Endogenous Pacemakers And Exogenous Zeitgebers

Question 1

What is the suprachiasmatic nucleus ?

Answer 1

• tiny bundle of nerves in the hypothalamus in each hemisphere
• primary endogenous pacemakers, maintains the circadian rhythm e.g. sleep/wake cycle
• SCN lies above the optic chiasm - receives information about light from this structure
• continuous when eyes are closed, allows biological clock to adjust to changing patterns of daylight whilst we are asleep

Question 2

Animal studies on the SCN

Answer 2

• DeCoursey et al; destroyed the CNS in chipmunks, returned to their natural habitat and observed for 80 days
• sleep/wake cycle disappeared, by the end of the study most were killed by predators
  … they were alive and active at the times they were supposed to be asleep

Question 3

The pineal gland and melatonin

Answer 3

• SCN passes info on day length and light it receives to the pineal gland (endogenous mechanism)
• during the night, pineal gland increases production of melatonin - chemical that induces sleep and is inhibited during periods of wakefulness

Question 4

Role of light as an exogenous zeitgeber

Answer 4

• it can reset the SCN and needed for the sleep wake cycle
• light effects processes in the body that control functions e.g. hormone secretion and circulation
• Murphy; light can be detected by skin receptor sites on the body when the same info is not received by the eyes
• light doesn’t rely on the eyes to exert its influence on the brain

Question 5

Entraining circadian rhythms - e.g. Social cues - babies and jet lag

Answer 5

• newborn babies sleep/wake cycle is random, at 6 weeks the circadian rhythm begins and by 16 weeks the rhythm has been entrained by the schedules of the parents, e.g. meal and bed times
• research on jet lag; adapting to local sleep and eat times is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances

Question 6

Evaluate endogenous pacemakers and the sleep/wake cycle

Answer 6

Limitation - obscured other body clocks
- research revealed that there are other circadian rhythms in other organs in the body
- et al; changing feeding patterns in mice could alter the circadian rhythms of cells in the liver up to 12 hrs, while the SCN rhythm was unaffected.

Limitation - endogenous pacemakers cannot be studied in isolation
- the cave study was rare, and Siffre’s had artificial light that may have reset his biological clock every time he put his lamp on
- in everyday life, pacemakers and zeitgebers interact, makes no sense to seperate them in studies

Question 7

Evaluation of exogenous zeitgebers and the sleep/wake cycle

Answer 7

Limitation - Exo.Zeit don’t have the same effect in all environments
- people who live in the arctic circle have similar sleep patterns all year round, despite spending 6 months in darkness
- sleep/wake cycle is primarily controlled by Endo.Pacemakers that can override environmental changes in light

Limitation - case study evidence against exo.zeit
- miles et al; man blind from birth, had an abnormal circadian rhythm of 24.9 hrs…
Despite exposure to social cues, his sleep/wake cycle couldn’t be adjusted
- suggests that social cues alone cant help reset a biological rhythm