12 - Endogenous Pacemakers and Exogenous Zeitgebers Flashcards
Define Endogenous Pacemakers
Internal biological rhythms
Define Exogenous Zeitgebers
External factors e.g. light
Environmental events that are responsible for maintaining the biological clock of an organism.
Why are endogenous pacemakers and and exogenous zeitgebers necessary?
Our internal biological rhythms must be finely tuned in order to stay in keeping with the outside world.
These reset our biological rhythms every day.
What is the most important endogenous pacemaker and what is its function?
Suprachiasmatic nuclei (SCN) - tiny cluster of nerve cells in the hypothalamus.
Plays an important role in generating circadian rhythms.
Acts as the master clock, linking other brain regions that control sleep and arousal, and controlling all other biological clocks throughout the body.
What is the mechanism behind the SCN?
Neurons within the SCN synchronise with each other, so that their target neurons in sites elsewhere in the body receive time-coordinated signals.
These peripheral clocks can maintain a circadian rhythm, but not for very long, which is why they are 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.
The SCN receives information about light levels through the optic nerve. If our biological clock is running slow then morning light shifts the clock.
How is the SCN linked to hormones?
Regulates manufacture and secretion of melatonin in the pineal gland via the interconnecting neural pathway.
SCN sends signal to pineal gland to increase production and secretion of the hormone melatonin at night and to decrease it as light levels increase in the morning.
Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness.
What are the advantages of endogenous pacemakers?
The SCN is of vital importance. Strain of hamster bred that had abnormal circadian rhythms of 20 hours. SCN neurons from these hamsters were transported to normal hamsters. The normal hamsters started to display the abnormal circadian rhythms of 20 hours.
University student, Kate Aldcroft, who spent 25 days in a laboratory studied. She had no access to daylight or other exogenous zeitgebers that might have reset the SCN. At the end of 25 days her core temperature rhythm was still at 24 hours.
What are the disadvantages of endogenous pacemakers?
Kate Aldcroft’s sleep-wake cycle extended to 30 hours, with periods of sleep as long as 16 hours. We do need external influences to maintain our circadian rhythms.
What is the most important exogenous zeitgebers?
Light
Describe the mechanism behind how light works as an exogenous zeitgeber
Receptors in the SCN are sensitive to changes in light levels during the day. They use this information to synchronise the activity of the body’s organs and glands.
Light resets the internal biological clock each day, keeping it on a 24-hour cycle. A protein in the retina of the eye called melanopsin, which is sensitive to natural light, is critical in this system.
How might someone’s biological rhythm get disrupted? What does this result in?
When people move to a night shift or travel to a country with a different time zone their endogenous pacemakers try to impose their inbuilt rhythm of sleep (circadian rhythm), but this is now out of synchrony with the exodengeous zeitgeber of light.
Disrupted biological rhythms lead to disrupted sleep patterns, increased anxiety and decreased alertness and vigilance.
What are the advantages of exogenous zeitgebers?
The vast majority of blind subjects who still have some light perception have normal circadian rhythms. People without light perception show abnormal circadian rhythms.
Exposure to bright light prior to an east-west flight decreased the time needed to readjust to local time on arrival.
Sleep-wake and activity-rest patterns of two groups of participants compared over a five-week period. One group remained in normal warm artificial light, other group experienced artificial blue light. All participants kept daily sleep log + wore devices that measured their movement. Participants under the warm light synchronised their circadian rhythms each day with natural light of dawn. Participants exposed to blue light synchronised their patterns to office hours.
