Endogenous pacemakers vs exogenous zeitgebers

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Laura Muth

Discuss the role of endogenous pacemakers and exogenous zeitgebers biological rhythms.

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Endogenous pacemakers and exogenous zeitgebers are used to control biological rhythms. The external environmental cues such as light or temperature are exogenous zeitgebers whereas the internal biological clocks that pace brain structures are endogenous pacemakers.

In mammals, the main endogenous pacemaker is the suprachiasmartic nucleus (SCN), which is a small group of cells located in the hypothalamus. The SCN obtains information about lights from the optic nerve. Evidence comes from studies of animals, for example, Morgan (1995) bred mutant hamsters so their circadian rhythm was 20 hours instead of 24 hours and then transplanted their SCN’s into normal hamsters. He found that the normal hamsters displayed the mutant rhythms of 20 hours, showing that the SCN controls the sleep/wake cycle in hamsters and therefore the role of endogenous pacemakers are very important. More evidence for the importance of the SCN comes from DeCoursey who removed the SCN in chipmunks and then returned them to their natural habitat. They found that significantly more of these chipmunks were killed by weasels, presumably because these chipmunks remained awake in their burrows and weasels could hear them. This shows that the SCN plays an important role as a pacemaker controlling biological rhythms such as the sleep/wake cycle. 

However there are several disadvantages with Morgan and DeCoursey’s research. One is the issue of harm to animals involved in the research, for example in DeCoursey’s research, most of the chipmunks died therefore there are ethical issues to consider Also as the research is based on animal studies , we cannot apply these findings to humans and therefore the experiments lack ecological validity as we cannot generalise the findings to humans. This is because humans are more complex organisms with different physiology so what affects chipmunks and hamsters may not affect us. As we do not know that systems differ from one animal to the next, we cannot solely rely on findings from animal studies but check any animal findings against research with humans. Also, the SCN seems to be the main clock, but the body temperature rhythm continues to exist when the SCNs are removed, suggesting another clock.

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 Endogenous rhythms are also related to the pineal gland. The pineal gland also contains light sensitive cells so when light levels drop, melatonin is produces and as light levels increase melatonin production is inhibited. Melatonin caused the raphe nucleus to produce serotonin which in turn inhibits the activity of the reticular activating system. This mechanism is involved in wakefulness when it shuts down; nerve signals to the limbs are repressed so movement decreases and a sleep state are reached. 

Supporting evidence comes from Potocki who found that levels of melatonin are inverted for those who suffer from difficulty to fall asleep. Taking ...

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