11 - Endogenous Pacemaker & Exogenous Zeitgebers

• The sleep/wake cycle:

  
  

  • Endogenous Pacemakers:

  (internal body clocks that regulate our biorhythms)

  
  

  • Exogenous Zeitgebers

  (external cues that may affect our biorhythms)
• SCN:


  The suprachiasmatic nucleus is a tiny bundle of cells located in the hypothalamus, which helps maintain circadian rhythms (e.g. sleep/wake cycle). It is one of the primary endogenous pacemakers in mammals. 

  • Nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the visual area of the cerebral cortex.
  • The SCN lies just above the optic chiasm (thus ‘supral’ meaning ‘above’).
• SCN - Part 2:

  • It receives information about light directly from this structure. This continues even when eyes shut, enabling the biological clock to adjust to changing patterns of daylight when we sleep.
  • SCN passes light information to the pineal gland which produces melatonin so it can use this information to increase/decrease production of melatonin dependent on daylight.
  • SCN is a primary endogenous pacemaker - The SCN is a tiny bundle of nerve cells
• Studies:

  • DeCoursey et al (2000)
  • Ralph et al (1990)

  Both Raph and DeCoursey emphasise the role of the SCN in establishing and maintaining the circadian sleep/wake cycle.
• DeCoursey et al (2000):

  • Influence of SCN showed
  • She destroyed SCN connections in brains of 30 chipmunks who were then returned to their natural habitat & observed for 80 days.
  • Found that the sleep/wake cycle disappeared and by the end a significant proportion had been killed by predators because they had been awake and vulnerable to attack - when they should have been asleep.
  • Shows how powerful SCN is at influencing sleep/wake cycle
• Ralph et al (1990):

  • Bred ‘mutant’ hamsters with a 20 hour sleep/wake cycle.
  • When SCN cells from the foetal tissue of mutants were transplanted into brains of normal hamsters, cycles of the 2nd group defaulted to 20 hours.
  • SCN is more powerful than the sun in this.
• Both Ralph & DeCoursey...

  Both Raph and DeCoursey emphasise the role of the SCN in establishing and maintaining the circadian sleep/wake cycle. - done gizmo not TB

  • Studies: Influence of SCN on the sleep/wake cycle demonstrated with chipmunks and hamsters
• Pineal Gland and melatonin are endogenous mechanisms:


  • 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 wakefulness. It has also been suggested as a causal factor in SAD.
• External zeitgebers and the sleep/wake cycle:
• Endogenous pacemakers = CONTROLS 

  Exogenous zeitgebers = ENTRAINS
• External Zeitgebers


  • External environmental cues/factors that may affect our biorhythms that reset biological clocks which is a process known as entrainment (they ENTRAIN our internal biological clock).
  • The German word zeitgebers means ‘time giver’
  • Without external cues, the free-running biological clock continues to ‘tick’ in a cyclical pattern.
  • Zeitgebers reset the sleep/wake cycle (interaction of internal and external factors).
  • Sunlight is the main zeitgeber but social cues can also have a strong effect.
• Sunlight:


  • Main zeitgeber
  • Can reset SCN & indirectly control hormone secretion & blood circulation.
  • Thus it plays a role in the maintenance of the sleep/wake cycle.
  • Bodies main endogenous pacemaker (SCN).
  • Light is a key exogenous zeitgeber that influences the sleep/wake cycle - 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.
• Who & Sunlight?

  • Campbell & Murphy (1998)
• Campbell & Murphy:


  • Campbell & Murphy (1998) woke 15 participants at various times and shone a light pad on the backs of their knees.
  • Producing a deviation in the usual sleep/wake cycle of up to 3 hours in some cases.
  • Demonstrated light may be detected by skin receptors on the body even when same information is not received by the eyes.
  • Fact that skin could detect light shows how powerful light is and does not necessarily rely on eyes to influence the SCN.
  • Suggesting light is a powerful exogenous zeitgeber that need not necessarily rely on the eyes to exert its influence on the brain.
• Social Cues:


  • Social cues also have an important influence on the sleep/wake cycle:
  • Sleep/wake cycle fairly random in human newborns, circadian rhythms start at 6 weeks & are fully entrained by 16 weeks.
  • Schedules imposed by parents are a key influence, including adult-determined mealtimes & bedtimes - Parents likely to be the responsible for entrainment through enforced:
  • Meal & Bed Times
  • Meals eaten at socially determined times & we go to bed & wake up at times designated for our age. Thus our sleep wake cycle could be ENTRAINED by social conventions 
  • KNOWLEDGE HELPFUL IN JETLAG
• Jetlag & Social Cues


  • Research on jet lag shows adapting to local times for eating and sleeping (not responding to one’s one feelings of hunger and fatigue) entrains circadian rhythms and tackles jet lag. 
• Who & Yr - social cues?

  Davidson (2006)

  • Found the zeitgeber for cells in the liver and heart is likely to be mealtimes because these cells are reset by eating.
• Practical Applications:

  Practical Applications:

  • Implications for economy 
  • Help parents
  • Jetlag research