Endogenous pacemakers and Exogenous zeitgebers

Question 1

Endogenous pacemakers (EP)

Answer 1

Internal body clocks regulate many of our biological rhythms, such as the influence of the suprachiasmatic nucleus (SCN) on the sleep/wake cycle.

Question 2

Exogenous zeitgebers (EZ)

Answer 2

External cues may affect or entrain out biological rhythms, such as the influence of light on the sleep/wake cycle.

Question 3

EP - suprachiasmatic nucleus (SCN)

Answer 3

A tiny bundle of nerve cells is located in the hypothalamus in each hemisphere.
One of the primary EP in mammals and is influential in maintaining circadian rhythms - sleep/wake cycle.
Nerve fibres connected to eye cross in an area - optic chiasm - on way to the visual area of the cerebral cortex.
SCN lies just above optic chiasm; received information about light directly and continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we’re asleep.

Question 4

EP - The influence of SCN has been demonstrated in…

Answer 4

animal studies.

Question 5

EP - What did Patricia DeCoursey et al (2000) do?

Answer 5

Destroyed the SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days.

Question 6

EP - Findings of Patricia DeCoursey et al (2000)

Answer 6

The sleep/wake cycle of the chipmunks disappeared and by the end of the study, a significant proportion of them had been killed by predators.

Question 7

EP - Martin Ralph et al (1990)

Answer 7

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

Question 8

EP - What do both the studies emphasise?

Answer 8

The role of the SCN in establishing and maintaining the circadian sleep/wake cycle.

Question 9

EZ - entrainment

Answer 9

When the external factors in the environment that reset our biological clocks through the process.

Question 10

EZ - What happens in the absence of external cues?

Answer 10

The free-running biological clock that controls the sleep/wake cycle continues to ‘tick’ in a distinct cyclical pattern.
Thus, sleeping and wakefulness would seem to be determined by an interaction of internal and external factors.

Question 11

EZ - light

Answer 11

A key zeitgeber in humans.
Resets the body’s main endogenous pacemaker, SCN, and thus plays a role in the maintenance of the sleep/wake cycle.
Light - indirect influence on key processes

Question 12

EZ - Campbell and Murphy (1998)

Answer 12

Demonstrated that light may be detected by skin receptor sites on the body even when the same information isn’t received by the eyes.
15 ppts were woken at various times and a light pad was shone on the back of their knees.
The researchers managed to produce a deviation in the ppts’ usual sleep/wake cycle of up to 3 hrs in some cases.
This suggests that light is a powerful EZ that need not necessarily rely on the eyes to exert its influence on the brain.

Question 13

EZ - Social cues

Answer 13

Infants are seldom on the same sleep/wake cycle as the rest of the family.
In infants, sleep/wake cycle is random.
At 6 weeks, the circadian rhythm begins, and by 16 weeks, most babies are entrained.
The schedules imposed by parents are likely to be a key influence.
Research also suggests that adapting to local times for eating and sleeping is an effective way of entraining circadian rhythms and beating jet lag when travelling long distances.

Question 14

Weakness - Methodological issues in studies

Answer 14

The findings from the Campbell and Murphy study have yet to be replicated. Other psychologists have been critical of the manner in which the study was conducted and have suggested that there may have been some limited light exposure to the participants’ eyes - a major confounding variable. Also, isolating one exogenous zeitgeber (light) in this way does not give us insight into the many other zeitgebers that influence the sleep/wake cycle, and the extent to which these may interact.

Question 15

Weakness/Strength - interactionist system

Answer 15

Only in exceptional circumstances are endogenous pacemakers free-running and unaffected by the influence of exogenous zeitgebers.
Total isolation studies, such as Siffre’s cave study, are extremely rare and could be judged as lacking validity for this reason. In real life, pacemakers and zeitgebers interact, and it may make little sense to separate the two for the purpose of research.

Question 16

Weakness - influence of exogenous zeitgebers may be overstated.

Answer 16

Laughton Miles et al (1977) recount 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 couldn’t be adjusted, and consequently, he had to take sedatives at night and stimulants in the morning to keep pace with the 24-hour world. Similarly, studies of individuals who live in Arctic regions show normal sleep patterns despite the prolonged exposure to light. Both these examples suggest that there are occasions when exogenous zeitgebers may have little bearing on our internal rhythm.

Question 17

Weakness - Ethics in animal studies

Answer 17

There are issues in generalising findings of the sleep/wake cycle from animal studies to humans. A more disturbing issue, however, particularly in relation to the DeCoursey et al study, is the ethics involved in such research. The animals were exposed to considerable harm, and subsequent risk, when they were returned to their natural habitat. Whether what we learn from investigations such as these juveniles the aversive procedures involved is a matter of debate.

Question 18

Weakness/ strength - beyond the master clock

Answer 18

Research has revealed there are numerous circadian rhythms in many organs and cells of the body. These are called peripheral oscillators and are found in the adrenal gland, esophagus, lungs, liver, pancreas, spleen, thymus, and skin. Although these peripheral clocks are highly influenced by the actions of the SCN, they can act independently. Francesa Damiola et al (2000) demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected. This suggests that there may be other complex influences on the sleep/wake cycle, aside from the master clock - SCN.