六级英语听力训练资料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 neural 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 stimulus, for example extreme heat, which creates an impulse that travels along sensory nerves and neurons to the spinal cord. Once it reaches the spinal cord, the impulse is routed speedily back to the muscle originally stimulated. 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 regularity 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？ 假如你曾由于突如其来的灯光耀眼，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，你会发现这些动作并非我们故意识地打算过的，而是不由自主发生的。 这些忽然、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。 故意的行为总是包含着意识的参与，比方要把一种勺子送到嘴边，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，它们是迅速、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，最终抵达脊髓。 一旦抵达脊髓，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毕的，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，但它会伴随时间发生变化，这要取决于刺激发生的频率。假如某种刺激不停地反复，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长。 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。
