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1、新东方在线[www.koolearn.com]网络课堂电子教材系列GMAT阅读讲义GMAT阅读讲义主讲：王鹏欢迎使用新东方在线电子教材GMAT核心阅读Passage1Caffeine,thestimulantincoffee,hasbeencalled“themostwidelyusedpsychoactivesubstanceonEarth.”Synder,DalyandBrunshaverecentlyproposedthatcaffeineaffectsbehaviorbycounteringtheactivityin(5)thehumanbraino
2、fanaturallyoccurringchemicalcalledadenosine.Adenosinenormallydepressesneuronfiringinmanyareasofthebrain.Itapparentlydoesthisbyinhibitingthereleaseofneurotransmitters,chemicalsthatcarrynerveimpulsesfromoneneurontothenext.(10)Likemanyotheragentsthataffectneuronfiring,adenosinemustfir
3、stbindtospecificreceptorsonneuronalmembranes.Thereareatleasttwoclassesofthesereceptors,whichhavebeendesignatedA1andA2.Snyderetalproposethatcaffeine,whichisstruc-(15)turallysimilartoadenosine,isabletobindtobothtypesofreceptors,whichpreventsadenosinefromattachingthereandallowstheneur
4、onstofiremorereadilythantheyotherwisewould.Formanyyears,caffeine’seffectshavebeenattri-(20)butedtoitsinhibitionoftheproductionofphosphodi-esterase,anenzymethatbreaksdownthechemicalcalledcyclicAMP.AnumberofneurotransmittersexerttheireffectsbyfirstincreasingcyclicAMPconcentra-tionsin
5、targetneurons.Therefore,prolongedperiodsat(25)theelevatedconcentrations,asmightbebroughtaboutbyaphosphodiesteraseinhibitor,couldleadtoagreater34新东方在线[www.koolearn.com]网络课堂电子教材系列GMAT阅读讲义amountofneuronfiringand,consequently,tobehav-ioralstimulation.ButSnyderetalpointoutthatthecaffein
6、econcentrationsneededtoinhibittheproduction(30)ofphosphodiesteraseinthebrainaremuchhigherthanthosethatproducestimulation.Moreover,othercom-poundsthatblockphosphodiesterase’sactivityarenotstimulants.Tobuttresstheircasethatcaffeineactsinsteadbypre-(35)ventingadenosinebinding,Snyderet
7、alcomparedthestimulatoryeffectsofaseriesofcaffeinederivativeswiththeirabilitytodislodgeadenosinefromitsreceptorsinthebrainsofmice.“Ingeneral,”theyreported,“theabilityofthecompoundstocompeteatthereceptors(40)correlateswiththeirabilitytostimulatelocomotioninthemouse;i.e.,thehigherthe
8、ircapacitytobindattherecep
