Editing Suprachiasmatic nucleus (section)

==Circadian clock==
{{Main|Circadian clock}}
Different organisms such as bacteria,<ref>{{cite journal|vauthors=Clodong S, Dühring U, Kronk L, Wilde A, Axmann I, Herzel H, Kollmann M|title=Functioning and robustness of a bacterial circadian clock | journal = Molecular Systems Biology|volume=3|issue=1|pages=90|year=2007|pmid =17353932|pmc=1847943|doi = 10.1038/msb4100128|author8= Clodong S, Dühring U, Kronk L, Wilde A, Axmann I, Herzel H, Kollmann M }}</ref> plants, fungi, and animals, show genetically based near-24-hour rhythms. Although all of these clocks appear to be based on a similar type of genetic feedback loop, the specific genes involved are thought to have evolved independently in each kingdom. Many aspects of mammalian behavior and physiology show circadian rhythmicity, including sleep, physical activity, alertness, hormone levels, body temperature, immune function, and digestive activity. Early experiments on the function of the SCN involved lesioning the SCN in hamsters.<ref name=":4">{{Cite journal |last1=Ralph |first1=Martin R. |last2=Foster |first2=Russell G. |last3=Davis |first3=Fred C. |last4=Menaker |first4=Michael |date=1990-02-23 |title=Transplanted Suprachiasmatic Nucleus Determines Circadian Period |url=http://dx.doi.org/10.1126/science.2305266 |journal=Science |volume=247 |issue=4945 |pages=975–978 |doi=10.1126/science.2305266 |pmid=2305266 |issn=0036-8075|url-access=subscription }}</ref> SCN lesioned hamsters lost their daily activity rhythms.<ref name=":4" /> Further, when the SCN of a hamster was transplanted into an SCN lesioned hamster, the hamster adopted the rhythms of the hamster from which the SCN was transplanted.<ref name=":4" /> Together, these experiments suggest that the SCN is sufficient for generating circadian rhythms in hamsters.

Later studies have shown that skeletal, muscle, liver, and lung tissues in rats generate 24-hour rhythms, which dampen over time when isolated in a dish, where the SCN maintains its rhythms.<ref>{{Cite journal |last1=Yamazaki |first1=Shin |last2=Numano |first2=Rika |last3=Abe |first3=Michikazu |last4=Hida |first4=Akiko |last5=Takahashi |first5=Ri-ichi |last6=Ueda |first6=Masatsugu |last7=Block |first7=Gene D. |last8=Sakaki |first8=Yoshiyuki |last9=Menaker |first9=Michael |last10=Tei |first10=Hajime |date=2000-04-28 |title=Resetting Central and Peripheral Circadian Oscillators in Transgenic Rats |url=http://dx.doi.org/10.1126/science.288.5466.682 |journal=Science |volume=288 |issue=5466 |pages=682–685 |doi=10.1126/science.288.5466.682 |pmid=10784453 |issn=0036-8075|url-access=subscription }}</ref> Together, these data suggest a model whereby the SCN maintains control across the body by synchronizing "slave oscillators," which exhibit their own near-24-hour rhythms and control circadian phenomena in local tissue.<ref>{{cite journal|vauthors=Bernard S, Gonze D, Cajavec B, Herzel H, Kramer A|title = Synchronization-induced rhythmicity of circadian oscillators in the suprachiasmatic nucleus | journal = PLOS Computational Biology | volume = 3 | issue = 4|pages=e68|date=April 2007|pmid =17432930|pmc=1851983 | doi = 10.1371/journal.pcbi.0030068 | bibcode = 2007PLSCB...3...68B |doi-access = free }}</ref>

The SCN receives input from specialized [[photosensitive ganglion cell]]s in the retina via the [[retinohypothalamic tract]].<ref name=":5">{{Cite journal |last1=Miller |first1=Joseph D. |last2=Morin |first2=Lawrence P. |last3=Schwartz |first3=William J. |last4=Moore |first4=Robert Y. |date=1996 |title=New Insights Into the Mammalian Circadian Clock |journal=Sleep |volume=19 |issue=8 |pages=641–667 |doi=10.1093/sleep/19.8.641 |pmid=8958635 |issn=1550-9109|doi-access=free }}</ref> Neurons in the ''ventrolateral SCN'' (vlSCN) have the ability for light-induced gene expression. [[Melanopsin]]-containing [[retinal ganglion cell|ganglion cell]]s in the [[retina]] have a direct connection to the ventrolateral SCN via the retinohypothalamic tract.<ref name=":5" /> When the retina receives light, the vlSCN relays this information throughout the SCN allowing ''[[entrainment (chronobiology)|entrainment]]'', synchronization, of the person's or animal's daily rhythms to the 24-hour cycle in nature.<ref name=":5" /> The importance of entraining organisms, including humans, to exogenous cues such as the light/dark cycle, is reflected by several [[circadian rhythm sleep disorders]], where this process does not function normally.<ref name="pmid15087208">{{cite journal | vauthors = Reid KJ, Chang AM, Zee PC | title = Circadian rhythm sleep disorders | journal = The Medical Clinics of North America | volume = 88 | issue = 3 | pages = 631–51, viii | date = May 2004 | pmid = 15087208 | doi = 10.1016/j.mcna.2004.01.010 | pmc = 3523094 }}</ref>

Neurons in the ''dorsomedial SCN'' (dmSCN) are believed to have an endogenous 24-hour rhythm that can persist under constant darkness (in humans averaging about 24&nbsp;hours 11&nbsp;min).<ref>{{Cite web|url=https://news.harvard.edu/gazette/story/1999/07/human-biological-clock-set-back-an-hour/|title=Human Biological Clock Set Back an Hour|date=1999-07-15|website=Harvard Gazette|language=en-US|access-date=2019-01-28}}</ref> A GABAergic mechanism is involved in the coupling of the ventral and dorsal regions of the SCN.<ref>{{cite journal |last1=Azzi |first1=A |last2=Evans |first2=JA |last3=Leise |first3=T |last4=Myung |first4=J |last5=Takumi |first5=T |last6=Davidson |first6=AJ |last7=Brown |first7=SA |title=Network Dynamics Mediate Circadian Clock Plasticity. |journal=Neuron |date=18 January 2017 |volume=93 |issue=2 |pages=441–450 |doi=10.1016/j.neuron.2016.12.022 |pmid=28065650 |pmc=5247339 }}</ref>