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  3. The would-be sleeper who refights his daily battles in bed——or rehearses tomorrow’s problems——finds it hard to fall asleep. Then

The would-be sleeper who refights his daily battles in bed——or rehearses tomorrow’s problems——finds it hard to fall asleep. Then

admin2009-02-10  40
问题      The would-be sleeper who refights his daily battles in bed——or rehearses tomorrow’s problems——finds it hard to fall asleep. Then he starts worrying about his inability to sleep, which increases his insomnia, which increases his worries, which... In a new development that may help the insomnia to break this vicious cycle, Dr.  Werner P. Koella of the Worcester Foundation for the Experimental Biology has discovered a chemical in the brain that may control normal sleep.
     The substance, known as serotonin, is one of a number of so-called neurohormones in the brain that researchers suspect play an important part in controlling the mind and the emotions. Such chemicals, researchers have learned, assist in transmitting nerve impulses from one nerve cell to another. Serotonin, Koella notes, is produced in particularly high concentrations in the hypothalamus, the "primitive" lower part of the brain and the brain stem, which joins the brain to the top of the spinal cord and is known to contain the centers controlling the level of transmitter substance in the brain stem and hypothalamus that regulate sleep.
     In preliminary experiments, Koella found that giving serotonon to cats produced signs of sleep. Electrode leads were implanted in the cats’ brains and attached to an EEG (electroencephalogram) machine to record the brain waves; next, the serotonin was injected directly into the brain or an artery in the neck. The pupils of the animals’ eyes narrowed and the electroencephalograms showed slow waves characteristic of deep sleep within five to ten minutes.
     More recently, Koella deprived cats of serotonin. The animals, again equipped with implanted electrodes, were given PCPA, a drug that blocks the formation of serotonin. They were then placed in small compartments fitted with one-way mirrors and watched round the clock.
     Normally cats sleep about 15 hours a day; but Koella’s cats, after receiving PCPA, spent about 30 minutes of each day sleeping. Most of the time, their EEG’s showed the brainwave patterns of arousal. Occasionally the cats would curl up as if to go to sleep, but would soon get back on their feet to wander about. The animals showed signs of irritability and often meowed complainingly after a few days of sleep deprivation, but had normal reflexes. The effects of the PCPA wore off eight days to two weeks after administration of the drug; the cats returned to their normal sleeping patterns as serotonin levels in their brains rose again.
     Koella believes that at least some types of chronic insomnia may be caused by a drop in brain - serotonin levels. The Worcester physiologist is now working on chemical ways to raise the brain’s serotonin levels and produce, in his words, "a truly physiological sleeping pill." Synthetic sleeping pills, such as barbiturates, bring sleep, but at a price: they depress the central nervous system, reduce heart action and respiration——and they can become habit-forming or even addictive.
     In one promising experiment toward the goal of letting the body "make its own pill," Koella has found that administration of 5-hydroxytryptophan, the chemical substance from which the body derives serotonin, will quickly restore a normal sleeping pattern to cats lacking in serotonin.
The brain stem is located (Paragraph 2 ) _______.
选项 A、in the hypothalamus
B、between the brain and the spinal cord
C、in the transmitter nerve cells
D、between the hypothalamus and the brain
答案B
解析 第二段中指出“．．．the brain stem，which joins the brain to the top of the spinal cord．．．”即脑干连接brain 与 spinal cord，所以B项正确。
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