Endogenous pacemakers

• Endogenous pacemakers are mechanisms within the body that govern the internal, biological bodily rhythms
• These pacemakers are most likely to be the product of inherited genetic characteristics, and allow our bodies to keep up with changes in the environment
• The most important of these is the suprachiasmatic nucleus (SCN) which is the ‘master clock’ of the body, linking to other brain regions and other biological clocks in the body
• The SCN is a tiny cluster of nerve cells in the hypothalamus, which constantly receives signals about external light levels via the optic chiasm
• The SCN receives information about light when we are awake, and even when we’re asleep, through the thin membrane of the eyelid
• From the information the SCN receives about light levels, it regulates the sleep/wake cycle. The SCN also regulates the production and release of the hormone melatonin from the pineal gland
• It has the ability to reset our biological rhythms too. For example, if our body clock is running slow, morning light automatically adjusts the clock, putting it back in rhythm with the world outside
• The SCN was removed from genetically abnormal hamsters who had a circadian cycle of just 20 hours. They then transplanted these cells into adult hamsters who didn't have such an abnormality, and functioned on the normal 24-hour cycle. Following the transplant, the hamsters‘ circadian rhythm shortened to 20 hours. This suggests the SCN is crucial in regulating the internal biological clock
• DeCoursey et al (2000) destroyed SCN connections in 30 chipmunks. These chipmunks were returned to their natural habitats and observed for 80 days. Their sleep/wake cycle disappeared. By the end of the observation, a significant proportion of the chipmunks had been killed by predators. This further emphasises the crucial role of the SCN in establishing and maintaining the sleep/wake cycle
• The SCN may not be as big of an influence on biological rhythms as suggested by theory and research. There are many other ‘body clocks’ and rhythms is other cells in our body, controlling the function of particular areas
• Damiola et al (2000) changed the feeding patterns in mice, and found that the circadian rhythms in their liver cells also altered by up to twelve hours
• Their SCNs were unaffected, and remained entrained to their usual 24-hour rhythm which suggests that there may be many other complex influences on our biological rhythms