Endogenous Pacemakers & Exogenous Zeitgebers

Question 1

Endogenous Pacemakers:

Answer 1

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

Question 2

Exogenous zeitgebers:

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

Sleep/wake cycle:

Answer 3

A daily cycle of the biological activity based on a 24 hour period (circadian rhythm) and that is influenced by regular variations in the environment, such as the alternation of night and day.

Question 4

SCN:

Answer 4

1. Tiny bundle of nerves located in the hypothalamus in each hemisphere of the brain.
2. It is one of the primary endogenous pacemakers in mammalian species and is influential in maintaining circadian rhythms.
3. The SCN lies just above the optic chiasm. It receives information about light directly from this structure.
4. This continues even when our eyes are closed.

Question 5

SCN: Animal Studies PDeC

Answer 5

1. Patricia DeCoursey et al. (2000) destroyed the SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days.
2. The SWC of the chipmunks disappeared and by the end of the study a significant proportion of them had been killed predators.

Question 6

SCN: Animal Studies MR

Answer 6

1. Martin Ralph et al. (1990) bred ‘mutant’ hamsters with a 20 hour SWC.
2. 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.
3. Both of these studies emphasise the role of the SCN establishing and maintaining the circadian the SWC.

Question 7

The pineal gland and melatonin:

Answer 7

1. The SCN passes the information on day length and light that it receives to the pineal gland behind the hypothalamus.
2. During the night, the pineal gland increases production of melatonin- a chemical that induces sleep and is inhibited during periods of wakefulness.
3. Melatonin has also been suggested as a causal factor in SAD.

Question 8

EZ: Light

Answer 8

1. Scott Campbell and Patricia Murphy (1998) demonstrated that light may be detected by skin receptors sites on the body even when the same information is not received by the eyes.
2. 15 participants were woken at various times and a light pad was shone on the back of their knees.
3. The researchers managed to produce a deviation in participants’ usual SWC of up to 3 hours.
4. This shows that light is a powerful EZ that does not rely on the eyes.

Question 9

EZ: Social Cues

Answer 9

1. In human infants, the SWC is pretty random.
2. At about 6 weeks of age, the CR begin and by about 16 weeks most babies are entrained.
3. The schedules imposed by parents are likely a key influence here, including adult determined mealtimes and bedtimes. Research also suggests that adapting to local times for eating and sleeping is an effective way of entraining CRs and beating jet lag.

Question 10

E: Beyond the Master Clock

Answer 10

1. Research has shown that there are many CRs in the organs and cells of the body.
2. They are called peripheral oscillators and are found in adrenal gland, oesophagus, lungs, liver e.t.c.
3. They are highly dependent on the actions of the SCN, they can act independently.
4. Francesca Damiola et al, (2000) demonstrated how changing feeding patterns in mice could alter the CR of cells in the liver by up to 12hrs, whilst leaving the rhythm of the SCN unaffected.
5. This shows that there are factors beyond the SCN.