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Caffeine, the stimulant in coffee, has been called “the most widely used psychoactive substance on Earth .” 背景介绍，提出讨论对象 Synder, Daly and Bruns have recently（新观点） proposed that主题句，提出新观点，下面是具体内容要详细读 caffeine affects behavior by countering（作用方式，注意是相反） the activity in (5) the human brain of a naturally occurring chemical called adenosine.（简化Ad） Adenosine normally depresses作用方式，注意是相反neuron firing in many areas of the brain. It apparently does this by inhibiting作用方式，注意是相反 the release of neurotransmitters（简化nt）, chemicals that carry nerve impulses from one neuron to the next. (10) 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 propose that caffeine, which is struc- (15) turally similar to adenosine（Ca与Ad的关系）, 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.
以上是新观点的具体内容，要尽量理解，看到此处要停下来想想，Ca 反作用 Ad ，和 specific receptors 结合，刺激兴奋
For many years,（老） caffeine’s effects have been attri- (20) buted to its inhibition of the production of phosphodi- esterase,（老观点） an enzyme that breaks down the chemical called cyclic AMP.A number of neurotransmitters exert their effects by first increasing cyclic AMP concentra- tions in target neurons. Therefore, prolonged periods at (25) the elevated concentrations, as might be brought about by a phosphodiesterase inhibitor, could lead to a greater amount of neuron firing and, consequently, to behav- ioral stimulation.具体的作用机理
But（提出反对，要注意如何反对的） Snyder et al point out that the caffeine concentrations needed to inhibit the production (30) of phosphodiesterase in the brain are much higher than those that produce stimulation.（理由）
Moreover,（递进） other com- pounds that block phosphodiesterase’s activity are not stimulants.
一段结束，想这段主旨：以前老观点是什么，Sny这些鸟人如何反对的
To buttress（正评价，支持） their case that caffeine acts instead（这个词是与前面的方法对应） by pre- (35) venting adenosine binding, Snyder et al compared（对比） the stimulatory effects of a series of caffeine derivatives with their ability to dislodge（逐出） adenosine from its receptors in the brains of mice. “In general,（有总结的意思了）” they reported, “the ability of the compounds to compete at the receptors (40) correlates with（关联） their ability to stimulate locomotion in the mouse; i.e.（下面是具体说明）, the higher their capacity to bind at the receptors, the higher their ability to stimulate locomo- tion.” Theophylline, a close structural relative of caffeine and the major stimulant in tea, was one of the most (45) effective compounds in both regards举例说明.
想：syn做试验来证明，用替代物
There were some apparent exceptions（想，什么例外？） to the general correlation observed between adenosine-receptor binding and stimulation. One of these was a compound called 3-isobuty1-1-methylxanthine(IBMX), which bound very (50) well but actually depressed mouse locomotion.（例外的表象）Snyder et al suggest that（大N又发话了，辩解） this is not a major stumbling block（态度） to their hypothesis. The problem is that the compound has mixed effects（新名词，后面肯定有解释） in the brain, a not unusual occurrence with psychoactive drugs. Even caffeine, which is generally (55) known only for its stimulatory effects, displays this property, depressing mouse locomotion at very low concentrations and stimulating it at higher ones.这就是那个mixed
effects
想：提出了一个例外，也被syn 驳斥了！
逻辑简图： 1， 提出新观点，具体内容：Ca 反作用 Ad ，和 specific receptors 结合，刺激兴奋 2， 以前老观点是什么：Ca 抑制 P，Sny这些鸟人如何反对的：浓度要高，且抑制了也不一定有用 3， syn做试验来证明，用替代物屡试不爽 4， 提出了一个例外IBM，也被syn 驳斥了：mixed
effect
1. The primary purpose of the passage is to (A) discuss a plan for investigation of a phenomenon that is not yet fully understood (B) present two explanations of a phenomenon and reconcile the differences between them (C) summarize two theories and suggest a third theory that overcomes the problems encountered in the first two (D) describe an alternative hypothesis and provide evidence and arguments that support it（D） (E) challenge the validity of a theory by exposing the inconsistencies and contradictions in it
recently proposed syn做试验来证明 Sny这些鸟人的反对和驳斥 1. D is the best answer. This question requires you to identify the primary concern of the passage as a whole. The first paragraph presents a recent hypothesis about how caffeine affects behavior. The second paragraph describes an earlier and widely accepted hypothesis about how caffeine affects behavior, and then presents evidence that is not consistent with that hypothesis. The third and fourth paragraphs return to the newer hypothesis introduced in the first paragraph and provide “evidence and arguments” that support this alternative hypothesis.
2. According so Snyder et al, caffeine differs from adenosine in that caffeine (A) stimulates behavior in the mouse and in humans, whereas adenosine stimulates behavior in humans only (B) has mixed effects（段落不对应） in the brain, whereas adenosine has only a stimulatory effect (C) increases cyclic AMP concentrations（段落不对应） in target neurons, whereas adenosine decreases such concentrations (D) permits release of neurotransmitters when it is bound to adenosine receptors, whereas adenosine inhibits such release（D） (E) inhibits both neuron firing and the production of phosphodiesterase when there is a sufficient concentration in the brain, whereas adenosine inhibits only neuron firing 此题对应第一段，为此出现别段落都不好，答案是对理论的理解 2. D is the best answer. Lines 11-23 state that adenosine “depresses neuron firing” by binding to specific receptors on neuronal membranes, which in turn inhibits the release of neurotransmitters. Lines 27-35 describe Snyder et al’s hypothesis about caffeine. They propose that caffeine binds to specific receptors on neuronal membranes, which prevents adenosine from binding to those receptors and “allows the neurons to fire more readily that they otherwise would”. Therefore, according to Snyder et al, caffeine differs from adenosine in that caffeine permits neurotransmitter release when it is bound to adenosine receptors, whereas adenosine inhibits neruotransmitter release.
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发表于 2009-5-20 00:04:00
