Control of Body Rhythms
The regular, rhythmic responses of organs and systems, as controlled by reverberating nerve circuits, may be lengthened or shortened by cues from the environment. An external cue, which resets the
Human physiologic processes that exhibit circadian rhythms during daily activities and periods of inactivity or sleep Filled circles represent the time of peak function. Horizontal bars represent the range of measurements confidence limits.
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Adapted, by permission, from A. Reinberg and M.H. Smolensky,Introduction to chronobiology. In Biological rhythms and medicine. Cellular, metabolic, physiopathologic, and pharmacologic aspects New York: Springer Verlag, -. duration of a biological rhythm, is named either a zeitgeber a German word meaning time giver, synchronizer, or entraining agent by chronobiologists those who study biological clocks. Besides light, which is the dominant external cue, human zeitgebers include stressful situations, group living, shift work meal timing, specific dietary nutrients, temperature, social interactions, and exercise. The hypothalamus, which is the site of control for many biological responses temperature, thirst, hunger, reproductive hormones, was for many years believed to be the primary body clock. Presently, the suprachiasmatic nucleus SCN is considered by many to be the brain’s master clock because it regulates important neuroendocrine circadian rhythms such as ACTH, thyroid- stimulating hormone, prolactin, insulin, and glucagon. However, because obliteration of the SCN in experimental animals leaves ultradian rhythms those with a duration less than intact, it is unlikely that the SCN acts alone to control biological rhythms It is now believed that the body contains numerous circadian oscillators, at distinct sites, that are driven by signals such as sunrise onset of light and/or sunset cessation of light. And, it is known that the microscopic structure and biochemical function of individual cells and cell components demonstrate variations. Examples of this include circadian rhythms in cell division mitosis; mitochondrial enzyme activity succinate dehydrogenase; lysosome number, size, and activity; Golgi apparatus activity; and the spatial arrangement of organelles structures within each cell that have specific functions. This also explains why isolated organs show circadian rhythmicity. For example, an independent circadian oscillator has been identified in the hamster retina, and a similar biological clock has been hypothesized in humans.
Further, genes sections of the DNA molecule in chromosomes that code for specific traits are known to control at least one complex circadian behavior in mice wheel running activity in the dark versus sleep during daylight hours. Genes that control other circadian rhythms have been discovered in hamsters, lizards, bacteria, mold, and fruit flies. Apparently, these genes produce peptides that regulate biological rhythms. Interestingly, periodic phenomena that are genetic in origin are not restricted to h, days, or days. Biological rhythms of approximately h, min, h, days, and days have been detected that have no correspondence to any known cosmic or environmental periodicities.
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