endogenous pacemakers and exogenous zeitgebers Flashcards
EP-The suprachiasmatic nucleus
The suprachiasmatic nucleus (SCN) was introduced on page 46. It is a tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain. It is one of the primary endogenous pacemakers in mammalian species (including humans) and 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 (supra means ‘above*). 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.
Animal studies and SCN
The influence of the SCN has been demonstrated in studies involving animals. 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).
In another study, Martin Ralph et al. (1990) bred ‘mutant’ hamsters with a 20-hour sleep/wake cycle.
When SCN cells from the foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters, the cycles of the second group defaulted to 20 hours.
pineal gland and melatonin
The SCN passes the information on day length and light that it receives to the pineal gland (a pea-like structure in the brain just behind the hypothalamus). This is another endogenous mechanism guiding the sleep/wake cycle. During the night, the pineal gland increases 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 seasonal affective disorder (see previous spread).
EZ-light
The German word zeitgeber means ‘time giver. Exogenous zeitgebers are external factors in the environment that reset our biological clocks through a process known as entrainment. We have 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 - see page 46). This free-running cycle is then ‘brought into line (i.e. entrained) by environmental cues, so there is an interaction of internal and external factors.
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.
In an innovative study, 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.
Fifteen participants were woken at various times and a light pad was shone on the back of their knees. The researchers managed to produce a deviation in the participants usual sleep/wake cycle of up to three 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.
EZ-social cues
As every parent knows, babies are seldom on the same sleep/wake cycle as the rest of the family, in fact 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.
Limitation-beyond master clock
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.
Limitation-interactionist system
Another limitation is that endogenous pacemakers cannot be studied in isolation.
Total isolation studies, such as Michael Siffre’s cave study (see page 46) are extremely rare.
Remember also that 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 the influence of internal pacemakers,
the lower the validity of the research.
Limitation-environmental determinism
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, people who live within the Arctic Circle (e.g. the Inuits of Greenland) have similar sleep patterns all-year round, despite spending around six months in almost total darkness.
This suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light.
limitation-case study evidence
Another limitation is evidence challenges the role of exogneous 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.
