Lesson 12 - Endogenous Pacemakers and Exogenous Zeitgebers Flashcards
What are endogenous pacemakers?
Internal biological rhythms
What are exogenous zeitgebers?
External cues and factors
E.g. light
What is the role of endogenous pacemakers and exogenous zeitgebers?
To reset our biological rhythms every day
What is the most important endogenous pacemakers?
SCN
What is the SCN?
A tiny cluster of nerve cells in the hypothalamus
What role does the SCN play?
An important role in generating circadian rhythms
What does the SCN do?
Acts as the master clock
Linking other brain regions controlling sleep and arousal
And controlling all other biological clocks throughout the body
What do the neurons in the SCN do?
Neurons in the SCN synchronise with each other
So that their target neurons elsewhere in the body receive time-coordinated signals
Why are the peripheral clocks controlled by the SCN?
They can maintain a circadian rhythm but not for very long
Why can the SCN maintain a circadian rhythm?
It has a built in circadian rhythm
Only needs resetting when external light levels change
How does the SCN receive information about light levels?
Optic nerve
What happens when the biological clock is running slow?
Morning light shifts the clock
What does the SCN also regulate?
Regulates the manufacture and secretion of melatonin in the pineal gland
Via the interconnecting neural pathway
How does the SCN control melatonin levels?
SCN sends a signal to pineal gland -> increase production and secretion of melatonin at night and decrease it as light levels increase in morning
How does melatonin work?
Induces sleep by inhibiting the brain mechanisms that promote wakefulness
Folkard
1996
Endogenous pacemakers
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Folkard studied university student Kate Aldcroft
Spent 25 days in laboratory with no access to exogenous zeitgebers which RESET SCN
e.g. daylight
Core temperature rhythm was still at 24 hours after 25 days
Folkard
1996
Kate Aldcroft’s sleep-wake cycle extended to 30 hours
Periods of sleep as long as 16 hours
Shows we do need external influences to maintain our biological rhythm
What does exogenous mean?
Anything with origins outside the organism
What is the long definition for exogenous zeitgebers?
Environmental events that are responsible for maintaining the biological clock of an organism
What is the most important zeitgeber for animals?
Light
How does light synchronise the activity of the body’s organs and glands?
Receptors in the SCN are sensitive to changes in light levels during the day
Use this information to synchronise the activity of the body’s organs and glands
What resets the biological clock each day?
Light
Keeps it on a 24 hour cycle
What is melanopsin?
A protein in the retina of the eye
What does melanopsin do?
Sensitive to natural light
Critical for resetting internal biological clock
What happens when people travel to a different country or do night shift?
Endogenous pacemakers try to impose their inbuilt rhythm of sleep, circadian rhythm
This is out of synchrony with exogenous zeitgeber, light
What do disrupted biological rhythms lead to?
Disrupted sleep patterns
Increased anxiety
Decreased alertness and vigilance
Skene and Arendt
2007
Exogenous zeitgebers
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Found vast majority of blind subjects with some light perception have normal circadian rhythms
People without light perception show abnormal circadian rhythms
Burgess et al
2003
Exogenous zeitgebers
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Exposure to bright light prior to an east-west flight
Decreased time needed to adjust to local time in arrival
Vetter et al
2011
Exogenous zeitgebers
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Compared sleep-wake cycle and activity-rest patterns of 2 groups over a five-week period
One group in normal warm artificial light; other group experienced artificial blue light
All participants kept a daily sleep log and wore movement-measuring devices
Participants under the warm light synchronised their circadian rhythms with the natural light of dawn
Participants exposed to blue light synchronised their patterns to office hours
