Discuss the ROLE of exogenous zeitgebers and endogenous pacemakers in biological rhythms

Revision and essay plan for psychology A2 biological rhythms

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Discuss the ROLE of exogenous zeitgebers and endogenous pacemakers in biological
AO1 ­ Endogenous pacemaker
­ In circadian rhythms ­ SCN, Pineal gland, melatonin
­ In Infradian rhythms ­ Pituitary gland, oestrogen, progesterone
AO2 ­ Research evidence to support
­ Siffre [cave]
­ Miles [Blind man]
­ DeCoursey et al [Chipmunks SCN removed]
­ Reductionist
­ Describe HOW they showed the endogenous pacemaker ­ link internal clock
AO1 ­ Exogenous zeitgebers
­ Describe what they are ­ external in environment / influence rhythms / examples
AO2 ­ Research evidence to support
­ Russell [pheromones]
­ Campbell & Murphy [light on knee]
­ Stephen & Zucker [damaged SCN in rats]
AO1 ­ Endogenous pacemaker
1] In Circadian rhythms
Some of our biological rhythms are set by genetically determined biological structures
and mechanisms within the body.
There is a free running cycle controlled by an endogenous pacemaker working as a
body clock which is internal to our biological system
For CIRCADIAN RHYTHMS the suprachiasmatic nucleus [SCN], part of the
hypothalamus, seems to act as an internal clock to keep the body on an approximate
24hour sleep/wake cycle.
It is sensitive to light and dark which regulates the pineal gland, which releases
melatonin [a hormone which seems to induce sleep]
When there is less light, more melatonin is produced. When there is more light, less
melatonin is produced and waking occurs.
Melatonin causes the body to produce serotonin, a chemical which slows the central
nervous system causing us to become sleepy.
2] In Infradian rhythms
In INFRADIAN RHYTHMS the endogenous pacemakers are hormones such as
oestrogen and progesterone.
The Pineal gland releases hormones which cause one ovary to ripen an egg which
triggers the release of oestrogen.
Progesterone is then secreted causing the lining of the womb to prepare for pregnancy.
About 2 weeks after ovulation, if there is no pregnancy, progesterone is reduced,
causing the womb lining to shed.
AO2 ­ Endogenous pacemaker

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- Siffre ­ First cave study found that his sleep/wake cycle continued to maintain a
sleep/wake cycle of around 24hours despite having no external cues such as a clock or
This supports that there is an internal control of the circadian rhythm, since even in the
absence of external cues we are able to maintain a regular daily cycle.…read more

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Campbell & Murphy ­ Found that shining a light on the back of participant's knees
shifted their circadian rhythms. This supports that the exogenous zeitgeber of light as
having an influence biological rhythms. However there was a very small sample size and so
the findings cannot be reliable in generalising to the population.
- Siffre ­ Siffre' second cave study into circadian rhythms can also be seen to provide
evidence for exogenous zeitgebers.…read more


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