endogenous pacemakers

• Endogenous pacemakers definition

  • internal biological mechanisms that regulate biological rhythms, particularly the circadian sleep-wake cycle
  • and while they are internal, they can be influenced by external factors
• the SCN is a primary endogenous pacemaker

  • The suprachiasmatic nucleus (SCN) = a tiny bundle of nerve cells in the hypothalamus which helps maintain circadian rhythms (eg sleep/wake cycle)
  • Nerve fibres from the eye cross at the optic chains on their way to the right and left visual fields
  • The SCN lies just above the optic chains and receives information about light from this structure
• influence of SCN on the sleep/wake cycle:

  • DeCoursey et al (chipmunks):
  • Destroyed SCN connections in the brains of 30 chipmunks which were returned to their natural habitat and observed for 80 days
  • Their sleep/wake cycle disappeared and many were killed by predators
  • Ralph et al (hamsters):
  • Bred ‘mutant’ hamsters with a 20-hour sleep/wake cycle
  • SCN cells were transplanted from the foetal tissue of these hampered into the brains of normal hamsters
  • Which then developed cycles of 20 hours
• pineal gland and melatonin are endogenous pacemakers

  • The SCN passes information on day length to the pineal gland which increases production of melatonin during the night
  • melatonin = a hormone that induces sleep and is inhibited during periods of wakefulness (has also been suggested as a causal factor in seasonal affective disorder)
• limitation = SCN research may obscure other body clocks

  • Body clocks (peripheral oscillators) = are found in many organ and cells (eg lungs, skin)
  • They are highly influence by the actions of the SCN but can act independently
  • Damiola et al = showed how changing feeding patterns in mice altered circadian rhythms of cells in the liver for up to 12 hours, leaving the SCN unaffected
  • Suggests there may be other complex influence on the sleep/wake cycle, aside from the master clock (SCN)
• limitation = EPs cannot be studied in isolation

  • Only in exceptional circumstances are EPs ‘free running‘ and unaffected by the influence of exogenous zeitgebers
  • Total isolation studies (eg Siffre’s cave study) are rare
  • In everyday life = pacemakers and zeitgebers interact so it may make little sense to separate the two
  • Suggests the more researchers attempt to isolate the influence of internal pacemakers, the lower the validity of the research
• extra evaluation = ethics

  • Animal studies of the sleep/wake cycle are justified becuase there are similar mechanisms in all mammals - so generalisation can be made to the human brain
  • BUT = a disturbing issue is the ethics involved
  • Animals were exposed to considerable risk in the DeCoursey et al study and most died as a result
  • Suggests that studies like these cannot be justified and researchers should find alternative ways of studying EPs
• strength = real world value for both EPs and EZs

  • Research has become useful in helping people cope with jet lag and seasonal depression (SAD therapy)