Exogenous zeitgeber: Flashcards
Exogenous zeitgeber:
An environmental cue, such as light, that helps to regulate the biological clock in an organism.
The term ‘exogenous’ refers to anything whose origins are outside the organism.
Exogenous zeitgebers are environmental events that are responsible for entraining the biological clock of an organism.
The most important zeitgeber for most animals is light.
Light:
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 resets the internal biological clock each day, keeping it on a 24-hour cycle.
Rods and cones in the retina of the eye detect light to form visual images.
However, there is a third type of light-detecting cell in the retina that gauges overall brightness to help reset the internal biological clock.
A protein called melanopsin, which is sensitive to natural light, is critical in this system.
A small number of retinal cells contain melanopsin and carry signals to the SCN to set the daily body cycle.
Social cues:
Social stimuli, such as mealtimes and social activities, may also have a role as zeitgebers.
Aschoff et al. (1971) showed that individuals are able to compensate for the absence of zeitgebers such as natural light by responding to social zeitgebers instead.
One of the earliest studies on jet lag (Klein and Wegmann, 1974) found that the circadian rhythms of air travellers adjusted more quickly if they went outside more at their destination.
This was thought to be because they were exposed to the social cues of their new time zone, which acted as a zeitgeber.
Likewise, the circadian rhythms of blind people were thought to be no different to sighted people as both groups were exposed to the same social cues.
We now know that both examples can better be explained in terms of light exposure acting as a zeitgeber.
The sleep-wake cycle of most blind people is still influenced by light during the day, even though they have no visual perception.
This is because connections exist between the eye and the SCN that do not involve those parts of the visual system on which the perception of light depends.
EVALUATION OF EXOGENOUS ZEITGEBERS:
Support for the role of melanopsin
The role of artificial light as a zeitgeber:
Support for the role of melanopsin:
The important role played by melanopsin in setting the circadian rhythm is demonstrated in studies of blind people.
Some blind people are still able to reliably entrain their circadian rhythm in response to light despite a total lack of image-forming visual perception (i.e. non-functioning rods and cones).
Skene and Arendt (2007) estimate that the vast majority of blind subjects who still have some light perception have normally entrained circadian rhythms.
This suggests that the pathway from retinal cells containing melanopsin to the SCN is still intact.
As further evidence for the importance of this pathway in setting the biological clock, people without light perception show abnormal circadian entrainment.
The role of artificial light as a zeitgeber:
Vetter et al. (2011) demonstrated the importance of light in the regulation of the sleep-wake and activity-rest patterns of two groups of volunteer participants over a five-week study period.
One group remained in normal warm’ artificial light over the five weeks while the other group experienced artificial ‘blue-enriched” light that had a spectral composition close to daylight
All participants kept a daily sleep log and wore devices that measured their movement over each 24-hour period.
Participants working under the warmer light synchronised their circadian rhythms each day with the natural light of dawn.
Over the course of the study, sunrise advanced by 42 minutes.
The participants who were exposed to blue-enriched light did not show the same 42-minute adjustment and instead synchronised their rhythms to office hours.
The results confirm that light is the dominant zeitgeber for the SCN and that its effectiveness depends on its spectral composition.
