Biological rhythms. In order to say that such a clock exists, it is essential to show that periodic behaviors really are internal not triggered by external cues such as the changes in light levels, temperature, and relative humidity.

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10F 20217


Teacher: Şirin Güntürkün



Many squirrels hibernate every year, many birds migrate south for the winter, and crickets start their mating calls each day before sunset. Many plant activities, like transpiration and synthesis of certain enzymes, happen within the course of a day. But what drives these cycles of behavior is the organisms’ internal clocks that help govern behavior on time scales that run from a few minutes to a year.

In order to say that such a clock exists, it is essential to show that periodic behaviors really are internal – not triggered by external cues such as the changes in light levels, temperature, and relative humidity. To prove that these conditions are not effective on internal clocks, biologists have done their researches on lab conditions, where the animals can be isolated from any environmental cue. Even in this artificially constant conditions, many physiological processes continue to oscillate with their periodicities. For instance, many legumes lower their leaves in the evening and raise them in the morning. A bean plant will continue these “sleep movements” even if kept in constant light or constant darkness; the leaves are not simply responding to sunrise and sunset. Another example is the male teleogryllus cricket, who continue to chirp nearly 11 hours per day and start every bout of calling approximately 25 or 26 hours after the end of the previous bout, in lab conditions where temperature is held constant and light kept on about a clock. The point here that this biorhythmic behaviors are prompted by an internal clock and will continue in a fairly fixed way in the absence of any environmental cause.

Such physiological cycles with a frequency of about 24 hours and are not directly  paced by any known environmental variable are called circadian rhythms. (From the Latin  circa, approximately, and dies, day)

Figure 1: Circadian rhythm, a graphic depiction of cortisol values over a 24-hour period.

From E.D. Weitzman et al., “Twenty-four-hour Patterns of the Episodic Secretion of Cortisol in Normal Subjects,” Journal of Clinical Endocrinology and Metabolism, vol. 33, pp. 13–22, © by The Endocrine Society, 1971

Figure 2:

Five-day segments of simultaneous records of rectal temperature, plasma urea concentration, and plasma cholesterol concentration of a female goat (Capra hircus). The horizontal bars at the top indicate the timing of the light-dark cycle. Notice that the rhythms of rectal temperature and urea concentration have similar phases (peaking in the middle of the night) but the rhythm of cholesterol concentration has the opposite phase (peaking in the middle of the day).

But can we say that circadian rhythms are only a consequence of internal clock, can’t there be any effect of environment on the organism? Organisms, including plants and humans, continue their rhythms  when placed in the deepest mine shafts or when orbited in satellites but all research thus far indicates that the oscillator for circadian rhythms is endogenous (internal) . This internal clock, however, is entrained (set)to a period of precisely 24 hours by daily signals from the environment. When an organism is kept under constant conditions, its circadian rhythm deviates from a perfect 24-hour.  Note that the cricket’s internal clock does not keep a strict 24-hour time, but takes approximately 25 or 26 hours to complete a full cycle. This time may vary from 21 hours to 27 depending on the nature of the response. In humans, researchers suggest that the internal clock operates on a 25-hour cycle, which means we would have a 25-hour sleep-wake cycle if we were isolated in a dark room. This frequency is considerably variable, some individual have 28-hour sleep-wake cycles whereas some others have less than 24-hours.

This deviation doesn’t indicate that these free-running periods are imperfect. Free-running periods are still keeping perfect time but they are not synchronized with the outside world.

Figure 3: Some of the key properties of a circadian oscillator

Besides these, just how our body controls its internal clocks are still mysterious. Researches suggest that that the brain, particularly one region, suprachiasmatic nucleus, has the major role in coordinating several core rhythms. The suprachiasmatic nucleus is tissue of nerve cells in the region called  hypothalamus. It may regulate other centers of control, therefore is often referred to as the “master clock”. Ultimate control of the SCN is thought to reside in a gland in the brain known as the pineal gland. The pineal gland secretes melatonin. As soon as our brains receive  sensory signals from our eyes about the waning light at sunset, for instance, the pineal gland steps up its rate of melatonin secretion. Molecules of melatonin are transported to certain brain neurons which are involved in sleep behavior, a lowering of body temperature, and possibly other physiological events. When sun rises, as the eye detects the light of a new day, melatonin production slows down. The temperature of our bodies increases, so we wake up and become active.

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Figure 5 (Next Page):

Photomicrograph of the master circadian clock, the suprachiasmatic nucleus (SCN), and examples of overt circadian rhythms in several mammalian species. Upper left panel: coronal section of Nissl stained fetal sheep bilateral suprachiasmatic nuclei (SCN; one is indicated by white arrows); V: third ventricle, bar: 500 µm. Upper right panel: melatonin rhythm in pregnant ewes and their fetuses (closed and open circles, respectively); (mean ± S.E.; n=4). Lower left panel: double plot of locomotor activity rhythm in a rat. Each line represents the recordings of two ...

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