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   257篇
  免费   14篇
  271篇
  2022年   5篇
  2021年   3篇
  2020年   3篇
  2019年   8篇
  2018年   4篇
  2017年   6篇
  2016年   2篇
  2015年   11篇
  2014年   17篇
  2013年   15篇
  2012年   11篇
  2011年   16篇
  2010年   9篇
  2009年   7篇
  2008年   19篇
  2007年   12篇
  2006年   12篇
  2005年   11篇
  2004年   15篇
  2003年   5篇
  2002年   10篇
  2001年   11篇
  2000年   12篇
  1999年   5篇
  1998年   5篇
  1996年   1篇
  1995年   4篇
  1992年   2篇
  1991年   6篇
  1990年   4篇
  1989年   4篇
  1988年   4篇
  1987年   2篇
  1985年   1篇
  1983年   1篇
  1982年   1篇
  1977年   1篇
  1975年   1篇
  1974年   2篇
  1971年   2篇
  1963年   1篇
排序方式: 共有271条查询结果,搜索用时 15 毫秒
31.
Aquatic macroinvertebrates in drainage ditches may alter rates of nutrient cycling and decomposition of organic matter but have not been accounted for in studies of ditch biogeochemistry. We collected sediment cores from four pairs of field (intermittent) and collection (perennial) ditches on Maryland’s Eastern Shore monthly from March 2011 to February 2012 to determine how taxonomic and functional group composition varies among different ditch types. We identified 138 taxa and assigned them to functional groups according to trophic position and modes of burrowing. There was no difference in mean abundance of invertebrates (5821 ind./m2) between seasons or types of ditches, and species richness peaked in winter (20 taxa/site) compared to other seasons (15 taxa/site), but did not vary between ditch types. Assemblage composition differed between field and collection ditches, but functional group composition did not. Field ditches flow intermittently which may limit the assemblage to early colonists and taxa adapted to survive desiccation. The benthic macroinvertebrate assemblage was dominated by the collector–gatherer functional feeding group (83.6%) and burrowing taxa (97.1%). Bioturbation by burrowing macroinvertebrates is likely an important process contributing to ecosystem-scale functions of drainage ditches, including regulation of biogeochemical processes occurring at the sediment–water interface.  相似文献   
32.
Control in vitro tick (Boophilus microplus; Acari: Ixodidae) through plant extracts. Hydro-alcohol extracts of ten tropical plants were tested by the immersion method on mature ticks (Boophilus microplus Canestrini 1887). The biological evaluation was made with raw hydro alcoholic extracts (R-OH) to determine the most promissory compounds, which later were fractioned in their polar and non-polar parts. The polar parts showed a significantly inferior effect. The species that produced the highest mortality were Zizigium aromaticum, Morus alba, Piper nigrum and the mixture of Allium sativum- Z. aromaticum (all non-polar); on oviposition: M. alba, P. nigrum and the mixture of A. sativum - Z. aromaticum (all non-polar); on eclosion: P. nigrum, Z. aromaticum, Echinacea angustiofilia, Poligonum punctatum, M. alba and the mixture of A. sativum-Z. aromaticum (all non-polar): on control percentage: P. nigrum, E. angustifolia, P. punctatum and M. alba (all non-polar, except for M. alba in which both fractions showed important activity); and finally, on the percentage of oviposition inhibition: P. nigrum, E. angustifolia, P. punctatum, Gliricidia sepium and M. alba.  相似文献   
33.
RBE of X rays of different energies: a cytogenetic evaluation by FISH   总被引:1,自引:0,他引:1  
Mammography using 26-30 kVp X rays is routinely used in breast cancer screening. Discussion about the radiation-related risk associated with this methodology is ongoing. For radioprotection purposes, a quality factor of 1 has been assigned for all photon energies. However, the relative biological effectiveness (RBE) could increase as the photon energy decreases. Analyzing different biological parameters, for 30 kVp X rays, RBE values from 1 to 8 have been estimated. In the present study, a cytogenetic FISH evaluation of the RBE of 30, 80 and 120 kVp X rays has been done. Blood samples were irradiated with 10 doses from 0.05 to 3 Gy for each energy studied. The yields of translocations and dicentrics were determined by fluorescence in situ hybridization (FISH) using whole chromosome probes for chromosomes 1, 4 and 11 together with a pancentromeric probe. The alpha coefficients of the dose-effect curves for dicentrics, minimum number of breaks needed to produce exchange-type aberrations, and apparently simple translocations were used to estimate the RBE. Using the curves obtained for 120 kVp as a reference, the RBE values for dicentrics were 1.08+/-0.43 and 1.73+/-0.59 for 80 and 30 kVp X rays, respectively; for minimum number of breaks these values were 1.38+/-0.39 and 1.42+/-0.41, and for apparently simple translocations they were 1.26+/-0.40 and 1.51+/-0.47, respectively. Moreover, the induction of complex aberrations by these energies was compared. The percentage of complex aberrations relative to total aberrations showed a significant tendency to increase as X-ray energy decreased: 7.8+/-1.19, 9.8+/-1.6 and 14.1+/-1.9 for 120, 80 and 30 kVp, respectively (P<0.02).  相似文献   
34.
Large‐scale proteomic approaches have been used to study signaling pathways. However, identification of biologically relevant hits from a single screen remains challenging due to limitations inherent in each individual approach. To overcome these limitations, we implemented an integrated, multi‐dimensional approach and used it to identify Wnt pathway modulators. The LUMIER protein–protein interaction mapping method was used in conjunction with two functional screens that examined the effect of overexpression and siRNA‐mediated gene knockdown on Wnt signaling. Meta‐analysis of the three data sets yielded a combined pathway score (CPS) for each tested component, a value reflecting the likelihood that an individual protein is a Wnt pathway regulator. We characterized the role of two proteins with high CPSs, Ube2m and Nkd1. We show that Ube2m interacts with and modulates β‐catenin stability, and that the antagonistic effect of Nkd1 on Wnt signaling requires interaction with Axin, itself a negative pathway regulator. Thus, integrated physical and functional mapping in mammalian cells can identify signaling components with high confidence and provides unanticipated insights into pathway regulators.  相似文献   
35.
New 4-phenylamino-3-quinolinecarbonitriles with a 7-ethynyl group substituted by a pyridine, phenyl or thiophene ring containing basic water solubilizing groups were prepared and evaluated as Src kinase inhibitors. Of these new analogs, potent activity was observed with compounds having a (2,4-dichloro-5-methoxyphenyl)amino group at C-4, a methoxy or ethoxy group at C-6, and a pyridyl group bearing a dimethylamine or N-methylpiperazine on the ethynyl group at C-7.  相似文献   
36.
Obesity is associated with numerous risk factors and comorbidities such as hypertension, metabolic syndrome, type 2 diabetes, and cardiovascular diseases. However, numerous studies have reported an obesity paradox; the overweight and obese patients with established cardiovascular disease have better prognosis than those with a BMI <25 kg/m2. This study was designed to assess potential differences in the clinical profile and management of hypertensive outpatients with chronic ischemic heart disease in obese and lean patients that could explain these two apparently contradictory points. Overweight and obesity were defined as a BMI 25–29.9 kg/m2 and ≥30 kg/m2, respectively. Cardiovascular risk factors goals were considered according to European Society of Hypertension‐European Society of Cardiology 2003, National Cholesterol Education Program Adult Treatment Panel III and American Diabetes Association 2005 guidelines. A sample of 2,024 patients (66.8 ± 10.1 years; 31.7% women) was included. Of these, 0.1% had a BMI <20 kg/m2; 17.1% BMI 20–24.9 kg/m2; 53.7% BMI 25–29.9 kg/m2; 23.7% BMI 30–34.9 kg/m2; 4.3% BMI 35–39.9 kg/m2; and 1.1% BMI ≥40 kg/m2. The subgroup of patients with BMI ≥30 kg/m2 had a higher proportion of women, diastolic dysfunction, diabetes, dyslipidemia, left ventricular hypertrophy, and heart failure. There was an inverse relationship between risk factors control rates and BMI (all comparisons BMI 20–24.9 kg/m2 vs. 25–29.9 kg/m2 vs. ≥30 kg/m2): blood pressure (BP) control (51.7% vs. 42.4% vs. 29.2%, P < 0.001); low‐density lipoprotein cholesterol (LDL‐C) control (35.2% vs. 30.5% vs. 27.9%, P = 0.03) and diabetes control (38.6% vs. 27.6% vs. 22.2%, P = 0.023). In conclusion, in patients with hypertension and chronic ischemic heart disease, as BMI increases, the clinical profile worsens as well as risk factors control rates.  相似文献   
37.
38.
39.
Individual differences in cognitive performance are partly dependent, on genetic polymporhisms. One of the single‐nucleotide polymorphisms (SNP) of the CNR1 gene, which codes for cannabinoid receptor 1 (CB1R), is the rs2180619, located in a regulatory region of this gene (6q14–q15). The alleles of the rs2180619 are A > G; the G allele has been associated with addiction and high levels of anxiety (when the G allele interacts with the SS genotype of the 5‐HTTLPR gene). However, GG genotype is observed also in healthy subjects. Considering G allele as risk for ‘psychopathological conditions’, it is possible that GG healthy subjects do not be addicted or anxious, but would have reduced performance, compared to AA subjects, in attentional control and working memory processing. One hundred and sixty‐four healthy young Mexican‐Mestizo subjects (100 women and 64, men; mean age: 22.86 years, SD=2.72) participated in this study, solving a task where attentional control and working memory were required. GG subjects, compared to AA subjects showed: (1) a general lower performance in the task (P = 0.02); (2) lower performance only when a high load of information was held in working memory (P = 0.02); and (3) a higher vulnerability to distractors (P = 0.03). Our results suggest that, although the performance of GG subjects was at normal levels, a lower efficiency of the endocannabinoid system, probably due to a lowered expression of CB1R, produced a reduction in the performance of these subjects when attentional control and working memory processing is challenged .  相似文献   
40.
After stroke, white matter integrity can be affected both locally and distally to the primary lesion location. It has been shown that tract disruption in mirror’s regions of the contralateral hemisphere is associated with degree of functional impairment. Fourteen patients suffering right hemispheric focal stroke (S) and eighteen healthy controls (HC) underwent Diffusion Weighted Imaging (DWI) and neuropsychological assessment. The stroke patient group was divided into poor (SP; n = 8) and good (SG; n = 6) cognitive recovery groups according to their cognitive improvement from the acute phase (72 hours after stroke) to the subacute phase (3 months post-stroke). Whole-brain DWI data analysis was performed by computing Diffusion Tensor Imaging (DTI) followed by Tract Based Spatial Statistics (TBSS). Assessment of effects was obtained computing the correlation of the projections on TBSS skeleton of Fractional Anisotropy (FA) and Radial Diffusivity (RD) with cognitive test results. Significant decrease of FA was found only in right brain anatomical areas for the S group when compared to the HC group. Analyzed separately, stroke patients with poor cognitive recovery showed additional significant FA decrease in several left hemisphere regions; whereas SG patients showed significant decrease only in the left genu of corpus callosum when compared to the HC. For the SG group, whole brain analysis revealed significant correlation between the performance in the Semantic Fluency test and the FA in the right hemisphere as well as between the performance in the Grooved Pegboard Test (GPT) and theTrail Making Test-part A and the FA in the left hemisphere. For the SP group, correlation analysis revealed significant correlation between the performance in the GPT and the FA in the right hemisphere.  相似文献   
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