Endogenous pacemakers and exogenous zeitgebers

• What is an endogenous pacemaker?

  Mechanism within the body that governs the internal biological bodily rhythms
• What is the main endogenous pacemaker in the body?

  The suprachiasmatic nucleus
• where is the suprachiasmatic nucleus located?

  hypothalamus
• what type of bodily rhythm does the SCN play an important role in?

  circadian rhythms
• details about the functions of the SCN?

  • neurones within the SCN synchronise spontaneously with each other
  • so target neurones in sites elsewhere receive correctly timed coordinated signals
  • These peripheral clocks can maintain a circadian rhythm, but not for long, so they are controlled by the SCN
  • SCN regulates manufacture and secretion of melatonin in the pineal gland by sending signals about light levels to pineal gland (pineal gland another example of endogenous mechanism/pacemaker)
• how does the SCN work?

  • recieves info about levels via the optic nerve
  • happens even when our eyes are shut as light penetrates the eyelids
  • if biological clock is running slow, the morning light automatically adjusts the clock
• How does melatonin produce sleep?

  inhibits brain mechanisms that promote wakefulness
• what is an exogenous zeitbeger?

  environmental cue, like light, helping to regulate the biological clock in an organism
• How does light act as an exogenous zeitberger?

  • Light resets the internal biological clock each day, keeping it on a 24 hour cycle
  • Rods and cones in the retina of the eye detect light to form visual images
  • Third type of light detecting cell in the retina, a protein called melanopsin is involved with this cell- sensitive to natural light
  • retinal cells with melanopsin carry signals to the SCN to set the daily body cycle
• What is the role of melanopsin?

  • receptor for light
  • Helps reset internal biological clock
• where is melanopsin found?

  in a small number of retinal cells
• how do social cues act as exogenous zeitgebers

  Individuals can compensate for the absence of zeitgebers like natural light by responding to social zeigebers instead (Aschoff et al)
• Study to prove the existence of the effect of exogenous zeitgebers on circadian rhythms?

  • Jet lag- (klein and Wegmann) found circadian rhythms of air travellers adjusted faster if they spent more time outside upon arrival at their destinations
  • as they are exposed to the social cues of the new time zone, acting as a zeitgeber
• evaluation of endogenous pacemakers and exogenous zeitbegers?

  • 😊 real world applications- research shows young people look too much at devices before sleep the light levels can decrease the volume of melatonin produced, less sleep, more detrimental to health
  • 😊 Siffre- free running study- no external cues to guide his rhythms, body clock became longer, 24.9 hours
  • 😊 role of SCN- Morgan- SCN neurones from abnormal hamsters transplanted into normal ones. Normal hamsters showed same abnormal circadian rhythm of 20hrs
  • 😊 One of the earliest researches on jet lag- Klein and Wegman- support that being out more upon arrival at a destination for air travellers helps circadian rhythms to adjust quicker because of exposure to social cues of the time zone