AQA Psychology for A Level Year 2 - Student Bk

The specification says… endogenous pacemakers and exogenous zeitgebers The effect of endogenous pacemakers and exogenous zeitgebers on the sleep/wake cycle. Biological rhythms are influenced by two things: internal body clocks (endogenous pacemakers) and external cues in the environment (exogenous zeitgebers). As with nature and nurture in psychology, it is very difficult to separate the relative influence of pacemakers and zeitgebers as they are so closely linked. However, we shall attempt to examine the effect of each on one type of circadian rhythm: the sleep/wake cycle. Key terms Endogenous pacemakers – Internal body clocks that regulate many of our biological rhythms, such as the influence of the suprachiasmatic nucleus (SCN) on the sleep/wake cycle. Exogenous zeitgebers – External cues that may affect or entrain our biological rhythms, such as the influence of light on the sleep/wake cycle. Sleep/wake cycle – A daily cycle of biological activity based on a 24-hour period (circadian rhythm) that is influenced by regular variations in the environment, such as the alternation of night and day. If you’re a fan of chipmunks, the DeCoursey study (above right) is probably best avoided. Endogenous pacemakers and the sleep/wake cycle The suprachiasmatic nucleus (SCN) The suprachiasmatic nucleus (SCN) 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 visual area of the cerebral cortex . The SCN lies just above the optic chiasm (thus ‘supra’, which 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 the 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 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. Both of these studies emphasise the role of the SCN in establishing and maintaining the circadian sleep/wake cycle. The 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). 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). Exogenous zeitgebers and the sleep/wake cycle 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). Thus, sleeping and wakefulness would seem to be determined by 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 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. Social cues As every parent knows, infants are seldom on the same sleep/wake cycle as the rest of the family. In human infants, the initial sleep/wake cycle is pretty much random. At about 6 weeks of age, the circadian rhythms begin and by about 16 weeks, most babies are entrained. The schedules imposed by parents are likely to be a key influence here, including adult-determined mealtimes and bedtimes. Research also suggests that adapting to local times for eating and sleeping (rather than responding to ones own feelings of hunger and fatigue), is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances. Concepts: Use of studies You shouldn’t be short of studies to illustrate the effects of endogenous pacemakers and exogenous zeitgebers. As well as the studies presented on this spread, you can also discuss those in relation to circadian and infradian rhythms featured on previous spreads. Question Explain how the following studies could be used to support the influence of endogenous pacemakers and exogenous zeitgebers: ⦁ Siffre ⦁ Aschoff and Wever ⦁ Stern and McClintock @ookx hs @ookx hs Chapter 2 Biopsychology 50 •