问题 单项选择题 A1/A2型题

关于微终板电位的叙述,正确的是()。

A.表现“全或无”特性

B.有不应期

C.是个别囊泡的自发释放在终板膜上引起的微小的电变化

D.是大量囊泡的自发释放在终板膜上引起的较大的电变化

E.是神经末梢单个动作电位引起的终板膜上的电变化

答案

参考答案:C

解析:在静息状态下,接头前膜也会发生约每秒钟1次的ACh量子的自发释放,并引起终板膜电位的微小变化。这种由一个ACh量子引起的终板膜电位变化称为微终板电位。每个MEPP的幅度平均仅0.4mV。当接头前膜产生动作电位和Ca内流时,大量的突触小泡几乎同步释放ACh;ACh量子所引起的MEPP此时会发生叠加,形成平均幅度约50mV的EPP。

单项选择题

In considering how the American family is changing, the starting point is the traditional family. a form which has developed over time on the basis of a number of assumptions. It is assumed that the family is heterosexual (异性的) institution, with prescriptions about how a man and a woman ought to be joined together and live together. The proper family form is assumed to be the nuclear family, that is, a family composed of a married man and woman and their children. It is assumed that the husband is the head of the family, with ultimate authority over wife and children, and that in their clearly separated roles the husband is the income-earner and the wife is the homemaker and provider of child care. It is assumed that the family lives by itself in its own house or residence.

Observers of family life have suggested that numerous changes are taking place in this traditional American family form. Included are the following general observations about trends:

More men and women seem to be living together before getting married.

Women and men seem to be marrying at a later age.

Married couples are having fewer children.

Unmarried women appear to be having more children.

Wives, even mothers with small children, are increasingly likely to be employed outside the home.

Marriages are more likely to end with divorce.

Single-parent families are more prevalent.

Remarriage is likely to follow divorce rather than widowhood.

Remarriage rates are declining, especially for women.

Some trends, viewed in a long-range historical context, appear not to be trends at all. The age at which men and women normally marry is the same now as it was 100 years ago, though it has fluctuated (变动,波动); a short-range increase in the 1940s and 1950s due to widespread postponement of marriage in war time made it appear that a change had occurred. Other trends appear to be following established patterns rather than representing a sharp break with tradition. Divorce rates have been increasing and families have been having fewer children for well over a century. Single-parent families and stepfamilies were very common in the past, although the reason for them was different. Many marriages formerly are dissolved and followed by remarriage because of the death of a parent or partner. Today single parenthood and remarriage are more likely to be a response to divorce.

Some social scientists see in these changes the breakdown of the family, to the detriment of the society. We take the position that family institutions and systems, like all human systems, are open, take many forms, and are constantly changing.

The major topic of the passage is ().

A. assumptions about traditional family

B. recent trends in traditional family form

C. new problems in family

D. reasons for breakdown of family

多项选择题