Circadian rhythm
From Freepedia
Circadian rhythm is the name given to the roughly 24 hour cycles shown by physiological processes in plants and animals. (The term circadian comes from the Latin circa, meaning "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. For a description of circadian rhythms in plants by de Mairan, Linnaeus, and Darwin see http://www.hhmi.org/biointeractive/museum/exhibit00/02_1.html The formal study of biological temporal rhythms (such as daily, weekly, seasonal, etc.) is called chronobiology.
The circadian rhythm is neither fully dependent on nor fully independent of external cues such as sunlight and temperature. Early researchers identified 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 will eventually adjust their internal clock to a new pattern (if it is sufficiently regular).
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 24 hour cycle.
The circadian 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. The human free-running circadian rhythm is close to 25 hours - cues from our environment help keep us on a 24 hour track. Free running organisms still have a consolidated sleep/wake cycle when in environment shielded from external cues, but the rhythm is not entrained and may become out of phase with other circadian, or ultradian rhythms (e.g. 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 primarily 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 light receptors found in the retina which have a pathway, (called the retinohypothalamic tract), leading to the SCN. Interestingly, if cells from the SCN are removed and cultured, they will 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 of the SCN "master clock." Liver cells, for example, 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 have 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 sleep disorders are associated with irregular or pathological functioning of the circadian rhythms.
Plant circadian rhythms
Plants are sessile organisms and thus they are intimately associated with their environment. This ability to anticipate daily changes in temperature and light period is of great advantage to plants. At the most basic level, circadian rhythms are the cyclical expression of genes. This cyclical expression is controlled by a central clock, which responds to light as well as temperature inputs. For example, as the 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 to prepare for winter. The study of circadian rhythms is of particular interest for plant biologists. Many of the circadian controlled genes are involved in chilling and freezing tolerance. A better understanding of these genes will allow the creation of stress tolerant plants, better able to survive in cold temperatures. This will allow the expansion of both growing seasons and the growth range for many economically important crops.
See also
Also have a look at the Reticular activating system in the Reticular formation.
