KEY TERMS:
ENDOGENOUS PACEMAKERS - Internal body clocks that regulate many of our biological rhythms,
e.g. the influence of the suprachiasmatic nucleus (SCN) on the sleep/wake cycle
EXOGENOUS ZEITGEBERS - External cues that may affect on entrain out biological rhythms e.g.
influence of light on the sleep/wake cycle
SLEEP/WAKE CYCLE - a daily cycle of biological activity based on a 24 hour period (circadian
rhythms) that's influenced by regular variations in the environment, e.g. as the alteration of night
and day
ENDOGENOUS PACEMAKER AND THE SLEEP/WAKE CYCLE:
● THE SUPRACHIASMATIC NUCLEUS (SCN) -
❏ SCN is a tiny bundle of nerve cells located in the hypothalamus in each hemisphere
of the brain
❏ Primary endogenous pacemaker in mammalian species (including humans) and is
influential in maintaining circadian rhythms e.g. sleep/wake cycle
❏ Nerve fibres connect to the eye cross in an area called the “Optic chiasm” on their
way to the visual area or the cerebral cortex
❏ SCN is just above optic chiasm ( “supra” meaning above)
❏ Receives info about light directly from this structure. This continues even when eyes
are closed enabling the biological clock to adjust to changing patterns of daylight
whilst we are sleep
● ANIMAL STUDIES AND THE SCN -
❏ Patricia DeCoursey et al (2000) destroyed the SCN connections in the brain of 30
chipmunks who were then returned to their natural habitat and observed for 80 days
❏ sleep/wake cycle of the chipmunks disappeared and by the end of the study a
significant proportion of them had been killed by predators ( probably due to them
being awake and vulnerable to attack when they should have been asleep)
❏ Martin Ralph et al (1990) bred “mutant” hamsters with a 20 hour sleep wake cycle
❏ SCN cells from the foetal tissue of mutant hamsters were transplanted into the
brains of normal hamsters, cycle of second group defaulted to 20 hours
❏ Both of these studies emphasise the role of the SCN in establishing and maintaining
the circadian sleep/wake cycle.
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