Endogenous Pacemakers and Exogenous Zeitgebers

• What are biological rhythms coordinated by?

  Endogenous pacemakers and exogenous zeitgebers
• What is the role of endogenous pacemakers?

  They are genetic mechanisms that regulate cycles
• What is the master pacemaker in the body?

  Suprachiasmatic nucleus (SCN)
• How does the SCN maintain the sleep/wake cycle?

  By receiving light information from the eyes
• What is the primary function of the suprachiasmatic nucleus?

  To synchronize the body clock with the environment
• What are exogenous zeitgebers?

  Events in the external environment affecting biological clocks
• What is the primary zeitgeber for resetting the body clock?

  Light
• What is melanopsin's role in the body?

  It sends signals to the SCN to set the body clock
• Where is melanopsin located?

  In the retina of the eye
• How does melanopsin help maintain synchronization with the environment?

  By sending light signals to the SCN
• Social cues such as mealtimes or social events can also act as exogenous zeitgebers. A study on jet lag found that circadian rhythms adjusted quicker if individuals went outside as they were exposed to new social cues.
• After receiving information about low light levels, the SCN sends a signal to the pineal gland which is responsible for producing melatonin at night and decreasing it in the morning. Hence despite their endogenous nature, the activity of these pacemakers must be synchronised with external cues.
• Retinal cells within the eye contain the protein melanopsin and carry signals to the SCN in response to brightness levels of the environment.
• Strength of endogenous pacemakers - Hamter!!

  + Morgan bred some hamsters so they would have an abnormal circadian rhythms of 20 rather than 24 hours.

  + SCN neurons of these abnormal hamsters were placed into the brains of normal hamsters.

  + These ‘normal’ hamsters developed the 20 hour circadian rhythm and vice versa.

  + Confirms biological root and importance of the SCN.
• Limitation of endogenous pacemakers - danger of disruption

  -Exposure to artificial light at night disrupts circadian rhythm, affecting health.

  -Teenagers spend increased time on phones at night, blue light of phones suppresses melatonin secretion and disrupts circadian rhythms.

  -Sleep becomes delayed, shortened and irregular - also increases rates of mood disorders.
• Support for Exogenous Zeitgebers - melanopsin

  + Research claims that blind People who percieve light have normally entrained cycles, and those without have abnormal ones.

  + Support from Michel Siffre’s cave study, with no clocks or radios, he believed it to be a month earlier than it really was.

  + Suggests impact of lack of EZ’s on bodily rhythms.
• Limitation of Exogenous Zeitgebers - reductionist view.

  -Behaviourist approach suggests that rhythms are influenced by social norms and interactions, sleep occurs at night as it is socially acceptable.

  -Explanation could be criticised for environmental reductionism.

  -A more holistic approach is needed to understand the complexity of EZ’s.