The effect of endogenous pacemakers and exogenous zeitgebers on the sleep/wake cycle. Flashcards
Define endogenous pacemakers?
Internal body clocks that regulate many of our biological rhythms, such as the influence of the suprachiasmatic nucleus (SCN) on the sleep/wake cycle
Define exogenous zeitgebers
External cues that may affect or entrain our biological rhythms, such as the influence of light on the sleep/wake cycle.
What is the main endogenous pacemaker in humans?
The Suprachiasmatic nucleus (SCN)
What is the SCN?
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.
What does the SCN do?
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.
What is another endogenous pacemaker?
The pineal gland
Explain the Pineal gland as an endogenous pacemaker
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.
What is melatonin?
A hormone which makes us sleepy.
What does the pineal gland do?
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
Who did research into endogenous pacemakers?
Siffre
Ralph et al
Decoursey
What did Siffre do?
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.
How was the SCN been studied?
The influence of the SCN has been demonstrated in studies involving animals (DeCoursey + Ralph)
What did DeCoursey do?
Destroyed the SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days.
What did DeCoursey find?
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).
What did Ralph do?
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.
What did Ralph find?
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.
What is an exogenous zeitgeber?
An external factor that entrains our biological rhythms. Without such a cue our EPs are described as ‘free running’.
Which 2 EZs do we look at?
Light
Social cues
How is light a key zeitgeber in humans?
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.
Give an example of light as an EZ
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).
Explain how social cues is an EZ
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.
What has been found about social cues and jet lag?
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.
Who did research into EZs?
Campbell and Murphy
What did Campbell + Murphy do?
Demonstrated how light can entrain our biological rhythms through skin receptor sites, even without that light reaching your eyes.
Explain Campbell + Murphy’s procedure
15 ppts were woken at various times and light was shone on the back of their knees only.
What did Campbell + Murphy find?
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.
Describe the interaction of both EPs and EZs
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.
