Endogenous pacemakers and exogenous Zeitgebers

• end - Suprachiasmatic nucleus (SCN)

  • Tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain
  • One of the primary endogenous pacemakers
  • Influential in maintaining circadian rhythms e.g. sleep-wake cycle
  • Nerve fibres connected to eye cross in area called optic chiasm on their way to left and right visual area of cerebral cortex
  • SCN lies just above optic chiasm
• end - SCN's role in regulating sleep-wake cycle


  SCN lies just above optic chiasm

  1. Receives information about light directly from this structure

  2. Continues to receive light information even when eyes are closed

  3. Enables biological clock to adjust to changing patterns of daylight whilst we are asleep
• end - Animal studies - SCN

  Influence of SCN has been demonstrated in studies involving animals - Patricia Decoursey et al destroyed SCN connections in brains of 30 chipmunks who then returned to their natural habitat and observed for 80 days.

  Sleep/wake cycle disappeared and by end of study a significant proportion of them been killed by predators (probs bc awake, active, vulnerable to attack when should have been asleep)
• end - Marin Ralph SCN

  Bred mutant hamsters with a 20hr sleep wake cycle, when SCN cells from foetal tissue of mutant hamsters were transplanted into brains of normal hamsters - cycles of second group defaulted to 20 hours.
• End- Pineal gland and melatonin

  SCN passes info on day length and light it receives to pineal gland (pea like structure in brain just behind hypothalamus). This is another endogenous mechanism guiding the sleep/wake cycle.

  During night pineal gland increases production of melatonin - chemical that induces sleep and is inhibited during periods of wakefulness. Melatonin has also been suggested as causal factor in seasonal affective disorder.
• exogenous Zeitgebers and sleep/wake cycle

  External factors in environment that reset our biological clocks through process known as entrainment

  Seen that in absence of external cues, free running biological clock that controls sleep/wake cycle continues to tick in a distinct cyclical pattern (as in suffer study)

  This free running cycle is then brought into line (entrained) by environmental cues, so there is interaction of internal and external factors.
• Light - Key Zeitgeber

  Can reset the body's main endogenous pacemaker, SCN -thus plays role in maintenance of sleep wake cycle

  Also has indirect influence on key processes in body that control such functions as hormone secretion and blood circulation
• Innovative study - Campbell and Murphy demonstrated light may be detected by skin receptor sites on body even when same info is not received by eyes

  15 ppts woken up at various times and a light pad was shone on back of their knees - researchers managed to produce a deviation in ppts usual sleep wake cycle of up to 3 hrs in some cases

  --> suggests light is a powerful exogenous zeitgeber that need not necessarily rely on the eyes to exert its influence on the brain
• Social cues


  At abt 6 weeks age circadian rhythms begin - by about 16 weeks babies rhythms have been entrained by schedules imposed by parents including adult determined mealtimes and bedtimes.

  Research on jet lag suggests adapting to local times for eating and sleeping (rather than responding to ones own feelings of hunger and fatigue) is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances.
• W:ENDBeyond master clock

  Limitation of SCN research is it may obscure other body clocks - research revealed there are numerous circadian rhythms in many organs and cells in the body - peripheral oscillators found in organs including lungs, pancreas and skin which are influenced by actions of the SCN but also act independently.

  Damiola et al demonstrated how changing feeding patterns in mice could alter circadian rhythm of cells in liver by up to 12 hours whilst leaving the rhythm of the SCN unaffected

  --> suggests other complex influences on the sleep/wake cycle or SCN would've stayed in time
• W: end- interactionist system 


  EP cannot be studied in isolation - total isolation studies eg Siffre are extremely rare - also Siffre made use of artificial light which could have reset his biological clock every time he turned his lamp on

  In everyday life pacemakers and zeitgebers interact and it may make little sense to separate the two for the purpose of research

  --> the more researchers attempt to isolate the influence of internal pacemakers the lower the validity of the research.
• W: exogenous - 


  Exogenous Zeitgebers don't have same effect in all environments

  people who live within Artic circle have similar sleep patterns all year round despite spending around 6 months in almost total darkness

  --> sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light