Endogenous Pacemakers and Exogenous Zeitgebers

    Cards (17)

    • endogenous pacemakers
      mechanisms within the body that govern the internal, biological bodily rhythms
    • exogenous zeitgebers
      an environmental cue that helps to regulate the biological clock in an organism
    • two examples of endogenous pacemakers
      • the suprachiasmatic nucleus (SCN)
      • the pineal gland
    • two examples of exogenous zeitgebers
      • light
      • social cues
    • entrainment
      when our biological-clock is reset each day by cues in the environment (e.g. the light cues of sunrise and sunset)= opposite of free-running (where our biological clock works free of any exogenous cues)
    • suprachiasmatic nucleus (SCN)
      • a tiny cluster of nerve cells which lie in the hypothalamus
      • also known as the ‘master clock’
      • plays an important role in controlling out sleep-wake cycle
      • has its own built-in circadian rhythm which controls other biological clocks in the body
      • its circadian rhythm needs resetting and interacts with light in order to do this
      • receives light through the optic chiasm (even when our eyes are shut as penetrates the eyelids)
      • if out biological clock is running slow morning light automatically adjusts the clock to the rhythm of the outside world
    • the pineal gland
      • the SCN also regulates the manufacture and secretion of melatonin in the pineal gland
      • melatonin induces sleep in the body
      • the pineal gland is a pea-like structure in the brain just behind the hypothalamus
      • the SCN sends signals to the pineal gland directing it to either increase or decrease the levels of melatonin in the body
      • at night-time it will tell the pineal gland to increase the production and secretion of melatonin, whereas in the morning, as light levels increase the production and secretion of melatonin by the pineal gland will decrease
    • exogenous zeitgebers detailed 

      our endogenous pacemakers are reset each day by cues in the environment (e.g. light cues of sunrise and sunset). this is called entrainment. as demonstrated in Siffre’s cave study, if we did not have exogenous cues our free-running circadian rhythm continues to tick in a cyclical pattern of 25 hours. exogenous zeitgebers and everyday cues throughout the day help to entrain our circadian rhythm within 24 hours. our sleep-wake cycle is determined by an interaction of internal factors (endogenous pacemakers) and external factors (exogenous zeitgebers)
    • what morgan (1995) shows evidence for 

      endogenous pacemaker- SCN
    • morgan (1995) PROCEDURE 

      bred a strain of hamsters so that they had abnormal circadian rhythms of 20 hours rather than 24 hours
      1. SCN neurons from these abnormal hamsters were transplanted into the brains of normal hamsters
      2. he then transplanted SCN neurons from normal hamsters into the brains of abnormal hamsters
    • morgan (1995) FINDINGS
      • the normal hamsters then displayed the same abnormal circadian rhythm of 20 hours
      • rather than maintain their abnormal circadian rhythm the abnormal hamsters then changed to a circadian pattern of 24 hours
    • what campbell and murphy (1998) shows evidence for 

      exogenous zeitgeber- light
    • campbell and murphy (1998) PROCEDURE 

      15 participants were woken at various times and a light pad was shone on the back of their knees
    • campbell and murphy (1998) FINDINGS 

      this produced a deviation in their usual sleep-wake cycle of up to 3 hours in some cases. light is a powerful exogenous zeitgeber that does not always rely on the eyes to exert its influence on the brain
    • what klein and wegmann (1974) shows evidence for 

      exogenous zeitgeber- social cues
    • klein and wegmann (1974) PROCEDURE
      got air travellers to either venture out more when they arrived at their destination or stay inside to see whether this would influence their adjustment to the time zone
    • klein and wegmann (1974) FINDINGS 

      found that those that ventured out more adjusted more quickly to the time zone of their destination because they were exposed to the social cues of their new time zone, which acted as a zeitgeber