Conclusion: The mouse has an endogenous rest-activity cycle of less than 24 hours, but that cycle can be entrained by the 24-hour daily cycle of light and dark.

jftfyi 52.13 Circadian Rhythms The marks indicate times when a [V ^h mouse is running on an activity wheel. Two days of activity are ^ J recorded on each horizontal line, such that the data for each ' ' day are plotted twice, once on the right half of a line and again on the left half of the next line below; this double plotting makes patterns easier to see.The schedule of light and dark exposure is indicated by the solid bars running across the top of the figure. First the mouse experiences 12 hours of light and 12 hours of dark every day (top panel), then it is placed in constant darkness (middle panel), and finally it is given a 20-minute exposure to light each day (bottom panel). In constant darkness, the circadian rhythm is free-running, but a 20-minute flash of light at 24-hour intervals can entrain it.

rhythm is waking you up, making you sleepy, initiating activities in your digestive tract, and stimulating many other physiological functions at inappropriate times of the day.

the circadian clock. Where is the clock that controls the circadian rhythm? In mammals, the master circadian clock is located in two tiny groups of cells just above the optic chi asm, the place where the two optic nerves cross. These structures are called the suprachiasmatic nuclei (SCN). If a mammal's SCN are destroyed, it loses circadian rhythmici-ty. Under constant conditions, it is equally likely to be active or asleep at any time of day (Figure 52.14).

Remarkable experiments have shown that circadian rhythms of rest and activity can be restored in an animal whose SCN have been destroyed if it receives a transplant of those nuclei from another animal. Because the restored rhythm has the period of the donor animal, the transplanted tissue clearly controls the recipient's behavior.

Circadian rhythms are found in every animal group, as well as in protists, plants, and fungi, but only vertebrates have SCN. Thus, natural selection has produced a variety of circadian clocks. In most non-mammals, the master clock contains photoreceptors that directly sense changes in light and synchronize behavior with environmental cycles of light

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Essentials of Human Physiology

Essentials of Human Physiology

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