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   7119篇
  免费   889篇
  国内免费   7篇
  2022年   48篇
  2021年   93篇
  2020年   68篇
  2019年   71篇
  2018年   73篇
  2017年   79篇
  2016年   125篇
  2015年   206篇
  2014年   227篇
  2013年   325篇
  2012年   350篇
  2011年   385篇
  2010年   248篇
  2009年   209篇
  2008年   336篇
  2007年   355篇
  2006年   310篇
  2005年   308篇
  2004年   308篇
  2003年   337篇
  2002年   296篇
  2001年   176篇
  2000年   210篇
  1999年   177篇
  1998年   101篇
  1997年   84篇
  1996年   76篇
  1995年   77篇
  1994年   71篇
  1993年   62篇
  1992年   97篇
  1991年   90篇
  1990年   106篇
  1989年   111篇
  1988年   88篇
  1987年   86篇
  1986年   77篇
  1985年   102篇
  1984年   86篇
  1983年   78篇
  1982年   69篇
  1981年   62篇
  1980年   71篇
  1979年   64篇
  1978年   50篇
  1977年   60篇
  1975年   49篇
  1974年   75篇
  1973年   51篇
  1971年   57篇
排序方式: 共有8015条查询结果,搜索用时 15 毫秒
971.

Background

The present study provides an estimate of the prevalence of bacteriologially positive pulmonary tuberculosis in Jabalpur, a district in central India.

Methodology/Principal Findings

A community based cross-sectional survey was undertaken in Jabalpur District of the central Indian state of Madhya Pradesh. A stratified cluster sampling design was adopted to select the sample. All eligible individuals were questioned for pulmonary symptoms suggestive of TB disease. Two sputum samples were collected from all eligible individuals and were examined by Ziehl-Neelsen smear microscopy and solid media culture methods. Of the 99,918 individuals eligible for screening, 95,071 (95.1%) individuals were screened. Of these, 7,916 (8.3%) were found to have symptoms and sputum was collected from 7,533 (95.2%) individuals. Overall prevalence of bacteriologically positive PTB was found to be 255.3 per 100,000 population (95% C.I: 195.3–315.4). Prevalence was significantly higher (p<0.001) amongst males (355.8; 95% C.I: 304.4–413.4) compared with females (109.0; 95% C.I: 81.2–143.3). Prevalence was also significantly higher in rural areas (348.9; 95% C.I: 292.6–412.8) as compared to the urban (153.9; 95% C.I: 123.2–190.1).

Conclusions/Significance

The TB situation in Jabalpur district, central India, is observed to be comparable to the TB situation at the national level (255.3 versus 249). There is however, a need to maintain and further strengthen TB control measures on a sustained and long term basis in the area to have a significant impact on the disease prevalence in the community.  相似文献   
972.
973.
Obesity has been linked to the human gut microbiota; however, the contribution of gut bacterial species to the obese phenotype remains controversial because of conflicting results from studies in different populations. To explore the possible dysbiosis of gut microbiota in obesity and its metabolic complications, we studied men and women over a range of body mass indices from the Old Order Amish sect, a culturally homogeneous Caucasian population of Central European ancestry. We characterized the gut microbiota in 310 subjects by deep pyrosequencing of bar-coded PCR amplicons from the V1-V3 region of the 16S rRNA gene. Three communities of interacting bacteria were identified in the gut microbiota, analogous to previously identified gut enterotypes. Neither BMI nor any metabolic syndrome trait was associated with a particular gut community. Network analysis identified twenty-two bacterial species and four OTUs that were either positively or inversely correlated with metabolic syndrome traits, suggesting that certain members of the gut microbiota may play a role in these metabolic derangements.  相似文献   
974.
SHIFTING CULTIVATION IN THE HUMID TROPICS IS INCREDIBLY DIVERSE, YET RESEARCH TENDS TO FOCUS ON ONE TYPE: long-fallow shifting cultivation. While it is a typical adaptation to the highly-weathered nutrient-poor soils of the Amazonian terra firme, fertile environments in the region offer opportunities for agricultural intensification. We hypothesized that Amazonian people have developed divergent bitter manioc cultivation systems as adaptations to the properties of different soils. We compared bitter manioc cultivation in two nutrient-rich and two nutrient-poor soils, along the middle Madeira River in Central Amazonia. We interviewed 249 farmers in 6 localities, sampled their manioc fields, and carried out genetic analysis of bitter manioc landraces. While cultivation in the two richer soils at different localities was characterized by fast-maturing, low-starch manioc landraces, with shorter cropping periods and shorter fallows, the predominant manioc landraces in these soils were generally not genetically similar. Rather, predominant landraces in each of these two fertile soils have emerged from separate selective trajectories which produced landraces that converged for fast-maturing low-starch traits adapted to intensified swidden systems in fertile soils. This contrasts with the more extensive cultivation systems found in the two poorer soils at different localities, characterized by the prevalence of slow-maturing high-starch landraces, longer cropping periods and longer fallows, typical of previous studies. Farmers plant different assemblages of bitter manioc landraces in different soils and the most popular landraces were shown to exhibit significantly different yields when planted in different soils. Farmers have selected different sets of landraces with different perceived agronomic characteristics, along with different fallow lengths, as adaptations to the specific properties of each agroecological micro-environment. These findings open up new avenues for research and debate concerning the origins, evolution, history and contemporary cultivation of bitter manioc in Amazonia and beyond.  相似文献   
975.
Cancer is a leading cause of death of men and women worldwide. Tumor cell motility contributes to metastatic invasion that causes the vast majority of cancer deaths. Extracellular receptors modified by α2,3-sialic acids that promote this motility can serve as ideal chemotherapeutic targets. For example, the extracellular domain of the mucin receptor podoplanin (PDPN) is highly O-glycosylated with α2,3-sialic acid linked to galactose. PDPN is activated by endogenous ligands to induce tumor cell motility and metastasis. Dietary lectins that target proteins containing α2,3-sialic acid inhibit tumor cell growth. However, anti-cancer lectins that have been examined thus far target receptors that have not been identified. We report here that a lectin from the seeds of Maackia amurensis (MASL) with affinity for O-linked carbohydrate chains containing sialic acid targets PDPN to inhibit transformed cell growth and motility at nanomolar concentrations. Interestingly, the biological activity of this lectin survives gastrointestinal proteolysis and enters the cardiovascular system to inhibit melanoma cell growth, migration, and tumorigenesis. These studies demonstrate how lectins may be used to help develop dietary agents that target specific receptors to combat malignant cell growth.  相似文献   
976.
977.
The cerebellum is thought to implement internal models for sensory prediction, but details of the underlying circuitry are currently obscure. We therefore investigated a specific example of internal-model based sensory prediction, namely detection of whisker contacts during whisking. Inputs from the vibrissae in rats can be affected by signals generated by whisker movement, a phenomenon also observable in whisking robots. Robot novelty-detection can be improved by adaptive noise-cancellation, in which an adaptive filter learns a forward model of the whisker plant that allows the sensory effects of whisking to be predicted and thus subtracted from the noisy sensory input. However, the forward model only uses information from an efference copy of the whisking commands. Here we show that the addition of sensory information from the whiskers allows the adaptive filter to learn a more complex internal model that performs more robustly than the forward model, particularly when the whisking-induced interference has a periodic structure. We then propose a neural equivalent of the circuitry required for adaptive novelty-detection in the robot, in which the role of the adaptive filter is carried out by the cerebellum, with the comparison of its output (an estimate of the self-induced interference) and the original vibrissal signal occurring in the superior colliculus, a structure noted for its central role in novelty detection. This proposal makes a specific prediction concerning the whisker-related functions of a region in cerebellar cortical zone A(2) that in rats receives climbing fibre input from the superior colliculus (via the inferior olive). This region has not been observed in non-whisking animals such as cats and primates, and its functional role in vibrissal processing has hitherto remained mysterious. Further investigation of this system may throw light on how cerebellar-based internal models could be used in broader sensory, motor and cognitive contexts.  相似文献   
978.
Protein ubiquitination, which is highly selective, regulates many important biological processes including cellular differentiation and pathogenesis in eukaryotic cells. Here, we integrated pharmacological, molecular and proteomic approaches to explore the role of ubiquitination in Magnaporthe oryzae, the leading fungal disease of rice world-wide. Inhibition of ubiquitin-mediated proteolysis using the 26S proteasome inhibitor, Bortezomib, significantly attenuated conidia germination, appressorium formation and pathogenicity in M. oryzae. Gene expression analysis revealed that many genes associated with protein ubiquitination were developmentally regulated during conidia germination. Only a few, including a polyubiquitin encoding gene, MGG_01282, were more abundantly expressed during appressorium formation and under nitrogen starvation. Targeted gene deletion of MGG_01282, in addition to a significant reduction in protein ubiquitination as determined by immuno blot assays, resulted in pleiotropic effects on M. oryzae including reduced growth and sporulation, abnormal conidia morphology, reduced germination and appressorium formation, and the inability to cause disease. Mutants were also defective in sexual development and were female sterile. Using mass spectrometry, we identified 63 candidate polyubiquitinated proteins under nitrogen starvation, which included overrepresentation of proteins involved in translation, transport and protein modification. Our study suggests that ubiquitination of target proteins plays an important role in nutrient assimilation, development and pathogenicity of M. oryzae.  相似文献   
979.
Terrestrial long-distance migrations are declining globally: in North America, nearly 75% have been lost. Yet there has been limited research comparing habitat suitability and connectivity models to identify migration corridors across increasingly fragmented landscapes. Here we use pronghorn (Antilocapra americana) migrations in prairie habitat to compare two types of models that identify habitat suitability: maximum entropy (Maxent) and expert-based (Analytic Hierarchy Process). We used distance to wells, distance to water, NDVI, land cover, distance to roads, terrain shape and fence presence to parameterize the models. We then used the output of these models as cost surfaces to compare two common connectivity models, least-cost modeling (LCM) and circuit theory. Using pronghorn movement data from spring and fall migrations, we identified potential migration corridors by combining each habitat suitability model with each connectivity model. The best performing model combination was Maxent with LCM corridors across both seasons. Maxent out-performed expert-based habitat suitability models for both spring and fall migrations. However, expert-based corridors can perform relatively well and are a cost-effective alternative if species location data are unavailable. Corridors created using LCM out-performed circuit theory, as measured by the number of pronghorn GPS locations present within the corridors. We suggest the use of a tiered approach using different corridor widths for prioritizing conservation and mitigation actions, such as fence removal or conservation easements.  相似文献   
980.
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