Biological rhythms

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  • Biological rhythms
    • Endogenous pacemakers
      • Definition
        • Internal mechanisms that govern biological rhythms (circadian sleep/wake cycle).
      • Example
        • Biological clock left to its own devices without the influence of external stimuli (light).
    • Exogenous zeitgebers
      • Definition
        • External changes in the environment responsible for resetting the biological clock of an organism.
      • Example
        • Effect of daylight: we feel drowsy when its night-time and alert during the day.
      • suprachiasmatic nucleus (SCN)
        • Endogenous pacemakers
          • Definition
            • Internal mechanisms that govern biological rhythms (circadian sleep/wake cycle).
          • Example
            • Biological clock left to its own devices without the influence of external stimuli (light).
        • Main endogenous pacemaker (master clock)
        • Controls other biological rhythms, as it links to other areas of the brain responsible for sleep and arousal.
        • The SCN also receives information about light levels (an exogenous zeitgeber) from the optic nerve.
          • Sets the circadian rhythm so that it is in synchronisation with the outside world, e.g. day and night.
        • Example of how melatonin works.
          • 1. Low levels of light (retina)
            • 2. Melanopsin (protein in eye) carries signal to SCN.
              • 3. Axon pathway to pineal gland.
                • 4. Leads to an increase in the production of melatonin (at night).
                  • 5. Helps to induce sleep.
      • Studies
        • Aschoff and Wever (1976)
          • Procedure
            • A group of pp's spent 4 weeks in a WW2 bunker, which was deprived of natural light.
          • Aim
            • Examine how light influences free-running biological rhythms.
          • Findings
            • All but one pp displayed a circadian rhythm between 24 and 25 hours.
              • Suggests that natural sleep/wake cycle is about 24 hours but is influenced by exogenous zeitgebers (no.of daylight hours).
        • Siffre (1970s)
          • Procedure
            • Siffre spend long periods in a dark caves: Alps and Texas.
          • Findings
            • In each case study Siffre's free-running circadian rhythm settled down to 25 hours.
              • Suggests that our internal body clock is very effective in sticking to a 24 hour day.
          • Aim
            • Examine effects of free-running biological rhythms.

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