exogenous zeitgebers & endogenous pacemakers

Question 1

define endogenous pacemaker.

Answer 1

internal body clocks that regulate many of our biological rhythms, such as the influence of the SCN on the sleep/wake cycle.

Question 2

define exogenous zeitgeber.

Answer 2

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

Question 3

describe the function of the SCN.

Answer 3

located in the hypothalamus in each hemisphere of the brain.
one of the primary endogenous pacemakers in mammalian species.
maintains circadian rhythms, such as the sleep/wake cycle.
lies just above the optic chiasm, so receives info about light from it.
continues when our eyed are closed, enabling it to adjust to changing patterns of daylight when we are asleep.

Question 4

outline the sleep/wake cycle.

Answer 4

a daily cycle of biological activity based on a 24-hour-period (circadian) that is influenced by variations in the environment e.g. alternation of night and day.

Question 5

name two animal studies into the role of the SCN.

Answer 5

DeCoursey et al (2000) - chipmunks.
Ralph et al (1990) - ‘mutant’ hamsters.

Question 6

outline the study of DeCoursey et al (2000) into the role of the SCN.

Answer 6

destroyed SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days.
the sleep/wake cycle of the chipmunks disappeared and by the end of the study a significant proportion of then had been killed by predators.
supports the influence of the SCN on maintaining the sleep/wake cycle.

Question 7

outline the study of Ralph et al (1990) into the role of the SCN.

Answer 7

if ‘mutant’ hamsters are bred so they have a circadian rhythm of 20 instead of 24 hours and their SCN are transplanted into normal hamsters, the normal hamsters will displayed the mutant cycle of 20 hours.
supports the influence of the SCN on maintaining the sleep/wake cycle.

Question 8

outline the role of the pineal gland and melatonin in the maintenance of the sleep/wake cycle.

Answer 8

the SCN passes info on day length and light it receives to the pineal gland.
during the night, the pineal gland increases production of melatonin - induces sleep and is inhibited when awake.
also a causal factor in SAD.

Question 9

discuss research that limits the importance of the SCN.
(peripheral oscillators).

Answer 9

research has shown that there are numerous circadian rhythms in many organs and cells of the body - peripheral oscillators.
found in the adrenal gland, lungs, liver etc.
although highly influenced by the SCN, can function independently.
Damiola et al (2000) - showed how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, and leave the rhythms of the SCN unaffected.
suggests there may be other complex influences on the sleep/wake cycle, aside from the SCN. more complex than previously suggested.

Question 10

discuss ethics in animal studies as a limitation of research into endogenous pacemakers.

Answer 10

particularly in DeCoursey et al’s study, the animals were exposed to considerable harm, and subsequent risk when returned to their habitat.
violates ethical guidelines.
whether what we learn from investigations like this justify the aversive procedures is a matter of debate.

Question 11

name a key exogenous zeitgeber in humans.

Answer 11

light.

Question 12

name three other exogenous zeitgebers.

Answer 12

social cues (mealtimes, bedtimes).
temperature changes.
physical activity.

Question 13

outline light as an exogenous zeitgeber.

Answer 13

can reset the body’s main endogenous pacemaker - the SCN, thus plays a role in the sleep/wake cycle.
also has an indirect influence on key processes that control functions like hormone secretion.

Question 14

outline a study that supports the influence of light as an exogenous zeitgeber.

Answer 14

Campbell and Murphy (1998).
demonstrated that light may be detected by skin receptor sites on the body even when the same info isn’t received by the eyes.
15 ptps were awoken at various times and a light pad shone on the back of their knees.
produced a deviation in the ptps usual sleep/wake cycle of up to 3 hours.
suggests that light is a powerful exogenous zeitgeber that need not necessarily rely on eyes to exert influence on the brain.

Question 15

outline methodological issues in Campbell and Murphy’s study into light as an exogenous zeitgeber.

Answer 15

findings have yet to be replicated.
study had been criticised for the way it was conducted, suggested that there may have been some limited light exposure to the ptps eyes - a major confounding variable.
limits the validity of the experiment and the support it can give the theory.

also, isolating one exogenous zeitgeber (light) does not give insight into the many other zeitgebers that influence the sleep/wake cycle, and how they interact.
suggests the sleep/wake cycle is more complex than suggested in research.

Question 16

discuss how studies of individuals in arctic regions show a limitation of the influence of exogenous zeitgebers.

Answer 16

Miles et al - one man, who was blind from birth, had a circadian rhythm of 24.9 hours.
he was exposed to various exogenous cues, but could not regulate his internal pace. had to take stimulates in the morning and sedatives at night to set his rhythms with the rest of the world.

studies how that individuals in arctic regions (where the sun doesn’t set in summer months) show normal sleep patterns despite the prolonged exposure to sunlight.

both studies suggest there are occasions where exogenous zeitgebers may have little bearing on our internal rhythm.