, loss of memory for traumatic events, and lack of motivation or interest in activities that used to engage the person. With intrusive symptoms, the opposite happens. Instead of a lack of memory for the traumatic events, there is a flood of memories that cannot be turned off. There may be nightmares or flashbacks, in which the trauma returns as if it is happening all over again. There may also be intrusive emotional storms, such as crying spells, rage outbursts, or panic attacks. With autonomic hyper-arousal, the autonomic nervous system is on overdrive. The body is continually on guard, ready to spring into action at any sign of danger. There can be an exaggerated startle response, difficulty concentrating and difficulty sleeping, as well as rapid heartbeat, sweating, and continued muscle tension.