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   450625篇
  免费   51815篇
  国内免费   956篇
  503396篇
  2018年   4337篇
  2017年   4169篇
  2016年   5907篇
  2015年   8405篇
  2014年   9799篇
  2013年   13204篇
  2012年   15731篇
  2011年   16692篇
  2010年   10952篇
  2009年   10016篇
  2008年   14046篇
  2007年   14478篇
  2006年   13586篇
  2005年   12950篇
  2004年   13008篇
  2003年   12163篇
  2002年   11811篇
  2001年   19045篇
  2000年   18946篇
  1999年   15115篇
  1998年   5651篇
  1997年   5670篇
  1996年   5377篇
  1995年   5177篇
  1994年   4867篇
  1993年   4868篇
  1992年   12495篇
  1991年   12454篇
  1990年   12211篇
  1989年   11688篇
  1988年   10847篇
  1987年   10210篇
  1986年   9738篇
  1985年   9517篇
  1984年   7889篇
  1983年   6839篇
  1982年   5148篇
  1981年   4635篇
  1980年   4323篇
  1979年   7358篇
  1978年   5982篇
  1977年   5298篇
  1976年   4967篇
  1975年   5780篇
  1974年   6310篇
  1973年   6128篇
  1972年   5465篇
  1971年   5082篇
  1970年   4289篇
  1969年   4206篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
This paper presents a neural-network-based system that can generate and control movements of the eyes. It was inspired by a number of experimental observations on the saccadic and gaze systems of monkeys and cats. Because of the generality of the approach undertaken, the system can be regarded as a demonstration of how parallel distributed processing principles, namely learning and attractor dynamics, can be integrated with experimental findings, as well as a biologically inspired controller for a dexterous robotic orientation device. The system is composed of three parts: a dynamic motor map, a push-pull circuitry, and a plant. The dynamics of the motor map is generated by a multi-layer network that was trained to compute a bidimensional temporal-spatial transformation. Simulation results indicate (1) that the system is able to reproduce some of the properties observed in the biological system at the neural and movement levels and (2) that the dynamics of the motor map remains stereotyped even when the motor map is subject to abnormal stimulation patterns. The latter result emphasizes the role of the topographic projection that connects the motor map to the push-pull circuitry in determining the features of the resulting movements.  相似文献   
992.
993.
The deposition of amyloid protein aggregates in brain is the main pathological feature of Alzheimer's disease. Their principal constituent is a peptide termed beta A4, which comprises up to 43 amino acid residues. It is highly insoluble under physiological conditions and aggregates into filaments that form very dense clusters in vivo and in vitro. Based on a beta A4 prototype sequence spanning residues 10 to 42 or 43, we have designed analogues in which hydrophobic amino acid residues in position 17 to 20 were substituted by more hydrophilic residues. Depending on the kind of newly introduced amino acids and their position within the sequence, the substitution of only two residues led to variants exhibiting a broad spectrum of different properties. Common to them was a reduced beta-sheet content after solubilization in water and in the solid state. Some of the variants showed significantly reduced amyloidogenicity: although still forming filaments, they did not aggregate into the highly condensed depositions that are typical for amyloid. In addition, they could be solubilized in 200 mM-NaCl and KCl. When mixed with beta A4 peptides bearing the natural sequence, two of the analogues could inhibit the formation of filaments in vitro. These results demonstrate that a well-preserved hydrophobic core around residues 17 to 20 of beta A4 is crucial for the formation of beta-sheet structure and the amyloid properties of beta A4. The introduction of structural alterations within this region may guide the development of reagents for the therapy of Alzheimer's disease.  相似文献   
994.
Characterization of nifH mutations of Klebsiella pneumoniae.   总被引:4,自引:2,他引:2       下载免费PDF全文
  相似文献   
995.
996.
997.
Prophylactic activity and side action of antiinfluenza lactoglobulin were studied under conditions of controlled epidemiological trial in 6361 persons during the epidemiological rise of influenza incidence. The data obtained pointed to the absence of any protective effect of antiinfluenza lactoglobulin. Marked allergenic properties of lactoglobulin and development of sensitization in the persons under observation were found during the study of the side-effect of the preparation. Thus, the absence of prophylactic efficacy and the sensitizing action of antiinfluenza lactoglobulin pointed to the inexpediency of its use for prophylactic and therapeutic purpose.  相似文献   
998.
999.
1000.
Nonlymphoid, stromal cells in the mouse thymus are believed to be important in T cell maturation and have been proposed to play a central role in the acquisition of major histocompatibility complex (MHC) restriction and self-tolerance by maturing thymocytes. Both cortical and medullary epithelial cells in the thymus express high levels of class II (A) major histocompatibility antigens (MHC Ags). We show here that a specific subset of these A epithelial cells express a transformation-associated antigen (6C3Ag) found previously on the surfaces of Abelson murine leukemia virus-transformed pre-B cells and on those bone marrow-derived stromal cell clones which support normal and preneoplastic pre-B cell proliferation. Among solid lymphoid organs, only the thymus contains 6C3Ag1 cells and within the thymus, this antigen is found exclusively on A epithelial cells in cortical regions. It is striking that the expression of the 6C3Ag on thymic epithelium is developmentally regulated, suggesting a role for this lymphostromal antigen in the maturation of the thymic microenvironment.  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