Endogenous pacemakers and exogenous zeitgebers

Question 1

What are endogenous pacemakers?

Answer 1

Internal body clocks that regulate many of our biological rhythms

Question 1

What are exogenous zeitgebers?

Answer 1

External factors that affect or entrain our biological rhythms

Question 2

What is the suprachiasmatic nucleus (SCN)?

Answer 2

• Tiny bundle of nerve cells
• Located in the hypothalamus in each hemisphere of the brain
• One of the primary endogenous pacemakers
• Influences maintenance of circadian rhythms (e.g. sleep/wake)

Question 3

What happens to nerve fibres in the SCN?

Answer 3

• Nerve fibres= connected to the eye
• Cross in the optic chiasm on their way to the left and right visual area of the cerebral cortex

Question 4

Where does the SCN lie and what is its relation to this?

Answer 4

Just above the optic chiasm and receives information about light from this structure (still happens when our eyes are closed- enabling the bio clock to adjust to changing patterns of daylight whilst asleep)

Question 5

What did DeCoursey et al do?

Answer 5

• Destroyed SCN connections in brains of 30 chipmunks, who returned to neutral habitat and were observed for 80 days

Question 6

What are DeCoursey et al’s findings?

Answer 6

• Sleep/wake cycle of chipmunks disappeared and a significant proportion had been killed by predators

Question 7

What did Ralph et al do and find?

Answer 7

• Bred ‘mutant hamsters’ with a 20 hour sleep/wake cycle
• When SCN cells from foetal tissue of mutant hamsters were transplanted into brains of normal hamsters, the cycles of the 2nd group defaulted to 20 hours

Question 8

How is the SCN related to the pineal gland and melatonin?

Answer 8

• SCN passes information on day light to the pineal gland (pea-like structutre, behind hypthothalamus)
• During night, the pineal gland increases melatonin production (chemical inducing sleep)

Question 9

Limitation:
I- SCN research obscures other body clocks

Endogenous pacemakers

Answer 9

D- Numerous circadian rhythms in many cells/organs in the body. Peripheral oscillators are found in organs (i.e. lungs, pancreas, skin) and are influenced by the actions of the SCN, but also act independently. Damiola demonstrates how changing feeding patterns in mice after the circadian rhythm of cells in the liver by up to 12 hours leaves the SCN unaffected
E- Suggests other complex influences on the sleep/wake cycle

Question 10

Limitation:
I- Interactionist system

Endogenous pacemakers

Answer 10

D- Endogenous pacemakers cannot be studied in isolation. Total isolation studies (Siffre) are very rare. Siffre used artificial light, which could have reset his biological clock every time he turned the lamp on. In daily practice, pacemakers and zeitgebers interact and it may make little sense to separate the two for research purposes
E- Suggests the more researchers attempt to isolate the influence of internal pacemakers, the lower the research’s validity

Question 11

Evaluation extra:
I- Ethical issues

Endogenous pacemakers

Answer 11

Limitation: Animals in DeCoursey’s study were exposed to considerable risk when returned to their natural habitat as their sleep/wake cycle had been altered, and most were killed by predators as a result

Strength: Animal studies are justified as there are similar mechanisms at work across species. Existence of the SCN and pineal gland in animals means generalisation can be made to the human brain

Question 12

What does zeitgeber mean?

Answer 12

Time giver

Question 13

What is the role of light as an exogenous zeitgeber?

Answer 13

• Light can reset the body’s main endogenous pacemaker (SCN) so plays a role in maintenance of the sleep/wake cycle
• Light has an indirect influence on bodily processes that control functions (hormone secretion/ blood circulation)

Question 14

What did Campbell and Murphy do and find?

Answer 14

• Demonstrated that light may be detected by skin receptor sites on the body, even when the same information is not received by the eyes
• 15 participants were woken at various times and a light pad was shone on the backs of their kness
• Managed to produce deviation in participants’ usual sleep/wake cycle of up to 3 hours
• Suggests light is a powerful exogenous zeitgeber that does not need to rely on the eyes to exert its influence on the brain

Question 15

What is the role of social cues?

Answer 15

• Newborn babies sleep/wake cycle is random
• At 6 weeks old, the circadian rhythm begins and by 16 weeks, babies’ rhythms have been entrained by schedules imporsed by parents, including: adult-determined mealtimes and bedtimes
• Jet lag studies suggest adapting to local times for eating and sleeping is effective in entraining circadian rhythms and beating jet lag

Question 16

Limitation:
I- Environmental observations

Exogenous zeitgebers

Answer 16

D- Exogenous zeitgebers have different effects in different environments. People living in places with little darkness in summer and little light in winter face different experiences to the usual narrative. E.g. people living in the Artic Circle have simialr sleep patterns all year round depsite spending 6 months in nearly total darkness
E- Suggests the sleep/wake cycle is primarily controlled by endogenous pacemakers that can override environmental changes in light

Question 17

Limitation:
I- Case study evidence

Exogenous zeitgebers

Answer 17

D- Evidence challenges role of exogenous zeitgebers. Miles et al recounts a study of a young man, who experienced blindness from birth, who had an abnormal circadian rhythm of 24.9 hours. Despite exposure to social cues (e.g. regular mealtimes) his sleep/wake cycle could not be adjusted
E- Suggests social cues alone are not effective in resetting the biological rhythm

Question 18

Evaluation extra:
I- Age-related insomnia

Exogenous zeitgebers

Answer 18

Strength: Evidence suggests people have poorer quality sleep as they get older. 30% aged 60+ have chronic insomnia. This may be due to natural changes in the body’s circadian rhythm with age, which can result in falling asleep earlier and experience broken sleep. Treated by medication to increase melatonin levels

Limitation: Studies suggest exogenous factors may be responsible for the sleep pattern change in the elderly. Hood et al found the management of insomnia was improved if older people were more active and had more exposure to natural light