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Summary Endogenous pacemakers and exogenous zeitgebers AQA A level psychology Cornell style notes
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ENDOGENOUS PACEMAKERS AND EXOGENOUS ZEITGEBERSThe Endogenous pacemakers and the sleep/wake cycle
suprachiasmatic
nucleus The suprachiasmatic nucleus
• The suprachiasmatic nucleus(SCN) is a tiny bundle of nerve cells
Hypothalamus located in the hypothalamus in each hemisphere of the brain
Hemisphere • It is one of the primary endogenous pacemakers in mammalian
species(including humans) and is influential in maintaining circadian
Endogenous rhythms such as the sleep/wake cycle
pacemakers • Nerve fibres connected to the eye cross in an area called the optic
chiasm on their way to the left and right visual area of the cerebral
Circadian rhythms cortex.
• The SCN lies just above the optic chiasm('supra' means 'above'). It
Sleep/wake cycle receives information about light directly from this structure
• This continues even when our eyes are closed, enabling the biological
Visual area clock to adjust to changing patterns of daylight whilst we are asleep
Cerebral cortex Animal studies and the SCN
Sleep/wake cycle: a family cycle of biological activity based on a 24-hour
Sleep/wake cycle period(circadian rhythm) that is influenced by regular variations in the
environment, such as the alternation of night and day
Patricia DeCourcy
et al.(2000) • The influence of the SCN has been demonstrated in studies involving
animals
• Patricia DeCourcy et al(2000) destroyed the SCN connections in the
Martin Ralph et brains of 30 chipmunks who were then returned to their natural
al.(1990) habitat and observed for 80 days
• The sleep/wake cycle of the chipmunks disappeared and by the end of
the study a significant proportion of them had been killed by
predators(presumably because they were awake, active and vulnerable
to attack when they should have been asleep
• In another study, Martin Ralph et al.(1990) bred 'mutant' hamsters
with a 20-hour sleep/wake cycle
Melatonin • When SCN cells from the foetal tissue of mutant hamsters were
transplanted into the brains of normal hamsters, the cycles of the
Seasonal affective second group defaulted to 20 hours
disorder
The pineal gland and melatonin
Endogenous • The SCN passes the information on day length and light that it receives
pacemakers to the pineal gland(a pea-like structure in the brain just behind the
hypothalamus)
• This is another endogenous mechanism guiding the sleep/wake cycle
• During the night, the pineal gland increases production of melatonin- a
chemical that induces sleep and is inhibited during periods of
wakefulness
Zeitgeber(time • Melatonin has also been suggested as a causal factor in seasonal
giver) affective disorder.
Exogenous zeitgebers and the sleep/wake cycle
, Endogenous pacemakers: internal body clocks that regulate many of our
biological rhythms, such as the influence of the suprachiasmatic nucleus(SCN)
on the sleep/wake cycle
Exogenous zeitgebers: External factors that affect or entrain our biological
rhythms, such as the influence of light on the sleep/wake cycle
Hormone • The German word zeitgeber means 'time giver'
• Exogenous zeitgebers are external factors in the environment that rest
Scott Campbell our biological clock through. A process known as entrainment
and Patricia • In the absence of external cues, the free-running biological clock that
Murphy(1998) controls the sleep/wake cycle continues to 'tick' in a distinct cyclical
pattern(as in the Siffre study)
• This free-running cycle is then 'brought into line'(i.e., Entertained) by
environmental cues, so there is an interaction of internal and external
factors
Light
• Light is a key zeitgeber in humans
• It can reset the body's main endogenous pacemaker, the SCN, and thus
plays a role in the maintenance of the sleep/wake cycle
• Light also has an indirect influence on key processes in the body that
control such as hormone secretion and blood circulation
• In an innovative study, Scott Campbell and Patricia Murphy(1998)
demonstrated that light may be detected by skin receptor sites on the
body even when the same information is not received by the eyes
• Fifteen participants were woken at various times and light pad was
shone on the back of their knees
• The researchers managed to produce a deviation in the participant's
usual sleep/wake cycle of up to three hours in some cases
• This suggests that light is a powerful exogenous zeitgeber that need
not necessarily rely on the eyes to exert its influence on the brain
Social cues
• Babies are seldom on the same sleep/wake cycle as the rest of the
family
• New born babies' initial sleep/wake cycle is pretty much random
• At about 6 weeks of age, the circadian rhythms begin and, by about 16,
babies' rhythms have been entrained by the schedules imposed by
parents, including adult-determined mealtimes and bedtimes
• Research on jet lag suggests that adapting to local time for eating and
sleeping(rather than responding t one's own feelings of hunger and
Francesca fatigue) is an effective way of entraining circadian rhythms and beating
Damiola et jet lag when travelling a long distances.
al.(2000)
Evaluation
Strengths Limitations
Beyond the master clock
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