Biopsychology: Endogenous pacemakers and exogenous zeitgebers Flashcards
What are endogenous pacemakers?
Mechanisms within the body govern the internal biological body rhythms and allow us to keep pace with the changing cycles in the environment.
What is the primary endogenous pacemaker in humans and how does it work?
The SCN is a tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain. One of the primary endogenous pacemakers in mammalian species is influential in maintaining circadian rhythms such as the sleep/wake cycle. Nerve fibres connected to the eye cross in an area called the optic chiasm on their way to the left and right visual area of the cerebral cortex. The SCN lies just above the optic chiasm. It receives information about light directly from this structure. This continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep.
What are animal studies to do with the SCN?
The influence of the SCN has been demonstrated in animal studies. - Patricia DeCoursey et al, 2000, destroyed the SCN connections in 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 predators (presumably because they were awake, active and vulnerable to attack when they should have been asleep).
Endogenous pacemakers - The Pineal Gland and Melatonin:
The SCN passes the information on day length and light that it receives to the pineal gland. This is another endogenous mechanism guiding the sleep/wake cycle. During the night, the pineal gland increases the production of melatonin - a chemical that induces sleep and is inhibited during periods of wakefulness. Melatonin has also been suggested as a causal factor in SAD.
What are exogenous zeitgebers?
External factors affect or entrain our biological rhythms, such as the influence of light on the sleep/wake cycle. Exogenous zeitgebers are external factors in the environment that reset our biological clocks through a process known as entrainment. - Seen that, in the absence of external cues, the free-running biological clock that controls the sleep/wake cycle continues to ‘tick’ in a distinct cyclical pattern (as in the Siffre study).
Exogenous zeitgebers - Light
Light is a key zeitgeber in humans. It can reset the body’s main endogenous pacemaker, the SCN, and thus plays a role in the maintenance of the sleep/wake cycle. Light also has an indirect influence on key processes in the body that control such functions as hormone secretion and blood circulation.
Receptors in the SCN are sensitive to changes in light levels during the day and use the information to synchronise the activity of the body’s organs and glands. Light resets the internal biological clock every 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 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 the system. A small number of retinal cells called melanopsin to carry signals to the SCN to set the daily body cycle.
Exogenous zeitgebers - Light (Scott Campbell and Patricia Murphy, 1998)
Scott Campbell and Patricia Murphy, 1998, demonstrated that light may be detected by skin receptor sites on the body even when the same information is not received by the eyes. 15 participants were woken at various times and a light pad was on the back of their knees. The researchers managed to produce a deviation in the participants’ usual sleep/wake cycle of up to 3 hours in some cases. This suggests that light is a powerful exogenous zeitgeber that need not necessarily rely on the eyes to exert its influence on the brain.
Exogenous zeitgebers - Social Cues
Newborn babies’ initial sleep/wake cycle is pretty much random. At about 6 weeks of age, the circadian rhythms begin and, by about 16 weeks, babies’ rhythms have been entrained by the schedules imposed by parents, including adult-determined mealtimes and bedtimes. Research on jet lag suggests that adapting to local times for eating and sleeping (rather than responding to one’s own feelings of hunger and fatigue) is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances.
Evaluation of Exogenous zeitgebers:
+ Evidence suggests that people have poorer quality sleep as they get older.
- Don’t have the same effect in all environments.
- Evidence challenges the role of exogenous zeitgebers.
Evaluation of Endogenous pacemakers:
- SCN research is that it may obscure other body clocks.
- Endogenous pacemakers cannot be studied in isolation
- Animal studies of the sleep/wake cycle are justified because there are very similar mechanisms at work across species.
Evaluation of Exogenous zeitgebers: Evidence suggests that people have poorer quality sleep as they get older
Evidence suggests that people have poorer quality sleep as they get older. This may be due to natural changes in the circadian rhythm as we age, which means falling asleep earlier and breaking sleep at night (Duffy et al, 2015). However, studies have suggested that exogenous factors may be responsible for the changes in sleep patterns among older people. Bernadette Hood et al, 2004, found that the management of insomnia was improved if elderly people were generally more active and had more exposure to natural light.
Evaluation of Exogenous zeitgebers: Don’t have the same effect in all environments
One limitation is that exogenous zeitgebers do not have the same effect in all environments. The experience of people who live in places where there is very little darkness in summer and very little light in winter tell a different story from the usual narrative. For instance, the Inuits of the Arctic Circle are said to have similar sleep patterns all-year round, despite spending around 6 months in almost total darkness. This suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light.
Evaluation of Exogenous zeitgebers: Evidence challenges the role of exogenous zeitgebers
Another limitation is evidence that challenges the role of exogenous zeitgebers. Laughton Miles et al, 1977, recount the study of a young man, blind from birth, who had an abnormal circadian rhythm of 24.9 hours. Despite exposure to social cues, such as regular mealtimes, his sleep/wake cycle could not be adjusted. This suggests that social cues alone are not effective in resetting the biological rhythm.
Evaluation of Endogenous pacemakers: SCN research is that it may obscure other body clocks
One limitation of SCN research is that it may obscure other body clocks. Research has revealed that there are numerous circadian rhythms in many organs and cells in the body. These peripheral oscillators are found in the organs including the lungs, pancreas and skin. They are influenced by the actions of the SCN, but also act independently. Francesca Damiola et al, 2000, demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected. This suggests other complex influences on the sleep/wake cycle.
Evaluation of Endogenous pacemakers: Endogenous pacemakers cannot be studied in isolation
Another limitation is that endogenous pacemakers cannot be studied in isolation. Total isolation studies, such as Michael Siffre’s cave study are extremely rare. Also, Siffre made use of artificial light which could have reset his biological clock every time he turned his lamp on. In everyday life, pacemakers and zeitgebers interact, and it may make little sense to separate the two for the purpose of research. This suggests the more researchers attempt to isolate.
