Endogenous Pacemaker Flashcards
SCN- Primary Endogenous Pacemaker
-The suprachiasmatic nucleus 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 recieves information about light from this structure.
Influence Of SCN On The Sleep/Wake Cycle
-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 cycles of 20 hours.
Pineal Gland And Melatonin
-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 causal factor in seasonal affective disorder.
A03 (May Obscure Other Body Clocks)
E- Body clocks are found in many organs and cells (e.g. lungs, skin). They are highly influenced by the actions of the SCN but can act independently. Darmiola 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.
E- This suggests there may be many other complex influences on the sleep/wake cycle, aside from the master clock (SCN).
A03 (Endogenous Pacemakers Cannot Be Studied In Isolation)
E- Only in exceptional circumstances are endogenous pacemakers ‘free running’ and unaffected by the influence of exogenous zeitgebers. Total isolation studies (e.g. Siffre’s cave study) are rare. In everyday life, pacemakers and zeitgebers interact so it may make little sense to separate the two.
E- This suggests the more researchers attempt to isolate the influence of internal pacemakers, the lower the validity of the research.
A03 (Ethics)
E- Animal studies of the sleep/wake cycle are justified because there are similar mechanisms in all mammals, so generalisations can be made to the human brain. However, a disturbing issue is the ethics involved. Animals were exposed to considerable risk in the DeCoursey et al. study and most died as a result.
E- This suggests that studies like these cannot be justified and researchers should find alternative ways of studying endogenous pacemakers.
