Endogenous Pacemakers and exogenous zeitgebers

• What is an endogenous pacemaker?

  • internal body clock that regulate many of our biological rhythms such as the influence of the suprachiasmatic nucleus (SCN) on the sleep/wake cycle
• What is an exogenous zeitgeber?

  • External cues that may affect or entrain our biological rhythms such as the influence of light on the sleep/wake cycle
• Describe the SCN

  • tiny bundle of nerve cells located in the hypothalamus (in both hemisphere)
  • one of the primary endogenous pacemakers in mammalian species
  • influential in maintaining circadian rhythms such as the sleep/wake cycle
• How does the SCN receive information?

  • SCN lies just above the optic chiasm, it receives information about light directly from this structure
  • Nerve fibres connected to the eye cross in an area known as the ‘optic chiasm’ on their way to the visual area of the cerebral cortex
  • This continues even when our eyes are close
  • enables the biological clock to adjust to changing patterns of daylight whilst we are sleep
• Animal study (squirrels) to support the need on SCN in circadian rhythms

  • DE COURSEY ET AL. (2000)
  • destroyed the SCN connections in the brains of 30 chipmunks
  • 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 proportion of them had been killed by predators
  • (presumably because they were awake and vulnerable to attack when they should have been asleep)
  • SUGGESTS: Circadian rhythms are key in survival and disruptions increase vulnerability especially to predators
• Animal study- hamsters to support SCN role

  • RALPH ET AL (1990)
  • Bred ‘mutant’ hamsters with a 20-hour sleep-wake cycle.
  • When SCN cells from the foetal tissue of the mutant hamsters were transplanted into the brains of normal hamsters, the cycles of the second group defaulted to 20 hours
  • SUGGESTS: SCN cells determine the rhythm’s timing
• What do both animal studies suggest?

  • Both of these studies emphasise the role of the SCN in establishing and maintaining the circadian sleep/wake cycle and aligning behaviour with environmental demands
  • SCN role may be required for survival
• How is the SCN linked to the pineal gland and melatonin?

  • SCN passes the information on day length and light that it receives to the pineal gland
  • During the night, the pineal gland increases production of melatonin
  • a chemical that induces sleep and is inhibited during periods of wakefulness
  • Melatonin has also been suggested as a causal factor in seasonal affective disorder (SAD)
• How is light an exogenous zeitgeber?

  • Light is a key zeitgeber in humans
  • can reset the body’s main endogenous pacemaker, the SCN
  • plays a role in the maintenance of the sleep/wake cycle
  • Light also has an indirect influence on key processes in the body that control such functions as hormone secretion and blood circulation
• Research evidence to support light as an EZ

  • CAMPBELL AND MURPHY (1998)
  • 15 ppts, woken at various times and a light pad was shone on the back of their knees
  • This was to demonstrate that light may be detected by skin receptor sites on the body even when the same information is not received by the eyes
  • Caused a deviation in their sleep/wake cycle from up to 3 hours.
  • SUGGESTS: light is a powerful exogenous zeitgeber that doesn’t need to rely on the eyes to exert its influence on the brain
• More research evidence to support light as an EZ (AO3)

  • BURGESS ET AL (2003)
  • Volunteers participated in one of three treatments – continuous bright light, intermittent bright light or dim light
  • Each of which shifted their sleep-wake cycle one hour a day over a period of three days
  • SUGGESTS: Exposure to bright light prior to an east-west flight decreased the time needed to readjust to local time on arrival.
  • Light has a large impact on circadian rhythms as suggested and acts as an external cue to synchronise biological rhythms to environmental changes
• Burgess et al findings to support light as an EZ

  • Continuous bright light: 2.1 shift
  • Intermittent bright light: 1.5 hour shift
  • Dim light: 0.6 shift
  • participants in the first treatment group felt sleepier two hours earlier in the evening, and woke 2 hours earlier in the morning
  • (i.e. closer to the local time conditions they would find after an east-west flight)