L13- Endogenous pacemakers and wxogenous zeitgeibers

Question 1

intro

Answer 1

Biological rhythms are regulated by endogenous pacemaker = internal biological clocks, and exogenous zeitgebers =external cues (light) that help to regulate the internal biological
clocks

Question 2

endogenous pacemakers ao1

Answer 2

• most important endogenous pacemaker is the SCN (cluster of nerve cells in hypothalamus) = important role in generating circadian rhythms.
• master clock, linking other brain regions that control sleep and arousal, and has control over other biological
  clocks throughout the body.
• Neurons in SCN synchronise so target neurons in sites elsewhere in the
  body receive time-coordinated signals. - These peripheral clocks can maintain a circadian rhythm, but not for
  verylong, which is why they are controlled by the SC.
• This is possible because of the SC’s built in circadian rhythm, which only needs resetting when external light levels change.
• SC receives info about light levels through the optic nerve. If our biological clock is running slow then morning light shifts the clock.
  -The SN also regulates the manufacture and secretion of melatonin in the pineal gland via the interconnecting neural pathway
  -The SCN sends a signal to the pineal gland, directing it to increase production and secretion of
  the hormone melatonin at night and to decrease it as light levels increase in the morning. Melatonin induces
  sleep by inhibiting the brain mechanisms that promote wakefulness.

Question 3

strengths of endogenous pacemakers

Answer 3

+ The SCN is of vital importance. Morgan (1995) bred a strain of hamster that had abnormal circadian rhythms of 20 hours. SCN neurons from these
hamsters were transported to normal hamsters. The normal hamsters started to display the abnormal
circadian rhythms of 20 hours. This study shows the vital importance of SN and how endogenous pacemakers are important for biological circadian
rhythms.
+ Folkard (1996) studied a university student, Kate Aldcroft, who spent 25 days in a laboratory. She had no access to daylight or other exogenous zeitgebers that might have
reset the SCN. At the end of 25 days her core temperature rhythm was still at 24 hours. This study therefore shows the importance of endogenous pacemakers

Question 4

weaknesses of endogenous pacemakers

Answer 4

-studied on animals - unethical (breeding hamsters to have abnormal circadian rhythms), it is not
ecologically valid since in the real world, people do not have their circadian rhythms deliberately tampered with.

• However, Folkard (1996) also found that Kate’s sleep wake cycle had extended to 30 hours with periods of sleep as long as 16 hours being recorded. This suggests that
  exogenous zeitgebers may also be important in controlling
  the sleep-wake cycle.

Question 5

Exogenous zeitgeibers- light

Answer 5

• most important zeitgebers for most animals is light. Receptors in the SN are sensitive to changes in light levels during the day and use this info to synchronise activity of the body’s organs and glands.
• Light resets internal biological clock each day, keeping it on a 24h cycle.
• A protein in the retina of the eye called melanopsin, which is sensitive to natural light, is critical in this system.

-When people move to a night shift or travel to a country with a different time zone their endogenous pacemakers try to impose their inbuilt rhythm of sleep (circadian rhythm), but this is
now out of synchrony with the exogenous zeitgeber of light. This will lead to disrupted sleep
patterns, increased anxiety and decreased alertness and vigilance - this typically is known as jetleg.

Question 6

Exogenous zeitgeibers- social cues

Answer 6

• Social stimuli such as meal times and activities may also have a role as zeitgebers. For example,
  Aschoff et al. (1971) showed that individuals are able to compensate forthe absence of natural light
  by responding to social cues instead e.g. mealtimes.
• Like wise the circadian rhythms of blind people were thought to be no different that sighted people
  as both groups were exposed to the same social cues. This can be better explained in terms of light exposure acting as a zeitgeber. The sleep-wake cycle of most blind people is still influential by light during the day even though they can’tsee - because connections exist between the eye and SCN that do not involve the necessity of being sighted.
• Also, in human infants, the sleep/wake cycle is pretty random. The circadian rhythms begins at
  about 6 weeks and by 16 weeks, most babies have some sort of sleep/wake cycle - this is due to the
  parents imposing mealtimes and bedtimes showing how exogenous zeitgebers such as control of
  mealtimes and bedtimes can effect the sleep/wake cycle.
• research also suggests that adapting to local times for eating and sleeping (rather than respondine
  to one’s own feelings of hunger and fatigue) is an effecting way of entraining circadian rhythms and
  beating jetlag when travelling long distances.

Question 7

strengths of exogenous zeitgeibers

Answer 7

• The role of melanopsin in setting the circadian rhythms comes from studies of blind people since kene and
  Arendt (2007) estimate that the vast majority of blind subjects who still have some light perception have
  normal circadian rhythms. This suggests that the pathway from retinal cells containing melanopsin to the SCN
  is intact. Thus even for blind people, exogenous zeitgebers are still important in the sleep wake cycle.
• Using light exposure to avoid jet lag was studied by Burgess et al. (2003) who found that exposure to bright
  light prior to an east-west flight decreased the time needed to readjust to local time on arrival. This study therefore is an important application of the role of exogenous zeligebers because It suggests that light is an
  extremely influential exogenous zeitgeber.

Question 8

weaknesses of exogenous zeitgeibers

Answer 8

Vetter compared Wake and activity rest patterns or swo groups of participants over a five- Week period One group remained in
Nornal Warm light , while the othergroup experienced artificial blue light, All participants kept a dally
sleep log and wore devices that measured their movement. Participants under the warm light synchronised their circadian rhythms each day with the natural light of dawn. The participants exposed to blue light synchronised their patterns to office hours. This study suggests that the shade of light is an important
influencer of the sleep-wake cycle as well as whether the light is artificial or natural. Clearly natural light is a
more influential exogenous zeitgeber on the sleep/wake cycle compared to artificial light. So we should be
cautious about the findings of studies that look at light.