Editing Brain (section)

=== Homeostasis ===
[[File:LocationOfHypothalamus.jpg|thumb|right|Cross-section of a human head, showing location of the [[hypothalamus]]]]

For any animal, survival requires maintaining a variety of parameters of bodily state within a limited range of variation: these include temperature, water content, salt concentration in the bloodstream, blood glucose levels, blood oxygen level, and others.<ref name=Dougherty/> The ability of an animal to regulate the internal environment of its body—the [[milieu intérieur]], as the pioneering physiologist [[Claude Bernard]] called it—is known as [[homeostasis]] ([[Ancient Greek|Greek]] for "standing still").<ref>{{cite journal|last=Gross|first=Charles G.|year=1998|title=Claude Bernard and the constancy of the internal environment|url=http://www.princeton.edu/~cggross/Neuroscientist_98_Bernard.pdf|url-status=dead|journal=The Neuroscientist|volume=4|issue=5|pages=380–385|doi=10.1177/107385849800400520|archive-url=https://web.archive.org/web/20181208033601/http://www.princeton.edu/~cggross/Neuroscientist_98_Bernard.pdf|archive-date=2018-12-08 |s2cid=51424670}}</ref> Maintaining homeostasis is a crucial function of the brain. The basic principle that underlies homeostasis is [[negative feedback]]: any time a parameter diverges from its set-point, sensors generate an error signal that evokes a response that causes the parameter to shift back toward its optimum value.<ref name=Dougherty/> (This principle is widely used in engineering, for example in the control of temperature using a [[thermostat]].)

In vertebrates, the part of the brain that plays the greatest role is the [[hypothalamus]], a small region at the base of the forebrain whose size does not reflect its complexity or the importance of its function.<ref name="Dougherty">{{cite web|last=Dougherty|first=Patrick |title=Hypothalamus: structural organization|url=http://neuroscience.uth.tmc.edu/s4/chapter01.html|url-status=dead|archive-url=https://web.archive.org/web/20111117073211/http://neuroscience.uth.tmc.edu/s4/chapter01.html|archive-date=2011-11-17|access-date=2011-10-11|website=Neuroscience Online}}</ref> The hypothalamus is a collection of small nuclei, most of which are involved in basic biological functions. Some of these functions relate to arousal or to social interactions such as sexuality, aggression, or maternal behaviors; but many of them relate to homeostasis. Several hypothalamic nuclei receive input from sensors located in the lining of blood vessels, conveying information about temperature, sodium level, glucose level, blood oxygen level, and other parameters. These hypothalamic nuclei send output signals to motor areas that can generate actions to rectify deficiencies. Some of the outputs also go to the [[pituitary gland]], a tiny gland attached to the brain directly underneath the hypothalamus. The pituitary gland secretes hormones into the bloodstream, where they circulate throughout the body and induce changes in cellular activity.<ref>{{cite web|last=Dougherty|first=Patrick |title=Hypothalamic control of pituitary hormone|url=http://neuroscience.uth.tmc.edu/s4/chapter02.html|url-status=dead|archive-url=https://web.archive.org/web/20111117073249/http://neuroscience.uth.tmc.edu/s4/chapter02.html|archive-date=2011-11-17|access-date=2011-10-11|website=Neuroscience Online}}</ref>