The Role of Endogenous and Exogenous Factors in Bodily Rhythms

Annotations:

• Biological 'clocks' in the brain are controlling biological rhythms

1. DeCoursey: Removed the SCN in 30 chipmunks and then put back in their natural habitat. After 80 days those who had their SCN removed were more likely to die
2. Suprachiasmatic Nucleus in the hypothalamus is responsible for controlling bodily rhythm
   1. Morgan: bred mutant hamsters and gave them Circadian Rhythms of 20 hours then transplanted the SCN into normal hamsters. The normal hamsters displayed Circadian rhythms
3. Aschoff and Weaver: placed P's in a WWII bunker and denied them social or environmental cues. Most showed CR of 24 hours.
   1. Exogenous factors must help in CR as the P's lost a few minutes in their CR every day
4. The SCN sends signal to the pineal gland, which produces melatonin. Melatonin induces sleep and it is underneath the top of the skull so that light induces sleep.
   1. Light as the exogenous factor plays a role
5. Folkard: Kate Aldcroft spent time in a cave. Her temperature was on a 24 hour CR but her sleep on a 30
6. Kleitman: P's lived in a bunker, with no light or dark cues.The CR were 25-27 hours
   1. Lacks generalisability. Also, Siffre maintained a normal CR which shows that exogenous factors also play a part
7. Miles: Man was blind from birth but still had a CR of 24.9 hours. Even though social cues were experienced to him his internal pace did not slow down suggesting that CRs are biological

Annotations:

• External Stimuli that help towards regulating biological rhythms to the outside world

1. Can be in the form of objects (clocks) or natural occurences (light levels)
2. Michael Siffre went to live in a cave for 6 months. When he was awake the lights were on, and vice versa. His CR was 25-30 suggesting exogenous factors affect bodily rhythms
   1. Cszeliar: Artificial light does not effect the CR. 24 m/f lived for a month in subdued light, and tried to place them in an artificial CR. Their CR was still 24
3. To suggest we have a 25hr CR is an evolutionary disadvantage as animals survive with a 24hr CR
4. Reductionist - CRs are also controlled by ednogenous, not just exogenous
5. Davison: Social cues - We follow social cues as we grow up. For example, our liver might rest when it's meal time
6. Campbell and Murphy: Shining the light on the back of a P's knees shifted their CRs showing that exogenous factors affect bodily rhythms
7. Seasonal Defective Order: Individuals feel more depressed in the winter and elated in the summer
   1. Rosenzweig: Depression is mostly found in Northern and southern countries because they have less light.
8. Stevens: Artificial light can change CRs