Difference between revisions of "Circadian rhythm" - New World Encyclopedia
Alisa Opar (talk | contribs) (added Czeisler citation, edited punctuation, fact checked) |
Alisa Opar (talk | contribs) (Minor edits) |
||
| Line 1: | Line 1: | ||
{{Contracted}} | {{Contracted}} | ||
| − | A '''circadian rhythm''' is a roughly | + | A '''circadian rhythm''' is a roughly 24-hour cycle in the physiological processes of [[plant]]s, [[animal]]s, [[fungi]], and [[cyanobacteria]]. (The term "circadian" comes from the [[Latin]] ''circa'', "around," and ''dies'', "day," meaning literally "around a day.") It was initially discovered in the movement of plant leaves in the 1700s by the French scientist [[Jean-Jacques d'Ortous de Mairan]]. The formal study of biological temporal rhythms, including daily, weekly, seasonal, and annual patterns, is called [[chronobiology]]. |
The circadian rhythm partly depends on external cues such as [[sunlight]] and [[temperature]]. Early researchers observed that some sort of "internal" rhythm must exist because plants and animals did not react immediately to artificially-induced changes in daily rhythms. However, it has been well established that a mechanism for adjustment also exists, as plants and animals eventually adjust their internal clock to a new pattern (if it is sufficiently regular and not too far off the norm for the species). Overall, circadian rhythms are defined by three criteria: | The circadian rhythm partly depends on external cues such as [[sunlight]] and [[temperature]]. Early researchers observed that some sort of "internal" rhythm must exist because plants and animals did not react immediately to artificially-induced changes in daily rhythms. However, it has been well established that a mechanism for adjustment also exists, as plants and animals eventually adjust their internal clock to a new pattern (if it is sufficiently regular and not too far off the norm for the species). Overall, circadian rhythms are defined by three criteria: | ||
| − | # The rhythm persists in constant conditions (for example in constant light) with a period of about 24 hours | + | # The rhythm persists in constant conditions (for example, in constant light) with a period of about 24 hours |
| − | # The rhythm period can be reset by exposure to a light or dark pulse | + | # The rhythm period can be reset by changes in environmental conditions, such as exposure to a light or dark pulse |
| − | # The rhythm | + | # The period of circadian rhythm does not change with temperature variations. |
==Animal Circadian Rhythms== | ==Animal Circadian Rhythms== | ||
| − | Circadian rhythms are important in determining the sleeping and feeding patterns of all animals, including humans. There are clear patterns of [[brain wave]] activity, [[hormone]] production, cell regeneration and other biological activities linked to this daily cycle. | + | Circadian rhythms are important in determining the sleeping and feeding patterns of all animals, including humans. 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 sleep|free-running]]" rhythm. Each "day," their sleep cycle is pushed back or forward (depending whether they are [[nocturnal]] or [[Diurnal animal|diurnal]] animals) by approximately one hour. Free-running rhythms of diurnal animals are close to 25 hours. The human free-running circadian rhythm is just over 24 hours, not 25 hours, as many text books assert (Czeisler 1999). The environmental cues that reset the rhythms each day are called ''[[Zeitgeber]]s''. | The rhythm is linked to the light-dark cycle. Animals kept in total darkness for extended periods eventually function with a "[[Free-running sleep|free-running]]" rhythm. Each "day," their sleep cycle is pushed back or forward (depending whether they are [[nocturnal]] or [[Diurnal animal|diurnal]] animals) by approximately one hour. Free-running rhythms of diurnal animals are close to 25 hours. The human free-running circadian rhythm is just over 24 hours, not 25 hours, as many text books assert (Czeisler 1999). The environmental cues that reset the rhythms each day are called ''[[Zeitgeber]]s''. | ||
Revision as of 12:53, 19 June 2006
A circadian rhythm is a roughly 24-hour cycle in the physiological processes of plants, animals, fungi, and cyanobacteria. (The term "circadian" comes from the Latin circa, "around," and dies, "day," meaning literally "around a day.") It was initially discovered in the movement of plant leaves in the 1700s by the French scientist Jean-Jacques d'Ortous de Mairan. The formal study of biological temporal rhythms, including daily, weekly, seasonal, and annual patterns, is called chronobiology.
The circadian rhythm partly depends on external cues such as sunlight and temperature. Early researchers observed that some sort of "internal" rhythm must exist because plants and animals did not react immediately to artificially-induced changes in daily rhythms. However, it has been well established that a mechanism for adjustment also exists, as plants and animals eventually adjust their internal clock to a new pattern (if it is sufficiently regular and not too far off the norm for the species). Overall, circadian rhythms are defined by three criteria:
- The rhythm persists in constant conditions (for example, in constant light) with a period of about 24 hours
- The rhythm period can be reset by changes in environmental conditions, such as exposure to a light or dark pulse
- The period of circadian rhythm does not change with temperature variations.
Animal Circadian Rhythms
Circadian rhythms are important in determining the sleeping and feeding patterns of all animals, including humans. 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 whether they are nocturnal or diurnal animals) by approximately one hour. Free-running rhythms of diurnal animals are close to 25 hours. The human free-running circadian rhythm is just over 24 hours, not 25 hours, as many text books assert (Czeisler 1999). The environmental cues that reset the rhythms each day are called Zeitgebers.
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. Contributing to this clock are melanopsin-expressing retinal ganglion cells, photoreceptors found in the retina, known as melanopsin ganglia. These cells, which contain a newly-discovered 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. The SCN does not appear to be able to react rapidly to changes in the light/dark cues.
Recently, evidence has emerged that circadian rhythms are found in many cells in the body—outside the SCN "master clock." For example, liver cells appear to respond to feeding rather than 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 sleep disorder are associated with irregular or pathological functioning of the circadian rhythms.
Recent research suggests that circadian rhythm disturbances found in bipolar disorder are positively influenced by lithium's effect on clock[genes].
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 [1][2]. Moreover, genetic manipulations of clock genes profoundly affect cocaine's actions [3].
Circadian rhythms also play a part in the reticular activating system in 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, 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
Illuminance must be greater than 1000 lux to reset the circadian clock in humans, though much lower light levels have been shown to effectively reset the clocks of nocturnal rodents.
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).[1]
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. Remarkably, although the circadian systems of eukaryotes and prokaryotes have the same basic architecture: input - central oscillator - output, they do not share any homology. This may imply their probable independent origin.
Literature
Takahashi JS, Zatz M (1982) Regulation of circadian rhythmicity. Science 217:1104–1111
Ditty JL, Williams SB, Golden SS (2003) A cyanobacterial circadian timing mechanism. Annu Rev Genet 37:513-543
Dvornyk V, Vinogradova ON, Nevo E (2003) Origin and evolution of circadian clock genes in prokaryotes. Proc Natl Acad Sci USA 100:2495-2500
Aschoff J (eds.) (1965) Circadian Clocks. North Holland Press, Amsterdam
See also
- Human factors
- Human reliability
- Actigraphy
External links
- Biological Clocks A description of circadian rhythms in plants by de Mairan, Linnaeus, and Darwin
- Texas study on darkness' influence on fat metabolization
- 28 hour day
Notes
- ↑ Newman LA, Walker MT, Brown RL, Cronin TW, Robinson PR: "Melanopsin forms a functional short-wavelength photopigment", Biochemistry. 2003 Nov 11;42(44):12734-8.
Credits
New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.0 License (CC-by-sa), which may be used and disseminated with proper attribution. Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. To cite this article click here for a list of acceptable citing formats.The history of earlier contributions by wikipedians is accessible to researchers here:
The history of this article since it was imported to New World Encyclopedia:
Note: Some restrictions may apply to use of individual images which are separately licensed.
