Endogenous Pacemakers and Exogenous Zeitgebers

Question 1

Endogenous pacemakers (EP)

Answer 1

• internal body clock

- controls and regulates many biological rhythms, such as the influence of the SCN on the sleep/wake cycle

Question 2

EP - suprachiasmatic nucleus

Answer 2

• SCN is a very small collection of nerve cells in the hypothalamus
• a main endogenous pacemaker within mammals
• it is influential in maintaining circadian rhythm
• the SCN is just above the optic chiasm, which is where the nerve fibres that are connected to the eye cross over while on their way to the visual area of the cerebral cortex
• the SCN receives info about light directly from the optic chiasm, and this continues even when out eyes are closed
• biological clock can adjust to changing daylight patterns while asleep

Question 3

EP - DeCoursey et al. (2000)

Answer 3

• study on chipmunks
• SCN connections were removed from their brains
• they were then returned to nature and observed
• the sleep/wake cycle within these chipmunks seemed to vanish, and a considerable number if them were killed by predators
• this suggests that they were awake when they should have been asleep, m which made them vulnerable to predators

Question 4

EP - Ralph et al. (1990)

Answer 4

• one hamster with a 20-hour sleep/wake cycle was bred with another hamster
• he found that when the SCN cells from the foetal tissue of the 20-hour cycle hamsters was inserted into the normal hamsters, they to the same sleep/wake cycle
• both this study and DeCoursey’s emphasise the role of the SCN in the sleep/wake cycle

Question 5

EP - Pineal gland and melatonin

Answer 5

• info on light and day lengths which the SCN receives is passed to the pineal gland
• this is a tiny, pea-like structure in the brain behind the hypothalamus
• the pineal gland increases production of melatonin during the night, and this is a chemical inducing sleep
• this is then inhibited during periods of being awake
• melatonin is also said to be the cause of SAD

Question 6

Exogenous zeitgebers (EZ)

Answer 6

• external factors within the environment that reset out biological clocks
• this process is known as entrainment e.g. influence of social cues and light
• when there are no exogenous zeitgebers, the free running biological clock controlling the sleep/wake cycle continues in a distinct cyclical pattern
• an interaction between internal and external factors is seen be determine sleeping and being awake

Question 7

EZ - light

Answer 7

• this is one of the key zeitgebers
• it is able to reset the body’s principal endogenous pacemakers
• it maintains the sleep/wake cycle as well
• indirectly influences key processes in the body which control functions, such as blood circulation and hormone secretion

Question 8

EZ - Campbell and Murphy (1998)

Answer 8

• study to show that light may be detected by skin receptor sites on the body
• this was even the case when the same info warns being received by the eyes of the individual
• 15 participants took part
• they were all woken up at different times, followed by a light being shone at their knees
• this resulted in the production of a deviation in the participant’s sleep/wake cycle
• in some cases, this disruption was by up to 3 hours
• this study shows that light is a very powerful exogenous zeitgeber and that it doesn’t rely upon on the eyes to influence the brain

Question 9

EZ - social vues

Answer 9

• a person’s sleep/wake cycle is pretty random for the first 6 weeks of their life
• however, when it gets past this age circadian rhythms kick in and most babies are entrained into a usual cycle by 16 weeks
• schedules are most likely the key influence in this case, put in place by parents e.g. bedtimes and mealtimes
• there is research which suggests that adapting to localised times for sleep and eating rather than being influenced by one’s own desires to eat and sleep is effective as a way of entraining circadian rhythms
• this also helps with other things such as overcoming jet lag

Question 10

(+) EVAL - other influences of sleep/wake cycle

Answer 10

• it has been revealed that there are many circadian rhythms in a number of organs and cells within the body
• these are known as peripheral oscillators
• they are found in the adrenal glands, liver, lungs, oesophagus, pancreas, spleen, skin and thymus
• they are highly influenced by the SCN but can act independently as well
• Damiola et al. (2000) conducted a study to show how changing the feeding patterns in mice could affect their liver’s circadian rhythms by up to 12 hours
• however, the rhythm of the SCN was unaffected
• there may therefore be many other complex influences on the sleep/wake cycle aside from the SCN

Question 11

(-) EVAL - ethics in animal studies

Answer 11

• when using animal studies and not human studies there is an issue with generalisability
• but there is also the problem of ethics, particularly in DeCoursey’s study
• the chipmunks were exposed to significant harm and subsequent risk when returned to the wild
• this sparks the debate auto whether the value of the research outweighs the costs

Question 12

(-) EVAL - overstated influence of exogenous zeitgebers

Answer 12

• Miles et al. (1977) produced the story of a man who was blind from birth and had a 24.9hr sleep/wake cycle
• despite his exposure to social cues his sleep/wake cycle couldn’t be altered
• the had to take sedatives at night and stimulates in the morning to get his circadian rhythm to 24hrs
• another example of people disobeying exogenous zeitgebers is seen in arctic regions
• these people display normal sleeping patterns although the sun does set in the summer months
• these two examples suggest that the role of exogenous zeitgebers may be overstated