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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.
In most vertebrates, exposure to light sets off a chain reaction of enzymatic events within the pineal gland that regulates circadian rhythms. [64] In humans and other mammals, the light signals necessary to set circadian rhythms are sent from the eye through the retinohypothalamic system to the suprachiasmatic nuclei (SCN) and the pineal gland.
The Rev-Erb proteins are members of the nuclear receptor (NR) superfamily of intracellular transcription factors and key regulatory components of the circadian clock. There are two forms of the receptor, Rev-Erb alpha and Rev-Erb beta, which are each encoded by a separate gene (NR1D1 and NR1D2, respectively). [1] [2]
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]
In Drosophila, CRY1 and CRY2 bind to TIM, a circadian gene that is a component of the transcription-translation negative feedback loop, in a light dependent fashion and blocks its function. In mammals, CRY1 and CRY2 are light independent and function to inhibit the CLOCK-BMAL1 dimer of the circadian clock which regulates cycling of Per1 ...
The SCN of the hypothalamus contains an endogenous pacemaker that regulates circadian rhythms. [4] The zeitgeber found to have the most profound effect on the SCN is light, which is the form of stimulation of which conversion is needed for it to be processed by the brain. Neurotransmitters that travel the RHT are responsible for delivering this ...