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Summary Endogenous pacemakers and exogenous zeitgebers
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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
Sleep/wake cycle
A daily cycle of biological activity based on a 24 hour period (circadian rhythm) that
is influenced by regular variations in the environment, such as the alternation of
night and day
Endogenous pacemakers and the sleep/wake cycle
The suprachiasmatic nucleus
Tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the
brain
One of the primary endogenous pacemakers in mammalian species
Influential in maintaining circadian rhythms such as sleep/wake cycle
Nerve fibres connected to the eye cross in an area called the optic chiasm
Receives information about light directly from this structure
This continues even when our eyes are closed, enabling the biological clock to adjust
to changing patterns of daylight whilst we are asleep
Animal studies and the SCN
Influence of the SCN has been demonstrated in studies involving animals
DeCoursey et al (2000) destroyed SCN connections in the brains of 30 chipmunks
who where then retuned to their natural habitat and observed for 80 days
Findings: sleep/wake cycle of the chipmunks disappeared and by end of 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)
Ralph et al (1990) bred ‘mutant’ hamsters with a 20 hour sleep/wake cycle
When SCN cells from the foetal tissues of mutant hamsters were transplanted into
the brains of normal hamsters, the cycles of the second group defaulted to 20 hours
The pineal gland and melatonin
SCN passes information on day length and light that it receives to the pineal gland -
structure in the brain behind the hypothalamus. Another endogenous mechanism
guiding the sleep/wake cycle
During the night, the pineal gland produces melatonin - a chemical that induces
sleep and is inhibited during periods of wakefulness
Melatonin has also been suggested as a causal factor in seasonal affective disorder
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