BIOPSYCH - pacemakers and zeitgebers

Endogenous pacemakers

- endogenous refers to anything whose origins are within the organism
- pacemaker are probably the products of inherited genetic mechanisms that allows us to keep pace with changing cycles in the environment
- most important pacemaker being in the SCN

Superchiasmatic nucleus

- endogenous pacemaker in hypothalamus
- generating the body's circadian rhythm
- master clock which links other brain regions controlling sleep and arousal and controls other biological clocks throughout the body
- neurons in the SCN synchronise with each other so target neurons recite time-coordinated signals
- can maintain a circadian rhythms but not for long so needs SCN to control
- possible because of the SCN's built-in circadian rhythm, which only needs resetting when external light levels change, receives information about from the optic nerve, even when our eyes are shut.
- when biological clock is running slow, morning light automatically adjusts the clock so its in step with the outside world
- SCN also regulates and secretes melatonin in the pineal gland via an interconnecting neural pathway.

Pineal gland

- SCN sends signals to the pineal gland which directs it to increase production and secretion of melatonin at night and decrease it in the morning
- melatonin induces sleep by inhibiting brain mechanisms that promotes being awake
- pineal and SCN function together as pacemakers in the brain
- sensitivity of pineal gland and SCN to light and role of melatonin means that despite endogenous nature of clocks, their activity must be light-dark rhythm of the outside world.

Exogenous zeitgebers

- exogenous \= outside the organism
- environmental events that are responsible for entertaining the biological clock of an organism
- most important is light

Light

- receptors in SCN are sensitive to change in light levels during the decay and using this information to synchronise the activity of the body's organs and glands
- light resets the internal biological clock keeping it on a 24h cycle
- rods and cones in retina detect light to form visual images
- third type of light-detecting cell which gauges overall brightness to help reset the internal biological clock
- protein called melanopsin, which is sensitive to natural light, is critical in this system
- retinal cells contain melanopsin and carry signals to the SCN to set the daily body cycle

Social cues

- social stimuli as zeitgebers
- compensate for the absence of zeitgebers by using social zeitgebers instead.
- jet lag found that circadian rhythms of air travellers adjusted more quickly if they went outside more at their destination
- circadian rhythms of blind people were thought to rely on social cues
- light exposure, as there are connections between the eye and the SCN that do not involve parts of the visual system on which the perception of light depends.

EVAL - role of SCN with animal studies

- endogenous pacemaker has been demonstrated in animal studies
- Morgan bred a strain of hamsters so they had abnormal circadian rhythms of 20 hours
- SCN neurons from these abnormal hamsters were then transplanted into the brains of normal hamsters.
- normal hamsters then displayed the same abnormal circadian rhythm of 20 hours, showing that the transplanted SCN had imposed its pattern onto the recipients brains.
- reverse experiment, planting SCN neurons from normal hamsters into the brains of abnormal hamsters.

EVAL - separate rhythms

- normal conditions the 'master clock' (the SCN) coordinates all bodily rhythms
- can become out of step with each other
- university student, Kate Aldcroft, who volunteered to spend 25 days in the controlled environment of a laboratory
- no access to daylight or other zeitgebers that might have rest the SCN
- end of the 25 days her core temperature rhythm was still at 24 hours
- sleep-wake cycle had [extended to 30 hours, with periods of sleep as long as 16 hours being recorded

EVAL - support from blind people

- role of melanopsin in setting the circadian rhythm comings from blind people
- Some blind people are still able to reliably retrain their circadian rhythms in response to light, despite a total lack of image-forming visual perception
- vast majority of blind subjects who still have some light perception have normally entrained circadian rhythms
- pathway from retinal cells containing melanopsin to the SCN is still intact.
- importance of this pathway in setting the biological clock, people without light perception show abnormal circadian entrainment

EVAL - jet lag and light exposure

- exposure to bright light prior to an east-west flight decreased the time needed to readjust to local time on arrival.
- Volunteers participated in one of three treatments (continuous bright light, intermittent bright light, dim light), each of which shifted their sleep-wake cycle back by one hour a day over three days
- Participants exposed to continuous bright light shifted by 2.1 hours, intermittent by 1.5 hours and dim light 0.6 hours.
- participants in the first treatment group felt sleepier 2 hours earlier and woke 2 hours earlier (closer to the local time conditions they would find after their flight).