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A circadian rhythm is a roughly-24-hour cycle in the physiological processes of living beings, including plants, animals, fungi and cyanobacteria. The term "circadian", coined by Franz Halberg, comes from the Latin circa, "around", and dies, "day", meaning literally "about a day." The formal study of biological temporal rhythms such as daily, weekly, seasonal, and annual rhythms, is called chronobiology. In a strict sense, circadian rhythms are endogenously generated, although they can be modulated by external cues such as sunlight and temperature. The first endogenous circadian oscillation was observed in the 1700s by the French scientist Jean-Jacques d'Ortous de Mairan who noticed that twenty four hour patterns in the movement of plant leaves continued even when the plants were isolated from external stimuli. Circadian rhythms may be defined by three criteria:
Origin Circadian rhythms are believed to have originated in the earliest cells to provide protection for replicating DNA, from high ultraviolet radiation during day-time. As a result, replication was relegated to the dark. The fungus Neurospora, which exists today, retains this clock-regulated mechanism. The simplest known circadian clock is that of the prokaryotic cyanobacteria. Recent research has demonstrated that the circadian clock of Synechococcus elongatus can be reconstituted in vitro with just the three proteins of their central oscillator. This clock has been shown to sustain a 22 hour rhythm over several days upon the addition of ATP. Previous explanations of the prokaryotic circadian timekeeper were dependent upon a DNA transcription / translation feedback mechanism, and although this has not been shown to be the case, it is still believed to hold true for eukaryotic organisms. Indeed, although the circadian systems of eukaryotes and prokaryotes have the same basic architecture: input - central oscillator - output, they do not share any homology. This implies probable independent origins. Animal circadian rhythms Circadian rhythms are important in determining the sleeping and feeding patterns of all animals, including human beings. There are clear patterns of brain wave activity, hormone production, cell regeneration and other biological activities linked to this daily cycle. The rhythm is linked to the light-dark cycle. Animals kept in total darkness for extended periods eventually function with a "free-running" rhythm. Each "day," their sleep cycle is pushed back or forward (depending on whether the endogenous period is longer or shorter than 24 hours). The environmental cues that each day reset the rhythms are called Zeitgebers (German, literally "Time Givers"). Interestingly, totally blind subterranean mammals (e.g., blind mole rat Spalax sp.) are able to maintain their endogenous clock in absence of the external stimuli. Free running organisms still have a consolidated sleep-wake cycle when in an environment shielded from external cues, but the rhythm is not entrained and may become out of phase with other circadian, or ultradian rhythms such as temperature and digestion. This research has influenced the design of spacecraft environments, as systems that mimic the light/dark cycle have been found to be highly beneficial to astronauts. The circadian "clock" in mammals is located in the suprachiasmatic nucleus (SCN), a distinct group of cells located in the hypothalamus. Destruction of the SCN results in the complete absence of a regular sleep/wake rhythm. The SCN receives information about illumination through the eyes. The retina of the eyes contains not only "classical" photoreceptors but also photoresponsive retinal ganglion cells. These cells, which contain a photo pigment called melanopsin, follow a pathway called the retinohypothalamic tract, leading to the SCN. It is interesting to note that, if cells from the SCN are removed and cultured, they maintain their own rhythm in the absence of external cues. It appears that the SCN takes the information on day length from the retina, interprets it, and passes it on to the pineal gland (a pea-like structure found on the epithalamus), which then secretes the hormone melatonin in response. Secretion of melatonin peaks at night and ebbs during the day. Recently, evidence has emerged that circadian rhythms are found in many cells in the body outside the SCN "master clock." Furthermore, liver cells, for example, appear to respond to feeding rather than to light. Cells from many parts of the body appear to have "free-running" rhythms. Disruption to rhythms usually has a negative effect in the short term. Many travelers have experienced the condition known as jet lag, with its associated symptoms of fatigue, disorientation and insomnia. A number of other disorders, for example bipolar disorder and some sleep disorders are associated with irregular or pathological functioning of circadian rhythms. Recent research suggests that circadian rhythm disturbances found in bipolar disorder are positively influenced by lithium's effect on clock genes. Disruption to rhythms in the longer term is believed to have significant adverse health consequences on peripheral organs outside the brain, particularly in the development or exacerbation of cardiovascular disease. Timing of treatment in coordination with the body clock may significantly increase efficacy, and reduce drug toxicity, or adverse reactions. For example, timing treatment of angiotensin converting enzyme inhibitors(ACEi) may reduce nocturnal blood pressure, also benefit left ventricular (reverse) remodeling. In addition, circadian rhythms and clock genes expressed in brain regions outside the SCN may significantly influence the effects produced by drugs of abuse such as cocaine **. Moreover, genetic manipulations of clock genes profoundly affect cocaine's actions *. Circadian rhythms also play a part in the reticular activating system in the reticular formation. Plant circadian rhythms Plants are sessile organisms, and thus they are intimately associated with their environment. This ability to synchronize with daily changes in temperature and light is of great advantage to plants. For example, the circadian clock makes an essential contribution to photosynthesis, with the outcome that the clock is believed to increase plant growth and survival. As days grow shorter and cooler in the autumn, plants are able to change the expression of their genes to prepare for the end of the growing season and for winter. At the most fundamental level, circadian rhythms are the cyclical expression of genes in individual cells. This cyclical expression is controlled by a central clock, which responds to light and temperature inputs. The study of circadian rhythms is therefore of particular interest for plant biologists. Many of the circadian-controlled genes are involved in chilling and freezing tolerance, and photosynthesis. A better understanding of these genes could allow the creation of stress-tolerant plants that are better able to survive in cold temperatures and grow with increased vigour. This will allow the expansion of both growing seasons and the growth range for many economically important crops. Light and the biological clock The ability of light to reset the biological clock depends on the phase response curve (to light). Depending on the phase of sleep, the light can advance or delay the circadian rhythm. The required illuminance varies from species to species, much lower light levels being required to reset the clocks in nocturnal rodents than in humans. In addition to light intensity, wavelength (or color) of light is an important factor in the degree to which the clock is reset. Melanopsin is most efficiently excited by blue light (420-440 nm). Literature See also Notes Bold text | ||||||||
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