Endogenous pacemakers and exogenous zeitgebers 

Endogenous pacemakers 

• Endogenous = anything whose origins are within the organism 

• These pacemakers are most probably the products of inherited genetic mechanisms and allow us to keep pace with changing cycles in the environment 

• Most important pacemaker is the suprachiasmatic pacemaker 

The suprachiasmatic Pacemaker 

• Tiny cluster of nerves call the suprachiasmatic nucleus (SCN) which lies in the hypothalamus 

• Plays an important role in generating the body’s circadian rhythms

• ‘Master clock’ with links to other brain regions eg sleep and arousal + controls other body clocks 

• Neurons within the SCN spontaneously synchronise with each other → this means that their target neurons in sites elsewhere in the body receive correctly timed coordinated signals 

• These Peripheral clocks can maintain a circadian rhythm but not for very long which is why they must be controlled by the SCN 

• SCN has a built in circadian rhythm which only needs to be reset when external light levels change. 

• Signals about light levels are received via the optic nerve 

• If our bio clock is running slow (ie the sun rises earlier than on the previous day) then morning light automatically adjusts the clock 

• The SCN also regulates the manufacture and secretion of melatonin in the Pineal gland via interconnecting pathways 

The Pineal gland 

• The SCN send signals to the pineal gland directing it to produce and secrete melatonin at night and decrease when light levels increase 

• Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness 

• PG and SCN function jointly as endogenous pacemakers in the brain 

Exogenous Zeitgebers 

• Exogenous = anything whose origin is outside of the organism

• Zeitgebers = ‘time giver’ 

• EZ are environmental events that are responsible for entraining the biological clock of an organism  

LIght 

• Receptors in the SCN are sensitive to changes in light levels, and uses this info to synchronise the activity of the body’s organs and glands 

• Rods and cones in the retina detect light to form visual images

• Protein called melanopsin is sensitive to natural light. 

• Small amount of retina cells contain melanopsin and carry signals to the SCN to set the daily body cycle 

Social cues 

• Ashcoff et al found that individuals are abel to compensate for the absence of Zeitgebers such as natural light by responding to social zeitgebers instead 

• Jet lag research → circadian rhythms of air travelers adjusted more quickly if they went outside at their destination → they were exposed to social cues at their destination 

• Circadian rhythms of the blind were thought to be any different than ppl who could see as they were brought up around the same social cues 

• Blind ppl can still be influenced by light levels as the connection still exists between the SCN and the eye that do not involve the visual part of the eye  

Evaluations 

Research support (EP) 

• Morgan’s study of hamsters 

• Bred a strain of hamsters so they had an abnormal circadian rhythm of 20 hrs rather than 24 

• The abnormal SCN neurons were than transplanted into normal hamsters 

• The normal hamsters then displayed the same abnormal circadian rhythm 

• Then transplanted normal SCN neurons into abnormal hamsters and they changed their behaviour 

• Shows the importance of SCN in setting the sleep/wake cycle and circandian rhythms 

• Limited through ethic standards of animal studies → wouldnt be allowed t conduct on humans 

• Cant be generalised 

Support for the role of melanopsin 

• The important role played by melanopsin in setting the circadian rhythm is demonstrated in studies of blind ppl 

• Sme blind ppl are still able to reliably entrain their circadian rhythm in response to light 

• Skene and Arendt estimate that the cast majority of blind subjects who still have light perception have normally entrained circadian rhythms

• This suggests that the pathway from retinal cells containing melanopsin to the SCN is still intact 

Limitation to EZ

• Individuals who live in the arctic where the sun does not set in the summer show normal sleep wake cycles despite prolonged exposure to light