endogenous pacemakers and exogenous zeitgebers Flashcards
endogenous pacemakers and the sleep/wake cycle
the suprachiasmatic nucleus
tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain
one of the primary endogenous pacemakers in the mammalian species
influential in maintaining circadian rhythms such as the sleep/wake cycle
nerve fivers connected to the eye cross in an area called the optic chias on their way to the left and right visual area of the cerebral cortex
the SCN lies just above the optic chias
it receives info about light directly from this structure
continues even when our eyes are closed enabling the biological clock to adjust to changing patterns of daylight when we are asleep
animal studies and the SCN
the influence of the SCN has been demonstrated in studies involving animals
DeCoursey et al destroyed the SCN connection in the brian’s of 30 chipmunks who were then returned to their natural habitat and observed for 80 days
the sleep/wake cycle of the chipmunks disappeared and by the end of the study a significant proportion of them had been killed by predators
another study done by Ralph et al bred mutant hamsters with a 20 hour sleep/wake cycle
when SCN cells from the foetal tissues fo the mutant hamsters were transplanted into the brian’s of normal hamsters the cycles of the second group defaulted to 20 hours
the pineal gland and melatonin
SCN passes the information on day length and light that it receives to the pineal gland
this is another endogenous mechanism guiding the sleep/wake cycyke
during the night the pineal gland increases production of melatonin - a chemical that induces sleep and is inhibited during periods of wakefulness
melatonin has also been suggested as a causal factors in seasonal affective disorder
limitation
beyond the master clock
limitation of SCN research is that it may be obscure other body clocks
research has revealed that there are numerous circadian rhythms in many organs and cells in the body
these peripheral oscillators are found in the organs including the lungs pancreas and skin
they are influenced by the action of the SCN but also act independently
Damiola et al 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 rhythms of the SCN unaffected
suggest other complex influences on the sleep/wake cycle
limitation
interactionist system
endogenous pacemakers cannot be studied in isolation
total isolation studies such as Siffres cave study is extremely rare
Siffre made use of artificial light which could have resets his biological clock every tune he turned his lamp on
in everyday life pacemakers and zeitbergers interact and it may make little sense to separate the two for the purpose of research
suggests the more researchers attempt to isolate the influence of internal pacemakers the lower the validity of the research
exogenous zeitgebers and the sleep/wake cycle
exogenous zeitgebers are external factors in the environment that research out biological clocks through a process known as entertainment
in the absence of external cues the free-running biological clock that control the sleep/wake cycle continues to tik in a distant cyclical pattern
this free-running cycle us then brought into line by environmental cues so there is an interaction of internal and external factors
light
light is a key zeitgeber in humans
can reset the body’s main endogenous pacemaker the SCN and thus plays a role in the maintenance of the sleep wake cycyle
light also has an indirect influence on key processes in the body that control such functions as hormones secretion anf blood circulation
Innovative study - Campbell and Murphy demonstrated that light may ve detected by skin receptor sites on the body even when the same information is nit received by the eyes
15 participants were woken at various times and a light pas was shone on the back of their knees
researchers manages to produce a deviation in the participants usual sleep/wake cycle of up to three hours
suggest that light is a powerful exogenous zeitgeber that need not necessarily rely on the eyes to exert its influence on the brain
social cues
babies are seldom on the same sleep/wake cycle are they rest of the family
newborn babies initial sleep.wake cycyle is pretty much random
at about 6 weeks of age the circadian rhythms being anf by about 16 weeks babies rhythms have been entertained by schedules imposed by parents
including adult determined mealtimes and bedtimes
research on jet lag suggest that adapting to local times for eating and sleeping is an ffecitve way of entraining circadian rhythms and beating jet lag when travelling long distances
limitation
environmental observations
exogenous zeitgebers do not have the same effect in all environments
experiences of people who live in places where there is very little darkness in summer and very little light in winter show a different story
example - people who live within the artic circle have similar sleep patterns all year round despite spending around six months in almost total darkness
suggest that the sleep/wake cycyle is primarily controlled by endogenous pacemakers that can override environmental changes in light
limitation
case study evidence
evidence challenges the role of exogenous zeitgebers
Miles et al recount the study of a young blind man from birth who has an abnormal circadian rhythm of 24.9 hours
despite exposure to social cues such as regular mealtimes his sleep/wake cycyle could not be adjusted
suggest that social cues are not effective in resetting the biological rhythm
