Endogenous Pacemakers and Exogenous Zeitgerbers

Question 1

the suprachiasmatic nucleus

Answer 1

• bundle of nerve cells located in the hypothalamus in each hemisphere of the brain
• primary endogenous pacemaker in maintaining circadian rhythms
• nerve fibres connected to eye cross in the optic chasm on their way to the left and right visual areas of the cerebral cortex
• SCN lies just above the optic chiasm receiving information about light directly from this structure
• happens even when eyes are closed enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep

Question 2

animal studies and the SCN

Answer 2

• DeCoursey et al (2000) destroyed the SCN in 30 chipmunks who were then returned to their natural habitat and observed for 80 days
• sleep wake cycle of the chipmunks disappeared and a significant proportion of them have been killed because they were awake at night
• Ralph et al (1990) bred mutant hamsters with a 20 hour sleep wake cycle
• when the SCN cells from the mutant hamsters were transplanted into the brains of normal hamsters their cycles defaulted to 20 hours

Question 3

the pineal glands and melatonin

Answer 3

-SCN information is received by the pineal gland
- during the night the pineal gland increases production of melatonin which induces sleep and is inhibited when awake
- causal factor in SAD

Question 4

limitation of endogenous pacemaker and the sleep/wake cycle

Answer 4

1. beyond the master clock
   - SCN research may obscure other body clocks
   - research has revealed that there are numerous circadian rhythms in many organs and cells in the body
   - peripheral oscillators are found in the organs of the lungs, pancreas and skin
   - influenced by the actions of the SCN but also act independently
   - study showed how changing feeding patterns in mice could alter the circadian rhythm of cells in the liver by up to 12 hours whilst leaving the rhythms of the SCN unaffected
   - suggest that other complex infelnese on the sleep/wale cycle
2. interactionist system
   - endogenous pacemakers cannot be studied in isolation
   - total isolate studies like Siffre’s case study are extremely rare
   - siffre use of artificial light could have also reset his biological clock
   - in everyday life pacemakers and zeitgebers interact and it makes no sense to separate the 2 for the purpose of research
   - lovers the validity of research

Question 5

light as an exogenous zeitgeber

Answer 5

• resets the SCN so plays a role in the sleep/wake cycle
• indirect influence on key processes in the body that controls hormone secretion and blood circulation
• Campbell and Murphy (1998) demonstrated that light may be detected by skin receptor sight on he body even when the same information is not received by the eyes
• 15 participants were woken at various times as a light pad was shown on the back of their knees
• prodced a deviation in the particpants sleep wake cycle of up to 3 hours

Question 6

social cues as an exogenous zeitgeber

Answer 6

1. babies initial sleep/wake cycle is pretty random
   - by 16 weeks babies rhythms have been entrained by the schedule imposed by parent
2. research on jet lag suggests that adapting to local times for eating and sleeping is an effective way of entraining circadian rhythms and beating jet lags when travelling long distances

Question 7

limitation of exogenous zeitgerbers

Answer 7

1. environmental observations
   - exogenous zeitgebers do not have the same effect in all environments
   - people who live in the arctic circle have similar sleep patterns all year round despite sounding 6 months in almost complete darkness
   - suggest sleep wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light
2. case study evidence
   - Miles et al (1977) studied a young man blind from birth who had an abnormal circadian rhythm of 24.9 hours
   - despite exposure to social cues like regular mealtime his sleep wake cycle could not be adjusted