问题 单项选择题

下列关于神经纤维膜上Na+通道的叙述,哪一项是错误的

A.是电压门控的

B.在去极化达阈电位时,可引起正反馈

C.有开放和关闭两种状态

D.有髓纤维,主要分布在郎飞结处

E.与动作电位的去极相有关

答案

参考答案:C

解析:
Na+通道是电压门控的,静息状态下,处于关闭状态;当去极化达阈电位水平时,Na+通道开放(E对),通过正反馈机制使更多的Ns+通道开放,产生动作电位,形成动作电位的上升支。之后,Na+通道失活、关闭,形成动作电位的下降支。因此Na(上)+通道有备用、激活和失活三种状态(C错)。注意:Na+通道对离子的导通只有开放和关闭两种状态。在有髓纤维,两段髓鞘之间有1~2μm的轴突膜裸露区,称郎飞结,该处膜上的Na+通道密集,容易产生动作电位,使得有髓纤维的动作电位沿郎飞结跳跃式传导,因此其传导速度要比无髓纤维快得多。

单项选择题 A1/A2型题
单项选择题

When Dave was eighteen, he bought a secondhand car for 200 so that he could travel to and from work more (1) than by bus. It worked quite well for a few years, but then it got so old, and it was costing him (2) much in repairs that he decided that he had better (3) it.

He asked among his friends to see if anyone was particularly (4) to buy a cheap car, but they all knew that it was falling to pieces, so (5) of them had any desire to buy it. Dave’s friend Sam saw that he was (6) when they met one evening, and said, "What’s (7) , Dave"

Dave told him, and Sam answered, "Well, what about advertising it in the paper You may (8) more for it that way than the cost of the advertisement!" Thinking that Sam’s (9) was sensible, he put an advertisement in an evening paper, which read "For sale: small car, (10) very little petrol, only two owners. Bargain at 50. " For two days after the advertisement first appeared, there was no (11) . But then on Saturday evening he had an enquiry. A man rang up and said he would like to (12) him about the car. "All right, " Dave said, feeling happy. He asked the man whether ten o’clock the next morning would be (13) or not. "Fine, " the man said, "and I’ll (14) my wife. We intend to go for a ride in it to (15) it. "

The next morning, at a quarter to ten, Dave parked the car in the square outside his front door, (16) to wait there for the people who had (17) his advertisement. Even Dave had to (18) that the car really looked like a wreck. Then, soon after he had got the car as clean (19) it could be, a police car stopped just behind him and a policeman got out. He looked at Dave’s car and then said, "Have you reported this (20) to us yet, sir\

19()

A. as

B. that

C. so

D. such