Endogenous Pacemakers and Exogenous Zeitgebers

Question 1

What is the Suprachiasmatic Nucleus?

Answer 1

• tiny bundle of nerve cells located in the hypothalamus in each hemisphere, it is an endogenous pacemaker in governing the sleep wake cycle
• 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
• SCN lies just above the optic chiasm and receives light info, continues even when eyes are closed

Question 2

Who and what research using animals was done into the SCN?

Answer 2

• DeCoursey destroyed SCN connections in 30 chipmunks who were returned to habitat and observed for 80 days, sleep wake cycle disappeared and a lot were killed by predators (because awake when should have been asleep)
• Ralph bred mutant hamsters with a 20 hour sleep wake cycle, when SCN cells from foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters, the cycles of the second group defaulted to 20 hours

Question 3

What is the pineal gland?

Answer 3

• SCN passes light info to pineal gland, during the night it increases secretion of melatonin (hormone which induces sleep)

Question 4

What is an Exogenous Zeitgeber?

Answer 4

• External factors that ‘entrain’ endogenous pacemakers

Question 5

Light as an EZ?

Answer 5

• can reset the SCN and plays a role in sleep wake cycle maintenance also influences hormone secretion and blood circulation
• Campbell and Murphy thought light may be detected in skin receptor cells
• Fifteen pps were woken up and a light pad was shone on the back of their knee
• produced a deviation of up to 3 hours in pps sleep wake cycles

Question 6

Social Cues as an EZ?

Answer 6

• Newborn babies sleep wake cycles are effectively random at about 6 weeks circadian rhythm begins and by 16 weeks babies rhythms have been entrained by parent imposed schedules
• Jet lag research suggests that by adapting to local times for eating and sleeping is an effective way of entraining circadian rhythms and avoiding jet lag

Question 7

What is one limitation of SCN research?

Answer 7

• Other circadian rhythms in many organs (lungs, pancreas skin) and cells in the body, (peripheral oscillators) Influenced by SCN but also act independently
• Damiola showed how changing feeding patterns in mice altered circadian rhythms of liver by up to 12 hours while leaving SCN unaffected - shows other complex influences on sleep wake cycle.

Question 8

What is an additional limitation of research into SCN?

Answer 8

• Endogenous pacemakers cannot be studied in isolation. Isolation studies e.g Siffre’s are rare and even then he used a lamp (artificial exogenous zeitgeber?) in everyday the two constantly interact and so it is useless to seperate them

Question 9

What is one limitation of Exogenous Zeitgebers?

Answer 9

• do not have the same effect in all environments e.g people that live in the arctic circle have similar sleeping patterns all year round despite spending 6 months of the year in darkness, endogenous pacemakers can override exogenous zeitgebers

Question 10

What is an additional limitation of Exogenous Zeitgebers?

Answer 10

• evidence challenges the role of exogenous zeitgebers - Miles studied a blind man with an abnormal circadian rhythm of 24.9 hours, despite exposure to social cues it could not be adjusted. endogenous pacemaker more powerful than exogenous zeitgebers