Endogenous Pacemakers and Exogenous Zeitgebers Flashcards
What are Endogenous Pacemakers
Internal body clocks that regulate many of our biological rhythms
Such as the influence of suprachiasmatic nucleus on the sleep/wake cycle
What are Exogenous Zeitgebers
External cues that may affect or entrain our biological rhythms
Such as the influence of light on the sleep/wake cycle
EP - The Suprachiasmatic Nucleus
A tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain
It receives information about light directly from its structure
This continues, even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep
One of the primary endogenous pacemakers in mammalian species
Influential in maintaining circadian rhythms, such as the sleep/wake cycle
Nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the visual area of the cerebral cortex
The SCN lies just above the optic chiasm
EP - Animal Studies and the SCN
DeCoursey et al (2000)
Destroyed the SCN connections in the brains of 30 chipmunks and then returned them to their natural habitat
They were observed for 80 days and found that the sleep/wake cycle of the chipmunks disappeared
by the end of the study, a significant proportion of the chipmunks had been killed by predators, presumably because they were awake and vulnerable to attack when they should’ve been asleep
Ralph et al (1990)
Bred mutant hamsters with a 20 hour sleep/wake cycle
When SCN cells from the foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters, the cycles of the second group defaulted to 20 hours
Both studies emphasise the role of the SCN in establishing maintaining the circadian sleep/wake cycle
EZ - Light
Can reset the bodies main endogenous pacemaker (the SCN) and thus plays a role in the maintenance of the sleep/wake cycle
Has an indirect influence on key processing the body that control functions such as hormone secretion and blood circulation
Campbell and Murphy (1998)
Demonstrated that light may be detected by skin receptor sites on the body, even when the same information is not received by the eyes
15 participants were woken at various times, and a light pad was shown on the back of their knees
Researchers managed to produce a deviation in the participants usual sleep/wake cycle up to 3 hours in some cases
Suggests light is a powerful exogenous zeitgeber that did not necessarily rely on the eyes to exert its influence on the brain
EX - Social Cues
In human infants, the initial sleep/wake cycle is usually random
At about six weeks old, the circadian rhythms begin
At about 16 weeks old, most babies are entrained
Schedules imposed by parents are likely to be a key influence here, including adult-determined meal times and bedtimes
Research suggests adapting to local times for eating and sleeping is an effective way of entraining circadian rhythms and beating jetlag when travelling long distance
Evaluation (WEAKNESS) - Beyond the Masterclock
Research revealed there are numerous circadian rhythms in many organs and cells in the body
These are called peripheral oscillators and are found in the adrenal gland, oesophagus, lungs, liver, pancreas, spleen, thymus and skin
Damiola et al (2000)
Demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the SCN unaffected
Suggest there may be many other complex influences on the sleep/wake cycle aside from the master clock (SCN)
Evaluation (WEAKNESS) - Ethics in Animal Studies
DeCoursey et al
Animals were exposed to considerable harm and subsequent risk when they were returned to their natural habitat
Evaluation (WEAKNESS) - Influence of Exogenous Zeitgebers may be overstated
Miles et al (1977) -
Recount the story of a young man, blind from birth with a circadian rhythm of 24.9 hours
Despite exposure to social cues, his sleep/wake cycle could not be adjusted, and consequently he had to take sedatives at night and stimulants in the morning to keep pace with the 24 hour world
Studies of individuals who live in arctic regions, show normal sleep/wake patterns, despite the prolonged exposure to light
Suggest there are occasions when exogenous zeitgebers may have a little bearing on our internal rhythm
Evaluation (WEAKNESS) - Methodological Issues in Studies
Findings from the Campbell and Murphy study have yet to be replicated
Other psychologists have been critical of the manner in which the study was conducted, and have suggested there may have been some limited light exposure to the participants eyes which is a major confounding variable
Isolating one exogenous
zeitgeber in this way does not give us insight into the many other zeitgebers that influence the sleep/wake cycle and the extent to which these may interact
Evaluation (WEAKNESS) - Interactionist System
Only an exceptional circumstances are endogenous pacemakers free-running and unaffected by the influence of exogenous zeitgebers
Total isolation studies, such as Siffre’s cave study are extremely rare and could be judged as lacking validity
In real life, pacemakers and zeitgebers interact and it may make little sense to separate the two for the purpose of research
