Endogenous Pacemakers and Exogenous Zeitgebers

• The suprachiasmatic nucleus is a tiny cluster of nerve cells in the hypothalamus
• The SCN is the primary endogenous pacemaker in mammals
• The SCN is influential in maintaining circadian rhythms, such as the sleep/wake cycle
• Nerve fibres are connected to the eye cross in the optic chiasm
• The SCN lies above the optic chiasm and receives info about light from this structure
• Decoursey et al (2000) aimed to study the influence of the SCN in chipmunks
• Decoursey et al destroyed the SCN in 30 chipmunks and returned them back to their natural habitats for 80 days
• Decoursey et al found that the chipmunks had been killed due to their sleep/wake cycles being destroyed
• Decoursey et al concluded that no SCN equals no biological rhythms
• Ralph et al (1990) aimed to study the influence the SCN has in hamsters
• Ralph et al took the SCN out of genetically abnormal hamsters with a circadian cycle of 20 hours and transplanted them into hamsters with the usual 24 hour cycle
• Ralph et al found that the normal hamster's cycles shortened to 20 hours following the transplant
• Ralph et al concluded that the SCN is key in establishing sleep/wake cycles
• The SCN signals the pineal gland to produce melatonin at night to induce sleep
• Light and social cues help regulate our biological rhythms through entrainment
• Light can reset the body's endogenous pacemaker
• Campbell and murphy (1998) aimed to investigate how powerful light is as an exogenous zeitgeber and whether it can be detected by an other way besides the eyes
• Campbell and Murphy woke 15 participants up at various times by shining light at the back of their knees
• Campbell and Murphy found that there were changes in the sleep/wake cycle (up to 3 hours difference) by shining a light on the back of their knees
• Campbell and murphy concluded that light is a powerful exogenous zeitgeber, even when it isnt processed by our eyes, it can still influence our brain and SCN
• Social stimuli (meal times and social activities) can act as zeitgebers
• Aschoff et al (1971) showed that individuals can compensate for the absence of zeitgebers like natural light by responding to social zeitgebers
• Klein and Wegmann (1974) found that the circadian rhythms of air travelers can be adjusted quicker if they go outside more at their destination
• In infants the sleep/wake cycle is initially random. By 6 weeks the circadian rhythms begin and by 16 weeks most babies are entrained due to parents introducing social cues
• It may be the case that endogenous pacemakers and exogenous zeitgebers are an interactionist system
• LIMITATIONS:

  • too simplistic to suggest the SCN is the only endogenous pacemaker
  • case study evidence that challenges the role of exogenous zeitgebers (Miles et al (1977))
• STRENGTHS

  • practical application- jet lag