全文获取类型
收费全文 | 6322篇 |
免费 | 526篇 |
国内免费 | 5篇 |
专业分类
6853篇 |
出版年
2023年 | 26篇 |
2022年 | 64篇 |
2021年 | 100篇 |
2020年 | 73篇 |
2019年 | 82篇 |
2018年 | 103篇 |
2017年 | 126篇 |
2016年 | 162篇 |
2015年 | 281篇 |
2014年 | 388篇 |
2013年 | 428篇 |
2012年 | 547篇 |
2011年 | 483篇 |
2010年 | 321篇 |
2009年 | 280篇 |
2008年 | 333篇 |
2007年 | 367篇 |
2006年 | 345篇 |
2005年 | 325篇 |
2004年 | 300篇 |
2003年 | 292篇 |
2002年 | 273篇 |
2001年 | 64篇 |
2000年 | 52篇 |
1999年 | 59篇 |
1998年 | 75篇 |
1997年 | 56篇 |
1996年 | 57篇 |
1995年 | 45篇 |
1994年 | 58篇 |
1993年 | 48篇 |
1992年 | 68篇 |
1991年 | 40篇 |
1990年 | 45篇 |
1989年 | 41篇 |
1988年 | 28篇 |
1987年 | 33篇 |
1986年 | 20篇 |
1985年 | 27篇 |
1984年 | 39篇 |
1983年 | 31篇 |
1982年 | 28篇 |
1981年 | 33篇 |
1980年 | 17篇 |
1979年 | 14篇 |
1978年 | 15篇 |
1976年 | 17篇 |
1975年 | 13篇 |
1974年 | 23篇 |
1972年 | 18篇 |
排序方式: 共有6853条查询结果,搜索用时 11 毫秒
81.
Cellular aging leads to functional heterogeneity of hematopoietic stem cells: a modeling perspective
Hematopoietic stem cells (HSCs) are the source for the life-long supply of functional cells in peripheral blood while they simultaneously maintain their own reserve pool. However, there is accumulating evidence that HSCs are themselves subject to quantitative and qualitative exhaustion. Although several processes linked to mitotic activity can potentially account for the observed aging phenomena (e.g., DNA damage, telomere shortening, epigenetic modification), a precise understanding of HSC exhaustion is still missing. It is particularly unclear how individual aging processes on the single-cell level translate on the phenotypic level of the overall tissue and whether there is a functional implication of an age-structured HSC population. We address these issues by applying a novel mathematical model of HSC organization in which division-specific, cumulative alterations of stem cell quality determine the phenotypic and functional appearance of the overall cell population. Adapting the model to a number of basic experimental findings, we quantify the level of additional heterogeneity that is introduced by a population of individually aging cells. Based on this model, we are able to conclude that division-dependent processes of cellular aging explain a wide range of phenomena on HSC exhaustion and that HSC aging needs to be considered as a highly heterogeneous process. We furthermore report that functional heterogeneity between young and old HSCs appears closely similar to the phenomena described for long- and short-term repopulating cells. We speculate whether differential, division-coupled stem cell aging introduces an intra-animal variability that also accounts for heterogeneity with respect to the repopulation ability of HSCs. 相似文献
82.
Siegler JC Mermier CM Amorim FT Lovell RJ McNaughton LR Robergs RA 《Journal of strength and conditioning research / National Strength & Conditioning Association》2008,22(5):1394-1401
In the present study, we aimed to compare the thermoregulatory response and soccer-specific training performance aspects of two commercially available sport drinks, both of similar carbohydrate concentration, but one containing 5.2% glycerol. Ten players participated in two similar outdoor training sessions and were randomly assigned to each of two drinks: a carbohydrate (C) beverage or a carbohydrate-glycerol (CG) beverage. Players consumed 500 mL of C or CG 30 minutes pre-exercise and at half-time. Pre- and postexercise body mass, core temperature (CT), and heart rate (HR) were recorded, and urine and blood samples were taken. No difference was observed between days for wet bulb globe temperature (session 1: 17.0 +/- 1.1 degrees C, session 2: 16.9 +/- 1.1 degrees C; P = 0.944). The degree of dehydration (% Delta BM) was greater after the C trial (P = 0.041). Similarly, percent change in plasma volume was greater in the C trial (P = 0.049). No overall main affect was observed between CT and mean exercise HRs during either training session (CT: P = 0.350; mean HR: P = 0.256), and there was no difference observed between groups in time to failure during the session-ending fatigue test (P = 0.547). Ingestion of a CG beverage provided players with better hydration than C alone. However, if training sessions are short (<75 minute), with adequate time for recovery, both drinks are sufficient for maintaining performance intensities during soccer-specific training. 相似文献
83.
Time-frequency filtering of MEG signals with matching pursuit. 总被引:4,自引:0,他引:4
Maciej Gratkowski Jens Haueisen Lars Arendt-Nielsen Andrew C N Chen Frank Zanow 《Journal of Physiology》2006,99(1):47-57
Time-frequency signal analysis based on various decomposition techniques is widely used in biomedical applications. Matching Pursuit is a new adaptive approach for time-frequency decomposition of such biomedical signals. Its advantage is that it creates a concise signal approximation with the help of a small set of Gabor atoms chosen iteratively from a large and redundant set. In this paper, the usage of Matching Pursuit for time-frequency filtering of biomagnetic signals is proposed. The technique was validated on artificial signals and its performance was tested for varying signal-to-noise ratios using both simulated and real MEG somatic evoked magnetic field data. 相似文献
84.
Sung-Hwan Moon Chun-Hao Huang Shauna L. Houlihan Kausik Regunath William A. Freed-Pastor John P. Morris Darjus F. Tschaharganeh Edward R. Kastenhuber Anthony M. Barsotti Rachel Culp-Hill Wen Xue Yu-Jui Ho Timour Baslan Xiang Li Allison Mayle Elisa de Stanchina Lars Zender David R. Tong Carol Prives 《Cell》2019,176(3):564-580.e19
85.
Nociceptin-immunoreactive cellbodies were detected in the human trigeminal ganglion, while no such fibers were identified in the temporal artery or in dermal tissue from the neck region. In four healthy subjects receiving nociceptin into the temporal muscle in an open labeled design no pain was detected. In 10 healthy subjects who received 200pmol of nociceptin into tender non-dominant trapezius muscles in a placebo-controlled, randomized, balanced, and double-blinded design local tenderness increased (P=0.025) while no pain was noted. Thus, the action of nociceptin should be searched for in the trigeminal ganglion and/or in the central nervous system (CNS). 相似文献
86.
Thermoanaerobacter ethanolicus gen. nov., spec. nov., a new,extreme thermophilic,anaerobic bacterium
Two strains, JW 200 and JW 201, of an extreme thermophilic, non-spore-forming anaerobic bacterium were isolated from alkaline and slightly acidic hot springs located in Yellowstone National Park. Both strains were peritrichously flagellated rods. Cell size varied from 0.5–0.8 by 4–100 m; coccoid-shaped cells of about 1 m in diameter frequently occurred. Division was often unequal. Spheroplast-like forms were visible at the late logarithmic growth phase. The Gram reaction was variable. The DNA base composition of the two strains was between 37 and 39 mol% guanine plus cytosine as determined by buoyant density measurements and approximately 32% by the thermal denaturation method. The main fermentation products from hexoses were ethanol and CO2. Growth occurred between 37 and 78°C and from pH 4.4 to 9.8. The name Thermoanaerobacter ethanolicus gen. nov., spec. nov. was proposed for the two, new isolates. Strain JW 200 was designated as the type strain.A preliminary account of this work was presented at the annual meeting of the American Society for Microbiology, Los Angeles, CA, 1979 (J. Wiegel and L. G. Ljungdahl, Abstr. Annu. Meet. Am. Soc. Microbiol., 1979, 163, p. 105) and at the 27th IUPAC Congress Helsinki, 1979 (L. G. Ljungdahl and J. Wiegel, Abstracts p. 546) 相似文献
87.
We describe a Mycobacterium smegmatis mutant with impaired biofilm and smegma formation. A gene homologous to Escherichia coli bacA, which has been proposed to play a role as undecaprenyl phosphokinase (Upk) was unmarked in-frame deleted from M. smegmatis. Though Upk is involved in cell wall synthesis, the surface of the mutant strain appeared virtually comparable to that of the wild type by electron microscopy. The absence of Upk influenced colony morphology and bacitracin resistance. The M. smegmatis Deltaupk mutant developed a biofilm characterized by scattered islands of bacteria distinct from the completely covered biofilm surface observed for wild-type bacteria. We further demonstrate biological consequences of upk deletion for smegma development in an in vivo model. These results suggest the upk gene to be essential in biofilm and smegma development. 相似文献
88.
Microbial communities at soil-stream interfaces may be particularly important in regulating amounts and forms of nutrients
that leave upland soils and enter stream ecosystems. While microbial communities are thought to be responsible for key nutrient
transformations within near-stream sediments, there is relatively little mechanistic information on factors that control microbial
activities in these areas. In this study, we examine the roles of dissolved organic carbon (DOC) vs. particulate organic carbon
(POC) as potential controls on rates of bacterial productivity (measured as incorporation of [3H]thymidine into bacterial
DNA) and amounts of bacterial biomass (measured as fatty acid yield) in sediments along a transect perpendicular to a soil–stream
interface. We hypothesized that spatial patterns in bacterial productivity would vary in response to strong and persistent
patterns in pore-water concentrations of DOC that were observed along a soil-stream transect throughout a 2-year period. Our
results did not support the existence of such a link between pore-water DOC and bacterial productivity. In contrast, we found
bacterial productivity and biomass were related to small-scale spatial variations in sediment POC on 3 of 4 sample dates.
While our results indicate that total bacterial productivity in near-stream sediments is not consistently linked to spatial
variations in pore-water DOC, it is likely that DOC and POC are not mutually exclusive and the relative contribution of DOC
and POC to sedimentary microbes varies temporally and spatially in different riparian habitats.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
89.
A key hallmark of the vertebrate adaptive immune system is the generation of antigen-specific antibodies from B cells. Fish are the most primitive gnathostomes (jawed vertebrates) possessing an adaptive immune system. Vaccination of rainbow trout against enteric redmouth disease (ERM) by immersion in Yersinia ruckeri bacterin confers a high degree of protection to the fish. The immune mechanisms responsible for protection may comprise both cellular and humoral elements but the role of specific immunoglobulins in this system has been questioned and not previously described. The present study demonstrates significant increase in plasma antibody titers following immersion vaccination and significantly reduced mortality during Y. ruckeri challenge.Rainbow trout were immersion-vaccinated, using either a commercial ERM vaccine (AquaVac™ ERM vet) or an experimental Y. ruckeri bacterin. Half of the trout vaccinated with AquaVac™ ERM vet received an oral booster (AquaVac™ ERM Oral vet). Sub-groups of the fish from each group were subsequently exposed to 1x109 CFU Y. ruckeri/ml either eight or twenty-six weeks post vaccination (wpv). All vaccinated groups showed 0% mortality when challenged, which was highly significant compared to the non-vaccinated controls (40 and 28% mortality eight and twenty-six weeks post vaccination (wpv), respectively) (P<0.0001). Plasma samples from all groups of vaccinated fish were taken 0, 4, 8, 12, 16 and 26 wpv. and Y. ruckeri specific IgM antibody levels were measured with ELISA. A significant increase in titers was recorded in vaccinated fish, which also showed a reduced bacteremia during challenge. In vitro plasma studies showed a significantly increased bactericidal effect of fresh plasma from vaccinated fish indicating that plasma proteins may play a role in protection of vaccinated rainbow trout. 相似文献
90.
Saevarsdottir S Ding B Steinsson K Grondal G Valdimarsson H Alfredsson L Klareskog L Padyukov L 《Arthritis research & therapy》2011,13(2):R65