Quiz 4 BNA wk 11

• Cyclical patterns and rhythms

  Our natural environment is filled with cyclical patterns and rhythms
• Origin of cyclical patterns and rhythms

  • Earth revolves around the sun each year causing seasonal changes in temperature
  • Moon revolves around the earth every approx 29 days causing changes in the tides
  • Earth rotates on its axis every 24 hours which affects the availability and quality of light
• Chronobiology


  Study of rhythms or cycles in biological systems
• Virtually all aspects of human physiology show rhythmic changes over time, so chronobiology has provided a better understanding of a huge range of phenomena
• Variety of biological rhythms

  • Ultradian rhythms (period < 24 hours)
  • Circadian rhythms (period of about 24 hours)
  • Infradian rhythms (period > 24 hours)
• Ultradian rhythms


  • Alternation of deep sleep and REM sleep in humans (occurs about 90 minutes)
• Circadian rhythms


  • Sleep wake cycle, body temperature cycle, hormone secretion cycles
• Infradian rhythms


  • Menstrual cycle in women, migration cycles in birds
• Influence of day/night and seasonal changes

  Varies between polar and equatorial regions
• Influence of day/night and seasonal changes

  • Polar regions: large seasonal differences, animals living near the poles are more affected by seasonal changes
  • Equatorial regions: minimal seasonal differences, animals living near the equator more affected by day and night changes
• Humans are equatorial living beings, so we are affected more by daily cycles than seasonal changes
• Circadian rhythms


  Physiological, mental and behavioural changes that follow a 24 hour cycle
• Circadian rhythms respond primarily to light and dark and affect most living things (animals, plants, microbes)
• Physiological functions in humans that fluctuate with the time of day

  • Body temperature, blood flow, hormone levels, metabolic rate, eating, sleeping and even hair growth
• Some rhythms have similar phases, and others are different (e.g. alertness and core body temperature vary similarly, but growth hormone and cortisol levels in the blood are highest during sleep, although at different times)
• Different species have different phase relationship to the outside world

  • Diurnal and nocturnal animals
• Chronotype


  Describes individual differences in preferred rest and activity times
• Chronotype shifts later from childhood to adolescence and becomes earlier again during adulthood
• Rhythms undergo a gradual loss of amplitude with ageing
• Biological circadian clocks


  • Endogenous (keeps track of time persistently without external cues)
  • Continuously consulted
  • Capacity to be reset (entrainment)
  • Temperature independent
• Adaptive advantage of the circadian clock

  • Synchronise behaviour and body states to changes in the environment (light, temperature, availability of food)
  • Optimally coordinate the timing of different internal physiological processes
• During the 18th century Jean Jacques d'Ortous de Mairan studied mimosa plants and found that the leaves opened towards the sun during daytime and closed at night, and the same rhythmic pattern of leaf movements occurred when the plants were placed in constant darkness
• About a century later, augustin de candolle determined that in constant light, mimosa plants fold their leaves approx once every 22.5 hours, concluding that the plant had an internal biological clock
• Free running clock


  When environmental cues are eliminated, the clock is no longer reset each day by light and its endogenous free-running period can be determined
• The first human temporal isolation experiments were conducted in caves, where the temperature is constant and subjects are isolated from light and noise from the outside world
• Subjects' circadian rhythms persisted despite the isolation, indicating that humans have an 'endogenous clock'
• The free running clock cycle in humans is longer than 24 hours (approx 24.5 - 25.5 hrs)
• Once the cave experiments are over, subjects rapidly resynchronise their cycles to external time cues
• Entrainment


  The process of synchronising biological rhythms with external cues
• Zeitgeber


  Any cue that an animal uses to synchronise its activity with the environment
• Zeitgebers


  • Light
  • Temperature cycles
  • Social cues
  • Sound cues
  • Exercise/activity
  • Food availability cycles and eating/drinking patterns
• Phase response curve


  Measures the effect of a stimulus (e.g. light) on the circadian system, where the stimulus is applied at different times and the resulting change in the phase is measured
• There is a part of the brain that can maintain a basic, independent circadian rhythms even if the external cues of the cycle of day and night are eliminated
• To maintain its accuracy, this central biological clock resynchronises itself each day with external stimuli such as the brightness of the ambient light
• This information comes from special light sensitive cells in the retina to the brain through a neural tract
• Visible light


  Electromagnetic radiation with wavelengths 400-700nm
• Intrinsically photosensitive retinal ganglion cells (ipRGC)

  Responsive to light, contain a photopigment called melanopsin which is excited by blue light (480nm)
• ipRGCs project to the suprachiasmatic nucleus (SCN) via the retino-hypothalamic tract
• Suprachiasmatic nucleus (SCN)


  A subregion of the hypothalamus containing about 10,000 neurons that are among the smallest in the brain and are very densely packed
• Lesions to the SCN interfere with circadian rhythms of drinking, locomotion and hormone secretion