Endogenous Pacemakers & Exogenous Zeitgebers Flashcards
SCN - Primary Endo Pacemaker
Most important endo pacemaker, known as ‘master clock’.
1) The suprachiasmatic nucleus (SCN) is a tiny bundle of nerve cells in the hypothalamus which helps maintain circadian rhythms (e.g. sleep/wake cycle).
2) Nerve fibres from the eye cross at the optic chiasm on their way to the right and left visual areas. The SCN lies just above the optic chiasm and receives info about light from this structure.
Endogenous Pacemaker
Pineal Gland & Melatonin are endogenous mechanisms.
1) The SCN passes info on day length to the pineal gland:
- Less light, more melatonin
- More light, less melatonin
2) Melatonin is a hormone that induces sleep and is inhibited during periods of wakefulness. It has also been suggested a causal factor in seasonal affective disorder.
Exogenous Zeitgebers
External environmental factors (e.g. light, alarm, temp) that reset biological clocks.
1) Resetting biological clocks is a process known as entrainment.
2) Without external cues, the free-running biological clock continues to ‘tick’ in a cyclical pattern.
3) Zeitgebers reset the sleep/wake cycle (interaction of internal and external factors).
Light - Exogenous Zeitgeber
Light is a key exogeneous zeitgeber that influences the sleep/wake cycle.
1) Light can reset the body’s main endogenous pacemaker (SCN, & also has an indirect influence on key processes in the body controlling hormone secretion, blood circulation, etc).
Limitation of Exo Zeitgebers
EFFECTS OF EXOGENOUS ZEITGEBERS DIFFER IN DIFFERENT ENVIRONMENTS
1) Exogenous zeitgebers do not have the same effect on ppl who live in places where there is very little darkness in summer and very little light in winter.
E.g. the Innuit Indians of the Arctic Circe have similar sleep patterns all-year round, despite spending around 6 months in almost total darkness.
—> Suggests the sleep/ wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light.
Influence of Social Cues on Sleep/Wake Cycle
1) The sleep/wake cycle is fairly random in human newborns, but most babies are entrained by about 16 weeks.
2) Schedules imposed by parents are a key influence, including adult-determined mealtimes and bedtimes.
3) Research on jet lag shows adapting to local times for eating and sleeping (not responding to one’s own feelings of hunger & fatigue) entrains circadian rhythms and tackles jet lag.
Limitation of Exogenous Cues
CASE STUDY EVIDENCE UNDERMINES EFFECTS OF EXOGENOUS CUES
1) Miles et al. reported the case of a man, blind from birth, with an abnormal circadian rhythm of 24.9 hrs.
2) Despite exposure to social cues, such as mealtimes, his sleep/ wake cycle could not be adjusted.
—> Suggests that social cues alone are not effective in resetting the biological rhythm and the natural body clock is stronger.
Limitation of Endogenous Pacemakers
ENDOGENOUS PACEMAKERS CANNOT BE STUDIED IN ISOLATION
1) Only in exceptional circumstances are endogenous pacemakers ‘free running’ & unaffected by the influence of exogenous zeitgebers.
2) Total isolation studies (e.g. Siffre’s cave study) are rare. In everyday life, pacemakers and zeitgebers interact so it may make little sense to separate the 2.
—> Suggests the more researchers attempt to isolate the influence of internal pacemakers, the lower the validity of the research.
Research Support - Menaker et al.
Importance of SCN in circadian rhythms.
1) Bred a strain of hamsters with abnormal circadian rhythms of 20 hours rather than 24.
2) SCN neurons from abnormal hamsters were transplanted into the brains of normal hamsters.
FINDINGS:
3) Normal hamsters then displayed the same abnormal circadian rhythm of 20 hours.
4) Further confirmation from reverse experiment, planting SCN neurons from normal hamsters into the brains of abnormal hamsters –> Changed to a circadian pattern of 24 hours.
This shows the importance of the SCN in regulating circadian rhythms.
Limitation of Studies
ETHICAL ISSUES
1) Animal studies of the sleep/wake cycle are justified cause there are similar mechanisms in all mammals, so generalisations can be made to the human brain.
2) BUT, animals were exposed to considerable risk in the Decoursey et al. study and most died as a result.
—> Suggests that studies like these cannot be justified and researchers should find alternative ways of studying
endogenous pacemakers.
