The hypothalamic-pituitary-adrenal axis (HPA) is an important part of the stress response in the body. In essence, it is the linkage between the autonomic nervous system and the endocrine system.
The HPA system is a three-stage process. The hypothalamus is direct to release corticotropin-releasing hormone (CRF) by the autonomic nervous system from the sympathetic division to stimulate the body to respond to stress. The CRF bind to the anterior pituitary gland and releases adrenocorticotropic hormone (ACTH). ACTH binds to the adrenal cortex which stimulates the release of cortisol. When cortisol reaches a certain blood concentration it begins a negative feedback loop by inhibiting CRF, which slows the production of the other hormones allowing for the body to return to stasis.
A long term stress response can be caused by a number of different problems within the body. During a stress response glucose is a by product for two reasons; increased production and decreased use in the immune system. The glucose allows the muscles to perform with increased strength. However, a glucocorticoid feedback inhibition will not allow for the decrease in CRF. Continual exposure has been shown to cause damage to the hypothalamus gland.
If a continual stress response is needed, such as in sustained combat. The HPA will work to keep the blood concentration of cortisol at the peak percentage by renewed stimulus. This works like reving a car engine. When the gas is pushed it will increase the rpm's of the motor. This is similar to how the HPA increases cortisol. As you relax your foot, the rpm's drop, much like the negative feedback of cortisol will relax the CRF output. However, in a continual stress response when the CRF is inhibited the sympathetic nervous system will keep a continual contact to keep the CRF activated. This results in an eventual balance where the negative feedback is cancelled out due to reduced but constant production. The balance allows people to maintain a stress response and increased performance for a period of time.