Circadian clock

A circadian clock, or circadian oscillator, is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time. A circadian clock, or circadian oscillator, is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time. Such a clock's in vivo period is necessarily almost exactly 24 hours (the earth's current solar day). In most living things, internally synchronized circadian clocks make it possible for the organism to anticipate daily environmental changes corresponding with the day–night cycle and adjust its biology and behavior accordingly. The term circadian derives from the Latin circa (about) diem (a day), since when taken away from external cues (such as environmental light), they do not run to exactly 24 hours. Clocks in humans in a lab in constant low light, for example, will average about 24.2 hours per day, rather than 24 hours exactly. The normal body clock oscillates with an endogenous period of exactly 24 hours, it entrains, when it receives sufficient daily corrective signals from the environment, primarily daylight and darkness. Circadian clocks are the central mechanisms that drive circadian rhythms. They consist of three major components: The clock is reset as an organism senses environmental time cues of which the primary one is light. Circadian oscillators are ubiquitous in tissues of the body where they are synchronized by both endogenous and external signals to regulate transcriptional activity throughout the day in a tissue-specific manner. The circadian clock is intertwined with most cellular metabolic processes and it is affected by organism aging. The basic molecular mechanisms of the biological clock have been defined in vertebrate species, Drosophila melanogaster, plants, fungi, bacteria, and presumably also in Archaea. In 2017, the Nobel Prize in Physiology or Medicine was awarded to Jeffrey C. Hall, Michael Rosbash and Michael W. Young 'for their discoveries of molecular mechanisms controlling the circadian rhythm' in fruit flies. In vertebrates, the master circadian clock is contained within the suprachiasmatic nucleus (SCN), a bilateral nerve cluster of about 20,000 neurons. 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 photosensitive ganglion cells 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. 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.