Endogenous And Exogenous Zeitgebers

Question 1

What are endogenous pacemakers?

Answer 1

Mechanisms within the body that govern the internal, biological bodily rhythms.

Question 2

What are exogenous zeitgebers?

Answer 2

Environmental cues that help to regulate the biological clock in an organism.

Question 3

What are two examples of endogenous pacemakers?

Answer 3

1. The suprachiasmatic nucleus (SCN)
2. The pineal gland

Question 4

What are two examples of exogenous zeitgebers?

Answer 4

1. Light
2. Social cues

Question 5

What is entrainment?

Answer 5

When our biological clock is reset each day by cues in the environment, such as the light cues of sunrise and sunset.

Question 6

Describe how the pineal gland governs biological rhythms

Answer 6

Pineal gland= A pea-like structure in the brain just behind the hypothalamus
SCN sends signals to the pineal gland directing it to either increase the production and secretion of melatonin at night and to decrease it as light levels increase in the morning. Melatonin induces sleep.

Question 7

Describe how the SCN regulates biological rhythms

Answer 7

A tiny bundle of nerves in the hypothalmus found in each hemisphere. It receives light through the optic chasm and alerts the pineal gland to secrete melatonin to ensure that our biorhythms synchronise with the outside world.

Question 8

Research support for SCN

Answer 8

Morgan (1995)
Bred hamsters to have circadian rhythms of 20 hours.
SCN neurons of these abnormal hamsters transplanted into normal hamsters.
Repeated the other way round.
Normal hamsters displayed 20hr rhythms and abnormal changed to 24hr rhythms.

Question 9

Research support for light as an exogenous zeitgeber.

Answer 9

Campbell and Murphy (1998)
15 participants awoken at various times and a light pad shone on the back of their knee. This produced a deviation in their usual sleep wake cycle of up to 3 hours.

Question 10

Research support for exogenous zeitgebers - social cues

Answer 10

Klein and Wegmen (1974)
Got air travellers to venture out or stay in mor when thy arrived at their destination.
Found that those who ventured out adjusted more quickly to the time zone

Question 11

3 Strengths - edogenous pacemakers and exogenous zeitgebers

Answer 11

Research support from animal studies for the role of the SCN:
Morgan (1995) - see last lesson

Research support from humans for influence of light:
Campbell and Murphy (1998)- see last lesson

Practical applications for jetlag:
Burgess et al. (2003)- Those who were exposed to the continuous bright light prior to the flight needed less time to readjust to the local time on arrival.
This suggests that light exposure prior to a flight would allow travellers to arrive with their circadian rhythms already partially re-entrained to local time.

Question 12

1 limitation for endogenous ps and exogenous zs

Answer 12

Influence of exogenous zeitgebers may be overstated:
Miles et al. (1977) - 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 and 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 (where the sun does not set during the summer months) show normal sleep patterns despite the prolonged exposure to light.
Both of these example suggest that there are occasions when exogenous zeigebers may have little bearing on our internal rhythm.