Endogenous Pacemakers & Exogenous Zeitgebers

Endogenous Pacemakers AO1

Main endogenous is suprachiasmatic nucleus
Found in hypothalamus
Information about light
Light penetrates eyelids
SCN is a pair of structures
- Ventral SCN quickly reset by external cues
- Dorsal SCN not quite so quickly reset (ALBUS)
SCN sends signals to pineal glad to increase melatonin supply
- Induces sleep-like feeling as inhibits mechanisms that contribute to feeling awake

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MORGAN - Mutant hamsters bred with circadian rhythms of 20 hours & transplanted SCN into normal hamsters. Normal hamsters displayed mutants rhythms
- Cannot generalise as animal study
KATE ALDCROFT - separate oscillators (separate SCNs) can desynchronise, had temperature cycle of 24 hours yet sleep rhythm of 30 hours (FOLKARD)
IDA POINT - Evolutionist approach emphasises adaptive nature of system controlled by endogenous pacemakers
- ANTON - circadian clocks allow tight temporal scheduling of physiological and behavioural programmes
- DeCOURSEY - animals anticipate daily environmental patterns (mutant chipmunks, significantly more than control groups had been killed by weasels as awake in burrows when would've been asleep)
- A lot of research is done on animals (Ethically issues, problems with generalising)

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Affected by external factors
Light is dominant zeitgebers
Can reset other oscillators, not just SCN
- Cryptochrome (protein clock) is very sensitive
CAMPBELL AND MURPHY - can reset circadian rhythms by shining light on back of patients knee
Social Cues (ie, eating)
Previously thought to be main zeitgeber as rhythms appeared to be entrained by social convention
DAVIDSON - Zeitgeber for cells in liver and heart likely to be mealtimes, cells are reset by eating
Can be entrained by temperature as well in warm blooded animals (BUHR)

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Artifical lighting affects biological rhythms, as does dim, though bright is more effective (BOIVIN)
STEVENS - living in artificially lit world can have negative consequences. Disrupts melatonin production, ergo explaining why women in industrialised environment are more likely to develop breast cancer
When biological system fails, multitude of problems, ie. Sleep-phase syndrome (CHICUREL, link to defect in PER gene)
- Suggestion that can occur in adolescent years, which explains irregular sleep patterns then
Biological clock is really blend of exo and endo factors (not two systems, not such neat divisions)

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