Endogenous Pacemakers And Exogenous Zeitgebers

Question 1

What are endogenous pacemakers

Answer 1

Part of internal organism and are internal body clicks that regulate biological rhythms eg sleep/wake cycle

Question 2

What are exogenous zeitgebers

Answer 2

External cues in environ that may affect/entrain our biological rhythms eg influence of light on sleep/wake cycle

Question 3

What and where is the main endogenous pacemakers in mammals

Answer 3

A pair of tiny clusters of nerve cells called the suprachiasmic nucleus (SCN) in the hypothalamus, just above optic chiasm (where the optic nerves from each eye cross over)

Question 4

How does the SCN get info on light when eyes are shut

Answer 4

As light can penetrate the eyelids and special photoreceptors in the eye transfer light signals to the SCN

Question 5

What happens if endogenous clock running slow

Answer 5

The morning light automatically shifts the clock ahead so it’s in synchrony with world outside

Question 6

Outline De Coursey’s animal study

Answer 6

-destroyed SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days
-sleep/wake cycle of the chipmunks disappeared and by the end of the study a signif proportion had been killed by predators (presumably because they were awake and vulnerable to attack when should have been asleep)

Question 7

Outline Ralph’s animal study

Answer 7

-bred mutant hamsters with 20hr sleep/wake cycle
-when SCN cells from the foetal tissue of mutant hamsters were transplanted into brains of normal hamsters, the cycles of the second group defaulted to 20hrs

Question 8

What do the animal studies show

Answer 8

Emphasise the role of the SCN in establishing and maintaining the circadian sleep/wake cycle

Question 9

What does the pineal gland do

Answer 9

Contains light sensitive cells which when sense light melatonin production inhibited
-when light levels fall melatonin production increased which induces sleep by inhibiting brain mechanisms that promote the awake state

Question 10

Flow diagram for how sleep induced

Answer 10

-low levels of light (retina)
-melanopsin carries signals to SCN
-axon pathway to pineal gland
-melatonin produced
-induces sleep

Question 11

How does light act as exogenous zeitgeber

Answer 11

-can reset the body’s main endogenous pacemaker (SCN) and plays a role in maintenance of sleep/wake cycle

Question 12

Outline Campbell and Murphy’s study

Answer 12

-demonstrated that light may be detected by skin receptor sites on the body even when the same info is not received by the eyes
-15 ppts woken at various times and light pads shone on the back of their knees
-researchers managed to produce a devotion in ppt’s usual sleep/wake cycle of up to 3hrs in some cases suggesting light is powerful exogenous zeitgeber that does not necessarily rely on the eyes to exert its influence on the brain

Question 13

How do social cues act as exogenous zeitgeber

Answer 13

-in human infants the sleep/wake cycle pretty much random and by 16 weeks most babies entrained, schedule imposed by parents likely to be key influence including adult-determined mealtimes and bedtimes

Question 14

How to beat jet lag

Answer 14

Adapt to local times for eating and sleeping (rather than responding to one’s own feelings of hunger and fatigue) to entrain circadian rhythms

Question 15

Expand on strength for endogenous pacemakers that there are animal studies that show role of the SCN and however point

Answer 15

-demonstrated with animals through lesion studies and cellular implantation
-decoursay destroyed the SCN connections in 30 chipmunks and returned them to their natural habitat for 80 days, noted that many of them were killed by predation as they had ventured out of their nest at wrong time of day=normal sleep/wake cycle impaired
-Ralph implanted SCN cells from mutant hamsters who had sleep cycle of 20hrs into SCN of hamsters with normal cycle, for the hamsters who received the transplant their cycles changed to that of mutant donor(importance of SCN)
-manipulation of the SCN conducted under controlled conditions so results can be replicated
H: issues in generalising these findings to humans who have many complex social cues which may alter sleep/wake patterns, also unethical

Question 16

Expand on limitation of endogenous pacemakers that may be too simplistic to suggest SCN only endogenous pacemaker responsible for regulating sleep wake cycle

Answer 16

-research has revealed there are numerous circadian rhythms in many organs and cells of the body (peripheral oscillators) found in adrenal gland, lungs, liver and skin
-although these peripherals clocks are highly influenced by actions of the SCN they can act independently—> research demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in liver by up to 12hrs while leaving rhythm of the SCN unaffected
-suggests there may be many other complex influences on sleep/wake cycle aside from master clock SCN

Question 17

Expand on limitation that role of exogenous zeitgebers may be overstated

Answer 17

-there is evidence that although external environmental cues may vary, some individual’s pacemakers are set to withstand their influence
-case study of young man blind from birth with circadian rhythm of 25 hrs whose sleep wake cycle could not be adjusted regardless of changes to social cues
-similarly studies of individuals who live in arctic regions(where sun doesn’t set during summer months) show normal sleep patterns despite prolonged exposure to light
-suggests there are occasions where exogenous zeitgebers may have little bearing on our internal rhythm

Question 18

Expand on limitation that much of research looking at endogenous pacemakers and exogenous zeitgebers influence on sleep/wake cycle is case studies

Answer 18

-not possible to generalise these results to whole populations of people as there may be some individual differences in the research sample that makes these ppts respond to changes in sleep/wake cycle in atypical ways
-eg Siffre noted when he went into the cage at 60yo his body responded very differently with his body clock following more of a 48hr cycle
-suggests results cannot be generalised from these small samples onto everyone as factors eg age/gender may have signif impacts on our circadian rhythms