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   1364篇
  免费   146篇
  2021年   21篇
  2020年   18篇
  2019年   18篇
  2018年   25篇
  2017年   17篇
  2016年   25篇
  2015年   38篇
  2014年   50篇
  2013年   56篇
  2012年   85篇
  2011年   65篇
  2010年   58篇
  2009年   42篇
  2008年   56篇
  2007年   51篇
  2006年   47篇
  2005年   52篇
  2004年   52篇
  2003年   36篇
  2002年   37篇
  2001年   39篇
  2000年   39篇
  1999年   28篇
  1998年   15篇
  1996年   12篇
  1995年   14篇
  1994年   11篇
  1993年   14篇
  1992年   26篇
  1991年   25篇
  1990年   20篇
  1989年   17篇
  1988年   26篇
  1987年   23篇
  1986年   23篇
  1985年   23篇
  1984年   20篇
  1983年   17篇
  1981年   13篇
  1980年   12篇
  1979年   12篇
  1977年   17篇
  1976年   11篇
  1975年   12篇
  1974年   13篇
  1973年   17篇
  1972年   14篇
  1970年   12篇
  1969年   20篇
  1968年   13篇
排序方式: 共有1510条查询结果,搜索用时 15 毫秒
1.
Noninvasive genetic sampling enables biomonitoring without the need to directly observe or disturb target organisms. This paper describes a novel and promising source of noninvasive spider and insect DNA from spider webs. Using black widow spiders (Latrodectus spp.) fed with house crickets (Acheta domesticus), we successfully extracted, amplified, and sequenced mitochondrial DNA from spider web samples that identified both spider and prey to species. Detectability of spider DNA did not differ between assays with amplicon sizes from 135 to 497 base pairs. Spider and prey DNA remained detectable at least 88 days after living organisms were no longer present on the web. Spider web DNA as a proof-of-concept may open doors to other practical applications in conservation research, pest management, biogeography studies, and biodiversity assessments.  相似文献   
2.

Objectives

Smartphone-enabled ECG devices have the potential to improve patient care by enabling remote ECG assessment of patients with potential and diagnosed arrhythmias. This prospective study aimed to assess the usefulness of pediatric ECG tracings generated by the AliveCor device (Oklahoma City, OK) and to assess user satisfaction.

Study Design

Enrolled pediatric patients with documented paroxysmal arrhythmia used the AliveCor device over a yearlong study period. Pediatric electrophysiologists reviewed all transmitted ECG tracings. Patient completed surveys were analyzed to assess user satisfaction.

Results

35 patients were enrolled with the following diagnoses: supraventricular tachycardia (SVT, 57%), atrial fibrillation (AF, 11%), ectopic atrial tachycardia (EAT, 6%), atrial tachycardia (AT, 3%), and ventricular tachycardia (VT, 23%). A total of 238 tracings were received from 20 patients, 96% of which were of diagnostic quality for sinus rhythm, sinus tachycardia, SVT, and AF. 126 patient satisfaction surveys (64% from parents) were completed. 98% of the survey responses indicated that it was easy to obtain tracings, 93% found it easy to transmit the tracings, 98% showed added comfort in managing arrhythmia by having the device, and 93% showed interest in continued use of the device after the study period ended.

Conclusions

Smartphone-enabled ECG devices can generate tracings of diagnostic quality in children. User satisfaction was extremely positive. Use of the device to manage certain patients with AF and SVT showcases the future role of remote ECGs in the successful outpatient management of arrhythmias in children by potentially reducing Emergency Department visits and healthcare costs.  相似文献   
3.
The essentiality of roots to the short-term responses of leaf elongation to salinity was tested by removing the roots of maize (Zea mays L.) from the shoots and comparing the initial short-term response of leaf elongation to that with intact plants. Eightday-old seedlings growing in solution culture were treated with 80 millimolar NaCl and their leaf elongation rate (LER) was monitored with a linear variable differential transformer connected to a computerized data aquisition system. Initially, LER of intact plants was sharply reduced by salinity, then rose rapidly to reach a new steady-state rate about 1.5 hours after salinization. The new steady-state rate of salinized intact plants was about 80% of the control rate. When the roots of nonsalinized plants were excised under the surface of the nutrient solution, excision did not disturb the steady-state LER. When these shoots were salinized, they responded in a manner nearly identical to that of intact plants, indicating that roots are not essential for the modulation of short-term LER of salt-stressed plants.  相似文献   
4.
S100A8/9 and S100A12 are emerging biomarkers for disease activity of autoimmune and cardiovascular diseases. We demonstrated previously that S100A12 accelerates atherosclerosis accompanied by large cholesterol deposits in atherosclerotic lesions of apoE-null mice. The objective of this study was to ascertain whether S100/calgranulin influences cholesterol homeostasis in macrophages. Peritoneal macrophages from transgenic mice expressing human S100A8/9 and S100A12 in myeloid cells [human bacterial artificial chromosome (hBAC)/S100] have increased lipid content and reduced ABCG1 expression and [3H]cholesterol efflux compared with WT littermates. This was associated with a 6-fold increase in plasma interleukin (IL)-22 and increased IL-22 mRNA in splenic T cells. These findings are mediated by the receptor for advanced glycation endproducts (RAGE), because hBAC/S100 mice lacking RAGE had normal IL-22 expression and normal cholesterol efflux. In vitro, recombinant IL-22 reduced ABCG1 expression and [3H]cholesterol efflux in THP-1 macrophages, while recombinant S100A12 had no effect on ABCG1 expression. In conclusion, S100/calgranulin has no direct effect on cholesterol efflux in macrophages, but rather promotes the secretion of IL-22, which then directly reduces cholesterol efflux in macrophages by decreasing the expression of ABCG1.  相似文献   
5.
High levels of manganese (Mn) exposure decrease striatal medium spiny neuron (MSN) dendritic length and spine density, but the mechanism(s) are not known. The Huntingtin (HTT) gene has been functionally linked to cortical brain‐derived neurotrophic factor (BDNF) support of striatal MSNs via phosphorylation at serine 421. In Huntington's disease, pathogenic CAG repeat expansions of HTT decrease synthesis and disrupt transport of cortical–striatal BDNF, which may contribute to disease, and Mn is a putative environmental modifier of Huntington's disease pathology. Thus, we tested the hypothesis that changes in MSN dendritic morphology Mn due to exposure are associated with decreased BDNF levels and alterations in Htt protein. We report that BDNF levels are decreased in the striatum of Mn‐exposed non‐human primates and in the cerebral cortex and striatum of mice exposed to Mn. Furthermore, proBDNF and mature BDNF concentrations in primary cortical and hippocampal neuron cultures were decreased by exposure to Mn confirming the in vivo findings. Mn exposure decreased serine 421 phosphorylation of Htt in cortical and hippocampal neurons and increased total Htt levels. These data strongly support the hypothesis that Mn‐exposure‐related MSN pathology is associated with decreased BDNF trophic support via alterations in Htt.

  相似文献   

6.
7.
We have examined the structure of the vacuolar ATPase of Neurospora crassa using negatively stained preparations of vacuolar membranes and of detergent-solubilized and gradient-purified ATPase complexes. We also examined the peripheral sector (V1) of the enzyme after it had been removed and purified. Using different stains, vacuolar membranes displayed ball-and-stalk structures similar to those of the intact mitochondrial ATPase. However, the vacuolar ATPase was clearly different from the mitochondrial ATPase in both size and structural features. The vacuolar enzyme had a much larger head domain with a distinct cleft down the middle of the complex. This domain was held above the membrane by a prominent stalk. Most intriguing was the presence of basal components. These structures appeared to project from the vacuolar membrane near the base of the stalks. Detergent-solubilized, gradient-purified ATPases displayed the same head, stalk, and basal features as those found with the intact enzyme on vacuolar membranes. The mitochondrial ATPase was significantly smaller, and no clefted head domains or basal components were observed. When V1 and F1 particles were directly compared, a significant difference in size and shape between these two soluble ATPase sectors was apparent. V1 retained all of the features seen in the globular head of the intact complex: V-shaped, triangular, and square forms around a stain-filled core.  相似文献   
8.
Based on prior evidence of coordinated multiple leaf trait scaling, we hypothesized that variation among species in leaf dark respiration rate (R d) should scale with variation in traits such as leaf nitrogen (N), leaf life-span, specific leaf area (SLA), and net photosynthetic capacity (A max). However, it is not known whether such scaling, if it exists, is similar among disparate biomes and plant functional types. We tested this idea by examining the interspecific relationships between R d measured at a standard temperature and leaf life-span, N, SLA and A max for 69 species from four functional groups (forbs, broad-leafed trees and shrubs, and needle-leafed conifers) in six biomes traversing the Americas: alpine tundra/subalpine forest, Colorado; cold temperate forest/grassland, Wisconsin; cool temperate forest, North Carolina; desert/shrubland, New Mexico; subtropical forest, South Carolina; and tropical rain forest, Amazonas, Venezuela. Area-based R d was positively related to area-based leaf N within functional groups and for all species pooled, but not when comparing among species within any site. At all sites, mass-based R d (R d-mass) decreased sharply with increasing leaf life-span and was positively related to SLA and mass-based A max and leaf N (leaf N mass). These intra-biome relationships were similar in shape and slope among sites, where in each case we compared species belonging to different plant functional groups. Significant R d-massN mass relationships were observed in all functional groups (pooled across sites), but the relationships differed, with higher R d at any given leaf N in functional groups (such as forbs) with higher SLA and shorter leaf life-span. Regardless of biome or functional group, R d-mass was well predicted by all combinations of leaf life-span, N mass and/or SLA (r 2≥ 0.79, P < 0.0001). At any given SLA, R d-mass rises with increasing N mass and/or decreasing leaf life-span; and at any level of N mass, R d-mass rises with increasing SLA and/or decreasing leaf life-span. The relationships between R d and leaf traits observed in this study support the idea of a global set of predictable interrelationships between key leaf morphological, chemical and metabolic traits. Received: 23 May 1997 / Accepted: 16 December 1997  相似文献   
9.
 Radiolytic reduction at 77 K of oxo-/hydroxo-bridged dinuclear iron(III) complexes in frozen solutions forms kinetically stabilized, mixed-valent species in high yields that model the mixed-valent sites of non-heme, diiron proteins. The mixed-valent species trapped at 77 K retain ligation geometry similar to the initial diferric clusters. The shapes of the mixed-valent EPR signals depend strongly on the bridging ligands. Spectra of the Fe(II)OFe(III) species reveal an S=1/2 ground state with small g-anisotropy as characterized by the uniaxial component (g z g av /2<0.03) observable at temperatures as high as ∼100 K. In contrast, hydroxo-bridged mixed-valent species are characterized by large g-anisotropy (g z g av /2>0.03) and are observable only below 30 K. Annealing at higher temperatures causes structural relaxation and changes in the EPR characteristics. EPR spectral properties allow the oxo- and hydroxo-bridged, mixed-valent diiron centers to be distinguished from each other and can help characterize the structure of mixed-valent centers in proteins. Received: 27 June 1998 / Accepted: 25 February 1999  相似文献   
10.
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