Exogenous Zeitgebers and endogenous pacemakers essay

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Laura Muth
Discuss the role of endogenous pacemakers and exogenous zeitgebers biological rhythms.
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
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.
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
Supporting evidence comes from Potocki who found that levels of melatonin are inverted for those
who suffer from difficulty to fall asleep. Taking supplements of melatonin at night can help with
symptoms, thus supporting role of melatonin in the sleep/wake cycle. As this research is based on
humans, it is likely to be valid and reliable.
Exogenous zeitgebers are time-givers that are externally caused and entrain biological rhythms. The
process of resetting the biological clock with exogenous zeitgebers is known as entrainment. The

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Laura Muth
opposite of entrainment is `free-running' where the biological clock operates in the absence of any
exogenous cues.
Until fairly recently, biologists thought that social cues were the main zeitgebers for human circadian
rhythms due to us eating meals at socially determined mealtimes and going to bed and waking up at
times designated times as appropriate for our age. Our daily rhythms appeared to be entrained by
social convention, not internal biology.…read more

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Laura Muth
have replicated these studies and were not able to replicate these findings and so there is dispute to
as how reliable his theory is.
We need pacemakers, otherwise there would be problems. If animal's rhythm solely relied on
environmental cues we might sleep most of the day in winter exogenous zeitgebers allow animals to
respond to changes in the environment, without them an animal might be awake at unsuitable times.…read more


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