Endogenous pacemakers and exogenous zeitgeibers

Question 1

Endogenous pacemakers

Answer 1

• Internal body clocks.
• Centres in the brain that play a main role in controlling biological rhythms.
• Clock is endogenous meaning its a part of the organism not part of the environment.
• Clocks help organism to anticipate what is coming e.g night.
• Clock needs to be reset with changing season so that organism can adapt to changing levels of light.
• Clock reset every day by cues in environment.

Question 2

SCN

Answer 2

• A tiny cluster of nerve cells in hypothalamus where main endogenous pacemaker is located.
• SCN is a pair of structures on half situated in left hemisphere of brain one in the right above where optic nerves from each eye cross over.
• SCN receives info on light from optic nerve even when eyes are not open.
• If clock is slow, sun rises earlier than previous day morning light automatically shifts the clock ahead.
• Results in clock being in rhythm with outside world.

Question 3

Pineal Gland and Melatonin

Answer 3

• Pineal gland contains light sensitive cells.
• If light is detected the production of melatonin in pineal gland is inhibited.
• When light is low the pineal gland produces melatonin which induces sleep by stopping brain mechanisms that promote wakefulness.

Question 4

Evaluation of endogenous pacemakers - research for the role of the SCN and melatonin

Answer 4

• Wealth of it
• Morgan bred mutant hamsters that a circadian rhythm of 20 minutes instead of 24 hours. SCN were transplanted into normal hamsters who then displayed the mutant rhythms.
• Strength - studies support role of the SCN as a pacemaker as when SCN was altered the sleep wake pattern was altered.

Question 5

However - research for the role of the SCN and melatonin

Answer 5

• Ethical issues with animal research into SCN.
• Animals were exposed to harm and if they were not returned to their natural habitat after the study they may have been at risk. This is because they may no longer have been adapted to their environment.
• Weakness - provides an argument against using animal studies to support the role of the SCN.

Question 6

Evaluation of Endogenous Pacemaker - Smith Magenis Syndrome

Answer 6

• Individuals with Smith Magenis syndrome have difficulty falling asleep.
• Potocki et al suggested that in these people melatonin rises in the early morning instead of at night. Taking melatonin at night can help some of the symptoms.
• Strength - supports for the role of melatonin in sleep wake cycle.

Question 7

However - Smith Magenis Syndrome

Answer 7

• Sackett and Kornor found that conjoined twins do not share the same circadian rhythms. Questions role of melatonin in sleep wake cycle because melatonin circulates in the bloody and we would therefore expect conjoined twins to share circadian rhythms.

Question 8

Evaluation of Endogenous - Separate Rhythms

Answer 8

• SCN is master clock that co ordinates all other rhythms.
• Folkard studied Kate Aldcroft who volunteered to spend 25 days in a controlled lab. She had no access to daylight or other zeitgebers that might have reset the SCN. At the end of the experiment her core temp rhythm was still 24 hours but her sleep wake cycle had extended to 30 hours.
• Weakness - study casts doubt on the role of the SCN as the master clock as it demonstrated that in some circumstances the SCN is not in complete control as the different rhythms can become out of step with eachother.
• Due to research being a case study this is just one person. Dilutes validity of this researching disputing EP

Question 9

Exogenous Zeitgeber

Answer 9

• Environmental factors that help synchronize biological rhythms with the outside world.
• The best example is light which resets the circadian biological clock.

Question 10

Exogenous Zeitgeber - Light

Answer 10

• Receptors in SCN detect changes in light and use this info to synchronize the activity of organs and glands.
• Rods and cones in the retina of the eye detect light from visual images in addition a third type of cell in the retina gauges overall brightness to reset the internal biological clock.
• Light sensitive protein called melanopsin is important in this system, carrying signals to the SCN.

Question 11

Exogenous Zeitgeber - Social Cues

Answer 11

• Absense of natural light we can respond to social zeitgebers instead e.g. mealtimes, social activites
• Klein and Wegman found that if long distance travellers went outside more, exposing them to the social cues of the new time zone their circadian rhythms adjusted more quickly.
• Their social cues of their new time zone acted like a zeitgeber and helped with jet lag.
• Rhythms of slighted and blind people are the same. Was thought to be due to both individuals being exposed to same social cues. It is now known that the zeitgeber of light is a better explanation.
• Sleep wake cycle in the majority of blind people is still influenced by daylight because connections exist between the eye and SCN that dont involve parts of the visual system which the perception of light depends on.

Question 12

Evaluation of Exogenous Zeitgeber - evidence for light as an exogenous pacemaker

Answer 12

• Some blind people can entrain their circadian rhythm by detecting light despite not being able to form images.
• Skene and Arendt found that most blind people who still have some light perception have normally entrained circadian rhythms.
• Strength - evidence that their pathway from retinal cells containing melanopsin to the SCN is still intact. Individuals without perception show abnormal circadian rhythms. Further evidence for this pathway synchronising the biological clock.

Question 13

However - evidence for light as an exogenous pacemaker

Answer 13

• Research has been based on the EP of individuals that are blind, due to limited sample size it questions validity of its supporting EP.

Question 14

Evaluation of Exogenour Zeitgeber - Jet Lag

Answer 14

• Wealth of supporting research
• Burgess et al found exposure to bright light before a west east flight decreased the time needed to readjust to the local time when they arrived. Exposed volunteers to one of three treatments which shifted their sleep wake cycle. Continuous bright light group shifted rhythm by 2.1 hours. Intermittent bright light group shifted rhythm by 1.5 hours. Dim light group shifted rhythm by 0.6 hours. As a result p in the first group felt sleepier 2 hours earlier in the evening and woke 2 hours earlier in the morning.
• Strength - supports role of bright light as an exogenour zeitgeber as the sleeping patterns changed to being closer to local time conditions they would find after a west east flight.

Question 15

However - Jet Lag

Answer 15

• Study cannot account for individual differences for example some passengers may have taken a sleeping tablet that increases their melatonin.
• Therefore be considered an extraneous variable and would distort the value of the findings above.