全文获取类型
收费全文 | 1669篇 |
免费 | 130篇 |
国内免费 | 3篇 |
专业分类
1802篇 |
出版年
2023年 | 16篇 |
2022年 | 12篇 |
2021年 | 20篇 |
2020年 | 10篇 |
2019年 | 20篇 |
2018年 | 50篇 |
2017年 | 24篇 |
2016年 | 46篇 |
2015年 | 99篇 |
2014年 | 92篇 |
2013年 | 104篇 |
2012年 | 109篇 |
2011年 | 126篇 |
2010年 | 72篇 |
2009年 | 67篇 |
2008年 | 98篇 |
2007年 | 91篇 |
2006年 | 71篇 |
2005年 | 76篇 |
2004年 | 70篇 |
2003年 | 68篇 |
2002年 | 45篇 |
2001年 | 21篇 |
2000年 | 24篇 |
1999年 | 25篇 |
1998年 | 11篇 |
1997年 | 11篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1992年 | 21篇 |
1991年 | 25篇 |
1990年 | 19篇 |
1989年 | 22篇 |
1988年 | 25篇 |
1987年 | 16篇 |
1986年 | 19篇 |
1985年 | 25篇 |
1984年 | 17篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 7篇 |
1979年 | 14篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1976年 | 9篇 |
1974年 | 7篇 |
1973年 | 13篇 |
1972年 | 11篇 |
1971年 | 6篇 |
1970年 | 8篇 |
排序方式: 共有1802条查询结果,搜索用时 15 毫秒
171.
172.
Rapid adjustment of bird community compositions to local climatic variations and its functional consequences 下载免费PDF全文
The local spatial congruence between climate changes and community changes has rarely been studied over large areas. We proposed one of the first comprehensive frameworks tracking local changes in community composition related to climate changes. First, we investigated whether and how 12 years of changes in the local composition of bird communities were related to local climate variations. Then, we tested the consequences of this climate‐induced adjustment of communities on Grinnellian (habitat‐related) and Eltonian (function‐related) homogenization. A standardized protocol monitoring spatial and temporal trends of birds over France from 2001 to 2012 was used. For each plot and each year, we used the spring temperature and the spring precipitations and calculated three indices reflecting the thermal niche, the habitat specialization, and the functional originality of the species within a community. We then used a moving‐window approach to estimate the spatial distribution of the temporal trends in each of these indices and their congruency with local climatic variations. Temperature fluctuations and community dynamics were found to be highly variable in space, but their variations were finely congruent. More interestingly, the community adjustment to temperature variations was nonmonotonous. Instead, unexplained fluctuations in community composition were observed up to a certain threshold of climate change intensity, above which a change in community composition was observed. This shift corresponded to a significant decrease in the relative abundance of habitat specialists and functionally original species within communities, regardless of the direction of temperature change. The investigation of variations in climate and community responses appears to be a central step toward a better understanding of climate change effects on biodiversity. Our results suggest a fine‐scale and short‐term adjustment of community composition to temperature changes. Moreover, significant temperature variations seem to be responsible for both the Grinnellian and Eltonian aspects of functional homogenization. 相似文献
173.
Internalization of staphylococcal leukotoxins that bind and divert the C5a receptor is required for intracellular Ca2+ mobilization by human neutrophils 下载免费PDF全文
Mira Y. Tawk Gaëlle Zimmermann‐Meisse Jean‐Louis Bossu Cristina Potrich Tristan Bourcier Mauro Dalla Serra Bernard Poulain Gilles Prévost Emmanuel Jover 《Cellular microbiology》2015,17(8):1241-1257
A growing number of receptors, often associated with the innate immune response, are being identified as targets for bacterial toxins of the beta‐stranded pore‐forming family. These findings raise the new question of whether the receptors are activated or merely used as docking points facilitating the formation of a pore. To elucidate whether the Staphylococcus aureus Panton‐Valentine leukocidin and the leukotoxin HlgC/HlgB act through the C5a receptor (C5aR) as agonists, antagonists or differ from the C5a complement‐derived peptide, their activity is explored on C5aR‐expressing cells. Both leukotoxins equally bound C5aR in neutrophils and in stable transfected U937 cells and initiated mobilization of intracellular Ca2+. HlgC/HlgB requires the presence of robust intracellular acidic Ca2+ stores in order to evoke a rise in free [Ca2+]i, while the LukS‐PV/LukF‐PV directly altered reticular Ca2+ stores. Intracellular target specificity is conferred by the F‐subunit associated to the S‐subunit binding the receptor. Furthermore, internalization of the two leukotoxin components (S‐ and F‐subunits) associated to C5aR is required for the initiation of [Ca2+]i mobilization. Electrophysiological recordings on living cells demonstrated that LukS‐PV/LukF‐PV does not alter the membrane resistance of C5aR‐expressing cells. The present observations suggest that part of the pore‐forming process occurs in distinct intracellular compartments rather than at the plasma membrane. 相似文献
174.
Chételat G 《Médecine sciences : M/S》2011,27(2):193-198
Neuroimaging offers a promising tool for the priority goals of current researches in Alzheimer's disease (AD) including early diagnosis, monitoring the progression of the disease and understanding the underlying mechanisms. The brain profiles of atrophy and hypometabolism associated with AD are well known and they can be used as support for early diagnosis, although the accuracy of each of these biomarkers on its own is not sufficient. An increasing number of studies highlights the relevance of disconnection processes in the development and progression of AD. The recent development of PET tracers such as the Pittsburg compound (PiB) allowing to visualize in vivo one of the neuropathological lesions characterizing AD (i.e. beta-amyloid depositions) offers a unique opportunity to better understand the mechanisms underlying this multifaceted disease. 相似文献
175.
176.
177.
Meiotic recombination generates reciprocal exchanges between homologous chromosomes (also called crossovers, COs) that are essential for proper chromosome segregation during meiosis and are a major source of genome diversity by generating new allele combinations. COs have two striking properties: they occur at specific sites, called hotspots, and these sites evolve rapidly. In mammals, the Prdm9 gene, which encodes a meiosis-specific histone H3 methyltransferase, has recently been identified as a determinant of CO hotspots. Here, using transgenic mice, we show that the sole modification of PRDM9 zinc fingers leads to changes in hotspot activity, histone H3 lysine 4 trimethylation (H3K4me3) levels, and chromosome-wide distribution of COs. We further demonstrate by an in vitro assay that the PRDM9 variant associated with hotspot activity binds specifically to DNA sequences located at the center of the three hotspots tested. Remarkably, we show that mutations in cis located at hotspot centers and associated with a decrease of hotspot activity affect PRDM9 binding. Taken together, these results provide the direct demonstration that Prdm9 is a master regulator of hotspot localization through the DNA binding specificity of its zinc finger array and that binding of PRDM9 at hotspots promotes local H3K4me3 enrichment. 相似文献
178.
The Bicoid morphogen gradient directs the patterning of cell fates along the anterior-posterior axis of the syncytial Drosophila embryo and serves as a paradigm of morphogen-mediated patterning. The simplest models of gradient formation rely on constant protein synthesis and diffusion from anteriorly localized source mRNA, coupled with uniform protein degradation. However, currently such models cannot account for all known gradient characteristics. Recent work has proposed that bicoid mRNA spatial distribution is sufficient to produce the observed protein gradient, minimizing the role of protein transport. Here, we adapt a novel method of fluorescent in situ hybridization to quantify the global spatio-temporal dynamics of bicoid mRNA particles. We determine that >90% of all bicoid mRNA is continuously present within the anterior 20% of the embryo. bicoid mRNA distribution along the body axis remains nearly unchanged despite dynamic mRNA translocation from the embryo core to the cortex. To evaluate the impact of mRNA distribution on protein gradient dynamics, we provide detailed quantitative measurements of nuclear Bicoid levels during the formation of the protein gradient. We find that gradient establishment begins 45 minutes after fertilization and that the gradient requires about 50 minutes to reach peak levels. In numerical simulations of gradient formation, we find that incorporating the actual bicoid mRNA distribution yields a closer prediction of the observed protein dynamics compared to modeling protein production from a point source at the anterior pole. We conclude that the spatial distribution of bicoid mRNA contributes to, but cannot account for, protein gradient formation, and therefore that protein movement, either active or passive, is required for gradient formation. 相似文献
179.
Who has ever been surprised to return to the bowl of salted peanuts without realizing it, even after having eating a moderate number and deciding to stop? Using rapid event-related functional magnetic resonance imaging (fMRI) in healthy volunteers, we investigated the neural correlates of automatic processes induced by subliminal stimuli. We demonstrated that the automatic activation of motor programs elicited unconsciously in the medial premotor cortex was normally restricted to specific contexts set by the environment, but can occur below the threshold of awareness even when no movement was executed. This novel finding expands our view on brain mechanisms underlying unconscious motor control and provides new evidence that activation of the motor preparation system and consciousness are not obligatory linked. 相似文献
180.
Tkačik G Garrigan P Ratliff C Milčinski G Klein JM Seyfarth LH Sterling P Brainard DH Balasubramanian V 《PloS one》2011,6(6):e20409
Here we introduce a database of calibrated natural images publicly available through an easy-to-use web interface. Using a Nikon D70 digital SLR camera, we acquired about six-megapixel images of Okavango Delta of Botswana, a tropical savanna habitat similar to where the human eye is thought to have evolved. Some sequences of images were captured unsystematically while following a baboon troop, while others were designed to vary a single parameter such as aperture, object distance, time of day or position on the horizon. Images are available in the raw RGB format and in grayscale. Images are also available in units relevant to the physiology of human cone photoreceptors, where pixel values represent the expected number of photoisomerizations per second for cones sensitive to long (L), medium (M) and short (S) wavelengths. This database is distributed under a Creative Commons Attribution-Noncommercial Unported license to facilitate research in computer vision, psychophysics of perception, and visual neuroscience. 相似文献