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The role of endogenous pacemakers and exogenous zeitgebers in controlling circadian rhythms
Internal clocks
• Endogenous pacemakers
• Pace all animals and humans over a 24 hour clock
External clock
• As seasons change body clocks need to be reset or organisms will never adjust
• To reset environmental EXOGENOUS cues are used E.G. light
• These are called exogenous zeitgebers
• Synchronisation also know as entrainment
Endogenous pacemakers WHEN?
• Genetic mechanism
• Unborn foetus
• Must respond to zeitgebers
• Siffre- free running body clock settles at 25 hours
• Ticking body clock due to protein
Endogenous pacemakers WHERE?
• SCN- super chiasmatic nuclei • Tiny cluster of nerve cells called SCN in the
hypothalamus • SCN sits on both sides of the hemisphere (dorsal and
ventral) above optic nerve • Ventral is quickly reset by EXTERNAL CUES like light• Dorsal is more resistant to being reset• SCN gets information of light from the optic nerve • Light can penetrate through the eyelids and special
photo receptors pick it up • SCN synchronises to a 24 hour clock
Evidence of the SCNANIMAL
• Morgan- mutant hamsters bred on 20 hour cycle transplanted into normal hamsters –hamsters take on this clock- shows SCN as a pacemaker
• Stephan and Zucker- removing SCN from rats and losing cycle
Evidence for SCNHUMAN
• Friedman- proved at pathway called the retinohypothalamic tract between retina and SCN
• Fulton and bailey- people with brain tumours damaging the SCN causing sleep/waking disorders
Advantages of endogenous pacemaker studies
• Supports SCN and role of light
• Suggests the disruption of artificial light
• human studies
• Campbell and murphy- knee cap light-including blind people – individual differences
disadvantages of endogenous pacemaker studies
• A lot of research via animals
• Animal studies not generalisable
• Biological differences between animals and humans
• Is it ethical ? Removing animals survival mechanism
• Mainly biological explanations and ignoring evolutionary therefore reductionist
• Doesn’t explain the link of hibernation and temperature
• Ignores individual differences such as; stress, anxiety, sleeping disorders, social differences
Other body clocks
• Folkard- Kate Aldercroft spent 25 days in a cave
• Body temperature at 24 hour clock
• Sleep altered to 30 hour clock
• Gives support of more than on clock
• Temperature clock possibly near to the SCN
Melatonin
• SCN sends information to the pineal gland
• Detection of light slows the production of melatonin = wakefulness
• Low light allows more melatonin production = sleep
HORMONE OF DARKNESS
Sunrise
Enter eyes
Photoreceptors pick up light
Carried along optic nerve
SCN
Pineal gland
Decrease melatonin
Wakefulness
Evidence for melatonin
• Smith-magenis syndrome – inverted melatonin levels- melatonin supplement at night causes sleep – support the role of melatonin
Criticism of melatonin
• Sackett and Korner – believed that Siamese twin s would have the same circadian rhythm due to the same melatonin levels but this is not correct
• Luce and segal- people living in the artic sleep normally even during the summer – melatonin doesn’t cause sleepiness – sleep waking cycle is endogenous
• Massively over simplistic
Exogenous zeitgebers
• External stimuli
• Change slowly unless large change E.G. jet lag and shift work
• E.G. light, social cues , temperature
Entrainment
• When the body clock is reset by external cues
• Opposite of entrainment is free funning rhythms whereby the clock doesn’t change due to exogenous cues
• entrainment works best when the the change isnt large
• Hard to entrain due to jet lag and shift work
Synchronising with external cues
• Jet lag is worse west to east PHASE ADVANCE due to the body clock being behind time
• Shift work leads to a permanent state of jetlag increasing stress and decreasing performance
• Czeisler –workers at a chemical plant on short change overs suffered with health problems
When changes from 7- 21 day rotations occurred performance improved and health problems disappeared
Light
• Stopping melatonin = wakefulness
• Hall – light sensitive proteins throughout the body also detect light
• Campbell and murphy- shining light on back of the knees increase wakefulness
• Miles - Light is important due to cases of blind people, one man blind from birth could shift his 24.9 hour clock even with exogenous cues E.G. time – there fore light is very important
Evidence against light
• Submariners live by 18 hour artificial light but stay at a 24 hour clock
• Free running rhythm stronger than light
Social cues
• Zeitgeber for circadian rhythm
• Things expected in society :
• Time we eat
• Go to bed
• Wake up
Evidence for social cues
• Entrainment of the menstrual cycle in groups by pheromones leading to birth at the same time and shared childbearing
Temperature
• Can be entrained by temperature even though there is no evidence for this
• Temperature is the onset of hibernation
• Winter becomes darker therefore the limited light may be the only factor not temperature
Overall advantages
• Animals rely entirely environmental cues E.G. light and without and endogenous pacemaker would sleep all winter and be awake all summer
• Exogenous zeitgebers allow individual change E.G. going across time zones
• Decoursey- destroying SCN made chipmunks awake and night and more likely to get attacked –this links to evolution
• Without both everyone would have different clocks from there birth entrainment wouldn’t occur
Overall Disadvantages
• You can’t change your biological rhythm E.G travelling across time zones
• Impossible to override
• Can influence E.G. eating food at the socially expectable time in the country, not going to sleep at usual time but going to sleep at a time expectable to that country
• Stevens- if light resets our clock artificial light will become more and more damaging