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