Lesson 13: Endogenous Pacemakers And Exogenous Zeitgebers Flashcards
Endogenous pacemakers and exogenous zeitgebers
Our internal biological rhythms must be finely tuned in order to stay in keeping with the outside world. In order to achieve this we have endogenous pacemakers (internal) and exogenous zeitgebers (external). Endogenous pacemakers are body clocks (e.g. the suprachiasmatic nuclei) which regulate biological rhythms whereas exogenous zeitgebers are cues (e.g. light) than entrain our biological rhythms.
Endogenous pacemakers
he most important endogenous pacemaker is the suprachiasmatic nuclei (SCN). This is a tiny cluster of nerve cells in the hypothalamus. The SCN plays an important role in generating circadian rhythms. It acts as the master clock, linking other brain regions that control sleep and arousal, and controlling all other biological clocks throughout the body.
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.
The SCN also regulates the manufacture and secretion of melatonin in the pineal gland via the interconnecting neural pathway. The SCN sends a signal to the pineal gland, directing it 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.
Advantages of endogenous pacemakers
+ Folkard (1996) studied a university student, Kate Aldcroft, who spent 25 days in a laboratory. She had no access to the exogenous zeitgebers of light to reset the SCN. However, at the end of 25 days her core temperature rhythm was still at 24 hours. This indicates that we DO NOT need the exogenous zeitgebers of light to maintain our internal biological rhythms
Disadvantages of endogenous pacemakers
- Kate Aldcroft’s sleep-wake cycle extended to 30 hours, with periods of sleep as long as 16 hours. This suggests that we DO need the exogenous zeitgebers of light to maintain our internal biological rhythms
Exogenous zeitgebers
The term exogenous refers to anything whose origins are outside of the organism. Exogenous zeitgebers are environmental events that are responsible for maintaining the biological clock of an organism. The most important zeitgebers for most animals is light.
Receptors in the SCN are sensitive to changes in light levels during the day and 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.
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. Out of sync biological rhythms lead to disrupted sleep patterns, increased anxiety and decreased alertness and vigilance.
Advantages of Exogenous Zeitgebers
+ The vast majority of blind people who still have light perception have normal circadian rhythms. Blind people without light perception show abnormal circadian rhythms. This shows the vital role that the exogenous zeitgeber of light levels play in maintaining our internal biological rhythms.
+ Burgess et al. (2003) found that exposure to bright light prior to an east-west flight decreased the time needed to adjust circadian rhythms to local time
Disadvantages of Exogenous Zeitgebers
- Studies of individuals who live in Artic regions, where the sun does not set in the summer months, show normal sleeping patterns despite the prolonged exposure to light. This suggests that there are occasions where the exogenous zeitgeber of light may have very little bearing on our internal biological rhythms
