Here are my explanations of the functions of the Amygdala and Hippocampus in terms of exposure to traumatic events and highly stressful situations.
The Amygdala is nestled deep inside the limbic system - the part of the brain which seems primarily in control of memory formation and emotions.
The Amygdala is specifically important when it comes to processing emotions and it records our reactions, particularly when fear is involved. Essentially, it is there to keep us safe and alert us of danger and is key in triggering fight, flight or freeze responses to prepare us to take action against the threat.
The Amygdala is like a highly effective scanner, processing information in our environment and can react within 0.4 seconds.
Sometimes however, the Amygdala can fire off when there is no real danger in the here and now as it is acting on a memory of a past traumatic event triggered by something in the immediate environment - a sound, smell, sight or other sense. Our bodies are then flooded with adrenalin and we are needlessly prepared to react. Unused adrenaline can build up and cause stress and anxiety symptoms if not expelled (see my tips for managing stress).
The Hippocampus works alongside the Amygdala and date stamps and logs our memories - a bit like a librarian.
In a potentially traumatic situation however, the Hippocampus is not as effective and does not function at it best. It can be overwhelmed and log things in the wrong place with incorrect information which is why a memory of a traumatic time can feel as though it is happening again by way of intrusive thoughts, flashbacks and other re-experiencing and hyperarousal symptoms.