Outline and evaluate the role of endogenous pacemakers and of exogenous zeitgebers in the control of circadian rhythms (8+16)
Circadian rhythms are biological rhythms that occur every 24 hours, an example of a circadian rhythm is the sleep-wake cycle, another circadian rhythm is body temperature which is at its lowest at around 36 degrees at night and it peaks in the morning at 38 degrees. Endogenous pacemakers are internal body 'clocks' that will modulate biological rhythms, the SCN is an example of an endogenous pacemaker, it controls our body temperature cycle. Circadian rhythms can also be controlled by exogenous zeitgebers which are external stimuli such as light and temperature.
Free running is used to describe biological rhythms in the absence of exogenous zeitgebers, this free running cycle is controlled by an endogenous pacemaker working as an internal body clock such as the SCN. The SCN is in the hypothalamus and will receive signals from the retina on the light levels, as the light levels increase the SCN will send out signals which will travel to the pineal gland, the pineal gland responds by switching off production of melatonin but when it is dark the bodies level of melatonin increases making the person feel drowsy because melatonin stimulates the raphe nucleus which produces serotonin which causes sleep. Siffre (1975) spent 6 months in an underground cave without natural light (an example of free running), the cave was artificially lit. They found that his circadian rhythms had increased to 25-32 hours, when Siffre came out of the cave on the final day he though that only 151 days had past but as a matter of fact it was the 179th day this could suggest that even without zeitgebers our endogenous pacemakers can still modulate biological rhythms
Research into endogenous pacemakers has shown that the SCN play a key role in regulating our circadian rhythms. Stephan and Zucker (1972) destroyed the SCN inside of rats and compared them with a...
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