Endogenous Pacemakers and Exogenous Zeitgebers

• Endogenous pacemakers and internal body clocks that regulate many of our biological rhythms, such as the influence of the superchiasmatic nucleus (SCN) on the sleep/ wake cycle
• Exogenous zeitgebers are external cues that may affect or entrain our biological rhythms, such as the influence of light on the sleep/wake cycle
• The sleep/wake cycle is a daily cycle of biological activity base on a 24-hour period that is influenced by variation in the environment such as night and day
• Circadian rhythm is a type of biological rhythm, subject to 24-hour cycle, which regulates a number of body processes such as the sleep/wake cycle
• Suprachiasmatic nucleus (SCN):

  • Main endogenous pacemaker for sleep and arousal (in hypothalamus)
  • Receives information about light levels (exogenous zeitgebers) from optic nerve
  • Sends information to pineal gland, leads to increase in melatonin production at night, inducing sleep and supresses melatonin production during the day
• Influence of SCN on sleep/wake cycle (DeCoursey et al., 2000):

  • Destroyed SCN connection in the brains of 30 chipmunks, returned to their natural habit and observed for 80 days
  • Sleep/wake cycle of chipmunks disappeared, significant number killed by predators
• Influence of SCN on sleep/wake cycle (Ralph et al., 1990):

  • Bred 'mutant' hamsters with a 20-hour sleep/wake cycle
  • SCN cells from foetal tissue of mutant hamsters transplanted to normal hamsters
  • Sleep/wake cycle defaulted to 20 hours
• Endogenous Pacemakers:

  • Light- cells in the retina of the eye detect light intensity, small number of retinal cells contain the protein melanopsin which carries signals to the SCN to set the daily body clock
  • Sleep/wake cycle of most blind people is still influenced by light due to existing connections between the eye and SCN that do not involve the visual system; but blind people experience circadian desynchrony
• S- Importance of SCN (Endogenous PM):

  Ralph et al. bred hamsters to have a circadian rhythm of 20 hours; SCN neurons from abnormal hamsters were transplanted into the brain of normal hamsters; displayed the same abnormal circadian rhythm of 20 hours, showing transplanted SCN has imposed its pattern onto hamsters. Demonstrates the significance of SCN and how endogenous pacemakers are important for biological circadian rhythms.
• W- Animal studies (Endogenous PM):

  Ralph et al.'s research is flawed because of the use of hamsters. Humans would respond differently to manipulation of biological rhythms, not only biological differences, but differences between environmental contexts. Research on animals is unable to explain roles of endogenous pacemakers in the biological processes of human
• W- Ethical issues (Endogenous PM):

  DeCoursey et al.'s chipmunks were exposed to considerable harm and subsequent risk. Significant amounts of chipmunks were killed as a result of being returned to their natural habitat. Does outcome outweigh costs?
• Entrainment (EZ) is the adjustment/ moderation of one behaviour either to synchronise of be in rhythm with another behaviour; exogenous zeitgebers can influence/ entrain out biological rhythms
• Exogenous Zeitgebers:

  • Social cues- we are able to compensate for absence of natural zeitgebers by responding to social cues, such as mealtimes and social activities
  • Light- primary exogenous zeitgebers in the sleep/wake cycle, can reset the body's main endogenous pacemaker- SCN
  • Sleep and wakefulness is determined by interactions between both internal and external factors
• Influence of Exogenous Zeitgebers (Campbell & Murphy, 1998):

  • Light can be detected by skin receptors on the body even when same information is not received by eyes
  • 15 participants woken up at various times and light pad shone at back of their knees
  • Produced deviation in participants' usual sleep/wake cycle by up to 3 hours
  • Light is a powerful exogenous zeitgeber that does not necessarily rely on the eyes to exert its influence on the brain
• Influence of Exogenous Zeitgebers (Klein & Wegmann, 1974):

  • Circadian rhythms of air travellers adjusted more quickly if they went outside more and right after landing at their destination
  • Influenced by social cues that acted as zeitgebers
• W- Influence of exogenous zeitgebers (Exogenous ZG):

  Miles et al. (1977) noted a story of a blind man from birth having a circadian rhythm of 24.9 hours. Despite constant exposure to social cues his sleep/wake cycle could not be adjusted; had to take sedatives at night and stimulants in the morning. Individuals in the arctic regions show normal sleep patterns despite prolonged exposure to light. Suggest occasions when exogenous zeitgebers may have little effect on internal rhythms
• S- Age-related insomnia (Exogenous ZG):

  Natural changes in circadian rhythm means old people fall asleep early and have poorer sleep quality (Duffy et al.). However, Hood et al. (2004) found insomnia management improved if elderly people were more active and had more exposure to natural light in the day. Suggests exogenous changes in lifestyles may be just as likely to cause age-related insomnia as internal, biological changes