Sleep - Chronobiology

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  • Created by: Joanna
  • Created on: 04-12-13 16:05

Biological Rhythms

Biological rhythms

  • Activity : Daily Rhythms
  • Hypothalamus - endogenous circadian clock
  • Seasonal change - circannual rhythms 

Circadian rhythms

  • Rhythm = 24 hours
  • Physiological
  • Behavioural 
  • Biochemical

Diurnal - daylight active

Nocturnal - nightime active 

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Determining Circadian Rhythm

Phase markers

  • Growth hormone
  • Cortisol
  • Testosterone
  • Melatonin
  • Body temperature

Circadian rhythms

  • - generated by endogenous internal clock
  • - free running = maintain own cycle w/ no external cues e.g. light
  • - period / time b/ween sucessive cycles - not always 24 hours
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Measuring activity rhythms

Phase shift

- shift in activity - response to a stimulus

- e.g. light / food

Entrainment 

- process of shifting rhythm

- synchronisation w/ environment

- called a zeitgeber = time giver

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Suprachiasmatic Nucleus (SCN)

Location: above optic chiasm - in hypothalamus

Studies:- circadian rhythms disrupted in SCN-lesioned animals

Isolated SCNs:- maintain electrical activity synchronized to previous light cycle

Endogenous period:- generated in the SCN

e.g. hamsters w/ stopped circadian rhythms had an SCN tissue

transplant at short period = 20 hours 

rhythms:- restored but matched the shorter period of the donor

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Circadian rhythm

Circadian rhythm entrain to light

e.g. dark cycles - use diff pathways, some outside of the eye

Amphibians + birds - pineal gland sensitive to light

Mammals - light info into eye to the SCN via retinohypothalamic pathway

Retinohypothalamic pathway: retinal ganglion cells - project to the SCN

Retinal ganglion cells: has melanopsin = photopigment = light sensitive

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Studies on circadian rhythms

Drosophila - mutations of period gene - circadian clock in mammals

SCN - 2 proteins in mammals

- CLOCK

- CYCLE or BMAL1

Clock + Cycle/Bmal1 proteins = a dimer

CLOCK/BMAL1 heterodimer: promotes transcription of 2 genes:

- Period (per) + their paralogs

- Cryptochrome (cry) + their paralogs

1. Per and Cry proteins - bind to each other / 3rd one = Tau

2. PER/CRY/TAU - protein complex -> enters nucleus -> stops per + cry transcription

3.Thus no new proteins made until 1st set degrades 

4. Cycle begins again every 24 hours

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Light entrainment

Light entrains molecular clock in diff ways:

e.g.1.flies - light reaches brain directly + degrades a clock protein

e.g. 2. mammals - melanopsin cells - detect light -> release glutam8 in SCN

- Glutam8 triggers events that promote production of the Per protein

- turn shifts the clock and animal's bhvr. 

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Mutations

Gene mutations - clock bhvr in constant conditions vary:

- Tau mutations - period is shorter > normal

- Double clock mutants - arrhythmic

- Single clock mutants - period is longer > normal

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Biological Rhythms - time

Some are shorter:

e.g. activity such as feeding + hormone release

Ultradian rhythms :

- occur more than once/day

- period length: mins - hours

Longer biological rhythms 

e.g. body weight + reproductive cycles

- endogenous circannuam clock : separ8 from SCN - runs @ 365 days

- infradian rhythms occur less than once / day 

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