Endogenous pacemakers

• The SCN (suprachiasmatic nucleus) is a primary endogenous pacemakers. It is a tiny bundle of nerve cells in the hypothalamus which helps maintain circadian rhythms (e.g. sleep/wake cycle)
• Nerve fibres from the eye cross at the optic chiasm on their way to the visual cortex. The SCN lies just above the optic chiasm and received information about light from this structure.
• Key study: 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
• Key study: 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.
• 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