Editing Hypothalamic–pituitary–adrenal axis (section)

==Function==
Release of [[corticotropin-releasing hormone]] (CRH) from the hypothalamus is influenced by [[stress (medicine)|stress]], physical activity, illness, by blood levels of cortisol and by the sleep/wake cycle ([[circadian rhythm]]). In healthy individuals, cortisol rises rapidly after wakening, reaching a peak within 30–45 minutes. It then gradually falls over the day, rising again in late afternoon. Cortisol levels then fall in late evening, reaching a trough during the middle of the night. This corresponds to the rest-activity cycle of the organism.<ref name="isbn_9780444530400"/> An abnormally flattened circadian cortisol cycle has been linked with [[chronic fatigue syndrome]],<ref>{{cite journal |vauthors=MacHale SM, Cavanagh JT, Bennie J, Carroll S, Goodwin GM, Lawrie SM |title=Diurnal variation of adrenocortical activity in chronic fatigue syndrome |journal=Neuropsychobiology |volume=38 |issue=4 |pages=213–7 |date=November 1998 |pmid=9813459 |doi=10.1159/000026543|s2cid=46856991 }}</ref> [[insomnia]]<ref>{{cite journal |vauthors=Backhaus J, Junghanns K, Hohagen F |title=Sleep disturbances are correlated with decreased morning awakening salivary cortisol |journal=Psychoneuroendocrinology |volume=29 |issue=9 |pages=1184–91 |date=October 2004 |pmid=15219642 |doi=10.1016/j.psyneuen.2004.01.010|s2cid=14756991 }}</ref> and [[Occupational burnout|burnout]].<ref>{{cite journal |vauthors=Pruessner JC, Hellhammer DH, Kirschbaum C |title=Burnout, perceived stress, and cortisol responses to awakening |journal=Psychosom Med |volume=61 |issue=2 |pages=197–204 |year=1999 |pmid=10204973 |url=http://www.psychosomaticmedicine.org/cgi/pmidlookup?view=long&pmid=10204973 |doi=10.1097/00006842-199903000-00012|url-access=subscription }}</ref>

The HPA axis has a central role in regulating many [[Homeostasis|homeostatic systems]] in the body, including the [[metabolic system]], [[cardiovascular system]], [[immune system]], [[reproductive system]] and [[central nervous system]]. The HPA axis integrates physical and [[psychosocial]] influences in order to allow an organism to adapt effectively to its environment, use resources, and optimize survival.<ref name="isbn_9780444530400">{{cite book | editor-last=del Rey | editor-first=A. | editor-last2=Chrousos | editor-first2=G. P. | editor-last3=Besedovsky | editor-first3=H. O. | others=Berczi, I.; Szentivanyi A., series [[Editor-in-chief|EICs]] | title=The Hypothalamus-Pituitary-Adrenal Axis | publisher=Elsevier Science | publication-place=Amsterdam London | series=NeuroImmune Biology | volume=7 | year=2008 | isbn=978-0-08-055936-0 | oclc=272388790 | url=https://books.google.com/books?id=nJSYf879en4C | access-date=27 February 2022 | page= | archive-date=10 August 2023 | archive-url=https://web.archive.org/web/20230810072626/https://books.google.com/books?id=nJSYf879en4C | url-status=live }}</ref>

Anatomical connections between brain areas such as the [[amygdala]], [[hippocampus]], [[prefrontal cortex]] and hypothalamus facilitate activation of the HPA axis.<ref>{{Cite book|title=Discovering behavioral neuroscience : an introduction to biological psychology|last=Laura|first=Freberg|others=Freberg, Laura,, Container of (work): Freberg, Laura.|isbn=9781305088702|edition= Third|location=Boston, MA|pages=504|oclc=905734838|date = 2015-01-01}}</ref> Sensory information arriving at the lateral aspect of the [[amygdala]] is processed and conveyed to the amygdala's [[central nucleus]], which then projects out to several parts of the brain involved in responses to fear. At the hypothalamus, fear-signaling impulses activate both the [[sympathetic nervous system]] and the modulating systems of the HPA axis.

Increased production of cortisol during stress results in an increased availability of [[glucose]] in order to facilitate [[Fight-or-flight response|fighting or fleeing]]. As well as directly increasing glucose availability, cortisol also suppresses the highly demanding metabolic processes of the [[immune system]], resulting in further availability of glucose.<ref name="isbn_9780444530400"/>

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 [[glucocorticoid]]s. Deficiencies of the [[hippocampus]] may reduce the memory resources available to help a body formulate appropriate reactions to stress.<ref>{{cite journal |last1=Frankiensztajn |first1=Linoy Mia |last2=Elliott |first2=Evan |last3=Koren |first3=Omry |title=The microbiota and the hypothalamus-pituitary-adrenocortical (HPA) axis, implications for anxiety and stress disorders |journal=Current Opinion in Neurobiology |date=June 2020 |volume=62 |pages=76–82 |doi=10.1016/j.conb.2019.12.003|pmid=31972462 |s2cid=210836469 }}</ref>