Endogenous pacemakers and exogenous zeitgebers

• What is endogenous pacemakers?

  Internal body clocks that regulate many of our biological rhythms, such as the influence of the suprachiasmatic nucleus (SCN) on the sleep/wake cycle.
• What is exogenous zeitgebers?

  External factors that affect or entrain our biological rhythms, such as the influence of light on the sleep/wake cycle.
• What is the sleep/wake cycle?

  A daily cycle of biological activity based on a 24-hour period (circadian rhythm) that is influenced by regular variations in the environment, such as the alternation of night and day.
• Limitation of SCN research ?

  P- may obscure other body clocks.

  E- Research revealed that there are numerous circadian rhythms in many organs and cells in the body. These peripheral oscillators are found in the organs including the lungs, pancreas and skin and are influenced by the actions of the SCN, but also act independently. Damiola et al. demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected.

  L- suggests other complex influences on the sléep/wake cycle.
• limitation of endogenous pacemakers ?

  P- cannot be studied in isolation.

  E- Total isolation studies like Siffre's cave study are extremely rare. 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.

  L- suggests the more researchers attempt to isolate the influence of internal pacemakers, the lower the validity of the research.
• Limitation of exogenous zeitgebers ?

  P- do not have the same effect in all environments.

  E- The experience of people who live in places with very little darkness in summer and very little light in winter tell a different story from the usual narrative. For instance, people who live within the Arctic Circle have similar sleep patterns all-year round, despite spending around six months in almost total darkness.

  L- suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light.
• Another limitation of exogenous zeitgebers ?

  P- evidence challenges the role of exogneous zeitgebers.

  E- Miles et al. recount the study of a young man, blind from birth, who had an abnormal circadian rhythm of 24.9 hours. Despite exposure to social cues, such as regular mealtimes, his sleep/wake cycle could not be adjusted.

  L- suggests that social cues alone are not effective in resetting the biological rhythm.
• What is the suprachiasmatic nucleus (SCN) ?

  It is a tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain. one of the primary endogenous pacemakers in mammalian species and is influential in maintaining circadian rhythms.

  Neve fibres connected to the eye cross in the optic chiasm on their way to the left and right visual area of the cerebral cortex The SCN lies just above the optic chiasm, receives information about light directly from this structure.
• What was DeCoursey eat al study?

  destroyed the SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days, The sleep/wake cycle of the chipmunks disappeared and by the end of the study a significant proportion of them had been killed by predators
• What was Ralph et al study?

  Bred 'mutant hamstets with a 20-hour sleep/wake cde.

  When SCN cells from the foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters, the cycles of the second group defaulted to 20 hours.
• What was Campbell and Murphy‘s study?

  demonstrated that light may be detected by skin receptor sites on the body even when the same information is not received by the eyes.

  Fifteen participants were woken at various times and a light pad was shone on the back of their knees. The researchers managed to produce a deviation in the participants' usual sleep/wake cycle of up to three hours in some cases! This suggests that light is a powerful exogenous zeitgeber that need not necessarily rely on the eyes to exert its influence on the brain.