Endogenous Pacemakers & Exogenous Zeitgebers

Flash cards for Exo and Endo 

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Endogenous Pacemakers AO1

  • Main endogenous is suprachiasmatic nucleus
  • Found in hypothalamus
  • Information about light 
  • Light penetrates eyelids
  • SCN is a pair of structures
    • Ventral SCN quickly reset by external cues
    • Dorsal SCN not quite so quickly reset (ALBUS)
  • SCN sends signals to pineal glad to increase melatonin supply
    • Induces sleep-like feeling as inhibits mechanisms that contribute to feeling awake 
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Endogenous Pacemakers AO2 + IDA

  • MORGAN - Mutant hamsters bred with circadian rhythms of 20 hours & transplanted SCN into normal hamsters. Normal hamsters displayed mutants rhythms
    • Cannot generalise as animal study
  • KATE ALDCROFT - separate oscillators (separate SCNs) can desynchronise, had temperature cycle of 24 hours yet sleep rhythm of 30 hours (FOLKARD)
  • IDA POINT - Evolutionist approach emphasises adaptive nature of system controlled by endogenous pacemakers
    • ANTON - circadian clocks allow tight temporal scheduling of physiological and behavioural programmes
    • DeCOURSEY - animals anticipate daily environmental patterns (mutant chipmunks, significantly more than control groups had been killed by weasels as awake in burrows when would've been asleep)
    • A lot of research is done on animals (Ethically issues, problems with generalising) 
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Exogenous Zeitgebers AO1

  • Affected by external factors
  • Light is dominant zeitgebers
  • Can reset other oscillators, not just SCN
    • Cryptochrome (protein clock) is very sensitive
  • CAMPBELL AND MURPHY - can reset circadian rhythms by shining light on back of patients knee
  • Social Cues (ie, eating)
  • Previously thought to be main zeitgeber as rhythms appeared to be entrained by social convention
  • DAVIDSON - Zeitgeber for cells in liver and heart likely to be mealtimes, cells are reset by eating
  • Can be entrained by temperature as well in warm blooded animals (BUHR)
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Exogenous Zeitgebers AO2

  • Artifical lighting affects biological rhythms, as does dim, though bright is more effective (BOIVIN)
  • STEVENS - living in artificially lit world can have negative consequences. Disrupts melatonin production, ergo explaining why women in industrialised environment are more likely to develop breast cancer
  • When biological system fails, multitude of problems, ie. Sleep-phase syndrome (CHICUREL, link to defect in PER gene)
    • Suggestion that can occur in adolescent years, which explains irregular sleep patterns then
  • Biological clock is really blend of exo and endo factors (not two systems, not such neat divisions)
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