The effect of endogenous pacemakers and exogenous zeitgebers on the sleep/wake cycle. Flashcards
what are endogenous pacemakers
Endogenous pacemakers: Internal body ‘clocks’ that regulate biological rhythms.
what is the suprachiasmatic nucleus (SCN)
The suprachiasmatic nucleus (SCN) is a tiny cluster of nerve cells in the hypothalamus.
It is important in generating the body’s circadian rhythm.
It acts as the ‘master clock’, with links to other brain regions that control sleep and arousal, and has control over other biological clocks throughout the body.
Neurons within the SCN spontaneously synchronise with each other, so that their target neurons in sites elsewhere in the body receive correctly time-coordinated signals.
These peripheral clocks can maintain a circadian rhythm, but not for very long, which is why they must be controlled by the SCN.
This is possible because of the SCN’s built-in circadian rhythm, which only needs resetting when external light levels change.
Fill in the gaps
The SCN is thought to be the main ________________ _________________. The Suprachiasmatic Nucleus is located in the ____________________, just above the ___________ __________. The SCN obtains information on light from the optic nerve, even when our eyes are shut as light penetrates the eyelids. Special ___________________ in the eye pick up light signals and carry them to the SCN. If our endogenous clock is running slow, the morning light shifts the clock ahead so that the rhythm is in sync with the _______________ _____________ (photoentrainment). Our circadian rhythms are therefore ___________________ by the _____________. The SCN also regulates the manufacture and secretion of ___________________ in the _____________ ____________ via an interconnecting neural pathway.
what did Ralph do
Ralph et al. (1990) removed the SCN out of genetically abnormal hamsters which only had a circadian cycle of 20 hours.
They transplanted the SCN cells into hamsters, which had no such abnormality and functioned on a normal 24-hour cycle.
Ralph et al. (1990) findings
Following the transplant, the circadian rhythm of the hamsters shortened to 20 hours i.e. they adopted the rhythm of the implanted SCN.
This suggests the importance of the SCN in regulating circadian rhythms.
Ralph et al. (1990) criticism
Criticisms: Animal research – difficult to extrapolate the findings to humans. The SCN may have a different function in the hamster brain compared to the human brain.
what did DeCoursey et al. (2000)
do
Destroyed the SCN in 30 chipmunks and returned them to their natural habitat.
They were then observed alongside chipmunks whose SCN remained intact.
what were DeCoursey et al. (2000)
findings
It was found that the SCN-lesioned chipmunks were much more active at night than the normal, control group of chipmunks, putting them at risk from nocturnal predators.
After 80 days, a significantly higher number of the lesioned chipmunks had been killed by weasels in comparison to the control group.
This research suggests that the SCN is crucial in regulating the sleep-wake cycle. Without it, the sleep-wake cycle is severely disrupted.
how could DeCoursey et al. (2000)
research be criticised
different function in the chipmunk brain compared to the human brain.
Explain what Siffre did
Siffre (a French cave explorer) spent 6 months in a cave with no natural light or cues as to the day or time e.g. clocks or radio.
He simply woke, ate and slept when he felt it was appropriate to do so and his internal body clock was allowed to free-run.
Key findings: It settled into a sleep/wake cycle of 25-30 hours.
He lost track of how many days he had been in the cave, believing it to be one month less than he had actually stayed in.
Conclusions: This suggests that circadian rhythms persist despite isolation from natural light, which demonstrates the existence of an endogenous clock i.e. his SCN continued to regulate his sleep/wake cycle.
However, it also shows that external cues are important in terms of keeping the sleep-wake cycle in sync with the outside world, because his clock was not exactly 24 hours.
what are Exogenous zeitgebers
Environmental cues, such as light, that help to regulate the sleep/wake cycle in an organism.
describe lights effect on the eyes
Rods and cones in the retina of the eye detect light to form images.
However, there is a third type of light-detecting cell in the retina that gauges overall brightness to help reset the internal biological clock.
A protein called melanopsin, which is sensitive to natural light, is critical in this system.
A small number of retinal cells contain melanopsin and carry signals to the SCN to set the daily body cycle.
Support for the role of melanopsin in setting the circadian rhythms comes from studies of blind people as some blind people are still able to reliably entrain their circadian rhythm in response to light despite a total lack of image-forming visual perception (i.e. non-functioning rods and cones).
This suggests that the pathway from retinol cells containing melanopsin to the SCN is still intact and that there is therefore more than one visual pathway: one responsible for communicating light information and another for communicating images. Further support comes from the fact that people without light perception show abnormal circadian entrainment.
What did Campbell and murphy do
monitored the body temperatures of 15 volunteers who slept in a laboratory. They introduced light to them during the night by waking them at a series of intervals and shining a light pad onto the back of their knees.
What did Campbell and murphy find
Their circadian rhythms were disrupted by up to three hour
