‘Discuss the Role of Endogenous Pacemakers and Exogenous Zeitgebers in Biological Rhythms’

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... [continues]

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