endogenous pacemakers

• what is an endogenous pacemaker?

  internal biological cues
• what are the two main endogenous pacemakers?

  the suprachiasmatic nucleus and the pineal gland
• what is the suprachiasmatic nucleus?

  its a pair of tiny clusters of nerve cells in the hypothalamus, which have an inbuilt circadian rhythmic firing pattern. this rhythm works by cells producing a protein for a number of hours until the protein level is high enough that it inhibits further production. for a number of hours the protein level drops until it crosses a threshold where the SCN starts producing it again.
• where is the SCN?

  it sits just above where the optic nerves from each eye cross over.
• how does the SCN work?

  it gets information from light from the optic nerve, even when our eyes are shut as special photoreceptors in the eyes pick up light signals and carry them to the SCN. if our endogenous clock is running slow the morning light automatically shifts the clock ahead putting the rhythm in step with the world outside. this is the mechanism by which light re-sits the circadian clock in the SCN.
• evaluation of the SCN.

  there are other body clocks, suggesting there are complex influences on the sleep/wake cycle. Damiola et al (2000) demonstrated how changing feeding patterns in mice could after the circadian rhythms of cells in the liver by up to 12 hours, whilst leaving the rhythm of the SCN unaffected.
• what is the pineal gland?

  its an endogenous pacemaker in the brain
• what does the pineal gland do?

  it converts serotonin into melatonin. it contains light sensitive cells. when the level of light falls, melatonin is produced by the pineal gland. melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness. when light levels are high, the production of melatonin in the pineal gland is inhibited and we wake up