Endogenous pacemakers and Exogenous zeitgebers

Question 1

What are sometimes known as biological ‘clocks’?

Answer 1

Endogenous pacemakers.

Question 2

What are endogenous pacemakers?

Answer 2

Internal body clocks that regulate many of our biological rhythms.

Question 3

What are exogenous zeitgebers?

Answer 3

External cues that may affect/entrain our biological rhythms.

Question 4

What is the sleep/wake cycle?

Answer 4

A daily cycle of biological activity based on the 24-hour period that is influenced by regular variations in the environment.

Question 5

Is the influence of the suprachiasmatic nucleus (SCN) an endogenous pacemaker or exogenous zeitgeber?

Answer 5

Endogenous pacemaker.

Question 6

Is the alternation of light during the night and day an endogenous pacemaker or exogenous zeitgeber?

Answer 6

Exogenous zeitgeber.

Question 7

What is the suprachiasmatic nucleus (SCN)?

Answer 7

A tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain.

Question 8

Which factor is responsible for resetting the clock every day?

Answer 8

Exogenous zeitgebers.

Question 9

What is the optic chiasm?

Answer 9

Where optic fibres from each eye cross over.

Question 10

What is the role of the suprachiasmatic nucleus (SCN)?

Answer 10

It receives information of light levels directly from the optic nerve and sends information to the pineal gland to secrete melatonin.

Question 11

Why can we respond to light levels even when we are asleep?

Answer 11

Because the SCN is always receiving data even when the eyes are closed.

Question 12

What happens when the SCN receives information of low light levels?

Answer 12

It sends signals to the pineal gland and melatonin is secreted.

Question 13

Are social cues an endogenous pacemaker or exogenous zeitgeber?

Answer 13

Exogenous zeitgeber.

Question 14

What is the suprachiasmatic nucleus called this?

Answer 14

‘supra’ = ‘above’

above the optic chiasm.

Question 15

When does the pineal gland release melatonin?

Answer 15

When the SCN stimulates the gland as light levels dip.

Question 16

Why is it significant that the SCN can detect light levels all the time?

Answer 16

It allows us to enable the biological clock to adjust to changing patterns of daylight whilst we are asleep.

Question 17

Describe the method of Patricia DeCoursey et al. (2000).

Answer 17

• Destroyed the SCN connections in brains of 30 chipmunks.

- Chipmunks returned to natural habitat and observed for 80 days.

Question 18

Describe the findings of Patricia DeCoursey et al. (2000).

Answer 18

• The sleep / wake cycle disappeared.
• As a result a significant proportion were killed by predators as they were asleep when they should have been awake, thus vulnerable to attack.

Question 19

What is the relationship between animal studies and the SCN?

Answer 19

The influence of the SCN has been demonstrated in many animal studies; including DeCoursey (2000) and Ralph (1990).

Question 20

Who conducted a study where the SCN connections of chipmunks were destroyed?

Answer 20

Patricia DeCoursey et al. (2000).

Question 21

Describe the findings of Martin Ralph et al. (1990).

Answer 21

• When the SCN cells from mutant hamsters where transferred to the ‘normal’ hamsters, their cycle defaulted to 20 hours.

Question 22

Who conducted a study where they bred ‘mutant’ hamsters?

Answer 22

Martin Ralph et al. (1990).

Question 23

Describe the method of Martin Ralph et al. (1990).

Answer 23

• He bred ‘mutant’ hamsters which had a 20 hour sleep / wake cycle.

Question 24

Which gland is responsible for release of melatonin?

Answer 24

Pineal gland.

Question 25

What is the role of melatonin?

Answer 25

Melatonin induces sleep and is inhibited during wakeful periods.

Question 26

Which hormone has been suggested as a causal factor in Seasonal Affective Disorder (SAD)?

Answer 26

Melatonin.

Question 27

What does ‘zeitgeber’ mean?

Answer 27

‘Time giver’.

Question 28

What is the initial infant sleep-wake cycle?

Answer 28

It is more or less random.

Question 29

When does a babies circadian rhythms begin to appear?

Answer 29

By 6 weeks.

Question 30

By what age are most babies entrained by?

Answer 30

16 weeks.

Question 31

What role do social cues have in babies sleep / wake cycle?

Answer 31

Adult imposed patterns such as deciding when infant eats and sleeps are social cues which the babies become trained to.

Question 32

What role do social cues have in jet lag?

Answer 32

Eating and sleeping at new local times is the fastest way to entrain your self to a new clock.

Question 33

What is the influence of light as a zeitgeber for humans?

Answer 33

It resets the SCN as the body’s endogenous pacemaker and thus plays a role in maintaining the sleep / wake cycle.
It also has an indirect influence on hormone secretion and blood circulation.

Question 34

What did Siffre demonstrate about the effect of light as an exogenous zeitgeber?

Answer 34

Siffre demonstrated that when external cues are not available the free running body clock continues to work in a cyclical pattern but can extend beyond working in a 24 hour period.

Question 35

Does light rely on the eyes to exert its influence on the brain?

Answer 35

No, light can be detected by skin receptor sites even when the information is not received by the eyes.

Question 36

Describe the study of Campbell and Murphy (1998).

Answer 36

• 15 ptps were woken at various times by shining a light pad on the backs of the knees (not the eyes).
• Produced a 3 hour deviation in the normal cycle.

Question 37

What is the practical application of exogenous zeitgebers?

Answer 37

Burgess (2003):

• 28 healthy ptps due to travel east.
• Prior to travel, exposed to morning light using a light box to advance circadian rhythm (by 1 hour / day for 3 days).
• Melatonin samples were taken to determine ptps rhythms and side effects also recorded.
• Results showed exposure to morning did help advance ptps rhythms, reducing the effects of jet lag.

Question 38

What did Schwartz et al find with reference to the effects of light as an exogenous zeitgeber?

Answer 38

East coast baseball teams playing away on the west coast had a higher win rate than vice-versa.

Question 39

AO3 - Animal studies.

Answer 39

The studies of DeCoursey (2000) and Ralph (1990) are hard to generalise as they use animals which are not humans. These studies also had poor ethics as the animals were exposed to considerable harm, and subsequent risk when they were returned to their natural habitat.

Question 40

Who studied the effect of light as an exogenous zeitgeber?

Answer 40

Campbell and Murphy (1998).

Question 41

AO3 - Methodological issues / Confounding variables.

Answer 41

The findings of Campbell and Murphy have not been replicated, they have both been criticised of the manner whihc they conducted their studies and suggested there may have been some exposure of light to patients eyes (which is a major confounding variable).

Question 42

AO3 - Interactionist system.

Answer 42

There are few circumstances where endogenous pacemakers are free-running and unaffected by the influence of exogenous zeitgebers. Studies such as Siffre’s cave study are rare and can be said to lack validity for this reason. In real life the two interact together and cannot be separated.

Question 43

AO3 - SCN not the only factor.

Answer 43

Research has revealed there are numerous circadian rhythms in many organs and cells, these are called peripheral oscillators, and are found in the adrenal gland, oesophagus, lungs, liver, pancreas, spleen, thymus and skin. Although they are influenced by the SCN, these can act independently. Damiola (200) demonstrated how changing feeding patterns in mice could alter circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected, suggesting there are other complex influences on the sleep / wake cycle aside from the SCN.

Question 44

AO3 - EZ influence overstated.

Answer 44

Miles (1977) recounted the story of a young man, blind from birth, with a circadian rhythm of 24.9 hours. Despite exposure to social cues, his sleep/wake cycle could not be adjusted. Similarly, studies of individuals who live in Arctic regions show normal sleep patterns despite prolonged light exposure. These suggest that there are occasions when exogenous zeitgebers have little bearing on our internal rhythm.

Question 45

What are peripheral oscillators and where are they found?

Answer 45

circadian rhythms in many organs and cells, found in the adrenal gland, oesophagus, lungs, liver, pancreas, spleen, thymus and skin.