Circadian Rhythms

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  • Created by: rhallett
  • Created on: 11-10-15 10:18
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  • Circadian Rhythms
    • Approx. 24 hrs, inc sleep/wake cycle - 8 sleep 16 awake; regulated by the interaction of two things:
    • Endogenous Pacemakers; biological 'clocks' mainly in the brain
      • Main EP; small area in the hypothalamus named SCN. Neurons have an in built circadian firing patter which continues even when removed from the brain
        • A direct pathway links the SCN to the retina of the eye and its activity is regulated by light. The SCN regulates the secretion of melatonin in the pineal gland.
          • Melatonin is a hormone that helps induce sleepiness and is secreted during darkness; daylight suppresses the production of melatonin
    • Exogenous Zeitgebers; external factors like light; reset our biological clocks
      • In the morning, our clocks reset because the light causes the SCN to reduce melatonin production from the pineal gland
        • Zeitgebers are required to 'entrain' our rhythm to a regular 24 hr cycle, without light the rhythm gets longer and out of sync with the environment
    • Research; a lot shows the rhythms are regulation of EPs and EZS
      • Michel Siffre; 6 months in cave in Texas, isolated from external stimuli, his sleep/wake cycle settled down to about 24.5 hrs - EPs alone can control circadian rhythms but EZs needed to keep them to a 24 hr cycle
        • Case Study; difficult to generalise, he was exposed to dim light; could affect circadian rhythm as a result its unclear what this type of research shows
      • Aschoff & Weaver; RPs in a war bunker, isolated from external cues, sleep/wake cycle settled down to about 25 hrs - EPs alone can control the S/W cycle
        • More reliable than Siffre; a number of RPs in a more controlled and scientific environment
      • AR; Morgan; bred mutant hamsters with rhythms of 20 hours, when transplanted their SCNs to normal hamsters they developed the same rhythm
        • AR; Stephen & Zucker; destroyed the SCNs of rats and this disrupted their rhythms of drinking and activity; shows SCNs are important in animals
          • Decoursey; destroyed chipmunks SCNs, rhythms disrupted - suggests animals have evolved to respond to zeitgebers as it is adaptive to have rhythms that are influenced by external factors; were noisy and active when they were vulnerable to predators like weasles


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