Stress: Which comes first SAM or HPA?

We recently received the question above from Claire Matthews of The Castle School in Bristol and thought some of you would be interested in the answer: The SAM system concerns the adrenal medulla and the release of adrenaline/noradrenaline which has the effect of creating physiological (sympathetic) arousal and producing the fight or flight response. The HPA access concerns the adrenal cortex which produces corticosteroids such as cortisol, which help the body to recover after stress but also have negative effects e.g. reduced immune function. Now to the question – do they happen simultaneously or does the SAM system come first and the HPA second?

The answer is both. Essentially both systems are triggered by a stressor but one (SAM) is an instantaneous response like lighting a match, whereas the other (HPA) is much slower and doesn’t happen very easily (like lighting a fire). It is harder to achieve (needs a continuing stressor) and is only activated in more extreme circumstances. So in a sense the HPA response does come second because, even though it is triggered at the same time as the SAM, its response is seen some time later – well after the fight/flight response.