Pacemakers and Zeitgebers

• endogenous pacemakers - 'endogenous' refers to anything whose origins are within the organism
• pacemakers are the products of inherited genetic mechanisms and allows us to keep pace with changing cycles in environment
• the most important pacemaker in human beings is the suprachiasmatic nucleus
• suprachiasmatic nucleus (SCN) is a tiny cluster of nerve cells in the hypothalamus
• suprachiasmatic nucleus is the main endogenous pacemaker in mammals
• suprachiasmatic nucleus has a role in generating bodys circadian rhythms
• suprachiasmatic nucleus acts as 'master clock' with links to other brain regions that control sleep arousal and other biological clocks
• there are neurons within the SCN which synchronise with eachother so their target neurons in sites elsewhere in the body receive the correct time coordinated signals
• SCN also regulates manufacture and secretion of melatonin in pineal gland
• exogenous zeitgebers are environmental events that are responsible for entraining biological clock of an organism
• the most important zeitgeber for most animals is light
• receptors within the SCN are sensitive to changes in light levels during the day and they use info to synchronise activity of bodys organs and glands
• the light resets internal biological clock each day where the rods and cones in retina detect light to form visual images
• there is a 3rd light detecting cell in the retina which is a protein called melanopsin that gauges overall brightness to help reset internal biological clock each day
• evaluation - role of SCN - endogenous pacemaker - strength/limit

  • the importance of SCN as endogenous pacemaker has been demonstrated in animal studies
  • morgan bred a strain of hamsters so they had abnormal circadian rhythms of 20 hours - their SCN neurons were transplanted into brains of normal hamsters
  • normal hamsters then displayed same abnormal circadian rhythms of 20 hours - showing transplanted SCN had imposed its pattern onto receipts brain
  • this further justices research but can be limitation due to animal studies
• evaluation - separate rhythms

  • under normal conditions the 'master clock' coordinates our bodily rhythms but in some circumstances they can become out of step with each other
  • folkand studied UNI student who volunteers to spend 25 days in controlled environment
  • during time she had no access to daylight or zeitgebers - end of 25 days her core temp rhythm was still 24 hours but sleep wake cycle had extended to 30 hours
  • outlines how there must be separate rhythms
• evaluation - role of SCN - melanopsin

  • research support for role of melanopsin in setting circadian rhythm from study of blind people
  • some blind people are able to reliably retain circadian rhythms in response to light despite lack of image forming visual perception - skene and arendt estimate majority of blind subjects who still have light perception have normally entrained circadian rhythms
  • suggests pathway from retinal cues containing melanopsin to SCN is still intact
  • provides evidence for importance of this pathway in setting biological clock
• evaluation - artificial light as Zeitgeber

  • vetter et al investigated importance of light in regulation of sleep wake and activity rest patterns of volunteer groups over 5 week study period
  • one group remained in normal 'warm' artificial lighting and others were in artificial 'blue enriched' light - all participants kept daily log and wore devices to measure movement
  • participants working in warm light synchronised circadian rhythms with natural light each day but blue light group did not show adjustment and syncronised rhythms to office hours - showing dominant zeitgeber for SCN is light