Endogenous Pacemakers and Exogenous Zeitgebers

• What is an Endogenous Pacemaker?

  Our internal body clock
• What are Exogenous Zeitgebers?

  • External time cues that our bodies can detect and use to keep the internal clock in sync with the outside world. 
  • This process is known as entrainment.
• What is the main Exogenous Zeitgeber?

  Light
• What can also act as an Exogenous Zeitgeber?

  • Social Cues
  • Schedules such as set meal times and bedtimes help newborns settle into a circadian rhythm
  • Adapting to local times for eating and sleeping beats jet lag.
• What is our Internal Body Clock?

  The Superchiasmatic Nucleus
• What is the Superchiasmatic Nucleus?

  • A tiny bundle of nerve cells
  • Located in the hypothalamus in each hemisphere
  • Maintains circadian rhythms such as the sleep wake cycle
  • The SCN lies just above the optic chasm
  • It receives information about light even when eyes are closed
• Describe DeCoursey et al animal study:

  • Destroyed the SCN connections in the brains of 30 chipmunks
  • They 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 amount were killed by predators
  • Due to being awake and active when they should have been asleep
• Describe Ralph's animal study:

  • Bred mutant hamsters with a 20 hour sleep wake cycle
  • SCN foetal tissues were transplanted into the brains of normal hamsters
  • The cycle defaulted to 20 hours
• The Pineal Gland and Melatonin:

  • The SCN passes the information on day length and light it receives to the pineal gland
  • During the night the pineal gland increases the production of melatonin - a chemical that induces sleep and is inhibited during periods of wakefulness
• Campbell and Murphy on Exogenous Zeitgebers:

  • Light may be detected by skin receptors even when not received by the eyes
  • 15 Ps were woken at various times and a light pad was shone on the back of their knees
  • Manages to produce a deviation in the Ps usual sleep wake cycle by up to 3 hours
• Limitation of EZ's and EP's - Ethical Issues:

  • Animals were used to study the sleep wake cycle which could lead to their death or serious harm
  • E.g. DeCoursey et al and Ralph et al
• Strength of EZ's and EP's - Practical Application:

  Knowledge of the importance of light in sleeping and wakefulness has lead to useful advice and products.
• Evaluation of EZ's and EP's - Multiple body clocks:

  • Numerous cells in the body have their own circadian rhythms – known as “peripheral oscillators”.
  • They can act independently of the SCN.
  • There may be many complex influences on the sleep/wake cycle.
• Limitation of EZ's and EP's - How important are Exogenous Zeitgebers?

  • People in the arctic regions (where the sun does not set in the summer) show normal sleep patterns.
  • Case studies such as Siffre show that we have a relatively normal sleep/wake cycle without EZs