Discuss the role of endogenous pacemakers in the control of one or more rhythms (24 marks)

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Marco Ball-Albarran
Discuss the role of endogenous pacemakers in the control of one or more rhythms
(24 marks)
Endogenous pacemakers are internal biological clocks that manage our rhythms. In humans, the main
biological clock is the suprachiasmatic nucleus (SCN) which is located in the hypothalamus and controls
circadian rhythms.
The SCN is located above the optical chiasm. Light enters into the retina and is transferred through
the optical nerve to the SCN which stimulates the release of hormones such as cortisol and melatonin
in the pineal gland, thus maintaining our sleep-wake cycle.
It has been suggested that endogenous pacemakers have an adaptive value in controlling our
biological rhythms such as the sleep-wake cycle, suggesting it aids survival.
This is shown through research by DeCoursey (1998) who found that removing the SCN in 30
chimpmunks and returning them to their natural habitat proved costly for their survival. After 80 days
it was found that significantly more chipmunks without an endogenous pacemaker had died as
opposed to a control group, therefore suggesting that the SCN aids survival by maintaining circadian
rhythms alongside environmental cues- for instance going to sleep when predators such as foxes are
out.
Furthermore, research has assessed the effects of isolating participants from their external time cues
(exogenous zeitgebers) on 24 hour cycles, and suggesting that an internal body clock (endogenous
pacemakers) creates a free-running cycle of 25 hours.
This has been carried out by Siffre who reported a case study of his own experiences in an
underground cave for 2 months. Without any external zeitgebers to guide him, his circadian
slee-wake cycle generally adjusted to 25 hours, though sometimes changing dramatically up to 48.
Similarly, Ascoff and Weaver also designed a temporal isolation study by placing participants in a
WW2 bunker without any environmental cues. They found that a free-running cycle persisted with a
sleep/wake cycle of 25 hours, therefore stressing the existence of an endogenous pacemaker to
control circadian rhythms.
However, a weakness of these pieces of research is that they can be said to lack internal validity.
Although natural light sources had been removed by hiding underground, artificial light from torches
and cameras have been shown to reset endogenous pacemakers, therefore counting as
unaccounted extraneous variables.
This is exemplified through Czeisler et al (1999) who altered participants' circadian rhythms down to
22 hours and up to 28 using only dim lighting to alter the release of melatonin from the pineal gland.
As a result, the findings lack internal validity making them less supportive of the role of endogenous
pacemakers.
Similarly, Siffre's study is criticised on being a case study which reflects only his own personal
individual differences, such as his motivation to be isolated and bodily history, therefore the findings
cannot be generalised across to the wider public.
In addition, there is a view that endogenous pacemakers are limited in their control of circadian
rhythms (such as the sleep wake cycle) and that exogenous zeitgebers are needed to keep them in
line with the ever changing environment. For example, Miles et al (1977) found that a man who was
blind from birth had a circadian rhythm of 24.9, which suggests that light is a central exogenous

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Marco Ball-Albarran
zeitgeber which is important in reducing the natural 25 hour rhythm to 24 hours and work alongside
endogenous pacemakers.
On the other hand, sleep-wake studies supporting the existence of endogenous pacemakers have
reported a consistent finding of 25 hours; Miles found a blind man who had a circadian rhythm of 24.9
and Ascoff and Weaver reported findings of 25 hours in the absence of light.…read more

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