全文获取类型
收费全文 | 2684篇 |
免费 | 9篇 |
国内免费 | 25篇 |
专业分类
系统科学 | 45篇 |
丛书文集 | 3篇 |
教育与普及 | 11篇 |
理论与方法论 | 41篇 |
现状及发展 | 305篇 |
研究方法 | 363篇 |
综合类 | 1851篇 |
自然研究 | 99篇 |
出版年
2021年 | 9篇 |
2020年 | 4篇 |
2019年 | 6篇 |
2018年 | 25篇 |
2017年 | 17篇 |
2016年 | 30篇 |
2015年 | 24篇 |
2014年 | 22篇 |
2013年 | 44篇 |
2012年 | 202篇 |
2011年 | 356篇 |
2010年 | 74篇 |
2009年 | 12篇 |
2008年 | 225篇 |
2007年 | 236篇 |
2006年 | 222篇 |
2005年 | 249篇 |
2004年 | 268篇 |
2003年 | 192篇 |
2002年 | 244篇 |
2001年 | 7篇 |
2000年 | 23篇 |
1999年 | 14篇 |
1998年 | 4篇 |
1997年 | 7篇 |
1996年 | 9篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1993年 | 6篇 |
1992年 | 8篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 9篇 |
1982年 | 11篇 |
1981年 | 10篇 |
1980年 | 5篇 |
1979年 | 16篇 |
1978年 | 11篇 |
1977年 | 8篇 |
1974年 | 6篇 |
1973年 | 10篇 |
1972年 | 5篇 |
1971年 | 7篇 |
1970年 | 7篇 |
1969年 | 6篇 |
1968年 | 4篇 |
1967年 | 6篇 |
排序方式: 共有2718条查询结果,搜索用时 31 毫秒
911.
Pryor WR Rymer AM Mitchell DG Hill TW Young DT Saur J Jones GH Jacobsen S Cowley SW Mauk BH Coates AJ Gustin J Grodent D Gérard JC Lamy L Nichols JD Krimigis SM Esposito LW Dougherty MK Jouchoux AJ Stewart AI McClintock WE Holsclaw GM Ajello JM Colwell JE Hendrix AR Crary FJ Clarke JT Zhou X 《Nature》2011,472(7343):331-333
Although there are substantial differences between the magnetospheres of Jupiter and Saturn, it has been suggested that cryovolcanic activity at Enceladus could lead to electrodynamic coupling between Enceladus and Saturn like that which links Jupiter with Io, Europa and Ganymede. Powerful field-aligned electron beams associated with the Io-Jupiter coupling, for example, create an auroral footprint in Jupiter's ionosphere. Auroral ultraviolet emission associated with Enceladus-Saturn coupling is anticipated to be just a few tenths of a kilorayleigh (ref. 12), about an order of magnitude dimmer than Io's footprint and below the observable threshold, consistent with its non-detection. Here we report the detection of magnetic-field-aligned ion and electron beams (offset several moon radii downstream from Enceladus) with sufficient power to stimulate detectable aurora, and the subsequent discovery of Enceladus-associated aurora in a few per cent of the scans of the moon's footprint. The footprint varies in emission magnitude more than can plausibly be explained by changes in magnetospheric parameters--and as such is probably indicative of variable plume activity. 相似文献
912.
TSLP promotes interleukin-3-independent basophil haematopoiesis and type 2 inflammation 总被引:1,自引:0,他引:1
913.
Genetic risk and a primary role for cell-mediated immune mechanisms in multiple sclerosis 总被引:1,自引:0,他引:1
International Multiple Sclerosis Genetics Consortium;Wellcome Trust Case Control Consortium Sawcer S Hellenthal G Pirinen M Spencer CC Patsopoulos NA Moutsianas L Dilthey A Su Z Freeman C Hunt SE Edkins S Gray E Booth DR Potter SC Goris A Band G Oturai AB Strange A Saarela J Bellenguez C Fontaine B Gillman M Hemmer B Gwilliam R Zipp F Jayakumar A Martin R Leslie S Hawkins S Giannoulatou E D'alfonso S Blackburn H Martinelli Boneschi F Liddle J Harbo HF Perez ML Spurkland A Waller MJ Mycko MP 《Nature》2011,476(7359):214-219
Multiple sclerosis is a common disease of the central nervous system in which the interplay between inflammatory and neurodegenerative processes typically results in intermittent neurological disturbance followed by progressive accumulation of disability. Epidemiological studies have shown that genetic factors are primarily responsible for the substantially increased frequency of the disease seen in the relatives of affected individuals, and systematic attempts to identify linkage in multiplex families have confirmed that variation within the major histocompatibility complex (MHC) exerts the greatest individual effect on risk. Modestly powered genome-wide association studies (GWAS) have enabled more than 20 additional risk loci to be identified and have shown that multiple variants exerting modest individual effects have a key role in disease susceptibility. Most of the genetic architecture underlying susceptibility to the disease remains to be defined and is anticipated to require the analysis of sample sizes that are beyond the numbers currently available to individual research groups. In a collaborative GWAS involving 9,772 cases of European descent collected by 23 research groups working in 15 different countries, we have replicated almost all of the previously suggested associations and identified at least a further 29 novel susceptibility loci. Within the MHC we have refined the identity of the HLA-DRB1 risk alleles and confirmed that variation in the HLA-A gene underlies the independent protective effect attributable to the class I region. Immunologically relevant genes are significantly overrepresented among those mapping close to the identified loci and particularly implicate T-helper-cell differentiation in the pathogenesis of multiple sclerosis. 相似文献
914.
Nguyen KD Qiu Y Cui X Goh YP Mwangi J David T Mukundan L Brombacher F Locksley RM Chawla A 《Nature》2011,480(7375):104-108
All homeotherms use thermogenesis to maintain their core body temperature, ensuring that cellular functions and physiological processes can continue in cold environments. In the prevailing model of thermogenesis, when the hypothalamus senses cold temperatures it triggers sympathetic discharge, resulting in the release of noradrenaline in brown adipose tissue and white adipose tissue. Acting via the β(3)-adrenergic receptors, noradrenaline induces lipolysis in white adipocytes, whereas it stimulates the expression of thermogenic genes, such as PPAR-γ coactivator 1a (Ppargc1a), uncoupling protein 1 (Ucp1) and acyl-CoA synthetase long-chain family member 1 (Acsl1), in brown adipocytes. However, the precise nature of all the cell types involved in this efferent loop is not well established. Here we report in mice an unexpected requirement for the interleukin-4 (IL-4)-stimulated program of alternative macrophage activation in adaptive thermogenesis. Exposure to cold temperature rapidly promoted alternative activation of adipose tissue macrophages, which secrete catecholamines to induce thermogenic gene expression in brown adipose tissue and lipolysis in white adipose tissue. Absence of alternatively activated macrophages impaired metabolic adaptations to cold, whereas administration of IL-4 increased thermogenic gene expression, fatty acid mobilization and energy expenditure, all in a macrophage-dependent manner. Thus, we have discovered a role for alternatively activated macrophages in the orchestration of an important mammalian stress response, the response to cold. 相似文献
915.
916.
Saraux C Le Bohec C Durant JM Viblanc VA Gauthier-Clerc M Beaune D Park YH Yoccoz NG Stenseth NC Le Maho Y 《Nature》2011,469(7329):203-206
In 2007, the Intergovernmental Panel on Climate Change highlighted an urgent need to assess the responses of marine ecosystems to climate change. Because they lie in a high-latitude region, the Southern Ocean ecosystems are expected to be strongly affected by global warming. Using top predators of this highly productive ocean (such as penguins) as integrative indicators may help us assess the impacts of climate change on marine ecosystems. Yet most available information on penguin population dynamics is based on the controversial use of flipper banding. Although some reports have found the effects of flipper bands to be deleterious, some short-term (one-year) studies have concluded otherwise, resulting in the continuation of extensive banding schemes and the use of data sets thus collected to predict climate impact on natural populations. Here we show that banding of free-ranging king penguins (Aptenodytes patagonicus) impairs both survival and reproduction, ultimately affecting population growth rate. Over the course of a 10-year longitudinal study, banded birds produced 41% [corrected] fewer chicks and had a survival rate 16 percentage points [corrected] lower than non-banded birds, demonstrating a massive long-term impact of banding and thus refuting the assumption that birds will ultimately adapt to being banded. Indeed, banded birds still arrived later for breeding at the study site and had longer foraging trips even after 10?years. One of our major findings is that responses of flipper-banded penguins to climate variability (that is, changes in sea surface temperature and in the Southern Oscillation index) differ from those of non-banded birds. We show that only long-term investigations may allow an evaluation of the impact of flipper bands and that every major life-history trait can be affected, calling into question the banding schemes still going on. In addition, our understanding of the effects of climate change on marine ecosystems based on flipper-band data should be reconsidered. 相似文献
917.
Electrical stimulation of certain hypothalamic regions in cats and rodents can elicit attack behaviour, but the exact location of relevant cells within these regions, their requirement for naturally occurring aggression and their relationship to mating circuits have not been clear. Genetic methods for neural circuit manipulation in mice provide a potentially powerful approach to this problem, but brain-stimulation-evoked aggression has never been demonstrated in this species. Here we show that optogenetic, but not electrical, stimulation of neurons in the ventromedial hypothalamus, ventrolateral subdivision (VMHvl) causes male mice to attack both females and inanimate objects, as well as males. Pharmacogenetic silencing of VMHvl reversibly inhibits inter-male aggression. Immediate early gene analysis and single unit recordings from VMHvl during social interactions reveal overlapping but distinct neuronal subpopulations involved in fighting and mating. Neurons activated during attack are inhibited during mating, suggesting a potential neural substrate for competition between these opponent social behaviours. 相似文献
918.
In metazoans, the Ras-Raf-MEK (mitogen-activated protein-kinase kinase)-ERK (extracellular signal-regulated kinase) signalling pathway relays extracellular stimuli to elicit changes in cellular function and gene expression. Aberrant activation of this pathway through oncogenic mutations is responsible for a large proportion of human cancer. Kinase suppressor of Ras (KSR) functions as an essential scaffolding protein to coordinate the assembly of Raf-MEK-ERK complexes. Here we integrate structural and biochemical studies to understand how KSR promotes stimulatory Raf phosphorylation of MEK (refs 6, 7). We show, from the crystal structure of the kinase domain of human KSR2 (KSR2(KD)) in complex with rabbit MEK1, that interactions between KSR2(KD) and MEK1 are mediated by their respective activation segments and C-lobe αG helices. Analogous to BRAF (refs 8, 9), KSR2 self-associates through a side-to-side interface involving Arg?718, a residue identified in a genetic screen as a suppressor of Ras signalling. ATP is bound to the KSR2(KD) catalytic site, and we demonstrate KSR2 kinase activity towards MEK1 by in vitro assays and chemical genetics. In the KSR2(KD)-MEK1 complex, the activation segments of both kinases are mutually constrained, and KSR2 adopts an inactive conformation. BRAF allosterically stimulates the kinase activity of KSR2, which is dependent on formation of a side-to-side KSR2-BRAF heterodimer. Furthermore, KSR2-BRAF heterodimerization results in an increase of BRAF-induced MEK phosphorylation via the KSR2-mediated relay of a signal from BRAF to release the activation segment of MEK for phosphorylation. We propose that KSR interacts with a regulatory Raf molecule in cis to induce a conformational switch of MEK, facilitating MEK's phosphorylation by a separate catalytic Raf molecule in trans. 相似文献
919.
920.