Endogenous Pacemakers & Exogenous Zeitgebers

Question 1

SCN

Answer 1

• A bundle of nerves located in the hypothalamus in each hemisphere of the brain
• It is one of the primary endogenous pacemakers in mammals and is influential in maintaining circadian rhythms like the sleep/wake cycle
• Nerve fibres connected to the eye cross in the optic chiasm on their way to the visual area of the cerebral cortex; the SCN lies just above the optic chiasm
• It receives information about light directly from the optic chiasm – this continues even when our eyes are closed
• This enables the biological clock to adjust to changing patterns of daylight even whilst we are asleep

Question 2

Decoursey et al (2000)

Answer 2

• They 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)

Question 3

Ralph et al (1990)

Answer 3

• They bred mutant hamsters with a 20hr sleep/wake cycle
• When SCN cells from the foetal tissue of mutant hamsters was transplanted into the brains of normal hamsters, the cycles of the 2nd group defaulted to 20hrs

Question 4

Pineal Gland & Melatonin

Answer 4

• The SCN passes info on day length and light that it receives to the pineal gland, which is a pea-like structure in the brain just behind the hypothalamus
• During the night, the pineal gland increases production of melatonin, which is the chemical that induces sleep and is inhibited during periods of wakefulness; melatonin has been suggested as a causal factor in seasonal affective disorder (SAD)

Question 5

Exogenous Zeitgeber Info

Answer 5

• They are external factors in the environment that reset our biological clocks through entrainment
• Without external cues, the free running biological clock controls the sleep/wake cycle – this was seen in Siffre’s study
• This shows that the sleep/wake cycle is determined by an interaction of endogenous (internal) and exogenous (external) factors

Question 6

Light As A Zeitgeber

Answer 6

• Can reset the body’s main endogenous pacemaker, the SCN, so 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

Question 7

Campbell & Murphy (1998)

Answer 7

• They demonstrated that light may be detected by skin receptor sites on the body even when the same info is not received by the eyes
• 15 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 3hrs in some cases
• This suggests that light is a powerful zeitgeber that doesn’t need to rely on the eyes to exert influence on the brain

Question 8

Social Cues As A Zeitgeber

Answer 8

• Babies are rarely in the same sleep/wake cycle as the rest of the family – for infants, the sleep/wake cycle is apparently random
• Circadian rhythms begin around 6 weeks of age and by 16 weeks, most babies are entrained – the schedules imposed by parents are a key influence
• Research suggests that adapting to local times for eating and sleeping is an effective way of entraining biological rhythms and beating jet lag when travelling long distances