endogenous pacemakers and exogenous zeitgebers

• endogenous pacemakers

  internal body clocks that regulate many of our biological rhythms - such as the influence of the suprachiasmatic nucleus on the sleep/wake cycle
• exogenous zeitgebers

  external factors that affect or entrain our biological rhythms such as the influence of light on the sleep wake cycle
• sleep/wake cycle

  a daily cycle of biological activity based on a 24-hour period that is influenced by regular variations in the environment
• endogenous pacemakers and the sleep/wake cycle - the suprachiasmatic nucleus

  • tiny bundle of nerve cells located in the hypothalamus in each hemisphere
  • maintains circadian rhythms (sleep/wake cycle)
  • nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the left and right visual area of the cerebral cortex
  • SCN lies above the optic chiasm
  • receives information about light
  • carries on while eyes closed - enables biological clock to adjust to changing patterns of daylight whilst asleep
• DeCoursey and suprachiasmatic nucleus

  • DeCoursey - destroyed the SCN connections in the brain of 30 chipmunks who were then returned to their natural habitat and observed for 80 days
  • sleep/wake cycle disappeared and by the end of 80 days most had been killed by predators
• Ralph and suprachiasmatic nucleus

  • Ralph - bred mutant hamsters with a 20 hour sleep/wake cycle
  • when SCN cells from the foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters - defaulted to 20 hours
• the suprachiasmatic nucleus - the pineal gland and melatonin

  • SCN passes information on day length and light that it receives to the pineal gland
  • the pineal gland increases the production of melatonin - induces sleep
  • factor of seasonal affective disorder
• exogenous zeitgebers and the sleep-wake cycle

  • external factors in the environment that reset our biological clocks through entrainment
  • in the absence of external cues the free-running biological clock that controls the sleep/wake cycle continues to tick in a distinct cyclical pattern
  • environmental cues entrain the free-running cycle
• exogenous zeitgebers - light

  • resets the SCN and plays a role in the maintenance of the sleep/wake cycle
  • indirect influence on key processes in the body that control such functions as hormone secretion and blood circulation
• exogenous zeitgebers - light

  Campbell and Murphy - light may be detected by skin receptor sites on the body even when the eyes do not receive the same information

  • 15 ppts were woken at various times and a light pad shone on the back of their knees
  • researchers managed to produce a deviation in the ppts usual sleep/wake cycle of up to 3 hours
  • light powerful exogenous zeitgeber that doesn't rely on eyes
• exogenous zeitgebers - social cues

  • newborn babies' initial sleep/wake cycle is random
  • at 6 weeks of age = circadian rhythms begin
  • 16 weeks = babies rhythms have been entrained by the schedules imposed by parents - mealtimes and bedtimes
• exogenous zeitgebers - social cues

  jet lag: adapting to local times for eating and sleeping is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances
• two exogenous zeitgebers for the sleep wake cycle

  • light
  • social cues
• AO3 endogenous pacemakers - beyond the master clock

  • research revealed that there are numerous circadian rhythms in many organs and cells in the body
  • influenced by actions of the SCN but also act independently
  • Damiola - changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours - SCN unaffected
  • there are other influences on sleep wake cycle
• AO3 endogenous pacemakers - interactionist system

  • cannot be studied in isolation
  • total isolation - Siffre - rare
  • siffre made use of artificial light - may of reset biological clock
  • in everyday life pacemakers and zeitgebers interact - makes little sense to separate the two
  • isolate - lowers validity
• AO3 exogenous zeitgebers - environmental observations

  • do not have the same effect in all environments
  • people who live in places where there is little darkness in summer and little light in winter
  • Artic circle - similar sleep patterns all-year round despite spending 6 months in darkness
  • sleep/wake cycle is primarily controlled by endogenous pacemakers that override environmental changes
• AO3 exogenous zeitgebers - case study evidence

  • Miles- young man, blind from birth, abnormal circadian rhythm of 24.9 hours
  • despite exposure to social cues his sleep/wake cycle could not be adjusted
  • social cues alone are not effective in resetting the biological rhythm