While the complete story is not fully known, scientists understand much about how the response to stress works. The two main systems involved are the hypothalamic-pituitary-adrenal (HPA) axis and the SNS. (These systems are described later.) Triggered (activated) primarily by an area in the brain stem (lowest part of brain) called the locus coeruleus, the SNS secretes epinephrine and norepinephrine. The five most important concepts to remember about these two systems are that
قناة روش

1. they are governed by a feedback loop to regulate their response (In a feedback loop, increased amounts of a substance — for example, a hormone — inhibit the release of more of that substance, while decreased amounts of the substance stimulate the release of more of that substance.);

2. they interact with each other;

3. they influence other brain systems and functions;

4. genetic (inherited) variability affects the responses of both systems. (That is, depending on their genes, different people can respond differently to similar stresses.);

5. prolonged or overwhelming responses of these systems can be harmful to an individual.