endogenous pacemakers

• SCN is a tiny bundle of nerve cells in the hypothalamus which helps maintain circadian rhythms. Nerve fibres from the eye cross at the optic chiasm on their way to the right and left visual areas. The SCN lies just above the optic chiasm and receives information about light from this structure. It is also a primary endogenous pacemaker
• endogenous pacemakers are internal mechanisms that govern biological rhythms like the circadian rhythm
• DeCoursey et al. destroyed SCN connections in the brains of 30 chipmunks which were returned to their natural habitat and observed for 80 days. Their sleep/wake cycle disappeared and many were killed by predators.
• Ralph et al. bred 'mutant' hamsters with a 20 hour sleep/wake cycle. SCN cells were transplanted from the foetal tissue of these hamsters into the brains of normal hamsters, which then developed cycle;es of 20 hours
• The SCN passes information on day length to the pineal gland which increases production of melatonin during the night. Melatonin is a hormone that induces sleep and is inhibited during periods of wakefulness. It has also been suggested as a casual factor in seasonal affective disorder (SAD)
• One limitation of endogenous pacemakers is the SCN research may obscure other body clocks. Body clocks are found in many organs and cells like the lungs and skin. They are highly influenced by the actions of the SCN but can act independently. Damiola et al. showed how changing feeding patterns in mice altered circadian rhythms of cells in the liver for up to 12 hours, leaving the SCN unaffected. This suggests there may be many other complex influences on the sleep/wake cycle, aside from the master clock (SCN)
• One limitation of endogenous pacemakers is that they cannot be studies in isolation. Only in exceptional circumstances are endogenous pacemakers 'free running' and unaffected by the influence of exogenous zeitgbers. Total isolation studies like Siffre's cave study are rare. In everyday life, pacemakers and zeitgebers interact so it may make little sense to separate the two. This suggests the more researchers attempt to isolate the influence of internal pacemakers, the lower the validity of the research