The series of organs working together to form to stress response are called the hypothalamic-pituitary-adrenal (HPA) axis. This post is a reference to them. The major players are:
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What it does in the HPA: in response to input from other parts of the brain, releases cortocotropin-releasing hormone (CRH) into blood vessels which take it directly to the pituitary and not into the rest of the body
Emplacement de l'Hypophyse
Patrick J. Lynch, medical illustrator
The pituitaryWhat it is: a little gland hanging off the bottom of the brain. Some people consider it part of the brain and some don't.
What it does in the HPA: in response to hormones coming through the blood directly from the hypothalamus (not going out through the rest of the body first), sends adrenocorticotropic hormone (ACTH) out to the rest of the body
The adrenalsWhat they are: small organs next to the kidneys responsible for sending all kinds of important hormones out into the body
What they do in the HPA: in response to ACTH in the bloodstream, release cortisol into the bloodstream so that it can alert different organs and tissues around the body to the need to respond to a stressor
The minor playersThose are the three organs which are part of the name of the stress response: the hypothalamic-pituitary-adrenal (HPA) axis. But to some extent, humans just chose those three as the central parts of the axis because we understood their functions first. Other organs are important in the functioning of the stress system too.
The hippocampusWhat it is: a part of the brain associated with learning and memory
What it does in the HPA: assesses the amount of cortisol in the bloodstream and sends a negative feedback message to the hypothalamus to tell it to slow down the HPA axis (resulting, eventually, in the release of less cortisol from the adrenals). This is probably part of how socialization works: the hippocampus undergoes epigenetic changes early in life which make it more or less able to send the “slow down” message to the hypothalamus and put the brakes on the stress response.
The amygdalaWhat it is: a part of the brain associated with fear
What it does in the HPA: the amygdala is part of the system that sends that initial message of fear when an animal encounters something scary, triggering the initial HPA axis stress response.
The liverWhat it is: an organ that makes a lot of useful substances used for various things in the body
What it does in the HPA: makes corticosteroid-binding globulin (CBG), the little protein that carries cortisol around in the blood stream. CBG does more than just ferry cortisol about; it actively spits it out in locations where it's needed, and when an animal has very low levels of CBG, the entire HPA axis becomes less reactive. Very young animals have low levels of CBG, which may contribute to their early lack of fear.