Endogenous pacemakers & exogenous zeitgebers Flashcards
Suprachiasmatic nucleus (SCN)
Endogenous pacemakers
Tiny bundle of nerve cells located in hypothalamus in each hemisphere of brain
Primary endogenous pacemakers in mammalian species & is influential in maintaining circadian rhythms
Nerve fibres connected to eye cross in area called optic chiasm on their way to left & right visual area of cerebral cortex
SCN lies above optic chiasm
Receives info about light directly from this structure
This continues even when our eyes are closed, enabling bio clock to adjust to changing patterns of daylight whilst we sleep
Animal studies (DeCoursey)
Endogenous pacemakers
DeCoursey et al destroyed SCN connections in brains of 30 chipmunks who were then returned to their natural habitat & observed for 80 days
S/w cycle of chipmunks disappeared & sig proportion were killed by predators
Animal studies (Ralph)
Endogenous pacemakers
Ralph et al bred ‘mutant’ hamsters w/ 20 hour s/w cycle
When SCN cells from foetal rissue of mutant hamsters were transplanted into brains of normal hamsters, cycles of 2nd group defaulted to 20 hours
The pineael gland & melatonin
Endogenous pacemakers
SCN passes info on day length & light that it recieves to pinael gland (another endogenous pacemaker guiding s/w cycle)
During the night, pineal gland increases production of melatonin
Melatonin induces sleep & is inhibited during periods of wakefulness
Melatonin has been suggested as causal factor in SAD
Evaluation: Beyond the master clock
Endogenous pacemakers
Limit: SCN research may obscure other body clocks
Research revealed there are numerous circadian rhythms in many organs & cells in body
These peripheral oscillators are found in organs including lungs, pancreas & skin
They are influenced by actions of SCN but also act independently
Damiola et al demonstrated how changing feeding patterns in mice could alter circadian rhythms in liver by up to 12 hours, whilset leaving ehythm of SCN unaffected
Therefore, other complex influences on s/w cycle
Evaluation: Interactionist system
Endogenous pacemakers
Limit: endogenous pacemakers cannot be studied in isolation
Total isolation studies (i.e Siffre) are extremely rare
Also Siffre made use of artificial light which could have reset his biological clock
In everday life, pacemakers & zeitgebers interact & it may make little sense to separate the two for research purposes
Therefore, the more researchers attempt to isolate the influence of internal pacemakers, the lower the validity of research
Light
Exogenous zeitgebers
Can reset SCN and thus plays role in maintenance of s/w cycle
Light has indirect influence on key processes in body that control functions like hormone secretion
Campbell & Murphy demonstrated that light may be detected by skin receptor sites on body even when same info is not received by eyes
15 ppts woken at various times & a light was shone on back of their knees
The researchers managed to produce a deviation in ppts usual s/w cycle of up to 3 hours
Suggests light is powerful exogenous zeitgeber that need not necessarily rely on eyes to exert its influence on the brain
Social cues
Exogenous zeitgebers
Every parent knows, babies are rarely on same s/w cycle as rest of family
Newborn’s inital s/w cycle is random
At about 6 weeks, the circadian rhythms begin & by about 16 weeks, babies rhythms have been entrained by schedules imposed by parents, including adult-determined mealtimes & bedtimes
Research on jet lag suggests that adapting to local times for eating & sleeping is an effective way of entraining circadian rhythms & beating jet leg
Evaluation: Environmental observations
Exogenous zeitgebers
Limit: don’t have same effect in all environments
Experience of people who live in places where there is little darkness in summer tell a diff story from usual narrative
People who iive within Arctic Circle have similar sleep patterns all-year round, despite spending around 6 months in almost total darkness
Therefore, s/w cycle is primarily controlled by endogenous pacemakers that cna override environmental changes in light
Evaluation: Case study evidence
Exogenous zeitgebers
Limit: evidence challenges role of exogenous zeitgebers
Miles et al recount study of young man, blind from birth, who has abnormal circadian rhythm of 24.9 hours
Despite exposure to social cues, such as regular mealtimes, his s/w cycle could not be adjusted
Therefore, social cues alone aren’t effective in resetting biological rhythm
