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   138361篇
  免费   10088篇
  国内免费   8808篇
  2023年   2255篇
  2022年   3177篇
  2021年   4461篇
  2020年   4629篇
  2019年   6217篇
  2018年   5248篇
  2017年   4102篇
  2016年   4041篇
  2015年   4615篇
  2014年   8056篇
  2013年   10412篇
  2012年   6276篇
  2011年   7975篇
  2010年   6118篇
  2009年   6948篇
  2008年   7158篇
  2007年   7279篇
  2006年   6397篇
  2005年   5689篇
  2004年   5086篇
  2003年   4312篇
  2002年   3868篇
  2001年   2668篇
  2000年   2237篇
  1999年   2197篇
  1998年   2089篇
  1997年   1787篇
  1996年   1618篇
  1995年   1492篇
  1994年   1443篇
  1993年   1189篇
  1992年   1143篇
  1991年   1056篇
  1990年   839篇
  1989年   771篇
  1988年   733篇
  1987年   657篇
  1986年   633篇
  1985年   1016篇
  1984年   1393篇
  1983年   946篇
  1982年   1147篇
  1981年   989篇
  1980年   882篇
  1979年   713篇
  1978年   541篇
  1977年   524篇
  1976年   485篇
  1975年   398篇
  1973年   401篇
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
21.
《Developmental cell》2022,57(8):995-1008.e5
  1. Download : Download high-res image (144KB)
  2. Download : Download full-size image
  相似文献   
22.
There is a lack of data on fatigue changes within 24 h among patients with multiple sclerosis. The purpose of this study was to evaluate the effect of time of day on central and peripheral fatigue during a continuous 2-min maximal voluntary contraction of the quadriceps muscle in women and men with multiple sclerosis (MS). We studied age-matched MS patients (range, 40–50 years). The inclusion criteria for patients were: a Kurtzke Expanded Disability Status score and a Fatigue Severity Scale score. We found a significant gender difference in central activation ratio (CAR) in the evening. At the end of the 2-min maximal voluntary contraction (MVC), the voluntary torque decreased by about 65% in men and women with MS in both the morning and evening. We also observed that, in women, CAR decreased markedly during the first 30 s in the evening test. The most interesting finding of our study is that central fatigue increased, whereas peripheral fatigue decreased markedly in the evening only in women. It remains unclear why women’s central fatigue is greater in the evening than in the morning.  相似文献   
23.
In this paper very simple nonparametric classification rule for mixtures of discrete and continuous random variables is described. It is based on the method of nearest neighbor proposed by Cover and Hart (1967). The bounds on the limit of the nearest neighbor rule risks are given. Both lower and upper bound depend on the Bayes risk and the loss function. Finally the method is compared with other existing methods on some practical data set.  相似文献   
24.
《Cell reports》2020,30(1):98-111.e5
  1. Download : Download high-res image (124KB)
  2. Download : Download full-size image
  相似文献   
25.
《Cell metabolism》2020,31(2):351-362.e5
  1. Download : Download high-res image (181KB)
  2. Download : Download full-size image
  相似文献   
26.
27.
28.
Cripto-1 is a protein participating in tissue orientation during embryogenesis but has also been implicated in a wide variety of cancers, such as colon, lung and breast cancer. Cripto-1 plays a role in the regulation of different pathways, including TGF-β/Smad and Wnt/β-catenin, which are highly associated with cell migration both during embryonal development and cancer progression. Little is known about the detailed subcellular localization of cripto-1 and how it participates in the directional movement of cells. In this study, the subcellular localization of cripto-1 in glioblastoma cells was investigated in vitro with high-resolution microscopy techniques. Cripto-1 was found to be localized to dynamic and shed filopodia and transported between cells through tunneling nanotubes. Our results connect the refined subcellular localization of cripto-1 to its functions in cellular orientation and migration.  相似文献   
29.
Several unit-length minicircles from the kinetoplast DNA of Leishmania tarentolae were cloned into pBR322 and into M13 phage vectors. The complete nucleotide sequences of three different partially homologous minicircles were obtained. The molecules contained a region of approx. 80% sequence homology extending for 160–270 bp and a region unique to each minicircle. A 14-mer was found to be conserved in all kinetoplast minicircle sequences reported to date. The frequency distributions of various minicircle sequence classes in L. tarentolae were obtained by quantitative gel electrophoresis and by examination of the “T ladder” patterns of minicircles randomly cloned into M13 at several sites. By these methods we could assign approx. 50% of the total minicircle DNA into a minimum of five sequence classes. A sequence-dependent polyacrylamide gel migration abnormality was observed with several minicircle fragments both cloned and uncloned. The abnormality was dependent on the presence of a portion of the conserved region of the minicircle.  相似文献   
30.
The activities of mitochondrial type A and B monoamine oxidase were determined in the liver of rats fed a diet containing 2-acetylaminofluorene (AAF). Three days after the initiation of AAF-feeding, there was a significant decrease of type B monoamine oxidase activity without affect on type A enzyme. The decreased activity of type B monoamine oxidase, which reached a minimum after three weeks, was sustained for as long as AAF-feeding was continued. Sex-related difference in response to AAF was seen in the rat with respect to the onset and the intensity of the decreased type B monoamine oxidase activity, male rats being more sensitive to the carcinogen than female rats. In contrast to the in vivo effect, AAF showed a potent inhibitory effect on type A monoamine oxidase, rather than on type B enzyme, when added in vitro. The pI50 values were estimated to be 7.5 against type A monoamine oxidase and 4.1 against type B enzyme, respectively. The in vitro inhibition of both types of monoamine oxidase by AAF was competitive. The Ki values for AAF were calculated to be 9.51 · 10?9 M for type A monoamine oxidase and 1.30 · 10?5 M for type B enzyme, respectively. In accordance with the potent inhibitory effect of AAF on type A monoamine oxidase in vitro, a single administration of the carcinogen, at a dose of 50 mg/kg, resulted in a marked and temporal decrease of the enzyme activity in the mitochondria of male rat liver. Recovery of the decreased type B monoamine oxidase activity was slow, and the enzyme activity did not return to control levels, even if rats were fed the basal diet for 2 or 4 weeks after the cessation of AAF-feeding.  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