Discuss the Role of Endogenous Pacemakers and Exogenous Zeitgebers.

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There are 3 biological rhythms in the body. Circadian, which is our body clock which works to a 24-hour cycle and regulates our bodies biochemical, physiological and behavioural processes. Ultradian processes, which are smaller processes that happen inside our 24-hour circadian cycle. These Ultradian cycles are the sleep stages lasting 90-120 minutes, however some other ultradian cycles include hormone release, heart rate, nostril dilation and appetite. Lastly there is Infradian which are outside our circadian 24-hour cycle such as the menstrual cycle and hibernation. These cycles are endogenously controlled, however can be tainted or ‘entrained’ to the environment by exogenous factors. Endogenous pacemakers are biological pacemakers inside us that regulate our cycles. An endogenous pacemaker is the Suprachiasmatic nucleus located in the hypothalamus. It is situated directly above the optic chiasm (allowing it to respond to light) and it responsible for controlling circadian rhythms. At the presence of light, it stimulates the pineal gland to release melatonin. Melatonin is a naturally occurring hormone that promotes sleep. A higher level of melatonin will promote sleep, and thus a lower level will help us wake up and stay alert. It’s this process that endogenously regulates our circadian sleep-wake cycle. The SCN’s influence on our bodies has been demonstrated in Morgans animal study. He bred ‘mutant’ hamsters with entrained ’20-hour’ circadian rhythms. When their SCN was transplanted into their ‘normal’ hamsters they exhibited the mutant rhythm. This shows the role the SCN plays in our circadian cycles, showing it regulates our sleep patterns and that rhythm entraining is done through the SCN. However, this argument can be deeply flawed. The SCN affects other circadian rhythms such as hormone release and these could affect sleep itself as well as individual differences; therefore it may be deterministic to conclude that the SCN is the regulator of sleep....
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