六级英语听力训练资料

六级英语听力训练资料If youve everblinked at a suddenlight,jerked yourhand awayfromsomething hot,or jumpedataloud noise,then youknow that these arenotactions weconsciously plan.Rather,they happeninvoluntarily.Such sudden,unconscious movementsare akind ofsimple neuralactivityknown asa reflex.A deliberateaction,such asraising aspoon to the mouth,involvesconsciousness.We decideto raisethe spoon,then wedo it.Most reflexes,however,do notinvolve thebrain；they arefast,involuntary responsesthattravel overwhat isknown asa reflexarc.A reflexarc consistsofa stimulus,for exampleextreme heat,which createsan impulsethat travelsalongsensory nervesand neuronstothespinal cord.Once itreaches thespinal cord,the impulseis routedspeedily backto themuscleoriginally stimulated.This processoccurs almostinstantaneously,before weveeven hadtime toconsciously registerthatthereflex hashappened.Although areflex isautomatic,it canchange overtime,depending ontheregularity ofthe stimulus.If astimulus is repeated continuously,thereflex firstundergoes sensitization,meaning thatit increasesinresponse.But ifthe stimulusisrepeatedbeyond acertain point,the reflexbeginsto decreaseuntil itdisappears.In otherwords,you cantame areflexby,for instance,hearing asudden loudnoise repeatedlyuntil youstopjumping.什么是条件反射？假如你曾由于突如其来的灯光刺眼，遇到烫的东西立即把手缩回来，或是听到很响的声音吓了一跳，你会发觉这些动作并非我们有意识地打算过的,而是不由自主发生的这些突然、无意识的动作是一种简洁的神经活动，我们称之为条件反射有意的行为总是包含着意识的参加，比方要把一个勺子送到嘴边，我们先打算拿起勺子，然后再做这个动作但大局部的条件反射都与大脑无关，它们是快速、无意识的反响，这种反响沿着我们所熟知的反射弧传递反射弧包括某种刺激，比方人在极高的温度下会产生一种神经冲动，这种神经冲动会沿着感觉神经和神经元传递，最终到达脊髓一旦到达脊髓，神经冲动便会快速沿原路返回至最初受到刺激的肌肉整个过程几乎是在瞬间完成的，甚至在我们有时间意识到之前，条件反射就已经发生了虽然条件反射是自发形成的，但它会随着时间发生变化，这要取决于刺激发生的频率假如某种刺激不断地重复，条件反射最开头会对这种刺激变得敏感，换句话说，条件反射的反响次数会增加但假如这种刺激超过了某个临界点时，条件反射次数就会开头削减直至消逝换句话说，条件反射是可以克制的，比方，反复听突如其来的巨响，直到你不会被吓一跳为止。