问题 单项选择题

心肌细胞超常期内兴奋性高于正常的原因是

A.此期间膜内电位绝对值尚低于静息电位,距阈电位水平的差距较小
B.此期间膜内电位绝对值尚高于静息电位,距阈电位水平的差距较大
C.此期间内Na+通道开放能力已经恢复到正常
D.此期间内Na+通道完全失活

答案

参考答案:A

解析:[考点] 心肌细胞超常期的原因
在心肌细胞动作电位的复极相中,膜内电位由-80mV恢复到-90mV这一段时期,称为超常期,即细胞膜的兴奋性高于正常。这是因为膜电位差绝对值低于静息时膜内外电位差绝对值,膜内电位较静息电位更接近阈电位(与阈电位水平的差距较小),因此用以引起该细胞发生兴奋所需的刺激阈值比正常要低。另一方面,此时Na+通道也基本上恢复到可被激活的正常备用状态,但开放能力仍然没有恢复正常,产生的动作电位的0期去极的幅度和速度,兴奋传导的速度都仍然低于正常。

阅读理解

     "In only six days I lost seven pounds of weight."

     "Two full inches in the first three days!"

     These are the kinds of statements used in magazine, newspaper, radio and television ads, promising

new shapes and new looks to those who buy the medicine or the device. The promoters of products say

they can shape the legs, slim the face, smooth wrinkles, or in some other way to add to beauty or

desirability.

     Often such products are nothing more than moneymaking things for their promoters. The results they

produce are questionable, and some are dangerous to health.

     To understand how these products can be legally promoted to the public, it is necessary to

understand something of the laws covering their regulation. If the product is a drug, FDA(Food and

Drug Administration)can require proof under the Food, Drug, and Cosmetic Act that is safe and

effective before it is put on the market. But if the product is a device, FDA has no authority to require

premarketing proof of safety or effectiveness. If a product already on the market is a danger to health, 

FDA can request the producer or distributor to remove it from the market voluntarily, or it can take legal

action,  including seizure(查封)of the product.

     One notable case a few years ago involved an electrical device called the Relaxacisor, which had

been sold for reducing the waistline. The Relaxacisor produced electrical shocks to the body through

contact pads. FDA took legal action against the distributor to stop the sale of the device on the grounds

that it was dangerous to health and life.

     Obviously,  most of the devices on the market have never been the subject of court proceedings

(法律诉讼),  and new devices appear continually. Before buying,  it is up to the consumer to judge the

safety or effectiveness of such items.

1. It can be inferred that the ads mentioned in the text are________.

A. objective  

B. costly

C. unreliable  

D. illegal

2. Which of the following is TRUE according to the text?

A. The court is in charge of removing dangerous products.

B. New products are more likely to be questionable.

C. The production of a device must be approved by FDA.

D. The promoters usually just care about profits.

3. FDA can ask for the proof of safety and effectiveness of a product________.

A. if it is a drug

B. if it is a device

C. if its consumers make complaints

D. if its distributors challenge FDA's authority

4. The Relaxacisor is mentioned as________.

A. a product which was designed to produce electricity

B. a product whose distributor was involved in a legal case

C. a successful advertisement of a beauty product

D. an example of a quality beauty product

5. The author intends to________.

A. make consumers aware of the promoters' false promises

B. show the weakness of the law on product safety

C. give advice on how to keep young and beautiful

D. introduce the organization of FDA

单项选择题