2. 外语
  3. The primary purpose of the passage is to______ According so Snyder et al, caffeine differs from adenosine in that caffeine_____

The primary purpose of the passage is to______ According so Snyder et al, caffeine differs from adenosine in that caffeine_____

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问题 The primary purpose of the passage is to______
According so Snyder et al, caffeine differs from adenosine in that caffeine______
选项 A、stimulates behavior in the mouse and in humans, whereas adenosine stimulates behavior in humans only
B、has mixed effects in the brain, whereas adenosine has only a stimulatory effect
C、increases cyclic AMP concentrations 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
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
答案D
解析 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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