Endogenous pacemakers and exogenous zeitgebers are used to control biological rhythms. Biological rhythms are controlled by environmental factors such as light, temperature and food availability; these are known as exogenous (external) zeitgebers. However biological organisms are complex systems, where many different processes are taking place. This means that it would be useful to have something which would co-ordinate these processes, keeping everything in time – an endogenous pacemaker.
This is commonly referred to as an internal ‘clock’ which regulates biological rhythms in the absence of zeitgebers. With endogenous pacemakers being known as ‘internal body clocks’, we tend to assume that these pacemakers are innate or an inherited genetic mechanism. In mammals the main pacemaker is called the suprachiasmatic nucleus (SCN), found in the hypothalamus. The SCN obtains information about light from the eye via the optic nerve. This usually will happen even when our eyes are closed because light is able to penetrate the eyelids.
Our endogenous pacemakers are very advanced in controlling our biological rhythms, because they can detect if environmental factors are different from what they usually are and will adapt to these changes. For instance, if the sun rises earlier than the day before, morning light will automatically shift the clock and this they regulate the rhythm in step with the external environment. Albus et al. (2005) was also able to find that, with the SCN being a pair of structures with one in each hemisphere of the brain, each is further divided into a ventral and dorsal SCN.
Research by Albus showed that the ventral SCN is relatively quickly reset by external cues, whereas the dorsal SCN is much less affected by light and therefore more resistant to being reset. The electrical activity of the SCN has an inbuilt circadian rhythm, and this pattern can be maintained even when the SCN is isolated from the rest of the brain, suggesting that it is truly endogenous and probably genetic. The SCN is also able to send signals to the pineal gland, which increases the production of melatonin at night.
As a result of this hormone increase, it induces sleep by inhibiting the brain mechanisms that promote wakefulness. Stephan and Zucker (1972) investigated the role of Endogenous pacemakers and more prominently investigated the effects of damage to the SCN on circadian rhythms. The experiments included housing rats in a laboratory with 12 hours of light, followed by 12 hours of dark, showing a circadian rhythm of drinking and locomotor activity. Observations show that they drank more and were more active during the dark period. Stephen and Zucker than compared a group with damage to the SCN with a group of normal controls.
Findings showed that damage to the SCN eliminated the normal circadian patterns of drinking and activity. Therefore, from this study we can conclude that the SCN is one of the key pacemakers in the brain controlling circadian rhythms. However, there were both limitations and advantages of this study. For instance, as the experiment was a laboratory experiment, there was high control and so high internal validity. But this also resulted in low ecological validity as the study is less likely to be applicable to the ‘real world’.
The experimental also touches on the controversial area of using animals to experiment on. It seems that Stephan and Zucker’s study used highly unethical procedures, with only 11 out of 25 rats surviving the damage of the SCN. It therefore becomes an issue where society has to decide if sacrificing animals in scientific studies is justified by the value to society of the findings. It seems that given the extreme severity of the procedures (damaging other parts of the hypothalamus of rats to show that this did not affect circadian rhythms) they would not be permitted today.
As the study was carried out in 1972, regulations covering animal experimentation have been tightened since then. There is also an issue of generalizability, as we are generalizing findings based on rats to humans. One may assume it is likely that their endogenous pacemakers will be similar to humans, but it must be confirmed by studies on humans. The most dominant exogenous zeitgebers in human is light. Light can reset the body’s main pacemaker, the SCN and can also reset the other oscillators located throughout the body because of the protein CRY. This protein is part of the protein clock that is light sensitive.
Research conducted by Campbell and Murphy (1998) found that if you shine light on the back of participants knees, this shifted their circadian rhythms, which is an example of resetting the internal biological clock, known as entrainment. The advantage of this study is that is uses a combination of environmental/biological factors, showing how both endogenous pacemakers and exogenous zeitgebers are involved in controlling biological rhythms. However, the study has low mundane realism as this is unlikely to happen usually, due to artificial being used.
However, the use of artificial light in research has also led to the assumption that 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) furthered this assumption by suggesting that exposure to artificial lighting disrupts circadian rhythms, which in turn disrupts melatonin production and this could ultimately explain why women in industrialised/well-lit societies are more likely to develop breast cancer.
The relationship between endogenous pacemakers and exogenous zeitgebers is of great importance in trying to understand how biological rhythms are controlled. Although it sounds as though that there are two systems, one endogenous and the other exogenous, the divisions do not really exist apart from total isolation of one in experiments, the running of the biological clock is a combined endogenous-exogenous exercise. A good example of how endogenous pacemakers interact with exogenous zeitgebers is Binky’s (1979) study. He found that when chickens wake and become active as dawn breaks, melatonin secretion falls.
This means that although their waking is controlled by the biological clock in the pineal gland, it is adjusted to the actual time that morning begins, which varies throughout the year. Although it is clear, there are limitations in generalizability again, because the findings are based on animals, where the pineal gland is the most important endogenous pacemaker in the brain for birds and mammals, and this is the SCN in humans, it still suggests there is a link between the two systems. The findings are also incredibly s=useful in drawing together biological and psychological factors.
It is supported by the diathesis-stress model, which represents such an interaction between nature (biological factors) and nurture (environment). In conclusion, endogenous pacemakers and exogenous zeitgebers are extremely important in controlling biological rhythms. The interplay between the two systems are designed to keep physiological processes and behaviours such as sleep-walking cycles and hibernation perfectly in tune with the outside world. Without them, out biological rhythms would have no stability and control.
