今天遇到的魔鬼阅读，自信心严重受挫~

chrissylee

 GMAT阅读大全中第10篇，完全看不懂，崩溃~

Caffeine, the stimulant in coffee, has been called “the most widely used psychoactive substance on Earth.” Snyder, Daly and Bruns have recently proposed that caffeine affect behavior by countering the activity in the human brain of a naturally occurring chemical called adenosine. Adenosine normally depresses neuron firing in many areas of the brain. It apparently does this by inhibiting the release of neurotransmitters, chemicals that carry nerve impulses from one neuron to the next. Like many other agents that affect neuron firing, adenosine must first bind to specific receptors on neuronal membranes. There are at least two classes of these receptors, which have been designated A₁ and A₂. Snyder et al (et al: abbr. (Lat) 以及其他人，等人) propose that caffeine, which is structurally similar to adenosine, is able to bind to both types of receptors, which prevents adenosine from attaching there and allows the neurons to fire more readily than they otherwise would.

For many years, caffeine’s effects have been attributed to its inhibition of the production of phosphodiesterase, an enzyme that breaks down the chemical called cyclic AMP. A number of neurotransmitters exert their effects by first increasing cyclic AMP concentrations in target neurons. Therefore, prolonged periods at the elevated concentrations, as might be brought about by a phosphodiesterase inhibitor, could lead to a greater amount of neuron firing and, consequently, to behavioral stimulation. But Snyder et al point out that the caffeine concentrations needed to inhibit the production of phosphodiesterase in the brain are much higher than those that produce stimulation. Moreover, other compounds that block phosphodiesterase’s activity are not stimulants.

To buttress their case that caffeine acts instead by preventing adenosine binding, Snyder et al compared the stimulatory effects of a series of caffeine derivatives with their ability to dislodge adenosine from its receptors in the brains of mice. “In general,” they reported, “the ability of the compounds to compete at the receptors correlates with their ability to stimulate locomotion in the mouse; i.e., the higher their capacity to bind at the receptors, the higher their ability to stimulate locomotion.” Theophylline, a close structural relative of caffeine and the major stimulant in tea, was one of the most effective compounds in both regards.

There were some apparent exceptions to the general correlation observed between adenosine-receptor binding and stimulation. One of these was a compound called 3-isobutyl-1-methylxanthine (IBMX), which bound very well but actually depressed mouse locomotion. Snyder et al suggests that this is not a major stumbling block (stumbling block: n.障碍物, 绊脚石) to their hypothesis. The problem is that the compound has mixed effects in the brain, a not unusual occurrence with psychoactive drugs. Even caffeine, which is generally known only for its stimulatory effects, displays this property, depressing mouse locomotion at very low concentrations and stimulating it at higher ones.

[此贴子已经被作者于2007-9-15 16:41:49编辑过]

qianrene

这篇在OG11里有，应该先抓结构：这里是新旧观点解释结构

第一段里说Snyder, Daly and Bruns这三个人最近发现了一个咖啡因怎么起作用的理论，是通过抵消adenosine的作用，adenosine有什么作用呢？它能够阻止neurotransmitters的释放进而使大脑没那么兴奋。抵消的原理就是本来adenosine要跟neurotransmitters接轨的，结果咖啡因抢先一步了

第二段里说另外一个旧理论很多年来一直认为咖啡因抵消phosphodiesterase的作用，phosphodiesterase可以抑制AMP的产生，AMP是一种兴奋的东东， 然后第一段中的人就跳出来了But Snyder et al point out that 两个原因：1。咖啡不够抵消phosphodiesterase的作用。2。有一些抵消phosphodiesterase作用的化合物并不是咖啡因

第三段就说了这三个人怎么拿小白鼠做实验证明他们第一段中的理论

最后一段说有反驳他们观点的结果被他们给驳回了，说咖啡因在低的时候有两方面的作用，一个是使兴奋，一个是使郁闷，郁闷占了上风

chrissylee

谢谢LS的~

huangyh03

这篇还好吧，算是比较容易得了，mm还要努力啊

niswzzhn

OG10里面有的文章，千万不要想让自己都看明白，大概看明白逻辑结构就可以了，这么晦涩的文章，就是给外国人也看不明白，建议你看一下杨继的GMAT阅读精解，会消除你对阅读的误解。