Biopsychology - Controlling Biological Rhythms Flashcards
What are key features of endogenous pacemakers and the sleep/wake cycle?
- The suprachiasmatic nucleus is a primary endogenous pacemaker.
- Influence of the suprachiasmatic nucleus on the sleep/wake cycle demonstrated with chipmunks and hamsters.
- Pineal gland and melatonin are endogenous mechanisms.
What is the suprachiasmatic nucleus an endogenous pacemaker?
The SCN is a tiny bundle of nerve cells in the hypothalamus which helps maintain circadian rhythms (e.g. sleep/wake cycle).
Nerve fibres from the eye cross at the optic chiasm on their way to the visual cortex. The SCN lies just above the optic chiasm and receives information about light from this structure.
How do chipmunks and hamsters demonstrate the influence of SCN on the sleep/wake cycle?
DeCoursey et al. (2000) destroyed SCN connections in the brains of 30 chipmunks which were returned to their natural habitat and observed for 80 days. Their sleep/wake cycle disappeared and many were killed by predators.
Ralph et al. (1990) bred ‘mutant’ hamsters with a 20-hour sleep/wake cycle. SCN cells were transplanted from the foetal tissue of these hamsters into the brains of normal hamsters, which then developed cycles of 20 hours.
How are the pineal gland and melatonin endogenous mechanisms?
The SCN passes information on day length to the pineal gland which increases production of melatonin during the night.
Melatonin is a hormone that induces sleep and is inhibited during periods of wakefulness. It has also been suggested as a causal factor in seasonal affective disorder.
What are key features of exogenous zeitgebers and the sleep/wake cycle?
- External environmental factors that reset biological clocks.
- Light is a key exogeneous zeitgeber that influences the sleep/wake cycle.
- Social cues also have an important influence on the sleep/wake cycle.
What are zeitgebers?
External environmental factors that reset biological clocks. Resetting biological clocks is a process known as entrainment.
Without external cues, the free-running biological clock continues to ‘tick’ in a cyclical pattern. Zeitgebers reset the sleep/wake cycle: an interaction of internal and external factors.
How is light a key exogenous zeitgeber that influences the sleep/wake cycle?
Light can reset the body’s main endogenous pacemaker (SCN), and also has an indirect influence on key processes in the body controlling hormone secretion, blood circulation, etc.
Campbell and Murphy (1998) woke 15 participants at various times and shone a light on the backs of their knees - producing a deviation in the sleep/wake cycle of up to three hours. Light is a powerful exogenous zeitgeber detected by skin receptor sites and does not necessarily rely on the eyes to influence the SCN.
How do social cues have an important influence on the sleep/wake cycle?
The sleep/wake cycle is fairly random in human newborns, but most babies are entrained by about six weeks.
Schedules imposed by parents are a key influence, including adult-determined mealtimes and bedtimes.
Research also shows adapting to local times for eating and sleeping (not responding to one’s own feelings of hunger and fatigue) entrains circadian rhythms and tackles jet lag.
What are the weaknesses of research into endogenous pacemakers and exogenous zeitgebers?
- the suprachiasmatic nucleus may obscure other body clocks
- the use of animals
- the influence of exogenous zeitgebers may be overstated
- there are methodological issues in exogenous zeitgebers research
- endogenous pacemakers and exogenous zeitgebers interact
Why is one limitation of research into the SCN that it may obscure other body clocks?
Body clocks (peripheral oscillators) are found in many organs and cells (e.g. lungs, liver, skin, pancreas). They are highly influenced by the actions of the SCN but can act independently.
Damiola et al. (2000) showed how changing feeding patterns in mice altered circadian rhythms of cells in the liver for up to 12 hours, leaving the rhythm of the SCN unaffected.
This suggests there may be many other complex influences on the sleep/wake cycle, aside from the master clock (SCN).
Why is the use of animals a weakness?
There is an issue in generalising findings from research into the sleep/wake cycle from animal studies because cognitive factors may be more significant in humans.
A more disturbing issue (e.g. in the DeCoursey et al. study) is the ethics of such research - animals were exposed to great harm and potential risk when returned to their natural habitat.
Whether what we learn from investigations on biological rhythms justifies the aversive procedures involved is a matter of debate.
How may the influence of exogenous zeitgebers be overstated?
Miles et al. (1977) note the case of a man blind from birth with a circadian rhythm of 24.9 hours. His sleep/wake cycle could not adjust to social cues so he took sedatives at night and stimulants in the morning to aligh with the 24-hour world.
Similarly, studies of individuals who live in Arctic regions (where the sun does not set during the summer months) show normal sleep patterns despite prolonged exposure to light.
Both these examples suggest there are occasions when exogenous zeitgebers may have little bearing on our internal rhythms.
What methodological issues are there in exogenous zeitgebers research?
Campbell and Murphy’s study has yet to be replicated and is criticised because there may have been some light exposure to participants’ eyes - a major confounding variable.
Also, isolating one exogenous zeitgeber (light) in this way does not give insight into the many other zeitgebers that influence the sleep/wake cycle.
This suggests that some studies may have ignored or underplayed the way in which different exogenous zeitgebers interact.
How do endogenous pacemakers and exogenous zeitgebers interact?
Only in exceptional circumstances do circumstances do endogenous pacemakers free-run unaffected by exogenous zeitgebers.
Total isolation experiences (e.g. Siffre’s study) are extremely rare and present an unrealistic view of how the system works.
Endogenous pacemakers and exogenous zeitgebers interact in real life - it makes no sense to separate them just for research purposes.
How can you conclude an essay?
Biological pacemakers are inbuilt but external zeitgebers help us to synchronise and make it adaptable. However, long-term disruption can have a detrimental effect.
