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A circadian rhythm (/ s ər ˈ k eɪ d i ə n /), or circadian cycle, is a natural oscillation that repeats roughly every 24 hours.Circadian rhythms can refer to any process that originates within an organism (i.e., endogenous) and responds to the environment (is entrained by the environment).
Transcription-translation feedback loop (TTFL) is a cellular model for explaining circadian rhythms in behavior and physiology. Widely conserved across species, the TTFL is auto-regulatory, in which transcription of clock genes is regulated by their own protein products.
In vertebrates, the master circadian clock is contained within the suprachiasmatic nucleus (SCN), a bilateral nerve cluster of about 20,000 neurons. [10] [11] 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.
While circadian rhythms are defined as regulated by endogenous processes, other biological cycles may be regulated by exogenous signals. In some cases, multi-trophic systems may exhibit rhythms driven by the circadian clock of one of the members (which may also be influenced or reset by external factors).
Neurons in an intact SCN show coordinated circadian rhythms in electrical activity. [23] Neurons isolated from the SCN have been shown to produce and sustain circadian rhythms in vitro, [24] suggesting that each individual neuron of the SCN can function as an independent circadian oscillator at the cellular level. [25]
Bmal1's regulation in circadian rhythms influences reproductive physiology such as ovulation, fertilization, and embryonic and fetal development via maternal circadian communication. [59] Studies have suggested mice that lack Bmal1 display reproductive ineffectiveness such as irregular cycles and reduced fertility. [60]
Rev-erbα plays a role in the regulation of whole body metabolism through controlling lipid metabolism, bile acid metabolism, and glucose metabolism. [27] Rev-Erbα relays circadian signals into metabolic and inflammatory regulatory responses and vice versa, although the precise mechanisms underlying this relationship are not entirely ...
The circadian rhythm is a 24-hour cycle in the body, which has been shown to continue even in the absence of environmental cues. This is caused by projections from the SCN to the brain stem. This two process model was first proposed in 1982 by Borbely, [97] who called them Process S (homeostatic) and Process C (Circadian) respectively. He ...