Biological Rhythms (Chronobiology)

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Biological Rhythms(Chronobiology)

• Chapter 9

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Outline

• Definition and Characteristics of BRs
• Entrainment and Zeitgebers
• Evidence for an Endogenous Clock
• Location and Physiology of the Clock
• Ecological Adaptations
• Practical Significance

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Biological rhythms

• Behavioural and physiological characteristics of
  animals change over time and many of these change
  in a regular or rhythmic way.

Examples Sleep/wake patterns Activity
patterns Reproductive cycles
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Biological rhythms

• A biological rhythm is a self-sustaining cyclic
  change in a physiological process or behavioural
  function of an organism that repeats at regular
  intervals.

Biological rhythms are widespread among animals,
plants, protozoans and even microorganisms.
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Characteristics of Biological Rhythms

• Endogenous (continue to cycle w/o environmental
  cues freerun)
• Entrained by environmental cues
• Many coincide with external environmental rhythms
  (e.g., daily, tidal, lunar, yearly)
• Temperature-compensated (not affected by
  fever/illness/increased body temp)
• Unaffected by metabolic poisons or inhibitors
  that block pathways in cells

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Entrainment of Circadian Rhythms (CR)

• CRs dont match perfectly with 24-hr solar day,
  approximate it
• Because the biological rhythms of animals are
  synchronised by environmental cues, we say that
  the rhythms are entrained.
• Cues that entrain biological rhythms are called
  Zeitgebers
• For example, the light-dark cycle is an
  entraining agent for circadian rhythms of flying
  squirrels.

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Zeitgebers

• Definition
• Those cyclic environmental cues that can entrain
  (synchronise) free-running endogenous pacemakers

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Zeitgebers

• Reset biological clock
• Synchronise biological rhythms with environmental
  rhythms

• Environmental cycles Sunrise/sunset,
  photoperiod (day length), temperature, humidity,
  food availability/meal timing, social cues, tidal
  cycles, lunar cycles, etc. (pg. 133)

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Normal Cyclic (Cued) Conditions
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Constant Environmental Conditions
NO CUE our day gets longer

• Drift of activity onset (starts later each day)
  Aschoffs Rule

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Aschoffs Rule

• The direction and rate of drift of free-running
  rhythms away from a 24hr period are a function of
  light intensity and whether the animal is diurnal
  or nocturnal.
• Examples
• Flying squirrel constant dark free-running
  period of less than 24hrs (activity begins
  earlier each day)
• Diurnal animal, same condtions free-running
  period slightly longer (activity starts slightly
  later each day)

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Constant Environmental Conditions

• Isolation studies Typically, humans show
  freerunning rhythms with periods 25 hr
• E.g., Siffre (1964) speleologist (cave
  explorer)
• Lived in a cave for 2 months mentally lost 25
  days!
• His days were 30 hours long
• depressed and pessimistic
• Results not typical very cold, humid,
  uncomfortable

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Siffres Free-Running Rhythm
ACTOGRAM (Graph of activity patterns)
White active Black inactive
what it might have looked like
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Biological Rhythms

• For endogenous rhythms to persist there must be
  an internal biological clock to keep time.
• Why have endogenous rhythms?
• They provide an internal timing mechanism that
  allows temporal organisation of animals by
  modifying behavioural and physiological processes
  in a rhythmic way.

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Evidence for an Endogenous Clock

• Freerunning rhythms are different from 24 hours,
  do not match any known environmental cycle
• E.g., human CR cycle is 25 hr
• Genes identified that code for period length,
  mutations show lack of environmental control
• Cells from 22-hr cycle hamster cause a 24-hr
  cycle hamster to exhibit a 22-hr cycle.

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Evidence for an Endogenous Clock

• Translocation studies behave according to time
  zone of origin, adaptation is not instant (jet
  lag)
• E.g., bees trained to feed at 12 noon, released
  in new time zone, feed later for first couple of
  days (but at 12 in old time zone)
• Jet lag different rhythms adjust to new time
  zone at different rates internal
  desynchronization

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Location Physiology of the Clock

• In lower organisms, mechanism associated with the
  eyes (e.g., sea hares), or optic lobes (e.g.,
  cockroaches, crickets)
• Suprachiasmatic nuclei (SCN) in mammals (cells
  show 24-hr rhythmicity in molecular activity)
• Pineal gland important in birds and reptiles
• VMN (ventromedial nucleus)

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Location Physiology of Clock

• In humans and other mammals, other sites still
  unknown
• BRs generated by endogenous physiological
  oscillators biochemical feedback loops

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Model of Drosophila Circadian Oscillator (p. 136)

• Four regulatory proteins interact to create
  24-hr periodicity (period, timeless,
  cycle, clock)
• Binding of CLK-CYC activates genes that produce
  PER TIM
• PER TIM accumulate to levels that inactivate
  the CLK-CYC complex
• Production of PER TIM is slowedleads to
  binding of CLK-CYC and beginning of next cycle

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Model of Biological Time-keeping System
(p. 134) ENTRAINMENT PATHWAY BIOLOGICAL CLOCK
Input (e.g., light)
Receptor system (e.g., retinal cones)
Nervous system (e.g., brain or ganglion)
Biological oscillators (e.g., SCN cells) generate
endogenous rhythms (through feedback loops)
light
eye
neural pathway
SCN
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Model of Biological Time-keeping System
OUTPUT PATHWAY
Message to tissues and organs that execute
behaviour/physiology
Change in target cells (e.g., light cue causes
phase shift)
Message to other brain areas via
neurotrans-mitters
SCN cells
Behaviour rhythm reset to match external one
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Ecological Adaptations

• Biological rhythms allow animals to anticipate
  changes in their environment in order to survive

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Adaptation to Tidal Rhythms

• Fiddler crabs use circatidal rhythms to return to
  their burrows before the tide comes in

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Hibernation

• Black bears build up their body fat and find a
  suitable den prior to the onset of winter

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Hibernation

• 13-lined ground squirrels
• (Spermophilus tridecemlineatus)

Arctic ground squirrels (Spermophilus parryii )
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Migration

• Many bird species fly south to escape the harsh
  northern winters (anticipation of weather change)

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Circadian Rhythms and Learning

• Bees adapt their foraging based on the time of
  day that certain flowers are open
• Can learn to go to experimental feeding sites
  based on time of day food available
• They anticipate - arrive before food is available
  (i.e., absence of external cues)
• Evidence for internal timing mechanism

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Linnaeus Flower Clock
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Practical Significance Hypothetical example

• BRs provide an advantage based on time zone
• CR of athletic ability peaks late afternoon
• E.g., Monday night football, 9 p.m. E.S.T. 5
  p.m. P.S.T. (west)
• Western teams are still in the phase of optimal
  athletic ability and pain endurance, while
  eastern teams are hours ahead and that phase has
  passed

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Practical (Hypothetical) Uses

• Rodent control
• zoos
• Determine optimal periods of study time/physical
  activity/sleep
• Prepare for trips across time zones (reduce jet
  lag)
• Important for research treatment of S.A.D., jet
  lag, etc. Light therapy

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Lark or Owl?

• Its not just an excuse anymore There really are
  morning people and night people!
• Period 3 gene basis for our preference, long gene
  early riser, short gene late riser
• Of course, behaviour can override genetics

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• Knowing your personal preference (or genetic
  predisposition) can help you to optimize your
  study/physical activity, etc., within societal
  constraints

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Biological Rhythms

• Time is viewed differently in other cultures,
  more in tune with internal rhythms
• e.g., siesta in middle of workday in Mexico and
  parts of Europe
• Corresponds to natural trough in
  awareness/alertness cycle (an ultradian rhythm,
  4 hr cycle)
• Maybe we should take a lesson