Scientists long believed that two nerve clusters in the human hypothalamus, called suprachiasmatic nuclei (SCNs), were what controlled our circadian rhythms. Those rhythms are the biological cycles that recur approximately every 24 hours in synchronization with the cycle of sunlight and darkness caused by Earth's rotation. Studies have demonstrated that in some animals, the SCNs control daily fluctuations in blood pressure, body temperature, activity level, and alertness, as well as the nighttime release of the sleeppromoting agent melatonin. Furthermore, cells in the human retina dedicated to transmitting information about light levels to the SCNs have recently been discovered.

Four critical genes governing circadian cycles have been found to be active in every tissue, however, not just the SCNs, of flies, mice, and humans. In addition, when laboratory rats that usually ate at will were fed only once a day, peak activity of a clock gene in their livers shifted by 12 hours, whereas the same clock gene in the SCNs remained synchronized with light cycles. While scientists do not dispute the role of the SCNs in controlling core functions such as the regulation of body temperature and blood pressure, scientists now believe that circadian clocks in other organs and tissues may respond to external cues other than light—including temperature changes—that recur regularly every 24 hours.

The primary purpose of the passage is to

challenge recent findings that appear to contradict earlier findings

present two sides of an ongoing scientific debate

report answers to several questions that have long puzzled researchers

discuss evidence that has caused a long-standing belief to be revised

attempt to explain a commonly misunderstood biological phenomenon



1)科学家一直相信:SCNs 控制生物周期circadian rhythms(1. 解释CR是随昼夜每24小时变化的;2.SCNs的具体作用;3.给SCNs传递光的细胞也被发现)










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