The effect of endogenous pacemakers and exogenous zeitgebers on the sleep/wake cycle.

Question 1

Define endogenous pacemakers?

Answer 1

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

Question 2

Define 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

What is the main endogenous pacemaker in humans?

Answer 3

The Suprachiasmatic nucleus (SCN)

Question 4

What is the SCN?

Answer 4

this is the main EP in mammals and is a cluster of nerve cells in the hypothalamus. It has control other biological rhythms in the body.

Question 5

What does the SCN do?

Answer 5

It lies just above the optic chiasm. It recieves information about light via the optic nerve, adjusting the biological clock so that it is in step with the outside world.

Question 6

What is another endogenous pacemaker?

Answer 6

The pineal gland

Question 7

Explain the Pineal gland as an endogenous pacemaker

Answer 7

The SCN sends signals to the pineal gland: when light is sensed the production of melatonin in the pineal gland is inhibited. Therefore, we wake. When light levels fall, melatonin is produced and this makes us sleep. Therefore, this process regulates the sleep/wake cycle.

Question 8

What is melatonin?

Answer 8

A hormone which makes us sleepy.

Question 9

What does the pineal gland do?

Answer 9

The pineal gland contains light sensitive cells. When light is sensed the production of melatonin in the pineal gland is inhibited
When light levels fall, melatonin is produced which makes us feel sleepy
This process regulates the sleep-wake cycle

Question 10

Who did research into endogenous pacemakers?

Answer 10

Siffre
Ralph et al
Decoursey

Question 11

What did Siffre do?

Answer 11

Siffre’s studies of circadian rhythms show the importance of EPs. This is because the findings show that free running EPs, without the influence of EZs, are not successfully entrained to a 24hr cycle, but they still show a cycle nevertheless. They also show the importance of EZs, as we need them for the sleep wake cycle to be 24 hours.

Question 12

How was the SCN been studied?

Answer 12

The influence of the SCN has been demonstrated in studies involving animals (DeCoursey + Ralph)

Question 13

What did DeCoursey do?

Answer 13

Destroyed the SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days.

Question 14

What did DeCoursey find?

Answer 14

The sleep/wake cycle of the chipmunks had disappeared and by the end of the study a significant proportion of them had been killed by predators (presumably because they were awake, active and vulnerable to attack when they should have been asleep).

Question 15

What did Ralph do?

Answer 15

Bred a strain of mutant hamsters who had abnormal circadian rhuthms that lasted 20hrs rather than 24hrs. SCN neurons from these mutant hamsters were then implanted in normal hamsters.

Question 16

What did Ralph find?

Answer 16

These normal hamsters took on the unusual 20hr rhythm, indicating that the SCN (an EP) plays an essential role in establishing and regulating the circadian rhythm.

Question 17

What is an exogenous zeitgeber?

Answer 17

An external factor that entrains our biological rhythms. Without such a cue our EPs are described as ‘free running’.

Question 18

Which 2 EZs do we look at?

Answer 18

Light
Social cues

Question 19

How is light a key zeitgeber in humans?

Answer 19

It can reset the body’s main endogenous pacemaker (the SCN) and thus plays a role in the maintenance of the sleep/wake cycle. Light also has an indirect influence on key processes in the body that control such functions as hormone secretion and blood circulation.

Question 20

Give an example of light as an EZ

Answer 20

If our clock is running slow because e.g. the sun rises earlier than the day before, the morning light shifts the clock ahead so we are synchronised with the outside world
This process is known as entrainment (where two or more processes are synchronised).

Question 21

Explain how social cues is an EZ

Answer 21

These include shared eating times and traditional sleeping times, and these also reset our pacemakers. For example, there are pacemaker cells in the liver which reset upon eating food.

Question 22

What has been found about social cues and jet lag?

Answer 22

Jet lag can be overcome more easily if the traveller goes outside more and takes part in the social cues in the place they have travelled to.

Question 23

Who did research into EZs?

Answer 23

Campbell and Murphy

Question 24

What did Campbell + Murphy do?

Answer 24

Demonstrated how light can entrain our biological rhythms through skin receptor sites, even without that light reaching your eyes.

Question 25

Explain Campbell + Murphy’s procedure

Answer 25

15 ppts were woken at various times and light was shone on the back of their knees only.

Question 26

What did Campbell + Murphy find?

Answer 26

Some ppts experienced deviations in their sleep/wake cycle of up to 3 hours. This suggests that light is a powerful exogenous zeitgeber, even when light does not reach our eyes.

Question 27

Describe the interaction of both EPs and EZs

Answer 27

In our normal lives EPs and EZs work together. Our EPs, such as the SCN, are entrained each day by EZs such as natural light. This means our sleep/wake cycle is regulated to adapt to changing seasons and their longer and shorter days.