Editing Circadian clock (section)

== Vertebrate anatomy ==
In vertebrates, the master circadian clock is contained within the [[suprachiasmatic nucleus]] (SCN), a bilateral nerve cluster of about 20,000 neurons.<ref>{{cite web | vauthors = Fahey J |title=How Your Brain Tells Time |work=Forbes |date=2009-10-15 |url=https://www.forbes.com/2009/10/14/circadian-rhythm-math-technology-breakthroughs-brain.html}}</ref><ref>{{cite book | vauthors = Gumz ML | veditors = Gumz ML  |title=Circadian Clocks: Role in Health and Disease |date=2016 |publisher=Springer, New York, NY |isbn=978-1-4939-3450-8 |page=10 |edition=1 |language=en-gb|doi=10.1007/978-1-4939-3450-8 |s2cid=44366126 }}</ref> The SCN itself is located in the [[hypothalamus]], a small region of the brain situated directly above the [[optic chiasm]], where it receives input from specialized [[intrinsically photosensitive retinal ganglion cells|photosensitive ganglion cell]]s in the retina via the [[retinohypothalamic tract]].

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> Through intercellular signalling mechanisms such as [[vasoactive intestinal peptide]], the SCN signals other hypothalamic nuclei and the [[pineal gland]] to modulate [[body temperature]] and production of hormones such as [[cortisol]] and [[melatonin]]; these hormones enter the [[circulatory system]], and induce clock-driven effects throughout the organism.

It is not, however, clear precisely what signal (or signals) enacts principal entrainment to the many biochemical clocks contained in tissues throughout the body. See section "regulation of circadian oscillators" below for more details.