Endogenous Pacemakers and Exogenous Zeitgebers Flashcards
Endogenous Pacemakers
Endogenous pacemakers are internal biological clocks that govern biological rhythms, in particular, the circadian sleep-wake cycle. They can be altered and affected by the environment, for example, although the circadian sleep-wake cycle will continue to function without natural cues from light, research from Siffre suggests that light is required to reset the cycle every 24 hours.
What is the most important Endogenous Pacemaker?
The most important endogenous pacemaker is the suprachiasmatic nuclei, which is closely linked to the pineal gland, both of which are influential in maintaining the circadian sleep-wake cycle.
Explain the role of the Suprachiasmatic Nuclei
The suprachiasmatic nuclei (SCN), which lies on the hypothalamus is the main endogenous pacemaker and it controls other biological rhythms, as it links to other areas of the brain that are responsible for sleep and arousal.
The SCN also receives information about levels of light (an exogenous zeitgeber) from the optic nerve, which sets the circadian rhythm so that it is in synchronisation with the outside world, e.g., day and night. First, low levels of light are detected by the retina. Then, melanopsin (a light-sensitive protein in the retina) carries the signal regarding low light levels to the SCN. The SCN sends information along the axon pathway to the pineal gland. The pineal gland leads to an increase in the production of melatonin at night, helping to induce sleep.
Ultimately, the SCN and pineal gland work together as endogenous pacemakers, however, their activity is responsive to the external cue of light.
Exogenous Zeitgebers
Exogenous zeitgebers are external cues that help to regulate internal biological clocks. They are environmental events that are responsible for resetting the biological clock of an organism.
What is the most important Exogenous Zeitgeber?
The most important exogenous zeitgeber is light, which is responsible for resetting the body clock each day, keeping it on a 24-hour cycle.
Explain the role of light
The SCN contains receptors that are sensitive to light and this external cue is used to synchronise the body’s internal organs and glands. Melanopsin is also sensitive to light and carries the signals to the SCN to set the 24-hour body cycle.
Other Exogenous Zeitgebers
Social cues, such as mealtimes and social activities, can also act as exogenous zeitgebers. Humans can compensate for the lack of natural light, by using social cues instead.
