Is a part of the brain not quite working.
Normally, when a highly stressful/dangerous situation happens, the amygdala releases a flood of chemical signals to the rest of the body, preparing it for the flight-or-fight. There’s also a brain path that is etched in response to the situation-nature’s way of making sure that we pay such supreme attention to this that the next time it happens, we’ll know what to do and how we survived this.
After the incident is over, of course memories can trigger intense emotions and the equivalent body response, but the pre-frontal cortex will interfere, saying, “Okay, that was then, but now we’re not in danger. That’s past.” And hence tone down the stress response in the body.
PTSD is where the frontal cortex is unable to override the instinctive response: it may send the signals, but the body continues to behave exactly as if it were facing the situation again. And, the more that it happens, the deeper that that “groove” gets marked in the brain.
They’ve found that the use of beta-blockers as soon after trauma as possible helps to prevent the establishment of PTSD.