Endogenous pacemakers and exogenous zeitgebers

What does Endogenous mean

Anything whose origins are within the organism

What is the most important endogenous pacemaker

The Suprachiasmatic pacemaker (SCN)

What do Endogenous pacemakers help us do

Keep pace with changing cycles in the environment

What is the SCN and where is it located, and why is it needed

A tiny cluster of nerve cells in the hypothalamus that has a built in circadian rhythm which only needs to be reset when external light levels change. 

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

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

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

what rhythms do the SCN help create

Circadian rhythms

Where are signals about light levels received

The optic nerve

The Pineal gland in relation to the SCN

• 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

What does Exogenous mean

anything whose origin is outside of the organism

What are Exogenous Zeitgebers

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

What are 2 examples of EZ

Light

Social cues

Light in relation to EZ

• 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

What did Ashcoff find

•  individuals are able to compensate for the absence of Zeitgebers such as natural light by responding to social zeitgebers instead 

• Jet lag research → circadian rhythms of air travellers adjusted more quickly if they went outside at their destination → they were exposed to social cues at their destination (Klein and Wegmann)

Blind people and Circadian Rhythms

• 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  

Research to support endogenous pacemakers

• 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