Endogenous pacemakers and Exogenous zeitgebers

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  • Endogenous Pacemakers and Exogenous Zeitgebers
    • Endogenous pacemakers
      • Suprachiasmatic Nucleus
        • Main endogenous pacemaker
        • Located in hypothalamus
        • Obtains information about light via the optic nerve in the eye.
        • If our endogenous clock is running slow the morning light will shift it forward so it is in time with the world outside.
        • The role of the SCN has been demonstrated.
          • Morgan (1995) bred "mutant" hamsters so they had circadian rhythms of 20 hours instead of 24. They then transplanted the mutant SCN into normal hamsters and found that the the normal hamsters then displayed the mutant rhythm.
            • This shows that the SCN is genetic.
        • There are implications of using animals to test for the suprachiasmatic nucleus.
          • First of all, the animals could  have been harmed during the experiment and the researcher would not know how much harm the animal is coming to since they are not able to communicate with humans effectively. Second, the evidence that was collected is difficult to generalise to humans as we are a completely different species.
      • The pineal gland and Melatonin
        • SCN sends signals to the pineal gland directing it to increase production of melatonin at night.
        • Melatonin induces sleep.
    • Exogenous Zeitgebers
      • Light
        • Dominant zeitgeber in humans
        • Can reset the body's main pacemaker.
        • Can also reset other pacemakers throughout the body.
          • May explain why Campbell and Murphy found that if you shine a light on the back of participants knees it shifted their circadian rhythms.
        • Artificial lighting may also act as a zeitgeber.
          • Recent research has shown that artificial lighting does have an effect. Boivin et al found that circadian rhythms can be entrained by ordinary dim lighting although bright light was more effective.
            • However, if dim lighting does reset the biological clock then it may have some negative effects since we live in a world of artificial lighting. Stevens suggested that exposure to artificial lighting disrupts circadian rhythms .
              • This shows that artificial lighting does reset the biological clock and act as a zeitgeber but it does come with negative effects.
      • Social Cues
        • We eat meals at socially determined times.
        • We also go to bed and wake up at times that we think are appropriate for our age.
        • The zeitgeber for cells in the liver and heart is likely to be mealtimes because these cells are reset by eating.
      • Temperature
        • Leaves on trees change colour and drop off because of the change in temperature
        • In the absence of light temperature may be the dominant zeitgeber.
      • When the biological system fails many problems occur.
        • Familial advanced sleep-phase syndrome has been linked to an inherited defect in one of the PER genes. this disorder causes sleep onset at around 7:00 PM and spontaneous awakening.
          • This shows that a small malfunction or mutation in the biological clock can cause many problems to occur that may stop people from leading a normal life.


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