Endogenous pacemakers and exogenous zeitgebers Flashcards
(14 cards)
what are endogenous pacemakers?
Internal body clocks that regulate many of our biological rhythms such as the influence of our SCN on sleep wake cycle
What are exogenous zeitgebers?
External factors that affect or entrain our biological rhythms such as influence of light on sleep wake cycle
what is the sleep wake cycle?
a daily cycle of biological activity based on a 24 hour period (circadian rhythm)
it’s nfluenced by variations in the environment such as the alteration of night and day
where is the SCN?
Suprachiasmatic nucleus
Tiny bundle of nerves in hypothalamus in each hemisphere of brain. It is one of primary endo pacemakers in humans, influential in maintaining circadian rhythms. SCN lies just above optic chiasm, it receives information about light directly from this structure. Continues when our eyes are closed enabling biological clock to adjust to changing patterns of daylight whilst we are asleep.
What animal studies correlate with SCN?
Decoursey et al (2000) destroyed SCN connections in 3 chipmunk who then returned to natural habitat and observed for 80 days. The sleep wake cycle of the chipmunks had disappeared and most had been killed by predators.
Ralph et al (1990) bred mutant hamsters with a 20 hour sleep/wake cycle. when SCN cells from foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters, the normal hamsters defaulted to a 20 hour cycle
what role does the pineal gland have?
SCN passes info light that it receives to the pineal gland. pineal gland is another endogenous mechanism guiding sleep/wake cycle
during night, pineal gland increases production of melatonin - a chemical that induces sleep and is inhibited during periods of wakefulness
melatonin has been suggested as casual factor is SAD
evaluation of endogenous pacemakers (brief)
weakness - SCN research may obscure other body clocks
weakness - endo pacemakers cant be studied in isolation
evaluation of endogenous pacemakers
SCN research may obscure other body clocks
Research revealed numerous circadian rhythms in many organs & cells. Found in lungs, pancreas, skin. They are highly influenced by actions of SCN but also act independently. Damiola et al demonstrated how changing feeding patterns in mice can alter circadian rhythms of cells in liver by up to 12 hours, whilst leaving rhythm of SCN unaffected. Suggests there may be other complex influences on sleep/wake cycle aside from master clock (SCN).
endogenous pacemakers cannot be studied in isolation. total isolation studies such as Siffre’s cave study are extremely rare. Siffre also made use of artificial light which may have reset his biological clock. in everyday life, pacemakers and zeitgebers interact and it makes little sense to separate the two for the purpose of research. this suggests that the more researchers attempt to isolate the influence of internal pacemakers, the lower the validity of the research.
What are two exogenous zeitgebers?
Light
Social cues
How is light an exogenous zeitgeber?
It resets the body’s main endo pacemaker, SCN and thus plays a role in maintenance of sleep wake cycle. Light has an indirect influence on hormone secretion and blood circulation.
What did Campbell and Murphy do?
They demonstrated that light may be detected by skin receptors sites on the body even when the same information isn’t received by the eyes. 15 p’s were woken at various times and the researchers shone a light pad on the back of their knees. The researchers managed to produce a deviation in the p’s usual sleep/wake cycle of up to three hours.
suggests light is powerful exogenous zeitgeber that doesn’t need to rely on eyes to exert its influence on the brain
How are social cues an exogenous zeitgeber?
A newborns circadian rhythm is entrained by the schedules imposed by parents e.g. adapts to parents meal and bed times
research on jet lag suggests adapting to local times for eating and sleeping is an effective way of entraining circadian rhythms and beating jet lag
evaluation of exogenous zeitgebers (brief)
weakness - zeitgebers dont have same effect in all enviros
weakness - opposing evidence, Miles et al, blind man
evaluation of exogenous zeitgebers
exogenous zeitgebers don’t have the same effect in all environments. for instance, people who live within the Arctic Circle (sun doesn’t set in summer); show normal sleep patterns all year round despite prolonged exposure to light. This suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light
There is evidence to challenge the role of exogenous zeitgebers.
Miles et al recounted the study of a young man blind from birth with an abnormal 24.9 hour circadian rhythm. Despite exposure to social cues such as regular mealtimes, his sleep/wake cycle could not be adjusted. This suggests that social cues alone are not effective in resetting the biological rhythm
