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The Chemistry of Stress
Stress and the HPA axis | Hypothalamus releases CRH | Pituitary releases ACTH | Adrenal release of Cortisol and Adrenaline | Insulin prepares and stores energy | Body and brain arousal | Control loop | So what?
When we are stressed, by whatever causes of stress, including external things that happen to us and the internal thoughts that we have, there is a sequence of action that happen in our brain and body that lead to the visible stress response.
Stress initiators, supported by areas such as thinking in the pre-frontal cortex, the hippocampal formation and the emotional triggers from the amygdala, create a sequence of activities through the HPA axis, where HPA stands for Hypothalamus, Pituitary and Adrenal system.
When stimulated by stress, the hypothalamus releases Corticotropin-releasing Hormone (CRH).
CRH is released via the hypophyseal portal system into the anterior pituitary, which is a part of the hippocampal formation, which then in turn releases Adrenocorticotrophic-releasing Hormone (ACTH) into the blood stream.
ACTH not only acts on the adrenal system (as below), it also releases vasopresin which constrict artery walls to raise blood pressure, and stimulates the thyroid gland in the neck to increase the metabolism.
ACTH acts on the adrenal cortex, which lies around the edge of the adrenal gland and creates the stress response through production of mineralocorticoids such as aldosterone (which affects blood pressure), and glucocorticoids, in particular cortisol. The adrenal medulla is also stimulated to release catecholamines.
The adrenal system also releases adrenaline and noradrenaline into the blood stream.
As blood sugar level increases, insulin is produced in the pancreas and converts some of the glucose to glycogen, which a starch stored in the muscles and the liver that is used for energy.
If all the glycogen storage areas are filled and more glucose remains in the blood, insulin converts this to fatty tissue called triglycerides. It also leads to the production of LDL cholesterol, which can increase heart problems. These are key components in the build up of fatty deposits on the walls of the arteries.
Insulin also increases salt and water retention, which leads to continued hypertension and overweight. In particular it makes hypertension worse by increasing the responsiveness of arteries to adrenaline.
Cortisol has a number of functions, in particular ensuring the body has enough energy for the fight-or-flight response.
It binds preferentially to gluco-corticoid receptors in the hippocampus and prefrontal cortex, which increases Ca2+ access and consequent brain activity.
It depresses the immune system and reduces the activity of white blood cells (that attack foreign bodies). It also increases blood sugar and so speeds up the metabolism.
Catecholamines affect many parts of the brain, including the hippocampus and prefrontal cortex, which are particularly rich in receptors activated by stress hormones.
When the amgdala becomes involved, this can create an amplifying loop as the fear and other stress emotions created trigger further hormonal release.
Adrenaline increases oxygen and glucose to the brain and to muscles whilst suppressing non-essential bodily functions.
When cortisol remains high, it eventually loops back to the pituitary and hypothalamus, turning off the HPA axis stimulation of further cortisol production. This is the normal and healthy system of stress management.
When the HPA is repeatedly stimulated, it results in over-cortisol production to the point where the body is damaged.
Stress is intended to be beneficial in enabling rapid reaction in the short term. If this chemical chain is repeatedly triggered, however, it exhausts the body and pushes it chemically out of balance. Unused energy also gets stored as fat, including in arteries.