全文获取类型
收费全文 | 2628篇 |
免费 | 232篇 |
国内免费 | 1篇 |
专业分类
2861篇 |
出版年
2023年 | 13篇 |
2022年 | 44篇 |
2021年 | 73篇 |
2020年 | 31篇 |
2019年 | 43篇 |
2018年 | 52篇 |
2017年 | 53篇 |
2016年 | 89篇 |
2015年 | 162篇 |
2014年 | 155篇 |
2013年 | 178篇 |
2012年 | 251篇 |
2011年 | 229篇 |
2010年 | 130篇 |
2009年 | 120篇 |
2008年 | 160篇 |
2007年 | 163篇 |
2006年 | 162篇 |
2005年 | 139篇 |
2004年 | 129篇 |
2003年 | 125篇 |
2002年 | 108篇 |
2001年 | 11篇 |
2000年 | 12篇 |
1999年 | 18篇 |
1998年 | 17篇 |
1997年 | 14篇 |
1996年 | 13篇 |
1995年 | 12篇 |
1994年 | 14篇 |
1993年 | 14篇 |
1992年 | 6篇 |
1991年 | 8篇 |
1990年 | 11篇 |
1989年 | 7篇 |
1988年 | 8篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1984年 | 7篇 |
1983年 | 6篇 |
1982年 | 6篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 5篇 |
1976年 | 4篇 |
1974年 | 7篇 |
1973年 | 5篇 |
1971年 | 4篇 |
1967年 | 3篇 |
排序方式: 共有2861条查询结果,搜索用时 15 毫秒
141.
142.
Vikman Kristina Robertson Brita Grant Gunnar Liljeborg Anders Kristensson Krister 《Brain Cell Biology》1998,27(10):749-760
Summary Interferon-γ can facilitate the spinal nociceptive flexor reflex and elicit neuropathic pain-related behavior in rats and mice. Immunoreactivity for the interferon-γ receptor (IFN-γR) occurs in the superficial layers of the dorsal horn and the lateral spinal nucleus in the rat and mouse spinal cord, as well as in subsets of neurons in the dorsal root ganglia. The aim of the present study was to examine the cellular localization and origin of the IFN-γR in the spinal cord. As viewed by confocal microscopy, the immunopositivity for the IFN-γR was co-localized with that of the presynaptic marker synaptophysin and with neuronal nitric oxide synthase in the lateral spinal nucleus, whereas only a minor overlap with these molecules was observed in laminae I and II of the dorsal horn. There was no co-localization of the IFN-γR with markers for astrocytes and microglial cells. Ultrastructurally, the IFN-γR was found predominantly in axon terminals in the lateral spinal nucleus but also at postsynaptic sites in dendrites in laminae I and II. The IFN-γR expressed in neurons in dorsal root ganglia was transported in axons both centrally and peripherally. Hemisection of the spinal cord caused no reduction in immunolabelling of the IFN-γR in the dorsal horn or the lateral spinal nucleus. Since rhizotomy does not effect the immunolabelling in the lateral spinal nucleus, our observation indicates that the presynaptic receptors in this nucleus are derived from intrinsic neurons. The localization of the IFN-γR in the spinal cord differed from that of the AMPA glutamate receptor subunits 2 and 3 and the substance P receptor (NK1). Our results, showing localization of IFN-γR to pre- and postsynaptic sites in the dorsal horn and lateral spinal nucleus indicate that IFN-γ can modulate nociception at the spinal cord level. 相似文献
143.
Stephanie Lemaire Christian Derappe Valerie Pasqualetto Kristina Mrkoci Eric G Berger Michele Aubery Dominique Neel 《Glycoconjugate journal》1998,15(2):161-168
We previously showed that in vitro activated human T lymphocytes expressed increased amounts of -1,6-branched N-linked oligosaccharides (Lemaire S et al. (1994) J Biol Chem 269: 8069-74), which have been proposed to participate in the regulation of the immune process. In the present paper, we compared the activity and expression of -1,4-galactosyltransferase (GalT), one of the glycosyltransferases involved in the biosynthesis of these -1,6-branched N-linked oligosaccharides, before and after in vitro activation of T lymphocytes after a 40 h treatment with a mixture of phorbol 12-myristate 13-acetate and Phaseolus vulgaris lectin. After treatment, the enzymatic activity of the GalT was significantly increased and immunoblot experiments performed with a monoclonal antibody to human GalT showed an increased intensity of the GalT band at 49 kDa, attributable to an enhancement of GalT mRNA level, as shown by Northern blots. However, treatment of the same T-lymphocytes by phorbol ester alone, which is unable to induce mitosis, resulted in a comparable increase of the expression of GalT. Moreover, these phorbol ester-treated T lymphocytes, analysed by flow cytometry exhibited a two-fold increase in the expression of GalT. Finally, confocal fluorescence microscopy performed on all T lymphocytes (treated or not) showed that the flow cytometric signal of GalT originates from intracellular, Golgi-associated antigen only since no surface GalT was detected. 相似文献
144.
Robert A DiSilvestro Joshua Selsby Kristina Siefker 《Journal of trace elements in medicine and biology》2010,24(3):165-168
Marginal copper deficiency has been proposed to occur frequently, but the benefits of correction remain largely uncharacterized. Two benefits could be reduced oxidant stress and better crosslinking of collagen in bone. Copper intake was increased in 8 female university students by supplementation with copper glycinate (2 mg copper/day) for 8 weeks. Supplementation improved copper status based on serum activity of two copper enzymes, ceruloplasmin and diamine oxidase (9% and 75% mean increase, respectively). No effect was seen for erythrocyte copper–zinc superoxide dismutase. Supplementation produced a 39% mean decrease in plasma for F2α-isoprostanes (a marker of oxidant stress), and gave a 62% increase in the urine ratio of collagen crosslinks to a measure of total collagen. None of the supplementation effects were duplicated for 8 women given placebo. In conclusion, this pilot study found that in young adult women, increased copper intake can alter biochemical parameters relevant to copper function. 相似文献
145.
Kristina Cockle Gabriel Capuzzi Alejandro Bodrati Rob Clay Hugo del Castillo Myriam Velázquez Juan I. Areta Nestor Fariña Rodrigo Fariña 《Journal of Field Ornithology》2007,78(1):21-39
ABSTRACT. Vinaceous Amazons ( Amazona vinacea ) are endemic to the Atlantic forest of southeastern Brazil, eastern Paraguay, and the province of Misiones in Argentina. We searched for Vinaceous Amazons throughout the western part of its range in Argentina and Paraguay during 1639 days of fieldwork from 1997 to 2006. These parrots have disappeared from most areas where they were historically recorded in these countries, and are now limited to a few sites in northeastern Paraguay and central Misiones (Argentina). We estimate the minimum remaining populations at 220 individuals in Paraguay and 203 individuals in Argentina. Important sites for the species are (1) the farming area from San Pedro to Tobuna (Misiones, Argentina) and (2) the Itaipú reserves complex and Reserva Natural Privada Itabó (Paraguay). In our surveys, Vinaceous Amazons were absent from the largest tracts of forest in Misiones, and were most often observed feeding, roosting, and nesting in small forest remnants and in agricultural areas that included forest fragments and isolated trees. Threats to amazons in these areas include nest poaching, forest clearing, and being shot as a crop pest. We confirmed 40 Vinaceous Amazons kept as pets in 35 homes between San Pedro and Tobuna. Environmental education and law enforcement are urgently needed to reduce threats in populated areas, and subsistence farmers need technical and logistical support to slow or stop the conversion of forest into cropland. Finally, additional study is needed to determine this amazon's habitat preferences, nest site requirements, and demography in different habitats. 相似文献
146.
Helena Kopp Kallner Louise Thunell Jan Brynhildsen Mia Lindeberg Kristina Gemzell Danielsson 《PloS one》2015,10(5)
ObjectiveTo describe contraceptive use and attitudes towards contraceptive use in Sweden which has the highest abortion rate in Western Europe. Secondary objectives were to investigate knowledge of contraceptive methods and outcomes of unplanned and unwanted pregnancies.DesignTelephone survey.SettingNational survey of women living in Sweden.PopulationWomen between 16 and 49 years.MethodsThe survey contained 22 questions with free text and multi choice answers on demographics, contraceptive use, knowledge of and attitudes towards contraception, the importance of monthly bleeding and experience of unintended pregnancy.ResultsA total of 1001 women participated in the survey. Of all women, 721/1001 (72.1%) currently used contraception whereas 268/1001 (26.8%) women did not. Long acting reversible contraception, (LARC; implant and intra uterine contraception) was used by 24.3% of women. The unmet need of contraception in Sweden was estimated at 8.9% (89/1001 women). A total of 781 (78%) women had never experienced an unintended pregnancy whereas 220 (22%) women had had at least one unintended pregnancy. Users and non-users alike stated that one of the most important characteristics of a contraceptive method is its effectiveness.ConclusionsSweden has a large unmet need for contraception. Furthermore, a large proportion of women have experienced at least one unintended pregnancy. Increasing contraceptive use and promotion of LARC is a possible way forward in the effort to reduce the rates of unwanted pregnancies. 相似文献
147.
148.
Local, wild-collected seeds of native plants are recommended for use in ecological restoration to maintain patterns of adaptive variation. However, some environments are so drastically altered by exotic, invasive weeds that original environmental conditions may no longer exist. Under these circumstances, cultivated varieties selected for improved germination and vigor may have a competitive advantage at highly disturbed sites. This study investigated differences in early establishment and seedling performance between wild and cultivated seed sources of the native grass, Poa secunda, both with and without competition from the invasive exotic grass, Bromus tectorum. We measured seedling survival and above-ground biomass at two experimental sites in western Montana, and found that the source of seeds selected for restoration can influence establishment at the restoration site. Cultivars had an overall advantage when compared with local genotypes, supporting evidence of greater vigor among cultivated varieties of native species. This advantage, however, declined rapidly in the presence of B. tectorum and most accessions were not significantly different for growth and survival in competition plots. Only one cultivar had a consistent advantage despite a strong decline in its performance when competing with invasive plants. As a result, cultivated varieties did not meet expectations for greater establishment and persistence relative to local genotypes in the presence of invasive, exotic species. We recommend the use of representative local or regional wild seed sources in restoration to minimize commercial selection, and a mix of individual accessions (wild, or cultivated when necessary) in highly invaded settings to capture vigorous genotypes and increase the odds native plants will establish at restoration sites. 相似文献
149.
Kristina Svennerholm Michael Haney Bj?rn Biber Erik Ulfhammer Ott Saluveer Pia Larsson Elmir Omerovic Sverker Jern Niklas Bergh 《PloS one》2015,10(3)
The expression of the tissue plasminogen activator (t-PA) gene appears to be under epigenetic control and can be affected by histone deacetylation inhibition. The study aimed to test if histone deacetalyase inhibitor treatment lead to increased t-PA release or reduced exhaustion in t-PA release in response to stimulation, as well as change in plasminogen activator inhibitor-1 (PAI-1) in subjects with coronary disease. In this clinical study, 16 post-myocardial infarction subjects, the perfused forearm model was used with isoprenaline provocation during 20 minutes, to stimulate local t-PA release. Each subject was measured twice on the same day (repeated stimuli sequences) as well as on two different occasions, without treatment and after four weeks of treatment with valproic acid (500 mg, twice daily). Net forearm release for t-PA in response to isoprenaline at minutes 1.5, 3, 6, 9, 12, 15 and 18 was measured, allowing assessment of cumulative t-PA release. There was a reduction in the exhaustion of cumulative t-PA release during repeated and prolonged stimulation with valproic acid treatment compared to non-treatment. Plasma PAI-1 antigen was decreased following treatment compared to non-treatment (18.4 ± 10.0 vs. 11.0 ± 7.1 nanograms/ml respectively, mean with 95% confidence interval). These findings demonstrate that histone deacetylation inhibition increases the capacity for endogenous t-PA release in subjects with vascular disease. Furthermore, the fibrinolytic balance is favored with suppressed PAI-1 levels. More studies are needed to establish the clinical relevance of these findings.
Trial registration
EU Clinical Trials Register 2012-004950-27 相似文献150.
Aifen Zhou Kristina L Hillesland Zhili He Wendy Schackwitz Qichao Tu Grant M Zane Qiao Ma Yuanyuan Qu David A Stahl Judy D Wall Terry C Hazen Matthew W Fields Adam P Arkin Jizhong Zhou 《The ISME journal》2015,9(11):2360-2372
To investigate the genetic basis of microbial evolutionary adaptation to salt (NaCl) stress, populations of Desulfovibrio vulgaris Hildenborough (DvH), a sulfate-reducing bacterium important for the biogeochemical cycling of sulfur, carbon and nitrogen, and potentially the bioremediation of toxic heavy metals and radionuclides, were propagated under salt stress or non-stress conditions for 1200 generations. Whole-genome sequencing revealed 11 mutations in salt stress-evolved clone ES9-11 and 14 mutations in non-stress-evolved clone EC3-10. Whole-population sequencing data suggested the rapid selective sweep of the pre-existing polymorphisms under salt stress within the first 100 generations and the slow fixation of new mutations. Population genotyping data demonstrated that the rapid selective sweep of pre-existing polymorphisms was common in salt stress-evolved populations. In contrast, the selection of pre-existing polymorphisms was largely random in EC populations. Consistently, at 100 generations, stress-evolved population ES9 showed improved salt tolerance, namely increased growth rate (2.0-fold), higher biomass yield (1.8-fold) and shorter lag phase (0.7-fold) under higher salinity conditions. The beneficial nature of several mutations was confirmed by site-directed mutagenesis. All four tested mutations contributed to the shortened lag phases under higher salinity condition. In particular, compared with the salt tolerance improvement in ES9-11, a mutation in a histidine kinase protein gene lytS contributed 27% of the growth rate increase and 23% of the biomass yield increase while a mutation in hypothetical gene DVU2472 contributed 24% of the biomass yield increase. Our results suggested that a few beneficial mutations could lead to dramatic improvements in salt tolerance. 相似文献