Endogenous pacemakers Flashcards
What are endogenous pacemakers
Mechanisms within the body that govern the internal, biological bodily rhythms
what are exogenous zeitgebers?
an environmental cue, such as light that helps to regulate the biological clock in an organism
what is the SCN?
The primary endogenous pacemaker/masterclock
it is found in the hypothalamus
it consists of a cluster of nerve cells that receive light via optic nerve
Neurons within the SCN spontaneously synchronise with each other so that their target neurons in sites elsewhere in the body receive time-coordinated signals as well as linking to other regions in the brain that are connected with sleep and arousal.
Describe the pineal gland (exogenous)
SCN sends signals to the pineal gland directing it to increase production and secretion of melatonin at night and decrease its levels in the morning
Melatonin induces sleep by inhibiting the brain’s mechanisms that promote wakefulness
The sensitivity of the pineal gland and SCN to light and the role of melatonin in sleep means that despite the endogenous nature of these clocks their activity must be synchronised with light/dark rhythms in the outside world
explore light as an exogenous zeitgebers
receptors in the SCN are sensitive to changes in light levels during the day and use this information to synchronise the activity of the body’s organs
Light resets the internal biological clocks
rods and cones in the retina detect light to form visual images
Melansopin (in the retina) gauges over all brightness to help reset the internal biological clock
It is sensitive to natural light
A small number of retinal cells contain melansopin and carry signals to the SCN to set the daily cycle
explore social cues as an exogenous zeitgeber
social stimuli such as mealtimes and social activities have an active role as zeigebers
Aschoff in 1971 showed that individuals are able to compensate for the absence of zeigebers such as natural light by responding to social zeitgebers
Klein and Wegman 1974 found that circadian rhythms of air travellers adjusted more quickly if they went outside more at their destination
Likewise the circadian rhythm of the blind were thought to be no different. This can be explained in terms of natural light as a zeitgeber
Name an AO3 point
The SCN is important in setting circadian rhythms
- Morgan in 1995 bred two groups of hamsters
- One group had an abnormal circadian rhythm of 20 hours
- the other group had a normal circadian rhythm of 24 hours
- The SCN neurons of the abnormal hamsters were implanted into the brain of the normal hamsters
- the findings concluded that the hamsters who had been bred to have an abnormal circadian rhythm of 20 hours adopted a normal circadian rhythm of 24 hours. Similarly, the results of the other hamsters with the normal circadian rhythms after having been implanted with the abnormal circadian rhythms developed this same abnormal rhythm
Name a second evaluation point
There are dangers to circadian rhythms
Touitou et al in 2017 found that teenagers spend increasing amount of time on electronic devices. the LED bulb of the devices contain a blue component which is active on the circadian clock which leads to the suppression of melatonin secretion and circadian disruption
- as a result adolescent sleep becomes irregular, shortened and delayed.
Name an evaluation point for exogenous zeitgebers?
There is support for the role of melansopin in the circadian rhythm
Some blind people are still able to regularly entrain their circadian rhythm despite being unable to form visual imaging perception
Skene and Ardt 2007 estimate that the vast majority of blind subjects who still have light perception have normally entrained circadian rhythms
This shows that the pathway from retina cells containing melansopin in the retina are still intact
for further evidence, those with little to no light perception have abnormal circadian rhythms
Name a final evaluation point
Evidence as light as the dominant exogenous zeitgeber
Vetter at al 2011 demonstrated the importance of light in sleep
He split participants into two groups
One group remained in normal ‘warm’ artificial light over five weeks whilst the other group experienced an artificial ‘blue-enriched’ light
Participants kept a daily sleep log and wore devices that measured their movement over each 24-hour period
Participants under the warm light synchronised their circadian rhythms each day with the natural light of dawn
Over the course of the study sunrise advance by 42 minutes
Participants who were exposed to blue-enriched light did not show the same 42-minute adjustment and instead synchronised their rhythms to office hours
