Discuss the role of endogenous pacemakers and exogenous zeitgebers in circadian rhythms PDF

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discuss the role of endogenous pacemakers and exogenous zeitgebers in circadian rhythms...

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Discuss the role of endogenous pacemakers and exogenous zeitgebers in circadian rhythms (16 marks) Endogenous pacemakers are internal body clocks that regulate many of our biological rhythms, such as the influence of the suprachiasmatic nucleus (SCN) on the sleep/wake cycle. The SCN is a tiny bubble of nerve cells located in the hypothalamus in each hemisphere of the brain. It is one of the primary endogenous pacemakers in mammal species, including humans. It is influential in maintaining circadian rhythms such as the sleep/wake cycle. This is the daily cycle of biological activity based on 24-hour period, known as the circadian rhythm that is influenced by regular variations in the environment such as the alteration of night and day. The SCN lies just above the optic chiasm (the nerve fibres connected to the eye). This continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep. The SCN passes the information on day length and light that it receives to the pineal gland. During the night, the pineal gland increases production of melatonin which is a chemical that includes sleep and is inhibited during periods of wakefulness. Melatonin has also been suggested as a casual factor in SAD (seasonal affective disorder). The influence of the SCN has been demonstrated in studies involving animals. Patricia DeCoursey et al (2000) destroyed the SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days. The sleep/wake cycle of the chipmunks disappeared and by the end of the study a significant proportion of them had been killed by predators, presumably when they were awake and vulnerable to attack when they should have been asleep. Exogenous zeitgebers are external cues that may affect or entrain our biological rhythms, such as the influence of light on the sleep/wake cycle. In the absence of external cues, the free running biological clock that controls the sleep/wake cycle continues to tick in a distinct cyclical pattern. Showing sleeping and wakefulness can be determined by internal and external factors. Light is a key Zeitgeber in humans. It can reset the body’s main endogenous pacemaker, the SCN and plays a role in the maintenance of the sleep/wake cycle. Light also has an indirect influence on key processes in the body that control such functions as hormone secretion and blood circulation. In a study by Scott Campbell and Patricia Murphy in 1998 they demonstrated that light may be detected by skin receptor sites on the body even when the same information is not received by the eyes. Fifteen participants were woken at various times and a light pad was shone on the back of their knees. The researchers managed to produce a deviation in the participants’ usual sleep/wake cycle of up to 3 hours. This suggests that light is a powerful exogenous Zeitgeber that need not necessarily rely on the eyes to exert its influence on the brain. As every parent knows, infants are seldom on the same sleep/wake cycle as the rest of the family. In human infants, the initial sleep/wake cycle is pretty much random for younger children. At about 6 weeks of age, the circadian rhythms begin and by about 16 weeks, most babies are entrained. The schedules imposed by parents are likely to be a key influence here, including times for eating and sleeping is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances. In another study done by a French speleologist names Michael Siffre in 1962. He spent 7 months living in total isolation in a subterranean cave, without access to a clock, calendar or the sun. Sleeping and eating only when his body told him to, his aim was to discover how the natural rhythms of human life were to be affected by living “beyond time”. They found that he adjusted to a 25 hour sleep pattern. This supports the idea of exogenous zeitgebers and endogenous pacemakers. Endogenous pacemakers are internal factors that affect the circadian rhythms. These are factors such as the how internal body clocks work to wake us up when there is no light and our body telling us

when to eat even with no clock to know what the time is. Factors which are supported in the cave study as even with no light he was still able to adjust to a sleep schedule even though it was 25 hour rather than 24 hour. Research has revealed there are numerous circadian rhythms in many organs and cells of the body. These are called peripheral oscillators, and are found in the adrenal gland, oesophagus, lungs, liver, pancreas, spleen, thymus and skin. Although these peripheral clocks are highly influenced by the actions of the SCN, they can act independently. Francesca Damiola et al. (2000) demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst still leaving the SCN unaffected. This suggest that there may be many other complex influences on the sleep/wake cycle, aside from the master clock known as the SCN. Particularly in relation to the DeCoursey et al study, ethics is a huge issue. The animals were exposed to considerable harm, and subsequent risk, when they were returned to their natural habitat. Whether what we learn from investigations such as these justifies the procedures involved within these studies can be harmful to animals. Finally there is a huge influence of exogenous zeitgebers may be overstated. Laughton Miles et al. (1977) recounted the story of a young man who was blind from birth with a circadian rhythm of 24.9 hours. Despite exposure to social cues, his sleep/wake cycle could not be adjusted and consequently he had to take sedatives at night and stimulants in the morning to keep pace with the 24-hour world. Similarly, studies of individuals who live in artic regions show normal patterns despite the prolonged exposure to light, this is because the sun does not set during the summer months. Both these examples suggest that there are occasions when exogenous zeitgebers may have little bearing on our internal rhythm....