Endogenous Pacemakers and Exogenous Zeitgebers

Question 1

key terms

Answer 1

Endogenous = internal

Pacemaker = body clock

&

Exogenous = outside the organism

Zeitgeber = time-giver

Question 2

endogenous pacemakers

Answer 2

• scn
• pineal gland

Question 3

exogenous Zeitgebers

Answer 3

• light
• social cues

Question 4

scn

Answer 4

Main endogenous pacemaker: Suprachiasmatic nucleus (SCN) in the hypothalamus.
Located above the optic chiasm (where optic nerves cross).
SCN maintains circadian rhythms, like the sleep/wake cycle.
It receives light information via the optic nerve, even through closed eyelids.
Morning light adjusts the internal clock if it runs slow, syncing it with the external world.

Question 5

research for scn

Answer 5

DeCoursey et al. (2000) destroyed SCN connections in 30 chipmunks.
The chipmunks were observed in their natural habitat for 80 days.
Their sleep-wake cycles disappeared.
A significant number were killed by predators, likely due to being awake and vulnerable when they should have been asleep.

Question 6

pineal gland

Answer 6

Another endogenous pacemaker that works with the SCN is the Pineal Gland.

Pea-like structure that receives info from SCN

Contains light sensitive cells.

When light is sensed melatonin production is inhibited.

When light level falls melatonin production increases

This induces sleep by inhibiting brain mechanisms that promote the awake state.

Question 7

exogenous Zeitgebers

Answer 7

ChatGPT
Environmental factors, like light (e.g. sunrise and sunset), can reset our biological clock, a process called entrainment.

The sleep/wake cycle is influenced by both internal (endogenous pacemakers) and external (exogenous zeitgebers) factors.

Question 8

light as an exogenous zeitgeber

Answer 8

Light is a key zeitgeber that resets the SCN, helping maintain the 24-hour sleep-wake cycle.
SCN light receptors detect changes in light and synchronize body organs and glands.
Light indirectly affects processes like hormone secretion (e.g., melatonin, cortisol) and blood circulation.

Question 9

research into exogenous zeitgebers

Answer 9

Campbell and Murphy (1998) showed that light can be detected by skin receptors, not just the eyes.
15 participants were exposed to light on the back of their knees at different times.
This caused a deviation of up to 3 hours in their sleep-wake cycle

Question 10

social cues

Answer 10

Social stimuli, like mealtimes and activities, can act as zeitgebers.
Human infants have random sleep-wake cycles, but circadian rhythms start around 6 weeks and are typically entrained by 16 weeks.
Parent-imposed schedules (e.g., mealtimes, bedtimes) are key in this entrainment.
Adapting to local eating and sleeping times helps entrain circadian rhythms and reduce jet lag when traveling.

Question 11

strength

Answer 11

• P: The role of the SCN in the sleep/wake cycle can be shown through animal studies.
• E: DeCoursey et al. (2000) destroyed the SCN in 30 chipmunks and observed them in the wild for 80 days.
• E: The chipmunks’ sleep-wake cycles disappeared, and many were killed by predators due to increased vulnerability.
• L: This suggests the SCN plays a crucial role in regulating the sleep/wake cycle.

Question 12

strength 2

Answer 12

• P: Evidence supports the importance of zeitgebers in the sleep/wake cycle.
• E: Campbell & Murphy (1998) showed that light can be detected by skin receptors, even without eye involvement.
• E: Shining a light on participants’ knees caused deviations of up to 3 hours in their sleep-wake cycle.
• L: This shows the significant role of exogenous zeitgebers in regulating the sleep/wake cycle, beyond those linked to the eyes.

Question 13

limtation 1

Answer 13

• P: A limitation of research on exogenous zeitgebers is that their influence may be overstated.
• E: Miles (1977) reported a blind man with a 24.9-hour circadian rhythm that couldn’t be adjusted by social cues; he needed medication to regulate his cycle.
• E: However, some argue this case is exceptional, and for most, internal pacemakers interact with exogenous zeitgebers to set circadian rhythms.
• L: This suggests the role of exogenous zeitgebers may be overstated, as some individuals’ pacemakers resist their influence.

Question 14

limitation 2

Answer 14

P – Research into endogenous pacemakers is limited, as much of it relies on non-human animals.

E – This raises issues with applying findings to humans, given the differences in biological systems.

E – Additionally, this research often involves significant harm to animals, raising ethical concerns and impacting validity.

L – Therefore, findings should be interpreted cautiously, as human systems may differ, especially under less stressful conditions.