Circadian Rhythms

To help you out on exogenous zeitgebers and endogenous pacemakers

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  • Created by: Beth_Gee
  • Created on: 07-04-14 14:24

What is a circadian rhythm?

1 Cycle that lasts 24 hours - such as the sleep-wake cycle

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Evidence for Endogenous Packemakers

Aschoff (65): 

Student Study, underground bunker for a few weeks with no external stimuli therefore, choose sleep-wake in absense of exogenous zietgebers.

Sleep disrupted --> settled to 24.9 s-w cycle (sleep-wake)

Conclusion; There is a endogenous pacemaker and exogenous zietgebers have an effect on internal rhythms.

CASE STUDIES: Siffre (75), Falkard (96) and Miles et al (77):

S: 6 months in actual cave. c. rythm --> 25-32 hrs S-W cycle. Conclusion: There is a endogenous pacemaker.

F:30hr C.(circadian) rhythm BUT individual differences and genetic influences

M: completely blind since birth --> 24.9 hrs BUT no exogenous zietgebers

-->body clock doesn't reset?

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Evaluation of Evidence

Aschoff: -Only students, not representative of population

    -Ethics - shutting people off from the world for weeks!

Siffre: - CASE STUDY, 1 person. Not representative of the population.

     -Not a mundane situation

     -Supports Aschoff -->  similar findings and same S-W cycle.

     -Ethics - reported depression and suicidal thoughts after 80 days

Folkard and Miles: -CASE STUDY again (see above)

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SCN as the Endogenous Pacemaker


Morgan (95); experiment on hamsters

1. Removing the SCN

Conc: Controls the  C rhythm

2. Transplant foetal hamster --> SCN cells into brain --> regain SCN

Conc: SCN control centre of C.rhythm 

2b. Transplant mutant foetal hamster SCN into damaged hamster brain --> short C.rhythm gained

Conc: Controlled genetically

Silver et al (96) - Supports! Similar findings, other mammals. Increasing reliability 

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Evaluation of Morgan (95)

Morgan (95)

-Ethic issues

BUT cost benefit = cheap

-Nocturnal animal brains are different to humans.

-Hamsters are not rational - they do not choose when they eat/sleep

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How does the SCN control circadian rhythms?

Kalat (98)

Protein synthesis and breakdown = circadian rhythm

Recipe for a circadian rhythm

1. Make protein A, keep producing protein A

2. When threshold reached, stop making protein A

3. Protein A gradually breaks ddown 

4. Low threshold concentration reached, make more protein A

N.B Cells have internal rhythm and affect other brain regions and biochemistry.

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So how does it make us fall asleep?

1. Darkness - exogenous zietgeber from the eyes, via optic nerve chiasm --> darkness, production of SEROTONIN

2. SCN

3.Increase in MELOTONIN

4.Increase in SEROTONIN in pons, specifically in the raphe nucleus (Jouvet '67)

5.Causes RAS to shut down

6.Decrease in activity in celebral cortex

7.Decrease in alertness


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