Endogenous pacemakers and exogenous zeitgebers

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  • Endogenous pacemakers and the sleep/wake cycle:
  • The Suprachiasmatic nucleus (SCN) is a tiny bundle of nerve cells in the hypothalamuswhich helps maintain circadian rhythms (e.g. sleep/wake cycle). Nerve fibres from the eye cross at the optic chiasm on their way to the visual cortex. The SCN lies just above the visual cortex and recieves information about light from this structure. 
  • DeCoursey et al destroyed SCN connections in the brains of 30 chipmunks which were returned to their natural habitst and observed for 80 days. Their sleep/wake cycle disapperaed and many were killed by predators. Ralph et al bred 'mutant' hamsters with a 20-hour sleep/wake cycle. SCN cells were transported from the foeral tissue of these hamsters into the brains of normal hamsters, which then developed cycles of 20 hours. 
  • The SCN passes information on day length to the pineal gland which increases production of melatonin during the night. Melatonin is a hormone that induces sleep and is inhibited during periods of wakefullness. It has also been suggested as a casual factor in seasonal affective disorder (SAD). 
  • Exogenous zeitgebers and the sleep/wake cycle:
  • The german word zeitgeber means 'time giver'. Resetting biological clocks is a process known as entrainment. Without external cues, the free-running biological clock continues to 'tick' in a cyslical pattern. Zeitgebers reset the sleep/wake cycle: an interaction of internal and external factors. 
  • Light can reset the body's main endogenous pacemaker (SCN), and also has an indirect influence on key processes in the body controlling hormone secretion, blood circulation, etc. Campbell and Murphy woke 15 participants at various time during the night and shone a light on the backs of their knees - producing a deviation in the sleep/wake cycle of up to 3 hours. Light is…

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