A01 Bio Rhythms: Endogenous Pacemakers/Exogenous Zeitgebers

• Suprachiasmatic nucleus (SCN)


  A tiny bundle of nerve cells located in the hypothalamus in each hemisphere of the brain where the optic nerves from each eye cross. It is one the primary endogenous pacemaker in mammals and is influential in maintaining ciractian rhythms like the sleep/wake cycle.
• Location of the suprachiasmatic nucleus (SCN)

  Nerve fibres are connected to the eye in the optic chiasm on their way to the visual area of the cerebral cortex. The SCN lies just above the optic chiasm where it receives information about light to changing patterns of daylight whilst asleep. This continues even when the eyes are closed , enabling the biologcial clock to adjust to changing patterns of daylight whilst asleep.
• Animal studies and the SCN

  Influence of the SCN has been demonstrated by animal studies. DeCoursey et al destroyed the SCN connections in the brain of 30 chimpmunks who were returned to natural habitat and observed for 80 days.
• Findings of DeCoursey study

  The sleep/wake cycle of the chimpmunks disappeared and at the end of the study, a big proportion of them had been killed by predators.
• Ralph et al study


  Bred 'mutant' hamsters witha 20-hour sleep/wake style. When the SCN cells from the foetal issue of mutant hamsters transplanted into the brians of normal hamsters, the cycles of the 2nd group faulted to 20 hours.
• The pineal gland and melatonin


  The SCN passes the information on day length and light that it receives to the pineal gland. During the night, the pineal gland increases production of melatonin, a chemical that induces sleep and is inhibited during periods of wakefulness.
• Exogenous zeitgebers and the sleep/wake cycle

  External factors in environment that reset biological clocks through entrainment like light and social cues.
• Light


  Can reset the body's main endogenous pacemaker and is the key zeitgeber in humans. Plays a role in maintenance of sleep/wake cycle while having an indirect influence on key processes which control functions like hormones secretion and blood circulation
• Campbell and Murphy


  Demonstrated light may be defected by skin receptors on the body even when the same information is not received by the eyes. 15 ppts were woken at various times and light pad shone at the back of their knees. Researchers managed to produced deviation from the normal sleep/wake cycle of up to 3 hours.
• Social cues


  infants sleep/wake cycle is initially pretty random but around 6 weeks the circadian rhythm begins and at 16 weeks most babies are entrained. Schedules imposed by parents are a key influence. Reserach suggests that adapting to local eat and sleep times is effective at entraining CR and beating jet lag.