Endogenous pacemakers and exogenous zeitgebers

• Endogenous vs exogenous

  Endogenous Pacemakers

  Part of the internal organism. Internal body clocks that regulate biological rhythms such as the sleep wake cycle. 

  Exogenous Zeitgebers

  External cues in the environment that may affect our biological rhythms. E.g. the influence of light on the sleep/wake cycle.
• The Suprachiasmatic Nucleus (SCN)

  In mammals, the main 

  endogenous pacemaker is a 

  pair of tiny clusters of nerve 

  cells called the SCN, which lie 

  in the hypothalamus, just 

  above the optic chiasm 

  (where the optic nerves from 

  each eye cross over)
• Endogenous pacemakers

   Even when our eyes are shut the SCN gets info on light from the optic nerve.  Light can penetrate the eyelids and special photoreceptors in the eye transfer light signals to the SCN.

  If our endogenous clock is running slow – the morning light automatically shifts the clock ahead so it is in synchrony with the world outside
• The pineal gland

  Another endogenous pacemaker that works with the SCN is the pineal gland

  • Contains light sensitive cells.
  • When light is sensed melatonin production is inhibited.
  • When light level falls melatonin production increases

  This induces sleep by inhibiting brain mechanisms that promote the awake state.
• Animal studies and the SCN

  Ralph et al (1990) Mutant hamsters

  Ralph bred ‘mutant’ hamsters with a 20 hour sleep/wake cycle. 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.
• Exogenous zeitgebers

  Exogenous = outside the organism

  Zeitgeber = time-giver

  The biological-clock is reset each day by cues in the environment – like the light cues of sunrise and sunset.

  This process is called entrainment, (the opposite of free-running where the biological clock works free of any exogenous cues, including social cues like clocks).
• Light is a key zeitgeber for humans!

  It has been shown to reset the main endogenous pacemaker and have an indirect effect on processes implicated in hormonal responses and blood circulation.
• Exogenous zeitgebers - Light

  Campbell and Murphy (1998)

  Suggested that light may be detected by skin receptor cells on the body even when this light is not received by the eyes.

  • 15 participants were woken at various times during the night, and a light pad was shone on the back of the knees.
  • The researchers managed to produce a deviation of 3 hours in the participants’ usual sleep/wake cycle in some cases.
  • This suggests that light is a powerful exogenous zeitgeber even without the influence of the eyes on the brain!
• Social Cues!

  In infants the sleep/wake cycle is initially random but by 6 weeks the circadian rhythms begin and by 16 weeks most babies are entrained. 

  Why? Schedules and social cues determined by the parents – e.g. meal times and bed times. This is also applied to adults. Rather than responding to hunger and fatigue, conforming to local times for sleeping and eating will train your circadian rhythm (good for jet lag!)