Lesson 12 - Endogenous Pacemakers and Exogenous Zeitgebers Flashcards
Endogenous Pacemakers and Exogenous Zeitgebers
Internal biological rhythms must be finely tuned in order to stay in keeping with the outside world
In order to achieve this, we have endogenous pacemakers (internal biological rhythms) and exogenous zeitgebers (external cues/factors) which reset out biological rhythms every day
Endogenous Pacemakers
Suprachiasmatic Nuclei (SCN)
Most important pacemaker
Tiny cluster of nerve cells in hypothalamus
Generates circadian rhythm
Acts as master clock, linking other brain regions that control sleep and arousal, and controlling all other biological clocks in body
Endogenous Pacemakers
Neurons
Neurons within SCN synchronise so target neurons in sites in body receive time-coordinated signals
These peripheral clocks maintain a circadian rhythm, but not for long, which is why they are controlled by SCN
Possible because of the SCN’s built in circadian rhythm, which only needs resetting when external light levels change
SCN receives information about light levels through optic nerve
If biological clock is slow then morning light shifts clock
Endogenous Pacemakers
Melatonin
SCN regulates manufacture and secretion of melatonin in pineal gland via interconnecting neural pathway
SCN sends signal to pineal gland, directing to increase production and secretion of melatonin at night and to decrease as light levels increase in morning
Melatonin induces sleep by inhibiting brain mechanisms that promote wakefulness
Endogenous Pacemakers Evaluation Points
Folkard (1996) - Kate Aldcroft Maintained
Kate Aldcroft - Extreme
Endogenous Pacemakers Evaluation
Folkard (1996) - Kate Aldcroft Maintained
Positive
Folkard studies a university student, Kate, who spent 25 days in a lab
No access to exogenous zeitgeber of light to reset SCN
However, at end of 25 days, her core temperature rhythm was still at 24 hours
Indicates we do not need the exogenous zeitgebers of light to maintain our internal biological rhythms
Endogenous Pacemakers Evaluation
Kate Aldcroft - Extreme
Negative
Sleep/Wake cycle extended to 30 hours, which periods of sleep as long as 16 hours
Suggests we do need the exogenous zeitgebers of light to maintain our internal biological rhythms
Exogenous Zeitgebers
Refers to anything whose origins are outside of the organism
Environmental events that are responsible for maintaining the biological clock of an organism
Most important zeitgeber for most animals is light
Exogenous Zeitgebers
Light
Receptors in SCN are sensitive to changes in light levels during day => use information to synchronise activity of body’s organs and glands
Light resets internal biological clock each day, keeping it on a 24 hour cycle
Protein in retina of eye called melanopsin, which is sensitive to light, is crucial
Exogenous Zeitgebers
Jet Lag
When people move to a night shift or travel to a country with a different time zone, their endogenous pacemakers try to impose their inbuilt rhythm of sleep (circadian rhythm), but this is out of synchrony with exogenous zeitgeber of light
Out of sync biological rhythms lead to disrupted sleep patterns, increased anxiety and decreased alertness and vigilance
Exogenous Zeitgebers Evaluation Points
Blind People
Burgess et al. (2003) - Flights
Artic Studies
Exogenous Zeitgebers Evaluation
Blind People
Positive
Majority of blind people who still have light perception have normal circadian rhythms
Blind people without light perception show abnormal circadian rhythms
Shows the vital role the exogenous zeitgeber of light levels play in maintaining our internal biological rhythms
Exogenous Zeitgebers Evaluation
Burgess et al. (2003) - Flights
Found that exposure to bright light prior to an east-west flight decreased time needed to adjust circadian rhythms to local time
Exogenous Zeitgebers Evaluation
Artic Studies
Negative
Studies show normal sleeping patterns despite prolonged exposure to light
Suggests there are occasions where the exogenous zeitgeber of light may have very little bearing on our internal biological rhythms
