首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   11651篇
  免费   44篇
  国内免费   72篇
系统科学   61篇
丛书文集   29篇
教育与普及   21篇
理论与方法论   32篇
现状及发展   5438篇
研究方法   586篇
综合类   5472篇
自然研究   128篇
  2012年   212篇
  2011年   302篇
  2010年   76篇
  2009年   75篇
  2008年   204篇
  2007年   244篇
  2006年   247篇
  2005年   235篇
  2004年   202篇
  2003年   196篇
  2002年   247篇
  2001年   435篇
  2000年   427篇
  1999年   288篇
  1992年   236篇
  1991年   194篇
  1990年   215篇
  1989年   191篇
  1988年   201篇
  1987年   219篇
  1986年   190篇
  1985年   253篇
  1984年   226篇
  1983年   171篇
  1982年   176篇
  1981年   180篇
  1980年   170篇
  1979年   403篇
  1978年   341篇
  1977年   250篇
  1976年   290篇
  1975年   260篇
  1974年   272篇
  1973年   224篇
  1972年   244篇
  1971年   308篇
  1970年   374篇
  1969年   254篇
  1968年   307篇
  1967年   292篇
  1966年   250篇
  1965年   178篇
  1964年   99篇
  1959年   88篇
  1958年   162篇
  1957年   100篇
  1956年   91篇
  1955年   85篇
  1954年   77篇
  1948年   64篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
891.
Na channels in skeletal muscle concentrated near the neuromuscular junction   总被引:2,自引:0,他引:2  
K G Beam  J H Caldwell  D T Campbell 《Nature》1985,313(6003):588-590
Neuronal function depends crucially on the spatial segregation of specific membrane proteins, particularly the segregation associated with sites of synaptic contact. Understanding the factors governing this localization of proteins is a major goal of cellular neurobiology. A conspicuous example of synaptic specialization is the almost exclusive localization of vertebrate skeletal muscle acetylcholine (ACh) receptors to the subsynaptic membrane of the neuromuscular junction (for example, refs 1,2). The localization of other membrane proteins in skeletal muscle has been much less studied, but a knowledge of their distribution is crucial for understanding the factors governing regional specialization. We have explored the distribution in muscle of the voltage-gated Na channel responsible for the action potential using the loose patch-clamp technique, and have measured Na currents in 5-10 micron-diameter membrane patches as a function of distance from the end plate region of snake and rat muscle fibres. Here we report that the Na current density immediately adjacent to the endplate is 5-10-fold higher than at regions away from the endplate. The increased Na current density falls off rapidly with distance, reaching the background level 100-200 micron from the endplate. Although one might expect ACh receptors to be concentrated near the region of ACh release, such a concentration for Na channels, which propagate the impulse throughout the length of the cell, is surprising and suggests that factors similar to those responsible for concentrating ACh receptors at the endplate also operate to concentrate Na channels.  相似文献   
892.
Summary Bilateral lesions of the suprachiasmatic nucleus of the hypothalamus of rats abolished circadian rhythms of oxygen consumption and of the respiratory quotient (RQ). The RQ remained constant at a level intermediate between the maximum (about 1.0) and minimum (about 0.9) values in control animals.  相似文献   
893.
The availability of cloned cDNAs encoding the four subunits of the Torpedo acetylcholine receptor, which can be expressed to make functional receptors in Xenopus oocytes, has made possible a detailed investigation of the functions of the different structural components of the receptor. The functional analysis of receptors with alpha-subunits altered at specific sites by site-directed mutagenesis of the cDNA has allowed the location of specific regions of the alpha-subunit molecule involved in acetylcholine binding and forming a transmembrane ionic channel.  相似文献   
894.
Cloning and sequencing of the complementary DNA for platelet-derived endothelial cell growth factor indicates that it is a novel factor distinct from previously characterized proteins. The factor, a protein with a relative molecular mass of about 45,000, stimulates endothelial cell growth and chemotaxis in vitro and angiogenesis in vivo.  相似文献   
895.
用反相色谱法、电子天平法和ESCA对氧化前后炭纤维的表面性能作了研究。以炭纤维为固定相,正烷烃作探针分子进行了反相色谱实验,求得了炭纤维的吸附等温线。等温线属BrunauerⅡ型,比表面积因氧化而增大。用电子天平法,以正烷烃-甲酰胺-炭纤维体系间接地求得了炭纤维的表面自由能及其分量。表面自由能的极性分量因氧化处理而增大。ESCA的结果表明,经氧化处理后炭纤维表面的[O]/[C]比增大,但氧化影响仅限于表面,而对距表面5nm以下没有影响。  相似文献   
896.
Using isolated, internally perfused bullfrog dorsal root ganglion cells we have studied the dose-response curves for gamma-aminobutyric acid (GABA) in the presence of internally or externally applied GABA antagonists. With external application of antagonists the inhibition of the GABA current by bicuculline was competitive and that by picrotoxin was noncompetitive. Picrotoxin but not bicuculline blocked when internally perfused.  相似文献   
897.
Heterosexual chemoattraction studies on juvenile worms showed that 20- and 21-day-old worms do not attract each other, whereas 23- and 28-day-old worms are attracted to each other and pair with worms of the opposite sex. This onset of chemoattraction in vitro corresponds to the time when worms begin pairing in vivo. The ability of single worms to locate each other and mate is presumably mediated by chemoreceptors.  相似文献   
898.
899.
Summary Phospholipids were added to purified lipoamidase from porcine brain microsomal membranes, and changes in lipoamidase activity were examined. Approximately twofold activation of lipoamidase activity occurred upon the addition of phosphatidylethanolamine. On the other hand, phosphosphatidylserine, cardiolipin, and phosphatidic acid reduced the enzyme activity by approximately 80%. This pattern of the activation of lipoamidase by phosphatidylethanolamine and its inhibition by phosphatidylserine is similar to the pattern for adenylate cyclase, and contrasts with the pattern for ATPase.  相似文献   
900.
A single whole-body X-irradiation of pregnant Wistar rats at a dose of 1.05 Gy at 10.30, 12.30 and 14.30 h respectively, of gestational day 10 resulted in significantly high incidences of hydrocephalic offspring. No hydrocephalic offspring resulted from X-irradiation of pregnant rats with 1.05 Gy at 16.30 h, whereas a dose of 1.22 Gy at 16.30 h resulted in a low but statistically significant incidence of hydrocephalus. Neither 1.05 Gy nor 1.22 Gy X-irradiation of pregnant rats at 18.30 h resulted in any hydrocephalic offspring. Dysplasia of the subcommissural organ was noticed in all the hydrocephalic brains histologically examined.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号