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Endogenous Pacemakers, Exogenous Zeitgebers Essay Plan
1. Describe in detail endogenous pacemakers- suprachiasmatic nucleus (SCN) in the
hypothalamus, SCN obtains information about light from the eye via the optic nerve, happens
even when eyes are shut, as light penetrates the eyelids. If endogenous clock is running
slowly, morning light automatically shifts the clock ahead, making the rhythm correct with the
world outside again. SCN- pair of structures- one in each hemisphere of the brain, and each
of these is divided into a ventral and dorsal SCN. Albus found that the ventral SCN is relatively
quickly reset by external cues, but the dorsal SCN is much less affected by light and more
resistant to being reset. SCN sends signals to the pineal gland making it produce melatonin at
night, melatonin creates sleep because it inhibits the brain mechanisms that promote
2. Describe in detail exogenous zeitgebers- resetting the biological clock (entrainment). Light
resets the SCN, also resets other oscillators located throughout the body as the protein CRY,
part of the protein clock is light sensitive. Darlington- morning two proteins bind together
CLOCK and CYCLE, once joined, CLK-CYC produce two more proteins, PERIOD and TIME
(PER-TIM). PER-TIM makes CLK-CYC inactive, as PER-TIM increases CLK-CYC decreases and
PER-TIM decreases as well (takes about 24 hours).
3. Critical Point 1: This explanation for the role of the Suprachiasmatic Nucleus is reliable.
Morgan bred "mutant" hamsters so they had circadian rhythms of 20 hours instead of 24
hours and then transplanted their SCN's into normal hamsters. He found that the normal
hamster displayed the mutant rhythms. Therefore, this implies there is strong evidence to
support the theory.
4. Critical Point 2: However, there is a methodological flaw to the majority of the studies
carried out on this topic. This is they have been performed on non-human animals. As a result
of this, it is difficult to generalise the results displayed by the non-human animals to humans.
This is because human beings are far more complex creatures and also systems differ from
animal to animal. Also there are serious ethical issues to consider when talking about
non-human animals. Although, if we acknowledge that this research does have important
applications for human behaviour, then the harm to animals might be considered acceptable
as long as the key ethics of non-human animal research remain.
5. Critical Point 3: This explanation has a flaw. Folkard observed a women for 25 days living in a
cave. He found that her temperature rhythm was a 24 hour one, but the sleep rhythm was a
30 hour one. Therefore, this implies that body's separate oscillators can desynchronise which
can lead to symptoms comparable to jet lag.
6. Critical Point 4: This theory has good evidence. Campell and Murphy found that if you shine a
light on the back of the knee, then their circadian rhythms will shift. Consequently, this proves
the theory useful as it demonstrates proof that light is the main exogenous zeitgeber.
7. Critical Point 5: There is a negative consequence from the research into the power of
artificial lighting. Stevens implies that the exposure to artificial lighting disrupts circadian
rhythms and therefore disrupts melatonin production. This could essentially explain why
women in industrialised (and well-lit) societies are more likely to develop breast cancer. This
is a positive application because it means that we can warn people about the dangers of
having lots of lights on all the time.
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Critical Point 6: There is a negative implication of having a biologically determined system.
This is when it fails, it could cause many problems. Chicurel discovered one example of this is
Familial Advanced Sleep-Phase Syndrome (FASPS) which has been linked with an inherited
defect in one of the PER genes. It seems to cause sleep onset at around 7pm and
spontaneous awakening at around 2am in affected family members, and so they have a
great difficulty leading a normal life.…read more