endogenous pacemakers & exogenous zeitgebers

• Endogenous pacemaker - internal body clock
• The SCN is one of the key endogenous pacemakers in mammals and is responsible for maintaining the circadian rhythms
• The SCN is made up of a tiny bundle of nerve fibres and is found in the hypothalamus in the brain. It lies close to the optic chiasm
• The SCN receives information about light directly from the optic chiasm, even when our eyes are closed enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep
• Decoursey et al (2000) destroyed the SCN in the brains of 30 chipmunks who then returned to their natural environment and were observed for 80 days. The sleep/wake cycle had disappeared and a significant number had been killed by predators
• Ralph et al (1990) bred mutant hamsters who had a sleep/wake cycle of 20 hours. When SCN cells from foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters they also had a 20 hours sleep/wake cycle
• Both Ralph's and Decoursey's studies demonstrate the role of the SCN in establishing and maintaining the circadian sleep/wake cycle
• Exogenous zeitgeber - external change in the environment
• Light is a key zeitgeber for the sleep/wake cycle in humans
• Social cues are also a factor in the sleep/wake cycle
• Campbell and Murphy (1998) demonstrated that light may be detected by the skin even if information is not received by the eyes. 15 pps 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 sleep/wake cycle by up to 3 hours in some cases
• As infants, the initial sleep/wake cycle is random
• At about 6 weeks, the circadian rhythms begin and by about 16 weeks, most babies are entrained. Schedules imposed by parents are likely a key influence including adult determined meal times and bed times
• research has improved our understanding of the role of peripheral clocks in the regulation of circadian rhythms. Damiola et al (2000) showed that changing the feeding pattern in mice could change the circadian rhythm in the liver but leave the SCN unaffected
• A limitation of the research in this field is the ethics of animal studies e.g Decoursey's study
• Laughton Miles et al (1977) is a case study of a man, blind from birth, with a circadian rhythm of 24.9 hours. Because his sleep/wake cycle couldn’t be adjusted despite social cues, he had to take sedatives (night) and stimulants (morning) to keep pace with the 24 hour world
• studies of individuals living in arctic regions (where the sun doesn’t set during summer) show normal sleep patterns despite prolonged exposure to light, suggesting the role of exogenous zeitgebers have little effect on our internal rhythms
• The findings in Campbell and Murphy’s study have yet to be replicated
• Only in exceptional circumstances are endogenous pacemakers free-running and unaffected by exogenous zeitgebers. Total isolation studies such as Siffre’s cave study are extremely rare and could be judged as lacking external validity for this reason
• a criticism is that in real life pacemakers and zeitgebers interact and it may make little sense to separate them for the purpose of research