Outline and evaluate research into the role of endogenous pacemakes and exogenous zeitgerbers (8+16)

Endogenous pacemakers are biological clocks within organisms. In mammals the main pacemaker is the suprachiasmatic nucleus (SCN), located in the hypothalamus. The SCN receives information on light via the optic nerve, occurring even when our eyes are closed as light penetrates through our eyelids. This helps to keep circadian rhythms synchronised with the outside world. When the SCN receives information about light is sends a signal to the pineal gland which regulates the production of melatonin. In low light it increases production, as melatonin is the hormone which induces sleep through inhibiting brain mechanisms which provoke wakefulness. 

Exogenous zeitgebers are external cues that help to regulate biological rhythms, known as entrainment. The opposite of this is free running where the biological clock operates without external cues. Light is the dominate zeitgeber in humans. The light sensitive protein CRY responds to light, shifting biological rhythms and has been found to reset the SCN and other bodily oscillators. Social cues such as meal times play a role as the liver and the heart are seen to be reset as the cells respond to eating. This suggests environmental factors play a role in biological cycles. Temperature is also important for the onset of hibernation in some animals. In the absence of light, temperature may become the dominant zeitgeber that resets biological rhythms. 

Morgan et al bred 'mutant' hamsters with a circadian rhythm of 20 hours instead of 24. The mutant SCN was transplanted into normal hamsters,which then displayed the 20 hours rhythm. This highlights the importance of the SCN and the possible dominant role it plays in controlling our biological rhythms. However, this study does not explain how exogenous zeitgebers may have interacted with the SCN to maintain a 22 hour rhythm. It is reductionist to assume only the SCN plays a role as we know exogenous zeitgebers interact with endogenous pacemakers to keep rhythms synchronised. Ethical issues also arise as the animals were left permanently damaged. Research may be justified if it has an important application to human behaviour research. However, generalisation problems occur between non-human animals studies to humans, therefore limits the conclusions which can be drawn. 

Campbell et  al altered participants circadian rhythms through shining lights on the backs of participants' knees demonstrating the existence of other endogenous pacemakers. Their circadian rhythms shifted demonstrating other oscillators must exist to help keep the biological clocks synchronised. This supports the assumption that light acts as an exogenous zeitgeber through its interaction with the protein CRY to reset the biological clock. Highlighting that a relationship exists between exogenous zeitgebers and endogenous pacemakers. This also suggests humans do not solely need light to penetrate the eyes. Blood may also be a messenger carrying light signals from the sun to the brain. However, such explanations may be reductionist to assume light is the main exogenous zeitgeber as other external influences such as temperature and social cues work with endogenous pacemakers to maintain rhythms. 

Both Morgan's and Campbell's studies demonstrate the importance of interaction of endogenous pacemakers and exogenous zeitgebers. They also highlight that most mammals have similar endogenous pacemakers to influence rhythms. Both studies took a biological approach ignoring the role of free will we have in overriding internal clocks, therefore is deterministic. We are not governed completely by biological programming and we can override them if we wish. 

Also from such research we can identify that nurture has a strong influence. As, our environments interact with internal biological clocks suggesting both appear to be equally important. Our endogenous pacemakers can keep a rough measure biological rhythms but the environment and stimuli appear to be important in keeping it completely synchronised. However, we cannot determine the exact amount of influence endogenous pacemakers and exogenous zeitgebers have on our rhythms, we can only conclude they interact.