Leave No Trace (2018)
PTSD is caused by a "blown-out" hippocampus, which in turn is caused by an over-stimulated amygdala, the organ that produces terror. In a healthy brain, the hippocampus double-checks the source of an alarm and switches off the amygdala if not judged urgent. This is nearly instantaneous, so that the person may not even be aware a terror prompt has occurred before the hippocampus cancels it.
In PTSD sufferers, the hippocampus has been burned out by one or more intensely traumatic events, so that the amygdala keeps pumping terror prompts into the neural system over and over, with little or no interference. Hence the experience of flashbacks (sudden irrational terror originating from a trigger sensed by the amygdala, but possibly not even known to the victim).
"Hippocampus" is also the classic/poetic word for seahorse. The hippocampus in the brain is called that because it's shaped like a seahorse.