Discuss the role of endogenous pacemakers and exogenous zeitgebers in bodily rhythms [25]

Discuss the role of endogenous pacemakers and exogenous zeitgebers in bodily rhythms [25] 

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What are biological rhythms ?

Biological rhythms are cyclical changes in the way biological systems behave.

#internal biological “clocks” are also called endogenous pacemakers (Inside)

#external cues from the environment, also called exogenous (outside) zeitgebers (time-giver) these include sunlight, food, noise and social interaction. 

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Suprachiasmatic nucleus (SCN)

The main internal biological clock (endogenous pacemaker) in mammals is the suprachiasmatic nucleus (SCN) in the hypothalamus, and this SCN receives information about light from the eye so the morning light “sets” the clock to a circadian rhythm,

There are two SCNs each divided:

#one is the ventral SCN which quickly resets by external cues

#the second is doral SCN which is less effected by light so harder to reset. 

SCN directs the pineal gland to secrete melatonin at night, which induces sleep. Light inhibits the production of melatonin. Moreover, in birds and reptiles the pineal gland receives direct light through the skull. 

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“tick” of the biological clock

The basis of the circadian rhythm lies in interactions between certain proteins, creating the “tick” of the biological clock.

In the morning, two proteins called CLOCK and CYCLE bind together (CLK-CYC) producing two other proteins, PERIOD and TIME (PER-TIM). PER-TIM makes CLK-CYC inactive; as PER-TIM increases CLK-CYC decreases.

So PER-TIM decreases too by negative feedback, and this whole loop takes about 24 hours.

However, in humans the main protein pairs are CLOCK-BMAL1 and PER-CRY.

This mechanism is found in the SCN and peripheral oscillators throughout the body thereby explaining why different functions e.g. hormone secretion and blood circulation can have different rhythms.

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Morgan (1995) & Rusak & Morin (1976) - Hamsters

In relation to this, Morgan (1995) removed the SCN from hamsters and found that their circadian rhythms disappeared.

And further research comes from Rusak & Morin (1976) who showed that the SCN may have an indirect role in the control of long term rhythms.
They found that lesions to the SCN of hamsters also affected their circannual breeding cycle. Male hamsters usually produce testosterone during the summer when there are long periods of daylight. But without the SCN to detect day length they produced testosterone continuously.

which suggests that the SCN is the main pacemaker for our daily rhythms. 

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Ultradian Rhythms

ultradian Rhythms occur more than once during a 24hr period although they do not have to occur across the 24hr day. Most occur during either the waking or sleeping phase of the activity cycle.

 An example is the 90min cycle of REM (Dream) sleep and slow wave sleep that occurs during the night. Sleep is studied through the use of the EEG to measure electrical activity in the brain. Sleep has been shown to include several shorter cycles and is not a single state. 

 Other waking behaviours such as smoking and coffee drinking are also cyclical because of the need to maintain the effects of the stimulant. We also eat roughly every 4 hrs. Some cycles are ‘invisible’ such as the release of hormones from the liver

Brain systems have been identified which regulate ultradian rhythms.

# Lesions to nuclei in the hypothalamus disrupt or abolish them ergo it can be suggested that these nuclei have an important role to play. 

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Circadian Rhythms

Circadian Rhythms, occur on an approximately 24hr cycle. Endogenous rhythms are not precisely related to the 24hr clock. The most obvious example of a circadian rhythm is the sleep-wake cycle.

De Coursey (1960) found that an animal (flying squirrel) maintained in an artificial environment, with constant temperature & illumination, will more often than not maintain its usual day/night activity cycle. However in the absence of the usual cues (light/dark, temp) the rhythm can often run fast or slow.

In humans our body temperature falls during the night and reaches its peak during mid afternoon (Morris et al, 1990). This peak in temp is accompanied by peaks in other physiological functions such as heart rate, blood pressure & breathing. As a result this is often the most active part of the day. The same peaks occur when the daily activity cycle is reversed thus indicating that they are regulated by an internal ‘clock’. This is sometimes known as an oscillator. A change in one cycle can result in a change in others suggesting that they are controlled by the same oscillator. There are significant individual differences in circadian rhythms. Some of these follow a pattern e.g. our internal clocks slow as we get older.

However there are noticeable differences among people: Kerkof (1985) identified ‘Larks’ who work best during the morning and ‘Owls’ who are more productive during the late evening/night period.

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Infradian rhythms

Infradian rhythms are cycle over a period greater that 24hrs. E.g. the menstrual cycle. Although this varies between individual women (the time of the month is not the same for everyone!) the cycle itself is Lunar following a cycle of approx 28 days.

Sabbagh & Barnard (1984) women spend a lot of time together often find that their menstrual periods become synchronised. The exact cause of this is unknown; one theory is that it is connected to the unconscious detection of pheromones secreted at various times during the menstrual cycle (Russell et al, 1980).

#  A characteristic of this cycle for many women is Premenstrual syndrome (PMS) this is thought to affect up to 40% of women (Choi, 1999) and results in stress, irritability, poor concentration and headaches for days before the onset of menstrual bleeding.

# Early research focussed upon the negative social impact of PMS. Dalton (1964) reported that a large proportion of crimes were clustered in the pre-menstrual interval along with a rise in accidents and suicides. However more recent research reflects a change in attitude towards PMS. Choi & McKeown (1997) indicate that some women have positive premenstrual experiences, such as increased energy & creativity.

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Fuller et al (1981)

The different rhythms seem to be relatively independent. Damage to the hypothalamus that disrupts circadian rhythms in animals often leaves ultradian rhythms unaffected. Fuller et al (1981) found evidence to suggest that there might be more than one circadian ‘clock’.

 There are certain sensory cues that train or time our biological rhythms. If this control is endogenous (within us) then the cues are known as pacemakers. When control is exerted from external factors the cues are known as zeitgebers, German for timegivers

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Disrupted cycles e.g. Jet lag

Rapid adjustment to our circadian rhythms is required when we travel across world time zones. Daily cycles need to be trained to the demands of a new time zone as the new cycle of light/dark affects the internal clock. This is known as disrupted cycles.

 Small adjustments of 1-2 hours are not a problem. Moline et al (1992) suggest that it takes 1 day to adjust for every 1 hour of time zone crossed.

 The human body adapts more easily to gaining time (phase delay) than it does to losing time (phase advance). Therefore Jet lag is worse on a west-east journey than on an east-west journey.

 # People whose job involves working in shifts can experience similar adjustment problems to those of long haul travellers, for example shift workersHowever they face a different problem. Their zeitgebers (dawn & dusk) remain the same. As a result of this they are forced to adjust their natural sleep-wake patterns to the demands of shift work in spite of the external cues that suggest the opposite. Adjustments are made more difficult because night shift workers usually sleep less during the day than they would do at night. Shift work can result in: Sleep disturbance, fatigue, digestive Problems and lack of concentration

Pinel (1997) suggests that the above factors can lead to a reduction in productivity and job satisfaction and also an increase in accidents.



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Czeisler et al (1982)

Further research comes from, Czeisler et al (1982) who found that workers took 16 days to adjust fully to a new shift and thus recommended that shifts changed on a 21 day rotation i.e. once every three weeks.

They also suggested that workers should move forward rather than backwards when changing shifts. 

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One method of overcoming the effects of disrupting bodily rhythms is to use melatonin.

# Melatonin is a hormone released by the pineal gland and is thought to be vital in sleep regulation in humans.

Melatonin plays a crucial role in the feeling of Jet Lag. It is mainly released at night and after a long flight the release of melatonin remains on the day/night pattern of the departure country for several days. In the USA a synthetic version of melatonin is marketed as a cure for jet lag and insomnia this is banned in the UK because not enough is known about its effects. Blakemore (1988) did show that giving melatonin to jet lagged volunteers was more successful at relieving symptoms than a placebo. However melatonin has also been shown to affect other cycles, such as the reproductive cycle, and thus should be used with caution.


# Melatonin has also been linked with SAD. The darkness of winter can increase melatonin output leading to tiredness & depression. 

Tina smells :p 

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