L13 - Endogenous Pacemakers & Exogenous Zeitgebers On The Sleep Wake Cycle Flashcards
What are biological rhythms regulated by?
- endogenous pacemakers, which are the body’s internal biological clocks,
- exogenous zeitgebers, which are external cues, including light, that help to regulate the internal biological clocks.
- This means that the sleep wake cycle can be influence by both endogenous and exogenous factors.
Endogenous Pacemakers
- most important endogenous pacemaker is the suprachiasmatic nucleus (SCN) - a tiny cluster of nerve cells in the hypothalamus.
- it plays important role in generating circadian rhythms.
- acts as the master clock, linking other brain regions that control sleep and arousal, and has control over other biological clocks throughout the body.
- Neurons within the SCN synchronise with each other, so that their target neurons in sites elsewhere in the body 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 possible because of the SCN’s built in circadian rhythm, which only needs resetting when external light levels change. The SCN receives information about light levels through the optic nerve. If our biological clock is running slow then morning light shifts the clock.
What else does the SCN regulate
- The SCN also regulates the manufacture and secretion of melatonin in the pineal gland via the interconnecting neural pathway.
- SCN sends a signal to the pineal gland, directing it to increase production and secretion of the hormone melatonin at night and to decrease it as light levels increase in the morning.
- Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness.
Evaluation of Endgenous Pacemakers
strengths
- hamster study
- other animal study
- further research support
weaknesses
- unethical
- exogenous zeitgebers
Hamster study
- The SCN is of vital importance as an endogenous pacemaker and was demonstrated in Morgan’s study of hamsters.
- Morgan (1995) bred a strain of hamster that had abnormal circadian rhythms of 20 hours. SCN neurons from these hamsters were transported to normal hamsters.
- The normal hamsters started to display the abnormal circadian rhythms of 20 hours.
- Then transplanted SCN neurons from normal hamsters into the brains of abnormal hamsters and found that the recipient hamsters changed their abnormal circadian rhythm to a circadian pattern of 24 hours.
- study shows the importance of SCN and how endogenous pacemakers are important for biological circadian rhythms.
Other animal study
- further research support of the SCN as an important endogenous pacemaker on the sleep/wake cycle in other animal studies.
E.g. DeCoursey et al (2000) destroyed the SCN connections to the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days. - The sleep/wake cycle of the chipmunks disappeared and by the end of the study a significant proportion of them had been killed by their predators (presumably because they were awake, active and vulnerable to attack when they should have been asleep!).
- study shows the importance of the SCN as an important endogenous pacemaker on the sleep/wake cycle as if you don’t have the SCN you could have an abnormal sleep wake cycle and be more vulnerable to danger or ultimately death!
Research support
Another piece of research to support the importance of the SCN as an important endogenous pacemaker was by Folkard (1996) who studied a university student, Kate Aldcroft, who spent 25 days in a laboratory. She had no access to daylight or other exogenous zeitgebers that might have reset the SCN. At the end of 25 days her core temperature rhythm was still at 24 hours. This study therefore shows the importance of endogenous pacemakers.
Ethical issues
Unfortunately, many studies to study the SCN have been carried out on animals where unethical procedures are carried out to meet the requirements of the study, one could argue that not only is this unethical (breeding hamsters to have abnormal circadian rhythms), it is not ecologically valid since in the real world, people do not have their circadian rhythms deliberately tampered with.
Exogenous Zeitgebers (EP Eval)
However, Folkard (1996) also found that Kate’s sleep wake cycle had extended to 30 hours with periods of sleep as long as 16 hours being recorded. This suggests that exogenous zeitgebers may also be important in controlling the sleep-wake cycle. Therefore this study shows that exogenous zeitgebers such as light and social cues may have more effect on the sleep wake cycle than endogenous pacemakers like the SCN.
Exogenous Zeitgebers
The term exogenous refers to anything whose origins are outside of the organism. Exogenous zeitgebers are environmental events that are responsible for maintaining the biological clock of an organism. Examples of exogenous zeitgebers are light, clocks and social cues which have a significant impact on the sleep/wake cycle.
2 types of exogenous zeitgebers
- light
- social cues
Light
- The most important zeitgebers for most animals is 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 say, keeping it on a 24-hour cycle.
- A protein in the retina of the eye called melanopsin, which is sensitive to natural light, is critical in this system.
- 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 now out of synchrony with the exogenous zeitgeber of light.
- This will lead to disrupted sleep patterns, increased anxiety and decreased alertness and vigilance – this typically is known as jet leg.
Light
- The most important zeitgebers for most animals is 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 say, keeping it on a 24-hour cycle.
- A protein in the retina of the eye called melanopsin, which is sensitive to natural light, is critical in this system.
- 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 now out of synchrony with the exogenous zeitgeber of light.
- This will lead to disrupted sleep patterns, increased anxiety and decreased alertness and vigilance – this typically is known as jet leg.
Social cues
- Social stimuli such as meal times and activities may also have a role as zeitgebers.
E.g. Aschoff et al. (1971) showed that individuals are able to compensate for the absence of natural light by responding to social cues instead e.g. mealtimes. - Likewise the circadian rhythms of blind people were thought to be no different that sighted people as both groups were exposed to the same social cues - can be explained in terms of light exposure acting as a zeitgeber.
- The sleep-wake cycle of most blind people is still influential by light during the day even though they can’t see – because connections exist between the eye and SCN that do not involve the necessity of being sighted.
- in human infants, the sleep/wake cycle is pretty random.
- circadian rhythms begins at about 6 weeks and by 16 weeks, most babies have some sort of sleep/wake cycle – this is due to the parents imposing mealtimes and bedtimes showing how exogenous zeitgebers such as control of mealtimes and bedtimes can effect the sleep/wake cycle.
- Research also suggests that adapting to local times for eating and sleeping (rather than responding to one’s own feelings of hunger and fatigue) is an effecting way of entraining circadian rhythms and beating jet lag when travelling long distances.
Evaluation of exogenous zeitgebers
strengths
- role of melanopsin
- jet lag
weaknesses
- not all studies support it
