Editing Hypothalamus (section)

===Stimulation===
The hypothalamus coordinates many hormonal and behavioural circadian rhythms, complex patterns of [[neuroendocrine]] outputs, complex [[homeostasis|homeostatic]] mechanisms, and important behaviours. The hypothalamus must, therefore, respond to many different signals, some of which are generated externally and some internally. [[Delta wave]] signalling arising either in the thalamus or in the cortex influences the secretion of releasing hormones; [[GHRH]] and [[prolactin]] are stimulated whilst [[TRH]] is inhibited. {{cn|date=March 2025}}

The hypothalamus is responsive to:
* Light: daylength and [[photoperiod]] for regulating [[circadian]] and seasonal rhythms
* [[Olfactory]] stimuli, including [[pheromones]]
* [[Steroids]], including [[gonadal steroids]] and [[corticosteroids]]
* Neurally transmitted information arising in particular from the heart, [[enteric nervous system]] (of the [[gastrointestinal tract]]),<ref>{{cite journal | vauthors = Mayer EA | title = Gut feelings: the emerging biology of gut-brain communication | journal = Nature Reviews. Neuroscience | volume = 12 | issue = 8 | pages = 453–66 | date = July 2011 | pmid = 21750565 | pmc = 3845678 | doi = 10.1038/nrn3071 }}</ref> and the reproductive tract.{{citation needed|reason=Your explanation here|date=April 2016}}
* [[Autonomic Nervous System|Autonomic]] inputs
* Blood-borne stimuli, including [[leptin]], [[ghrelin]], [[angiotensin]], [[insulin]], [[pituitary hormones]], [[cytokines]], plasma concentrations of glucose and osmolarity etc.
* [[Stress (medicine)|Stress]]
* Invading microorganisms by increasing body temperature, resetting the body's thermostat upward.

====Olfactory stimuli====
Olfactory stimuli are important for [[sexual reproduction]] and [[neuroendocrine]] function in many species. For instance, if a pregnant mouse is exposed to the urine of a 'strange' male during a critical period after coitus then the pregnancy fails (the [[Bruce effect]]). Thus, during coitus, a female mouse forms a precise 'olfactory memory' of her partner that persists for several days. Pheromonal cues aid synchronization of [[oestrus]] in many species; in women, synchronized [[menstruation]] may also arise from pheromonal cues, although the role of pheromones in humans is disputed. {{cn|date=March 2025}}

====Blood-borne stimuli====
[[Peptide]] hormones have important influences upon the hypothalamus, and to do so they must pass through the [[blood–brain barrier]]. The hypothalamus is bounded in part by specialized brain regions that lack an effective blood–brain barrier; the [[Capillary#Types|capillary]] [[endothelium]] at these sites is fenestrated to allow free passage of even large proteins and other molecules. Some of these sites are the sites of neurosecretion - the [[neurohypophysis]] and the [[median eminence]]. However, others are sites at which the brain samples the composition of the blood. Two of these sites, the SFO ([[subfornical organ]]) and the OVLT ([[organum vasculosum of the lamina terminalis]]) are so-called [[circumventricular organs]], where neurons are in intimate contact with both blood and [[Cerebrospinal fluid|CSF]]. These structures are densely vascularized, and contain osmoreceptive and sodium-receptive neurons that control [[drinking]], [[vasopressin]] release, sodium excretion, and sodium appetite. They also contain neurons with receptors for [[angiotensin]], [[atrial natriuretic factor]], [[endothelin]] and [[relaxin]], each of which important in the regulation of fluid and electrolyte balance. Neurons in the OVLT and SFO project to the [[supraoptic nucleus]] and [[paraventricular nucleus]], and also to preoptic hypothalamic areas. The circumventricular organs may also be the site of action of [[interleukins]] to elicit both fever and ACTH secretion, via effects on paraventricular neurons.{{citation needed|date=February 2013}}

It is not clear how all peptides that influence hypothalamic activity gain the necessary access. In the case of [[prolactin]] and [[leptin]], there is evidence of active uptake at the [[choroid plexus]] from the blood into the [[cerebrospinal fluid]] (CSF). Some pituitary hormones have a negative feedback influence upon hypothalamic secretion; for example, [[growth hormone]] feeds back on the hypothalamus, but how it enters the brain is not clear. There is also evidence for central actions of [[prolactin]].{{citation needed|date=February 2013}}

Findings have suggested that [[thyroid hormone]] (T4) is taken up by the hypothalamic [[glial cells]] in the [[infundibular nucleus]]/ [[median eminence]], and that it is here converted into [[Triiodothyronine|T3]] by the type 2 deiodinase (D2). Subsequent to this, T3 is transported into the [[thyrotropin-releasing hormone]] ([[TRH]])-producing [[neurons]] in the [[paraventricular nucleus]]. [[Thyroid hormone receptor]]s have been found in these [[neurons]], indicating that they are indeed sensitive to T3 stimuli. In addition, these neurons expressed [[SLC16A2|MCT8]], a [[thyroid hormone]] transporter, supporting the theory that T3 is transported into them. T3 could then bind to the thyroid hormone receptor in these neurons and affect the production of thyrotropin-releasing hormone, thereby regulating thyroid hormone production.<ref>{{cite journal|vauthors=Fliers E, Unmehopa UA, Alkemade A|date=June 2006|title=Functional neuroanatomy of thyroid hormone feedback in the human hypothalamus and pituitary gland|journal=Molecular and Cellular Endocrinology|volume=251|issue=1–2|pages=1–8|doi=10.1016/j.mce.2006.03.042|pmid=16707210|s2cid=33268046}}<!--|access-date=7 July 2011--></ref>

The hypothalamus functions as a type of [[thermostat]] for the body.<ref name=Harrisons>{{cite book | author-link = Anthony Fauci | author = Fauci, Anthony | title = Harrison's Principles of Internal Medicine | url = https://archive.org/details/harrisonsprincip00asfa | url-access = limited | edition = 17 | publisher = McGraw-Hill Professional | year = 2008 | isbn = 978-0-07-146633-2 | pages = [https://archive.org/details/harrisonsprincip00asfa/page/n155 117]–121 | display-authors = etal }}</ref> It sets a desired body temperature, and stimulates either heat production and retention to raise the blood temperature to a higher setting or sweating and [[vasodilation]] to cool the blood to a lower temperature. All [[fever]]s result from a raised setting in the hypothalamus; elevated body temperatures due to any other cause are classified as [[hyperthermia]].<ref name=Harrisons /> Rarely, direct damage to the hypothalamus, such as from a [[stroke]], will cause a fever; this is sometimes called a ''hypothalamic fever''. However, it is more common for such damage to cause abnormally low body temperatures.<ref name=Harrisons />

====Steroids====
The hypothalamus contains neurons that react strongly to steroids and [[glucocorticoids]] (the steroid hormones of the [[adrenal gland]], released in response to [[ACTH]]). It also contains specialized glucose-sensitive neurons (in the [[arcuate nucleus]] and [[ventromedial hypothalamus]]), which are important for [[appetite]]. The preoptic area contains thermosensitive neurons; these are important for [[TRH]] secretion. {{cn|date=March 2025}}

====Neural====
[[Oxytocin]] secretion in response to suckling or vagino-cervical stimulation is mediated by some of these pathways; [[vasopressin]] secretion in response to cardiovascular stimuli arising from chemoreceptors in the [[carotid body]] and [[aortic arch]], and from low-pressure [[atrial volume receptors]], is mediated by others. In the rat, stimulation of the [[vagina]] also causes [[prolactin]] secretion, and this results in [[pseudo-pregnancy]] following an infertile mating. In the rabbit, coitus elicits [[Induced ovulation (animals)|reflex ovulation]]. In the sheep, [[cervix|cervical]] stimulation in the presence of high levels of estrogen can induce [[maternal bond|maternal behavior]] in a virgin ewe. These effects are all mediated by the hypothalamus, and the information is carried mainly by spinal pathways that relay in the brainstem. Stimulation of the nipples stimulates release of oxytocin and prolactin and suppresses the release of [[Luteinizing hormone|LH]] and [[Follicle-stimulating hormone|FSH]]. {{cn|date=March 2025}}

Cardiovascular stimuli are carried by the [[vagus nerve]]. The vagus also conveys a variety of visceral information, including for instance signals arising from gastric distension or emptying, to suppress or promote feeding, by signalling the release of [[leptin]] or [[gastrin]], respectively. Again, this information reaches the hypothalamus via relays in the brainstem. {{cn|date=March 2025}}

In addition, hypothalamic function is responsive to—and regulated by—levels of all three classical [[monoamine neurotransmitter]]s, [[noradrenaline]], [[dopamine]], and [[serotonin]] (5-hydroxytryptamine), in those tracts from which it receives innervation. For example, noradrenergic inputs arising from the locus coeruleus have important regulatory effects upon [[corticotropin-releasing hormone]] (CRH) levels. {{cn|date=March 2025}}