英语听力文摘 English Digest 580、什么是条件反射？

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　　If you’ve ever blinked at a sudden light, jerked your hand away from something hot, or jumped at a loud noise, then you know that these are not actions we consciously plan. Rather, they happen involuntarily.

 

　　Such sudden, unconscious movements are a kind of simple neural¹ activity known as a reflex.

 

　　A deliberate action, such as raising a spoon to the mouth, involves consciousness. We decide to raise the spoon, then we do it. Most reflexes, however, do not involve the brain; they are fast, involuntary responses that travel over what is known as a reflex arc. A reflex arc consists of a stimulus², for example extreme heat, which creates an impulse that travels along sensory³ nerves and neurons to the spinal⁴ cord.

 

　　Once it reaches the spinal cord, the impulse is routed speedily back to the muscle originally stimulated⁵. This process occurs almost instantaneously, before we’ve even had time to consciously register that the reflex has happened.

 

　　Although a reflex is automatic, it can change over time, depending on the regularity⁶ of the stimulus. If a stimulus is repeated continuously, the reflex first undergoes sensitization, meaning that it increases in response.

 

　　But if the stimulus is repeated beyond a certain point, the reflex begins to decrease until it disappears. In other words, you can tame a reflex by, for instance, hearing a sudden loud noise repeatedly until you stop jumping.

 

　　什么是条件反射？

 

　　如果你曾因为突如其来的灯光眨眼，碰到烫的东西立刻把手缩回来，或是听到很响的声音吓了一跳，你会发现这些动作并非我们有意识地计划过的，而是不由自主发生的。

 

　　这些突然、无意识的动作是一种简单的神经活动，我们称之为条件反射。

 

　　有意的行为总是包含着意识的参与，比如要把一个勺子送到嘴边，我们先决定拿起勺子，然后再做这个动作。但大部分的条件反射都与大脑无关，它们是快速、无意 识的反应，这种反应沿着我们所熟知的反射弧传递。反射弧包括某种刺激，比如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，最后到达脊髓。

 

　　一旦到达脊髓，神经冲动便会迅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。如果某种刺激不断地重复，条件反射最开始会对这种刺激变得敏感，换句话说，条件反射的反应次数会增加。

 

　　但如果这种刺激超过了某个临界点时，条件反射次数就会开始减少直至消失。换句话说，条件反射是可以克服的，比如，反复听突如其来的巨响，直到你不会被吓一跳为止。

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