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   2338篇
  免费   162篇
  国内免费   3篇
  2023年   16篇
  2022年   30篇
  2021年   54篇
  2020年   40篇
  2019年   35篇
  2018年   57篇
  2017年   69篇
  2016年   96篇
  2015年   134篇
  2014年   166篇
  2013年   152篇
  2012年   186篇
  2011年   186篇
  2010年   129篇
  2009年   91篇
  2008年   135篇
  2007年   116篇
  2006年   128篇
  2005年   122篇
  2004年   113篇
  2003年   96篇
  2002年   69篇
  2001年   21篇
  2000年   16篇
  1999年   26篇
  1998年   22篇
  1997年   16篇
  1996年   13篇
  1995年   14篇
  1994年   17篇
  1993年   3篇
  1992年   17篇
  1991年   9篇
  1990年   7篇
  1989年   4篇
  1988年   4篇
  1987年   11篇
  1986年   5篇
  1985年   5篇
  1984年   3篇
  1983年   6篇
  1982年   9篇
  1981年   6篇
  1980年   4篇
  1979年   6篇
  1978年   11篇
  1977年   4篇
  1976年   4篇
  1975年   4篇
  1974年   7篇
排序方式: 共有2503条查询结果,搜索用时 15 毫秒
61.
NDUFV1 mutations have been related to encephalopathic phenotypes due to mitochondrial energy metabolism disturbances. In this study, we report two siblings affected by a diffuse leukodystrophy, who carry the NDUFV1 c.1156C>T (p.Arg386Cys) missense mutation and a novel 42-bp deletion. Bioinformatic and molecular analysis indicated that this deletion lead to the synthesis of mRNA molecules carrying a premature stop codon, which might be degraded by the nonsense-mediated decay system. Our results add information on the molecular basis and the phenotypic features of mitochondrial disease caused by NDUFV1 mutations.  相似文献   
62.

Background

Limited knowledge on the prevalence and distribution of risk factors impairs the planning and implementation of cardiovascular prevention programs in the Latin American and Caribbean (LAC) region.

Methods and Findings

Prevalence of hypertension, diabetes mellitus, abnormal lipoprotein levels, obesity, and smoking were estimated from individual-level patient data pooled from population-based surveys (1998–2007, n = 31,009) from eight LAC countries and from a national survey of the United States (US) population (1999–2004) Age and gender specific prevalence were estimated and age-gender adjusted comparisons between both populations were conducted. Prevalence of diabetes mellitus, hypertension, and low high-density lipoprotein (HDL)-cholesterol in LAC were 5% (95% confidence interval [95% CI]: 3.4, 7.9), 20.2% (95% CI: 12.5, 31), and 53.3% (95% CI: 47, 63.4), respectively. Compared to LAC region’s average, the prevalence of each risk factor tended to be lower in Peru and higher in Chile. LAC women had higher prevalence of obesity and low HDL-cholesterol than men. Obesity, hypercholesterolemia, and hypertriglyceridemia were more prevalent in the US population than in LAC population (31 vs. 16.1%, 16.8 vs. 8.9%, and 36.2 vs. 26.5%, respectively). However, the prevalence of low HDL-cholesterol was higher in LAC than in the US (53.3 vs. 33.7%).

Conclusions

Major cardiovascular risk factors are highly prevalent in LAC region, in particular low HDL-cholesterol. In addition, marked differences do exist in this prevalence profile between LAC and the US. The observed patterns of obesity-related risk factors and their current and future impact on the burden of cardiovascular diseases remain to be explained.  相似文献   
63.

Background

The common or brinjal eggplant (Solanum melongena L.) belongs to the Leptostemonum Clade (the “spiny” solanums) of the species-rich genus Solanum (Solanaceae). Unlike most of the genus, the eggplant and its relatives are from the Old World; most eggplant wild relatives are from Africa. An informal system for naming eggplant wild relatives largely based on crossing and other biosystematics data has been in use for approximately a decade. This system recognises several forms of two broadly conceived species, S. incanum L. and S. melongena. Recent morphological and molecular work has shown that species-level differences exist between these entities, and a new species-level nomenclature has been identified as necessary for plant breeders and for the maintenance of accurately named germplasm.

Methodology/Principal Findings

We examined herbarium specimens from throughout the wild species ranges as part of a larger revision of the spiny solanums of Africa. Based on these morphological and molecular studies, we delimited species in the group to which the common eggplant belongs and constructed identification keys for the group. We also examined the monophyly of the group considered as the eggplant relatives by previous authors.

Conclusions/Significance

We recognise ten species in this group: S. aureitomentosum Bitter, S. campylacanthum A.Rich., S. cerasiferum Dunal, S. incanum L., S. insanum L., S. lichtensteinii Willd., S. linnaeanum Hepper & P.-M.L.Jaeger, S. melongena L., S. rigidum Lam. and S. umtuma Voronts. & S.Knapp. We review the history of naming and provide keys and character lists for all species. Ploidy level differences have not been investigated in the eggplant wild relatives; we identify this as a priority for improvement of crop wild relative use in breeding. The application of species-level names to these entities will help focus new collecting efforts for brinjal eggplant improvement and help facilitate information exchange.  相似文献   
64.

Background

Rickettsia typhi is the etiological agent of murine typhus (MT), a disease transmitted by two cycles: rat-flea-rat, and peridomestic cycle. Murine typhus is often misdiagnosed and underreported. A correct diagnosis is important because MT can cause severe illness and death. Our previous seroprevalence results pointed to presence of human R . typhi infection in our region; however, no clinical case has been reported. Although cats have been related to MT, no naturally infected cat has been described. The aim of the study is to confirm the existence of R . typhi in our location analyzing its presence in cats and fleas.

Methodology/Principal Findings

221 cats and 80 fleas were collected from Veterinary clinics, shelters, and the street (2001-2009). Variables surveyed were: date of collection, age, sex, municipality, living place, outdoor activities, demographic area, healthy status, contact with animals, and ectoparasite infestation. IgG against R . typhi were evaluated by indirect immunofluorescence assay. Molecular detection in cats and fleas was performed by real-time PCR. Cultures were performed in those cats with positive molecular detection. Statistical analysis was carried out using SPSS. A p < 0.05 was considered significant.Thirty-five (15.8%) cats were seropositive. There were no significant associations among seropositivity and any variables. R . typhi was detected in 5 blood and 2 cultures. High titres and molecular detection were observed in stray cats and pets, as well as in spring and winter. All fleas were Ctenocephalides felis. R . typhi was detected in 44 fleas (55%), from shelters and pets. Co-infection with R . felis was observed.

Conclusions

Although no clinical case has been described in this area, the presence of R . typhi in cats and fleas is demonstrated. Moreover, a considerable percentage of those animals lived in households. To our knowledge, this is the first time R . typhi is detected in naturally infected cats.  相似文献   
65.
Typification of the names Antirrhinum barrelieri Boreau, A. controversum Pau and A. litigiosum Pau (Antirrhineae, Plantaginaceae) is needed to clarify the use and concept of these three names. The origin of a misinterpretation of the name A. barrelieri, which conditioned the subsequent use of these three names, is discussed. The previous ‘lectotype’ of A. barrelieri designated by Rothmaler from a Barrelier's illustration is shown to be ineffective, being contrary to Art. 9.12 of the ICN, and therefore a lectotype is designated from a syntype preserved in the herbarium of the Muséum des sciences naturelles d'Angers (France) at ANG. The Pau's names A. controversum and A. litigiosum have not been typified before, and lectotypes are here designated from specimens preserved at P and MA, respectively. As a consequence, the name A. litigiosum should be treated as a later heterotypic synonym of A. barrelieri.  相似文献   
66.
Abstract

Biofilm fluoride reservoirs may be a source of fluoride to the fluid phase during a sugar challenge reducing tooth mineral loss. However, the evidence for that is conflicting and has not been studied in biofilms containing different fluoride levels. In order to test fluoride release from biofilms with distinct fluoride concentrations, biofilms were grown in situ exposed to a combination of placebo, calcium and fluoride rinses forming biofilms with no (fluoride-free rinses), low (fluoride-only rinses) or high (calcium followed by fluoride rinses) fluoride concentrations, and collected before and 5?min after a sucrose challenge. Rinsing with fluoride increased fluoride concentration in the biofilm (p?<?0.05), mainly when a calcium pre-rinse was used before the fluoride (p?<?0.05). However, after a sugar challenge, no significant increase in the biofilm fluid fluoride concentration was observed, even in the fluoride-rich biofilms (p?>?0.05). Fluoride-rich biofilms do not release fluoride to the fluid phase during a sugar challenge.  相似文献   
67.
68.
Species inhabiting mountain ecosystems are expected to be particularly vulnerable to environmental change, yet information on their basic ecology is often lacking. Knowledge from field-based empirical studies remains essential to refine our understanding of the impact of current habitat alterations and for the consequential development of meaningful conservation management strategies. This study focuses on a poorly investigated and vulnerable mountain bird species in Europe, the Ring Ouzel Turdus torquatus. Our aim was to identify the species’ key ecological requirements during the crucial period of nestling provisioning in the context of environmental change. We radiotracked and observed Alpine Ring Ouzels in a high-density population, investigating their pattern of foraging habitat selection in 2015 and 2017, and evaluated the transferability of these results over a wider geographical range across the SW Swiss Alps. Foraging birds selected, consistently in space and time, short grass swards (< 10 cm) with interspersed patches of accessible and penetrable soils, at intermediate moisture levels (around 40–65% volumetric water content). In Alpine ecosystems, this microhabitat configuration is typically widespread during the spring snowmelt, but extremely seasonal, with a rapid decrease in its availability over the course of the breeding season. This underlines the high vulnerability of the Ring Ouzel to environmental change: an earlier snowmelt could generate a temporal mismatch between the peak of the breeding effort and optimal foraging conditions; however, abandoning grazing activities on semi-wooded Alpine pastures may further decrease foraging habitat suitability through taller and denser grass swards, and subsequent woody vegetation encroachment. This study provides a mechanistic appraisal of the challenges Ring Ouzels will face in the future, as well as initial guidelines for targeted habitat management within timberline ecotones.  相似文献   
69.
The simultaneous effects of habitat traits and interspecific interactions determine the occurrence and habitat use of wildlife populations. However, little research has been devoted to examining spatial co-occurrence among closely related species while considering the effect of habitat variation and imperfect detectability of species in the field. In this study, we focused on migratory and resident 'wood-warblers' that coexist during the winter in a Neotropical working landscape in southern Mexico to understand if habitat occupancy of resident wood-warblers is influenced by habitat characteristics and by the presence of other species of resident and migratory wood-warblers. For this purpose, we implemented two-species occupancy models, which account for the imperfect detectability of these birds in the field. Our results revealed that habitat occupancy of resident wood-warblers was positively influenced by the presence of other closely related species (both migratory and resident). These positive relationships may be explained by the fact that different species of wood-warblers frequently participate in mixed-species flocks. However, these patterns of species co-occurrence were more evident among resident species than between migratory and resident species, which may be explained by micro-habitat segregation and differences in behaviours between resident and migratory wood-warblers. We also found that some habitat characteristics may mediate the observed patterns of species co-occurrence. Specifically, sites with larger trees were associated with the co-occurrence of some species of resident wood-warblers. In addition, we discuss the possibility that species co-occurrence might be the result of shared preferences for environmental factors that we did not consider. Our study highlights the importance of the interplay between species co-occurrence and habitat traits in determining the presence and habitat use of resident birds in Neotropical working landscapes.  相似文献   
70.
This article introduces a simulation model of rat behavior in the elevated plus-maze, designed through a Decision trees approach using Classification and Regression algorithms. Starting from the analysis of the behavior performed by a sample of 18 Sprague-Dawley male rats, probabilistic rules describing behavioral patterns of the animals were extracted, and were used as the basis of the model computations. The model adequacy was tested by contrasting a simulated sample against an independent sample of real animals. Statistical tests showed that the simulated sample exhibits similar behaviors to those displayed by the real animals, both in terms of the number of entries to open and close arms as well as in terms of the time spent by the animals in those arms. However, the performance of the model in parameters related to the behavioral patterns was partially satisfactory. Given that previous attempts in the literature have neither include this kind of patterns nor the time as a crucial model parameter, the present model offers a suitable alternative for the computational simulation of this paradigm. Compared with antecedent models, the present simulation produced similar or better results in all the considered parameters. Beyond the goal of establish an appropriate simulational model, extracted rules also reveal important regularities associated to the rat behavior previously ignored by other models, i.e. that specific rat behaviors in the elevated plus-maze are time dependent. These and other important considerations to improve the model performance are discussed.  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