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   2234篇
  免费   123篇
  2024年   4篇
  2023年   13篇
  2022年   44篇
  2021年   69篇
  2020年   63篇
  2019年   57篇
  2018年   89篇
  2017年   74篇
  2016年   105篇
  2015年   157篇
  2014年   174篇
  2013年   168篇
  2012年   193篇
  2011年   203篇
  2010年   142篇
  2009年   94篇
  2008年   112篇
  2007年   106篇
  2006年   96篇
  2005年   64篇
  2004年   47篇
  2003年   49篇
  2002年   39篇
  2001年   15篇
  2000年   8篇
  1999年   10篇
  1998年   9篇
  1997年   7篇
  1995年   7篇
  1994年   5篇
  1993年   4篇
  1992年   8篇
  1991年   9篇
  1990年   9篇
  1989年   8篇
  1988年   7篇
  1987年   6篇
  1986年   6篇
  1985年   8篇
  1984年   7篇
  1981年   3篇
  1980年   6篇
  1979年   7篇
  1978年   4篇
  1977年   4篇
  1976年   3篇
  1975年   4篇
  1973年   9篇
  1971年   3篇
  1968年   3篇
排序方式: 共有2357条查询结果,搜索用时 46 毫秒
141.

Objective

The present study aimed at comparing frontal beta power between long-term (LTM) and first-time meditators (FTM), before, during and after a meditation session. We hypothesized that LTM would present lower beta power than FTM due to lower effort of attention and awareness.

Methods

Twenty one participants were recruited, eleven of whom were long-term meditators. The subjects were asked to rest for 4 minutes before and after open monitoring (OM) meditation (40 minutes).

Results

The two-way ANOVA revealed an interaction between the group and moment factors for the Fp1 (p<0.01), F7 (p = 0.01), F3 (p<0.01), Fz (p<0.01), F4 (p<0.01), F8 (p<0.01) electrodes.

Conclusion

We found low power frontal beta activity for LTM during the task and this may be associated with the fact that OM is related to bottom-up pathways that are not present in FTM.

Significance

We hypothesized that the frontal beta power pattern may be a biomarker for LTM. It may also be related to improving an attentive state and to the efficiency of cognitive functions, as well as to the long-term experience with meditation (i.e., life-time experience and frequency of practice).  相似文献   
142.
143.
Investigation of the effect of endogenous and exogenous factors on the diet of animals is necessary for a better understanding of their feeding habits. This approach can provide relevant information on the autoecology of a species and its ecological interactions. We investigated the composition and intraspecific variation in the diet of the marsupial Gracilinanus agilis in areas of dry woodland forests (i.e., cerradão) in the Cerrado of Central Brazil, taking into consideration the availability of prey (arthropods) in the environment. We found insects, spiders, birds, and fruits in the scats of G. agilis. Insects (orders Hymenoptera, Isoptera, Hemiptera, and Coleoptera) and fruits were the most frequently consumed resources. Males fed more heavily on insects than females did, whereas during the warm–wet season (October to April), the reproductive females fed on insects more than the nonreproductive females did. On the other hand, the consumption of fruits and vertebrates did not vary between seasons, sexes, or according to female reproductive condition. Moreover, reproductive females fed more frequently on ants and beetles than nonreproductive females did. We also detected both positive (for Isoptera and Hemiptera) and negative (for Hymenoptera) selection of insects during the cool–dry season, whereas in the warm–wet season, these resources were consumed according to their availability in the environment. Our study revealed that G. agilis is an insectivore–omnivore species, but fruits also are a relevant part of its diet. This marsupial seemed to select their prey qualitatively according to its energy demands and nutritional requirements.  相似文献   
144.
145.
Rare Caenorhabditis elegans males arise when sex chromosome non-disjunction occurs during meiosis in self-fertilizing hermaphrodites. Non-disjunction is a relatively rare event, and males are typically observed at a frequency of less than one in five hundred wild-type animals. Males are required for genetic crosses and phenotypic analysis, yet current methods to generate large numbers of males can be cumbersome. Here, we identify RNAi reagents (dsRNA-expressing bacteria) with improved effectiveness for eliciting males. Specifically, we used RNAi to systematically reduce the expression of over two hundred genes with meiotic chromosome segregation functions, and we identified a set of RNAi reagents that robustly and reproducibly elicited male progeny.  相似文献   
146.

Background

It is increasingly evident that CD8+ T cells are involved in atherosclerosis but the specific subtypes have yet to be defined. CD8+CD25+ T cells exert suppressive effects on immune signaling and modulate experimental autoimmune disorders but their role in atherosclerosis remains to be determined. The phenotype and functional role of CD8+CD25+ T cells in experimental atherosclerosis were investigated in this study.

Methods and results

CD8+CD25+ T cells were observed in atherosclerotic plaques of apoE(−/−) mice fed hypercholesterolemic diet. Characterization by flow cytometric analysis and functional evaluation using a CFSE-based proliferation assays revealed a suppressive phenotype and function of splenic CD8+CD25+ T cells from apoE(−/−) mice. Depletion of CD8+CD25+ from total CD8+ T cells rendered higher cytolytic activity of the remaining CD8+CD25 T cells. Adoptive transfer of CD8+CD25+ T cells into apoE(−/−) mice suppressed the proliferation of splenic CD4+ T cells and significantly reduced atherosclerosis in recipient mice.

Conclusions

Our study has identified an athero-protective role for CD8+CD25+ T cells in experimental atherosclerosis.  相似文献   
147.
Franciscanas Pontoporia blainvillei are small dolphins endemic to Brazil, Uruguay and Argentina. During routine pathologic examinations, chronic pneumonia associated with cholesterol deposits was found in 16.7% of stranded and incidentally bycaught franciscanas (n = 60), and was more frequent in dolphins from the Brazilian state of S?o Paulo (Franciscana Management Area II) and frequently accompanied by splenic lymphoid hyperplasia. It is unclear why these otherwise uncommon lesions were relatively frequent in the studied dolphins, and further research is advised.  相似文献   
148.
Bladder cancer (BC) is the fourth most common cancer in the USA. In Brazil, BC represents 3% of the total existing carcinomas in the population and represents the second highest incidence among urological tumors. The majority of bladder cancer cell lines available were derived from Caucasians and established in the seventies or eighties. Thus, neoplasia development in these cells likely occurred in environment conditions vastly different than today. In the present study, we report the establishment and characterization of three Brazilian bladder cancer cell lines (BexBra1, BexBra2, and BexBra4). These cell lines may be helpful for dissecting the genetic and epigenetic aspects that trigger the progression of BC. Moreover, the development of a Brazilian representative of the disease will allow us to investigate the potential inter-racial differences of malignancy-associated phenotypes in bladder cancer.  相似文献   
149.
A set of 81 new microsatellite markers for Carica papaya L. previously identified by data mining using freely available sequence information from Genbank were tested for polymorphism using 30 germplasm accessions from the Papaya Germplasm Bank (PGM) at Embrapa Mandioca e Fruticultura Tropical (CNPMF) and 18 landraces. The data were used to estimate pairwise genetic distances between the genotypes. A neighbor-joining based dendrogram was used to define clusters and infer possible genetic structuring of the collection. Most microsatellites were polymorphic (73%), with an observed number of alleles per locus ranging from one to eleven. The levels of observed and expected heterozygosity for 51 polymorphic loci varied from 0.00 to 0.85 and from 0.08 to 0.82, averaging 0.19 and 0.59, respectively. Forty-four percent of microsatellites showed polymorphism information content (PIC) higher than 0.50. The compound microsatellites seem to be more informative than dinucleotide and trinucleotide repeats in average alleles per locus and PIC. Among dinucleotides, AG/TC or GA/CT repeat motifs exhibited more informativeness than TA/AT, GT/CA and TG/AC repeat motifs. The neighbor-joining analysis based on shared allele distance could differentiate all the papaya accessions and landraces as well as differences in their genetic structure. This set of markers will be useful for examining parentage, inbreeding and population structure in papaya.  相似文献   
150.
Today's intensive farming practices are known to have affected farmland biodiversity negatively in many different ways. As far as birds are concerned, they are known to have suffered during both summer and winter. Relatively little is known about the effects on birds during migration. We studied the stopover ecology of the Eurasian golden plover Pluvialis apricaria, a species listed in EU Birds Directive, in intensively farmed arable land in southernmost Sweden in the autumns of 2003–2007. We used key ecological variables (length of stay, fat deposition and moult) as fitness proxies to evaluate how the birds manage in this habitat. Eurasian golden plovers were present in large numbers mainly on arable fields from early August to November and radio‐tagged birds were found to stay in the area for up to three months. Adult birds carried out a substantial part of their flight feather moult during their stay. Body mass increased only somewhat during moult, but from the last stages of moult and onwards fuel loads corresponding to 24% above lean body mass (LBM) were accumulated at a rate of 0.5% of LBM per day, before the birds departed. Juveniles arrived later, from mid Sep., and had a similar pattern of fuel deposition. The fact that the birds choose to stay for long periods, moult in the area, and manage to store substantial fuel loads strongly suggests that Eurasian golden plovers do well in this intensively farmed arable land.  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