问题 单项选择题

当神经细胞处于静息电位时,电化学驱动力最大的离子是()

A.Na+

B.K+

C.Ca2+

D.Cl-

答案

参考答案:A

解析:

[解释] 当某种离子跨膜扩散时,它受到来自浓度差和电位差的双重驱动力,两个驱动力的代数和称为电化学驱动力。例如,当质膜只对溶液中的一种离子有通透性时,该离子将顺浓度差跨膜扩散,但扩散的同时也在膜两侧形成逐渐增大的电位差,且该电位差造成的驱动力与浓度差的驱动力的方向相反,成为阻止离子进一步跨膜扩散的力量,直至电位差驱动力增加到等于浓度差驱动力时达到稳态,此时的跨膜电位差称为该离子的平衡电位。可见,当膜电位处于某一离子的平衡电位时,该离子的电化学驱动力为零(最小),此时尽管膜对该离子有通透性,但没有离子的跨膜净移动。一般认为,膜对Cl-不存在原发性主动转运,因此,Cl-在膜两侧的分布是被动的;膜电位并不决定于Cl-平衡电位,相反,膜电位的大小可决定Cl-在膜内的浓度(可用Nernst方程式算出);并且,Cl-平衡电位总是等于或接近静息电位。Ca2+在细胞膜两侧的浓度都很低,且膜对Ca2+的通透性也很低,其作用可以忽略。在静息电位条件下,Na+受到很强的内向驱动力(最大),一旦膜对Na+的通透性增大,将出现很强的引起去极化的内向电流;而在锋电位期间,K+受到很强的外向驱动力。所以,电化学驱动力的概念理解是解题关键。

单项选择题
单项选择题

That rapscallion who leaps off the monkey bars, landing smack onto an innocent 3-year-old bystander, and skips off, giggling all the while According to a new paper out of Israel, he may not feel all that bad about the incident. The study, conducted by Dr. In bal Kivenson Bar-On at the University of Haifa, shows that high levels of fearlessness in 3-and 4-year-olds is ply associated with aggression and a lack of sympathy. This news will likely surprise risk-loving America, where parents typically beam with pride when their undaunted child mounts the big slide.

Fearlessness is a far-end point on the spectrum of what psychologists call the "approach and withdrawal dimension"—people’s tendency to approach new stimuli (to gain information and acquire new skills ) and withdraw from unfamiliar stimuli (to avoid danger). Striking the right balance is considered crucial to man’s survival. But what about preschoolers’ There’s a clear downside, Dr. Kivenson Bar-On discovered, after she observed lots of preschool play and machinations. In total, she documented 80 children at preschool, home and in the lab, measuring their propensity for fearlessness and other social and emotional characteristics at the beginning and end of one year. Fearlessness was measured by observing reactions to various fright-inducing situations: separation from parents, the roar of a vacuum cleaner, a jack-in-the-box and the like. Those who displayed greater levels of fearlessness, the study found, had no trouble recognizing facial expressions of anger, surprise, happiness and sadness in other children—but they had a hard time identifying fear.

Over all, they were "emotionally shallow" and showed lower levels of sympathy. They took advantage of friends and lacked regret over inappropriate conduct. "These findings," the paper explains, " suggest that fearlessness in preschool constitutes a clear risk factor for developmental pathways that lead to problems in morality, conscience development, and severe antisocial behaviors. " At the same time, fearless children tended to be highly sociable. "One of the most interesting findings was that we could discriminate between friendliness and sympathy," Dr. Kivenson Bar-On said. "These kids are curious, easygoing and friendly, but they have a hard time recognizing emotional distress in others. "

Jamie Ostrov, a psychology professor at the State University at Buffalo who studies aggression, says that children at the extreme end of the fearless spectrurn "may be charming, but they’re also highly manipulative and deceptive and skilled at getting their way—even at age 3 or 4. " It could be that fearless children need per distress cues to active their autonomic nervous systems, limiting their ability to detect distress cues in others. It seems to be, if I’m not worried about this, you can’t be, either. But should we be

The example of the first paragraph is to()

A. quote the research that the children of high levels of fearlessness are lack of empathy

B. illustrate the children ply associated with aggression and a lack of sympathy exist

C. criticize children in reckless behavior and moral deficiency

D. emphasize the necessity of the research