Endogenous Pacemakers and Exogenous Zeitgebers

Question 1

What is an endogenous pacemaker?

Answer 1

Part of the internal organism that regulate biological rhythms

Question 2

What are the 2 main endogenous pacemakers?

Answer 2

• Suprachiasmatic nucleus
• Pineal gland

Question 3

Describe the SCN

Answer 3

• In mammals, the main endogenous pacemaker is a tiny cluster of nerve cells, the SCN, which lies in the hypothalamus, above the optic nerve
• Neurons in the SCN synchronise with each other, so target neurons in sites elsewhere receive correct signals, which is why it’s the ‘master clock’

Question 4

Describe the pineal gland and melatonin

Answer 4

• SCN sends signals to the pineal gland, to increase melatonin at night or decrease it at night
• Sensitivity of pineal gland and the SCN to light, and the role of melatonin means that their activity must be synchronised with the light-dark rhythm of the world outside

Question 5

What is an exogenous zeitgebers?

Answer 5

External cues in the environment that may affect our biological rhythms

Question 6

What are the 2 main exogenous zeitgebers?

Answer 6

• Light
• Social cues

Question 7

Describe light as an EZ

Answer 7

• SCN has light sensitive cells to detect it, light can reset the SCN and internal biological clock each day
• Light also has an indirect effect on hormone secretion and blood circulation in the body

Question 8

Describe social cues as an EZ

Answer 8

• Social stimuli, like mealtimes and social activities, have a role as an EZ.
• e.g. infants’ the sleep-wake cycle is random but by 18 weeks most babies are entrained (resetting the biological clock). Schedules and social cues are determined by parents (e.g. mealtimes)

Question 9

Give a study supporting social cues as an EZ

Answer 9

• A study Klein and Wegmann on jetlad found that the circadian rhythms of air travellers adjusted more quickly if they went outside more at their destination
• Thought to be as they were exposed to the social cues of their new homes, which acted as a EZ

Question 10

Give evaluation for EPs (animal studies)

Answer 10

• The SCN role in the sleep-wake cycle can be shown in animal studies
• Decoursay destroyed SCN connections in 20 chipmunks and returned them to their natural habitat for 80 days. Many were killed by a predator after, as they left their nests at night, suggesting their normal sleep-wake cycle was impaired
• This study was done under controlled conditions so results can be replicated and shows a clear cause and effect relationship.
• However, generalising these findings to humans is different as we have many complex social cues which alter sleep-wake cycle. Furthermore, the study is unethical as rodents were exposed to harm, and it could said the findings don’t justify the means of obtaining them

Question 11

Give evaluation for EZs (peripheral oscillators)

Answer 11

• It may be too reductionist to say the SCN is the inly EP that regulates the sleep-wake cycle
• Campbell and Murphy showed that light can be detected by skin receptor cells. 15 patients were woken at various times in the night and light was shone onto the back of their knee. Some sleep-wake cycles were altered by 3 hours. Suggests light doesn’t need to enter the eyes to have an effect on the sleep-wake cycle
• However there is methodological limits, as participants may have had limited light exposure to their eyes. Also, they didn’t control for the influence of other EZs like meal times and temperature

Question 12

Give evaluation for EP and EZs (issues with case studies)

Answer 12

• Much of the research into EP and EZ influence on the sleep-wake cycle have been case studies
• It’s impossible to generalise these results to populations, due to the individuals differences that make others responded to changes to the sleep-wake cycle in atypical ways. e.g. Siffre noted when he was 60 in the cave, his body clock followed a 48hr cycle, while Czeisler found cycles can vary between 13-65hrs
• Suggests results can’t be generalised as age and gender can impact our circadian rhythm

Question 13

Give evaluation for EP and EZS (role of EZs may be overstated)

Answer 13

• The role of EZs may be exaggerated, as although environmental cues can vary, some individual’s pacemakers are set to withstand their influence
• A case study by Miles of a young man blind from birth with a circadian rhythm of 24.9 hours, whose sleep-wake cycle couldn’t be adjusted with social cues. He had to take medication to allow him to sleep and wake everyday to appropriate times
• However, this study is an exceptional circumstance and for most, there’s an interaction between our EP and EZs that sets our circadian rhythm. Isolating factors tells us little about our rhythms are set in real life