Hypothalamic-Pituitary- Adrenal Axis
The HPA axis is a complex set of direct influences and feedback interactions among the hypothalamus, the pituitary gland, and the adrenal glands. The interactions among these organs constitute a major part of the neuroendocrine system that controls reactions to stress and regulates many body processes, including digestion, the immune system, mood and emotions, sexuality, and energy storage and expenditure. Several monoamine neurotransmitters are important in regulating the HPA axis, especially dopamine, serotonin and norepinephrine. The body's response from the time it perceives a danger to the time it secretes the hormones to prepare to deal with it involves the following structures, in the following order: 1) the limbic system, 2) the hypothalamus, 3) the pituitary gland, and 4) the adrenal glands. The adrenal glands secrete glucocorticoids, which interact with the serotonin receptors in the brain. Glucocorticoids have many important functions, including modulation of stress reactions, but in excess they can be damaging. Atrophy of the hippocampus in humans and animals exposed to severe stress is believed to be caused by prolonged exposure to high concentrations of glucocorticoids. Deficiencies of the hippocampus may reduce the memory resources available to help a body formulate appropriate reactions to stress. When someone experiences a stressful event, the level of glucocorticoids in their blood rises. Via specific receptors in the hippocampus, this activates the hypothalamus, which then secretes corticotropin-releasing hormone (CRH). The CRH in turn causes the pituitary gland to release adrenocorticotropic hormone (ACTH) into the bloodstream, from which it enters the adrenal glands and causes them to secrete cortisol. This process creates a negative feedback loop in which the excess cortisol activates the brain's glucocorticoid receptors and suppresses the production of CRH. In depressed...
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