Endogenous Zeitgebers

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
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
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