L12 - Endogenous Pacemakers & Exogenous Zeitgebers

Question 1

What are Endogenous Pacemakers and Exogenous Zeitgebers?

Answer 1

Endogenous Pacemakers - Internal Biological rhythms

Exogenous Zeitgebers - External cues

That reset our biological rhythms everyday to keep us in time with the outside world

Question 2

Describe an Endogenous Pacemaker

Answer 2

• Most important one is the SCN in the hypothalamus. It controls all other biological clocks in the body
• Neurons in the SCN synchronise with each other so their target neurons elsewhere receive time-coordinated signals. (Not for long)
• The SCN has its own circadian rhythm only resetting when external lights change and it receives info about light levels through the the optic nerve.
• The SCN regulates manufacturing and secretion of melatonin in the lineal gland by the interconnecting neural pathway. The SCN sends a signal to the gland, making it increase production&secretion of melatonin at night.
• Melatonin indices sleep by inhibiting the brain mechanisms that promote wakefulness

Question 3

Evaluation of Endogenous Pacemaker

Answer 3

+ Kate Aldcroft spent 25 days in a lab. She had no access to light (exo zeit) to reset her SCN. At the end of her 25 days, her core temp rhythm was still at 24 hours, indicating that we DO NOT need exo zeits to maintain our internal biological rhythms

• Her sleep wake cycle extended to 30 hours, with periods of sleep as long as 16 hours, suggesting we DO need the exo zeits to maintain our internal biological rhythms

Question 4

Describe an Exogenous Zeitgebers

Answer 4

Exogenous Zeitgebers - environmental events that are responsible for maintaining the biological clock of an organism eg light.

• Receptors in the SCN are sensitive to changes in light during the day and use this info to synchronise the activity of the body’s organs and glands.
• Light resets the clock every day, keeping it a 24 hour cycle
• Melanopsin is found in the retina and is sensitive natural light.
• When people go to a different country, their endo pacemakers try to impose their inbuilt rhythm of sleep but this is now out of synchrony with exo zeits.
• Out of sync rhythms lead to disrupted sleep patterns, increased anxiety, and decreased a,dryness and vigilance

Question 5

Evaluation of Exogenous Zeitgebers

Answer 5

+ Blind people with light perception still have normal circadian rhythms, but those without light perception have abnormal circadian rhythms, suggesting exo zeits do play a role
+ Exposure to a bright light before an East-West flight decreased time needed to adjust circadian rhythms to local time

• Individuals who live in the Artic where the sun does not set for prolonged periods of time have normal sleeping patterns, suggesting exo zeits have little bearing on our internal biological rhythms