Describe and assess the consequences of disrupting biological rhythms

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Describe and assess the consequences of disrupting biological rhythms (24 marks)

All living organisms experience rhythmic changes, which tend to coincide with seasonal or daily environmental changes.  These rhythms are known as biological rhythms, which include circadian, ultradian and infradian types.  

Circadian rhythms are those that repeat themselves daily, those lasting ‘about one day’.  The best example of a circadian rhythm is the sleep-wake cycle, associated with which are many cyclical changes with active and dormant periods, for instance body temperature and urine production.  These rhythms allow animals to prepare for predictable daily environmental changes, such as night and day.

Ultradian rhythms are those cycles with less than one day.  Examples include levels of alertness throughout the day and the cycle of brain activity during sleep.

Infradian rhythms are those with a period of greater than a day.  The menstrual cycle is an example of an infradian rhythm.  Infradian rhythms that occur as a result of seasonal changes, for example, migration and hibernation are called circannual rhythms.

All biological rhythms are controlled by two different factors – internally (endogenous) through nature, and externally (exogenous) through nurture.   Most organisms have internal biological clocks, called endogenous pacemakers.  The main endogenous pacemaker in circadian rhythms is the suprachiasmatic nucleus (SCN), a small bundle of nerves in the hypothalamus, as suggested by Morgan (1995), and Kalat (1998).  Kalat suggested that low levels of light lead to an electrical stimulant, which activates the pineal gland in the SCN, located in the centre of the brain, to secrete a hormone called melatonin, which causes sleepiness.  Its production of melatonin varies with periods of light and darkness in the environment, and it obtains this information about light in the environment by means of nerve pathways originating in the eyes.  He summarised that light slows and darkness stimulates the pineal glands production of melatonin and, therefore, the gland tends to secrete small amounts of melatonin during the day and large amounts at night.  The main exogenous zeitgeber (‘time-giver’) that controls circadian rhythms is therefore light.

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Research that suggests the SCN is the main endogenous factor for circadian rhythms comes from Morgan in 1995.  His aim was to determine whether the SCN in hamsters is linked to the disappearance of their circadian rhythms.  He removed the SCN from hamsters and transplanted an SCN from mutant hamsters whose biological rhythms have shorter cycles than those of the recipients.  He found that their circadian rhythms had disappeared (when the SCN was removed).  Further to this he found that the hamsters with the transplanted SCNs took on the mutant rhythms.  However, this study can be questioned as the ...

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