Lesson 12 - Endogenous Pacemakers and Exogenous Zeitgebers Flashcards
What are endogenous pacemakers and exogenous zeitgebers?
Our internal biological rhythms must be finely tuned in order to stay in keeping with the outside world which is achieved through the use of endogenous pacemakers and exogenous zeitgebers which reset our biological rhythms everyday.
What are the difference between endogenous pacemakers and exogenous zeitgebers?
Endogenous pacemakers are internal biological rhythms. (eNdogenous, iNternal)
Exogenous zeitgebers are external factors eg light
(eXogenous, eXternal)
What is the most important endogenous pacemaker?
The Suprachiasmatic nuclei (SCN)
What is the SCN and where is it?
The SCN is a tiny cluster of nerve cells situated in the hypothalamus.
What are the 2 main functions of the SCN?
The SCN plays an important role in generating circadian rhythms by acting as a master clock which controls all other biological clocks throughout the body.
The SCN also regulates the manufacture and secretion of melatonin from the pineal gland.
How does the SCN successfully act as the master clock?
Neurons within the SCN synchronise with each other, so that target neurons elsewhere in the body can receive time coordinated signals - these peripheral clocks can maintain a circadian rhythm, but not for very long, which is why they are controlled by the SCN.
This is achieved because the SCN has a built in circadian rhythm which only needs resetting when external light levels change (if no change in light levelss, circadian rhythm should stay at 24 hours). The SCN receives information about light levels through the optic nerve where if the biological clock is running slow, morning light will shift the clock and reset it accordingly.
How does the SCN manufacture and secrete melatonin?
The SCN sends a signal to the pineal gland which prompt it to increase the production and secretion of melatonin at night and decrease it as light levels increase in the morning. Melatonin induces sleep by inhibiting the brain mechanisms which promote wakefulness.
What is an advantage of endogenous pacemakers?
Folkard (1996) studied a university student, Kate Aldcroft, who spent 25 days in a laboratory. She had no access to daylight or any other exogenous zeitgebers that might have reset the SCN where at the end of the 25 days, her core temperature rhythm was still at 24 hours (SCN has a built in circadian rhythm which is only affected by changes in light levels).
What is a disadvantage of endogenous pacemakers?
However, Kate Aldcroft’s sleep-wake cycle did increase to 30 hours where some periods of sleep were as long as 16 hours proving that we do in fact need external influences in order to maintain our circadian rhythms, SCN is not solely enough.
What are exogenous zeitgebers?
Exogenous Zeitgebers are environmental events that are responsible for maintaining the biological clock of an organism.
What is the most important Exogenous Zeitgeber?
Light
How does light help maintain biological clocks?
Receptors in the SCN are sensitive to changes in light levels during the day and use this information to synchronise the activity of the body’s organs and glands. Light also resets the internal biological clock each day, keeping it on a 24 hour cycle.
What is melanopsin?
Melanopsin is a protein in the retina of the eye which is sensitive to natural light and is essential for helping maintain biological clocks
What happens when people move to night shifts or travel to another country’s with different time zones?
Endogenous pacemakers will attempt to enforce their inbuilt rhythm of sleep (circadian rhythms) onto the individual, however, this will not be possible because it will be out of sync with the Exogenous Zeitgeber of light. This will ultimately cause disruption of sleep patterns, increased anxiety and decreased alertness and vigilance.
What are 3 advantages of exogenous zeitgebers?
+ Skene and Arendt (2007) found that the vast majority of blind patients who still have some light perception have normal circadian rhythms but completely blind patients had abnormal circadian rhythms (therefore, exogenous zeitgebers are essential for maintaining biological clocks)
+ Burgess et al. (2003) found that exposure to bright light before a flight to the east-west decreased the time needed to adjust to the local time of their destination upon arrival (Exogenous Zeitgebers have an affect on biological clocks)
+ Vetter et al. (2011) compared the sleep-wake cycle and activity-rest of two groups of participants over a five-week period. One group remained exposed to normal warm artificial light as the other group experienced blue artificial light. All participants were asked to keep a daily sleep log and wore devices that measured their movement. Participants who were exposed to the normal warm artificial light synchronised their circadian rhythms with the natural light of dawn. However, the participants who were exposed to the blue artificial light synchronised their circadian rhythms to office hours.
