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   8218篇
  免费   736篇
  国内免费   148篇
系统科学   1245篇
丛书文集   73篇
教育与普及   42篇
理论与方法论   287篇
现状及发展   878篇
研究方法   10篇
综合类   6567篇
  2024年   9篇
  2023年   29篇
  2022年   43篇
  2021年   66篇
  2020年   38篇
  2019年   18篇
  2018年   758篇
  2017年   777篇
  2016年   466篇
  2015年   114篇
  2014年   133篇
  2013年   92篇
  2012年   362篇
  2011年   1085篇
  2010年   911篇
  2009年   551篇
  2008年   639篇
  2007年   889篇
  2006年   123篇
  2005年   173篇
  2004年   220篇
  2003年   243篇
  2002年   141篇
  2001年   93篇
  2000年   84篇
  1999年   146篇
  1998年   118篇
  1997年   105篇
  1996年   115篇
  1995年   103篇
  1994年   86篇
  1993年   67篇
  1992年   75篇
  1991年   73篇
  1990年   48篇
  1989年   37篇
  1988年   40篇
  1987年   17篇
  1986年   10篇
  1985年   1篇
  1981年   3篇
  1967年   1篇
排序方式: 共有9102条查询结果,搜索用时 296 毫秒
991.
A dragonfly wing consists of membranes and both longitudinal and cross veins. We observed the microstructure cross-section at several locations in the dragonfly wing using environmental scanning electron microscopy (ESEM). The organic nature of the junction between the vein and the membrane was clearly identifiable. The membrane was divided into two layers, the upper epidermis and the lower epidermis. These layers extend around the sandwich structure vein, and combine with the adjacent membrane at a symmetrical location along the vein. Thus, we defined this as an organic junction between the vein and the membranes. The organic junction is able to form a tight corrugation angle, which dramatically increases both the warping rigidity and the strength of the wing, but not the torsional rigidity. The torsional deformation is primarily controlled by the microstructure of the longitudinal veins, and is based on the relative rotation angle between the epidermal layer and the inner layer of the vein that forms the zigzag section.  相似文献   
992.
Neuronal hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are known to modulate spontaneous activity, resting membrane potential, input resistance, afterpotential, rebound activity, and dendritic integration. To evaluate the role of HCN2 for hippocampal synaptic plasticity, we recorded long-term potentiation (LTP) in the direct perforant path (PP) to CA1 pyramidal cells. LTP was enhanced in mice carrying a global deletion of the channel (HCN2−/−) but not in a pyramidal neuron-restricted knockout. This precludes an influence of HCN2 located in postsynaptic pyramidal neurons. Additionally, the selective HCN blocker zatebradine reduced the activity of oriens-lacunosum moleculare interneurons in wild-type but not HCN2−/− mice and decreased the frequency of spontaneous inhibitory currents in postsynaptic CA1 pyramidal cells. Finally, we found amplified LTP in the PP of mice carrying an interneuron-specific deletion of HCN2. We conclude that HCN2 channels in inhibitory interneurons modulate synaptic plasticity in the PP by facilitating the GABAergic output onto pyramidal neurons.  相似文献   
993.
The skin is our primary shield against microbial pathogens and has evolved innate and adaptive strategies to enhance immunity in response to injury or microbial insult. The study of antimicrobial peptide (AMP) production in mammalian skin has revealed several of the elegant strategies that AMPs use to prevent infection. AMPs are inducible by both infection and injury and protect the host by directly killing pathogens and/or acting as multifunctional effector molecules that trigger cellular responses to aid in the anti-infective and repair response. Depending on the specific AMP, these molecules can influence cytokine production, cell migration, cell proliferation, differentiation, angiogenesis and wound healing. Abnormal production of AMPs has been associated with the pathogenesis of several cutaneous diseases and plays a role in determining a patient’s susceptibility to pathogens. This review will discuss current research on the regulation and function of AMPs in the skin and in skin disorders.  相似文献   
994.
We give a “direction for use” of the scale relativity theory and apply it to an example of spontaneous multiscale integration including four embedded levels of organization (intracellular, cell, tissue and organism-like levels). We conclude by an update of our analysis of the arctic sea ice melting.  相似文献   
995.
Autoreactive CD8+ regulatory T cells (Tregs) play important roles as modulators of immune responses against self, and numerical and functional defects in CD8+ Tregs have been linked to autoimmunity. Several subsets of CD8+ Tregs have been described. However, the origin of these T cells and how they participate in the natural progression of autoimmunity remain poorly defined. We discuss several lines of evidence suggesting that the autoimmune process itself promotes the development of autoregulatory CD8+ T cells. We posit that chronic autoantigenic exposure fosters the differentiation of non-pathogenic autoreactive CD8+ T cells into antigen-experienced, memory-like autoregulatory T cells, to generate a “negative feedback” regulatory loop capable of countering pathogenic autoreactive effectors. This hypothesis predicts that approaches capable of boosting autoregulatory T cell memory will be able to blunt autoimmunity without compromising systemic immunity.  相似文献   
996.
对青海野生大黄茶营养成分进行了分析,结果表明:野生大黄茶中含有蛋白质、脂肪、维生素C、胡萝卜素等营养物质,其中维生素C和胡萝卜素含量丰富,分别达到47.97mg/100g和0.41mg/kg,且氨基酸种类较多。青海野生大黄茶保持了大黄的有效药用成份-大黄素,是一种饮用方便的新型保健品。  相似文献   
997.
通过对深圳市滨河污水处理厂污不处理集散控制系统和办公自动化管理信息系统的分析,将先后建成的生产型和管理型两套系统,组成一个完整的统一网络,实现了异构型双网互联  相似文献   
998.
建立了HZY型振动运输机力学模型,运用相对运动原理对其振动规律进行了分析,探索了产生扭转振动的原因,推导出了稳定同步振动的运动方程,并提出了实现稳定同步运转的判据及相应的技术措施.这对采用双电机进行自同步振动机的设计具有指导意义.  相似文献   
999.
Modeling a business to match its customer perceived (and customer desired) outcomes, remains an on-going task. This paper considers shows how a service value networks (SVNs) approach may be engaged to model, and deliver, understanding of the front-end business and its direct engagement with its immediate (or front-end) customers. These front-end customers may be either: (1) its off-line, in-store customers - engaging directly with the business sales staff (or representative), or-(2) its on-line, virtual customers - engaging with the business via internet or remote access. In this situation, both the business, and the customer draw upon their respective viewpoints, and both sides influence the interaction. External to these business and customer influences are additional factors that capture the immediate and broader global effects - termed environmental effects. These three business-customer engagement areas of influence are captured by a SVN SEM approach.
Without a business-customer encounter of some kind the likelihood of a services business, and a prospecting customer, successfully engaging in an exchange process is reduced. This exchange may be a physical, and/or services exchange, and/or an information and/or ideas exchange. To the business, this encounter is, in effect, a trade, and as a result, the business targets acquiring an economic exchange that will ultimately deliver a net positive economic outcome. To the customer, external and internal information feeds, servicing, perceived value, and satisfaction, are key encounter drivers. SVNs offers a new way to understand the business-customer encounter, and to then utilize this acquired knowledge to either: (1) build a strategic management model, or (2) reengineer its business networks, and to then create a re-positioned, more customer-aligned business operation - one that is capable of delivering win - win, competitive business-customer solutions.  相似文献   
1000.
本书内容简练而全面,是一本很好的生物信息学教材。本书主要特点有:(1)尽可能避开繁琐的数学公式,详细介绍了算法的技术细节,免去了读者必须具备复杂的计算背景才能理解算法的前提;(2)恰当地使用图例,使读者更容易理解书中的内容;(3)广泛参阅并综合了生物信息学各关键领域的文献,帮助读者尽快掌握生物信息学的全貌。本书不仅介绍了结构生物信息学,还涵盖了比较基因组学和功能基因组学等较新领域的内容。  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