EPs and EZs

• Endogenous pacemakers (EP)


  EP's are internal mechanisms that govern biological rhythms, in particular, the circadian sleep-wake cycke.

  
  

  Although they are biological body clocks, they can be altered and affected by the environment.

  
  

  For example, although the circadian sleep-wake cycle will continue to function wihtout natural cues from light, research suggests that light is required to reset the cycle every 24 hours.
• Suprachiasmatic nucleus (SCN)


  The SCN, which lies in the hypothalamus, is the main endogenous pacemaker (or master clock).

  
  

  It is the central pacemaker in the circadian timing system, regulating most circadian rhythms in the body.

  
  

  It is closely linked with the pineal gland, both of which are influential in maintaining the sleep/wake cycle.
• SCN - sleep / wake cycle

  The SCN receives information about light levels (an exogenous zeitgeber) from the optic nerve, which sets the circadian rhythm so that it is in synchronisation with the rest of the world.

  
  

  The SCN sends signals to the pineal gland - and so regulates the timing of melatonin release.

  
  

  This triggers an increase in the production of melatonin at night, which helps induce feelings of sleepiness.

  
  

  The SCN and the pineal gland therefore work together as endogenous pacemakers. However, their activity is responsive to the external cue of light.
• Exogenous zeitgebers (EZ)


  EZ'S influence biological rhythms - can be described as environmental events that are responsible for resetting the biological clock of an organism.

  
  

  They can include social cues such as meal times or social activities.

  
  

  The most important EZ is light, which is responsible for resetting the body clock each day - keeping it on a 24 hour cycle.

  
  

  Humans can compensate for the lack of natural light, by using social cues instead.
• A03 - importance of SCN (E)

  Morgan bred hamsters so that they had circadian rhythms of 20 hours, rather than 24 hours.
• A03 - importance of SCN (E)

  SCN neurons from these abnormal hamsters were transplanted into the brains of normal hamsters, who subsequently displayed the same abnormal circadian rhythm of 20 hours.

  
  

  This showed that the transplanted SCN had imposed its pattern onto the hamsters.
• AO3 - importance of SCN (L)

  This research demonstrates the significance of the SCN and how endogenous pacemakers are important for biological circadian rhythms.
• Limitation with Morgan's study:

  It was conducted with hamsters. In contrast, humans would respond very differently to manipulations of their biological rhythms, not only because we are different biologically, but also because of the vast differences between environmental contexts.

  
  

  This makes it difficult to extrapolate findings directly to explain the role of endogenous pacemakers in the biological rhythms of humans.
• A03 - Siffre (E)

  When Siffre returned from spending 61 days underground in a cave without any clocks or natural light, he believed the date to be a month earlier than it actually was.
• A03 - Siffre (E)

  This suggests that his 24-hour sleep/wake cycle was increased by the lack of external cues, making him believe that one day was longer than it actually was.
• A03 - Siffre (L)

  This highlights the importance of external cues on biological rhythms. It demonstrates how changes in light exposure can trigger the desynchronisation of a 'pre-set' sleep/wake cycle.
• A03 - Biologically reductionist (P)


  Despite all the research support for the role of EPs and EZs in regulating biological rhythms, the argument could still be considered biologically reductionist.
• A03- Biologically reductionist (E)

  For example, the behaviourist approach would suggest that bodily rhythms are influenced by other people and social norms.

  - for example, sleep occurs when it is dark because that is the social norm.
• A03 - Biologically reductionist (L)

  The research discussed here could be criticised for being reductionist as it only considers a singular biological mechanism and fails to consider the other widely divergent viewpoints.