Endogenous Pacemakers, Exogenous Zeitgebers

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Endogenous Pacemakers

  • The Suprachiasmatic Nucleus
  • In animals, main endogenous pacemaker is a tiny cluster of nerve cells called the suprachiastmatic nucleus, in the hypothalamus.
  • The SCN obtains info about light from the eye via the optic nerve.
  • If our endogenous clock is running slow, morning light automatically shifts the clock ahead, putting the rhyth, in step with the world outside.
  • The SCN is a pair of structures, each is divided into a ventral and dorsal SCN.
  • Ventral is relatively quickly reset by external cues, the dorsal is much less affected by light and more resistant to being reset.
  • The Pineal Gland
  • SCN sends signals to the pineal gland, directing it to increase production of the hormone melatonin at night. Melatonin induced sleep by inhibiting brain mechanisms that promote wakefulness.
  • Directly regulated by light.

Exogenous Zeitgebers

  • Entrainment is resetting the biological clock with exogenous zeitgebers.
  • Light
  • Light is the main dominant zeitgeber in humans. It can reset the SCN. 
  • The protein CRY is light sensitive.
  • If you shine light on the back of someones knees, it can shift their circadian rhythm
  • Social Cues
  • Biologists thought that social cues were the main zeitgebers for human circadian rhythms until recently.
  • Our daily rhyth,s appeared to be entrained by social convention.
  • Today we know that light is the dominant zeitgeber, but its now understood that all parts of the body produce their own oscillating rhythms and some are not primarily reset by light
  • Temperature
  • In cold blooded animals, variation in external temperature affects the setting of the circadian rhythms rhythms


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