endogenous pacemakers

• The suprachiasmatic nucleus is a tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain. It is one of the primary endogenous pacemakers in mammalian species and is influential in maintaining circadian rhythms such as the sleep/wake cycle.
• The SCN lies just above the optic chiasm (where the nerve fibres connected to the eye cross). It receives information about light directly from this structure. This continues even when our eyes are closed, enabling the biological clock to adjust to changing patterns of daylight whilst we are asleep.
• Decoursey destroyed the SCN connections in the brains of 30 chipmunks who were then returned to their natural habitat and observed for 80 days. The chipmunk’s sleep/wake cycle disappeared and by the end of the study a significant proportion had been killed by predators.
• Ralph et al. bred ‘mutant’ hamsters with a 20-hour sleep/wake cycle. When SCN cells from the foetal tissue of mutant hamsters were transplanted into the brains of normal hamsters, the cycles of the second group defaulted to 20 hours.
• The SCN passed information on day length and light that it receives to the pineal gland. This is another endogenous mechanism guiding the sleep/wake cycle.
• During the night, the pineal gland increases production of melatonin- a chemical that induces sleep and is inhibited during periods of wakefulness. Melatonin has also been suggested as a factor in seasonal affective disorder.
• One limitation of SCN research it that it may obscure other body clocks. Research has revealed that there are numerous circadian rhythms in many organs and cells in the body. These peripheral oscillators are found in the organs including the lungs, pancreas and skin. They are influenced by the actions of the SCN but also act independently. Damiola et al. demonstrated how changing feeding patterns in mice could alter the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected. This suggests other complex influences on the sleep/wake cycle.
• Another limitation is that endogenous pacemakers cannot be studied in isolation. Total isolation studies, such as Michael Siffre’s cave study are extremely rare. In everyday life, pacemakers and zeitgebers interact, and it may make little sense to separate the two for the purpose of research. This suggests the more researchers attempt to isolate the influence of internal pacemakers, the lower the validity of the research.