endogenous pacemakers and exogenous zeitgebers

1. The role of endogenous pacemakers
and exogenous zeitgebers in
controlling circadian rhythms

2. Internal clocks
• Endogenous pacemakers
• Pace all animals and humans over a 24 hour
clock

3. 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

4. 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

5. 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

6. Evidence of the SCN
ANIMAL
• 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

7. Evidence for SCN
HUMAN
• 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

8. 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

9. 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

10. 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

11. 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

12. Sunrise
Enter eyes
Photoreceptors pick up light
Carried along optic nerve
SCN
Pineal gland
Decrease melatonin
Wakefulness

13. Evidence for melatonin
• Smith-magenis syndrome – inverted
melatonin levels- melatonin supplement at
night causes sleep – support the role of
melatonin

14. 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

15. Exogenous zeitgebers
• External stimuli
• Change slowly unless large change E.G. jet lag
and shift work
• E.G. light, social cues , temperature

16. 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

17. 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

18. 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

19. Evidence against light
• Submariners live by 18 hour artificial light but
stay at a 24 hour clock
• Free running rhythm stronger than light

20. Social cues
• Zeitgeber for circadian rhythm
• Things expected in society :
• Time we eat
• Go to bed
• Wake up

21. Evidence for social cues
• Entrainment of the menstrual cycle in groups
by pheromones leading to birth at the same
time and shared childbearing

22. 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

23. 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

24. 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