User:NotePad13

My views
Discuss the role of endogenous pacemakers and exogenous zeitgebers in biological rhythms.

Endogenous pacemakers are inherited genetic mechanisms, examples of this are the pineal gland and the suprachiasmatic nucleus function.

In mammals, SCN or suprachiasmatic nucleus is a small group of cells in the hypothalamus generating a circadian reset by light entering the eye. It is located above the optic nerves each eye cross over. SCN obtains information about light from the eye via the optic nerve. This happens even when our eyes are shut, because light penetrates the eyelids. If our endogenous clock is running slow, such as the un rising earlier than the day before, the morning light automatically shifts the clock ahead, putting the rhythm in step with the world outside. This also suggests how endogenous pacemakers interact with exogenous zeitgebers.

Another endogenous pacemaker in the brain is the pineal gland. This structure contains light receptors that respond to external light. These light receptors influence the activity of neurones in the pineal gland which converts neurotransmitter serotonin to melatonin. Melatonin is then released into the general circulation responsible for the rhythmic nature of many activities such as sleep/wake cycle. The production and release of melatonin is regulated by the amount of light falling on the pineal gland, decreasing as light increases. Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness.

These are supported by research studies. Binkley (1974) found chickens awake and become active when dawn breaks and melatonin secretion falls. Despite their waking being controlled by the biological clock in the pineal gland, it is adjusted to the actual time that morning begins, which changes throughout the year. This is a good example of how endogenous pacemakers interact with exogenous zeitgebers.

Another study is by Ralph et al (1990) took the SCN out of genetically abnormal hamsters with a circadian cycle of only 20 hours, transplanting them into hamsters with the usual 24 hour cycle and their cycle shortened to 20 hours, suggesting that the SCN is the main endogenous pacemaker.

Most of the research carried out involves animals, which cannot be generalised to the biological of humans. Ethical issues also occurs since there is an issue of harming animals.

It is claimed that biological rhythms have adaptive advantage to animals, as it allows them to anticipate daily environmental events such as patterns of light to dark. Decoursey (2000) investigated this by functionally removing SCN from 30 chipmunks. The chipmunks were than returned into their natural habitat. After 80 days, more SCN-lesioned chipmunk had been killed by weasels. This was probably because the chipmunks remained awaked in the borrows, creating noises making them easier to locate.

The process of resetting the biological with exogenous zeitgebers is known as entrainment. Zeitgebers play an important role in regulating biological rhythms, helping rest them and endogenous pacemakers need to respond to zeitgebers, cording the behavious they regulate with the external environment.

Examples of exogenous zeitgebers are light, social cues and temperature. Light is a dominant zeitgeber. It reset the body's main pacemaker, the SCN. Social cues are when we eat meals at socially determined mealtimes and go to bed and wake up at times designated as appropriate for our age. Biological rhythms can also be entrained by temperature, for example, leaves on trees change colour and drop off because of the changes in temperature. Temperature is also a factor in the process.

These are supported by research evidence. Boivin et al. (1996) found using 32 male subjects that circadian rhythm can be entrained by ordinary dim lighting, though bright lighting was more effective.

Klein et al (1993) studied a blind man with a circadian rhythm of 24.5 hours, which eventually got out of sync with the 24 hour day. Time cues such as clocks did not help and he had to take sedatives to regulate his sleep-wake cycle. This suggest that light acts as an exogenous zeitgebers in the form of a time cue.

Exogenous zeitgebers are important since it acts as an adaptive advantage for for survival in animals.

If dim lighting does reset the biological clock, then the fact that we live in an artificially lit world may have some negative consequences. Stevens (2006) suggests that exposure to artificial lighting disrupts circadian rhythms and thus disrupts melatonin production and this might explain why women in industrialised societies are more likely to develop breast cancer.