Distinct brain activity triggered by memories of trauma

It is well known that people who have lived through traumatic events like sexual assault, domestic abuse, or violent combat can experience symptoms of post-traumatic stress disorder (PTSD), including terrifying flashbacks, severe anxiety, and uncontrollable thoughts about the incident. But what exactly happens in the brains of PTSD patients as they recall these traumatic events? Are they remembered the same way as, say, the loss of a beloved pet -- or, for that matter, a relaxing walk on the beach?