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   410篇
  免费   32篇
  2022年   4篇
  2021年   2篇
  2020年   5篇
  2019年   5篇
  2018年   9篇
  2017年   6篇
  2016年   12篇
  2015年   16篇
  2014年   9篇
  2013年   25篇
  2012年   31篇
  2011年   54篇
  2010年   33篇
  2009年   17篇
  2008年   23篇
  2007年   17篇
  2006年   15篇
  2005年   13篇
  2004年   12篇
  2003年   8篇
  2002年   7篇
  2001年   9篇
  2000年   13篇
  1999年   8篇
  1998年   3篇
  1997年   2篇
  1995年   6篇
  1993年   2篇
  1992年   5篇
  1991年   5篇
  1990年   8篇
  1989年   5篇
  1988年   2篇
  1987年   4篇
  1986年   3篇
  1985年   6篇
  1984年   7篇
  1983年   5篇
  1982年   3篇
  1981年   1篇
  1980年   2篇
  1977年   1篇
  1976年   1篇
  1973年   1篇
  1971年   1篇
  1970年   2篇
  1969年   1篇
  1968年   4篇
  1966年   6篇
  1965年   2篇
排序方式: 共有442条查询结果,搜索用时 500 毫秒
111.
A clinical trial was undertaken to evaluate the efficacy of hypertonic solution (5% NaCl) in patients who have bullous keratopathy (BK). The aim of the study was to define the stage of the disease and the thickness of cornea in micrometers, which would be the threshold for therapeutic approach. This was a prospective study on 70 eyes of 55 patients. Patients were divided in two groups at the beginning of the study. The first group (n=33 eyes) included patients with initial stage of BK: only stromal component of corneal oedema was present. The second group (n=37 eyes) included patients with advanced stage of BK: the epithelial component of the disease with bullae on the corneal surface had already developed. Visual acuity, central and peripheral thickness of cornea and morphology of the disease was recorded before therapy, 7 days and 4 weeks after administration of hypertonic solution. Our results shown that the efficacy of hypertonic solution correlates with the severity of clinical picture in patients with BK. When 5% NaCl hypertonic solution was applied in the early stage of the disease, when only stromal component of corneal oedema was presented, visual acuity and pachymetry readings were significantly improved. The threshold pachymerty measurement of corneal thickness justifying the application of hypertonic solution was 613-694 microm (in the central corneal area), and 633-728 microm (at corneal periphery). It seems reasonable to apply hypertonic solution to the patients who have BK and whose pachymetric values are below mentioned range. In terminal stages of BK, when superficial bullae (epithelial component) had already developed, treatment with NaCl was not effective and patients had to be submitted to penetrating keratoplasty.  相似文献   
112.
Activation of postsynaptic alpha-calcium/calmodulin-dependent protein kinase II (alphaCaMKII) by calcium influx is a prerequisite for the induction of long-term potentiation (LTP) at most excitatory synapses in the hippocampus and cortex. Here we show that postsynaptic LTP is unaffected at parallel fiber-Purkinje cell synapses in the cerebellum of alphaCaMKII(-/-) mice. In contrast, a long-term depression (LTD) protocol resulted in only transient depression in juvenile alphaCaMKII(-/-) mutants and in robust potentiation in adult mutants. This suggests that the function of alphaCaMKII in parallel fiber-Purkinje cell plasticity is opposite to its function at excitatory hippocampal and cortical synapses. Furthermore, alphaCaMKII(-/-) mice showed impaired gain-increase adaptation of both the vestibular ocular reflex and optokinetic reflex. Since Purkinje cells are the only cells in the cerebellum that express alphaCaMKII, our data suggest that an impairment of parallel fiber LTD, while leaving LTP intact, is sufficient to disrupt this form of cerebellar learning.  相似文献   
113.
In Arabidopsis, different combinations of ABC organ identity proteins interact in the presence of SEPALLATA (SEP) proteins to regulate floral organ differentiation. Ectopic expression of SEP3 in combination with class A and B or B and C genes is sufficient to homeotically convert vegetative leaves into petal-like organs and bracts into stamen-like structures, respectively. Recently, it has been shown that the three MADS-box genes SEEDSTICK (STK), SHATTERPROOF1 (SHP1) and SHP2 act redundantly to control ovule identity. Protein interaction assays performed in yeast in combination with genetic studies demonstrated that these MADS-box factors only interact in the presence of SEP proteins to form complexes that determine ovule differentiation. Here, we address the question whether the ectopic co-expression of ovule identity proteins is sufficient to induce the homeotic conversion of vegetative leaves into carpel-like structures bearing ovules. We present the phenotypic characterization of Arabidopsis plants that ectopically express ovule identity factors under the regulation of the ethanol inducible gene expression system. These experiments indicate that the ectopic co-expression of SEP3 and SHP1 and/or STK is probably not sufficient to homeotically transform vegetative tissues into carpels with ovules. However, comparing the phenotypes obtained by ectopic expression of STK and/or SHP1 with or without SEP3 shows that co-expression of factors that are able to form complexes in yeast cause more extreme homeotic transformations, confirming the functional role of these complexes in vivo.  相似文献   
114.
Background, aim, and scope  Many recent studies on waste management have described in detail the potential impacts of recycling and final treatment of municipal waste. In public debates, the attention has also been focused on the choice of final disposal technologies (e.g. landfilling vs. incineration). However, a comprehensive assessment of the impacts of waste collection and transport was still lacking. In the present study, we use LCA to evaluate the potential impact of the provincial waste management plan of Varese (northern Italy). Particular attention is devoted to the estimation of environmental impacts generated during waste transport. Materials and methods  A detailed Life Cycle Inventory was built for the transportation phase, based on primary data collected by interviewing the agencies involved in waste collection. To model the recycling and final disposal phase we relied on the BUWAL 250 database. Impacts were evaluated with the Eco-Indicator 99 method in its egalitarian formulation. Results  The results of our analysis reveal that the major potential impacts of the plan are associated with waste collection and transport. These impacts are partially compensated by reduced resource consumption through recycling and energy recovery through incineration. Discussion  The outputs of the LCIA were compared with those obtained by using other ecoindicators (Eco-Indicator 99 hierarchist and individualist, CML2, EPS2000). Although not comparable on a quantitative basis, they are qualitatively consistent. Conclusions  Neglecting the effects of collection and transport might result in a severe underestimation of the environmental impacts of a waste management system, especially as refers to depletion of fossil fuels, emission of respiratory inorganics and climate change. To reduce the environmental impact of waste management systems, an accurate optimisation of waste transport is required. Recommendations and perspectives  Effective waste management planning requires the explicit inclusion of waste collection and transport when comparing alternative management policies.  相似文献   
115.
116.
CK2 is a highly conserved protein kinase controlling different cellular processes. It shows a higher activity in proliferating mammalian cells, in various types of cancer cell lines and tumors. The findings presented herein provide the first evidence of an in vivo modulation of CK2 activity, dependent on growth rate, in Saccharomyces cerevisiae. In fact, CK2 activity, assayed on nuclear extracts, is shown to increase in exponential growing batch cultures at faster growth rate, while localization of catalytic and regulatory subunits is not nutritionally modulated. Differences in intracellular CK2 activity of glucose- and ethanol-grown cells appear to depend on both increase in molecule number and kcat. Also in chemostat cultures nuclear CK2 activity is higher in faster growing cells providing the first unequivocal demonstration that growth rate itself can affect CK2 activity in a eukaryotic organism.  相似文献   
117.
Diagnosability is the ability to discriminate between similar taxa, including sibling or cryptic taxa. We have developed an explicit test of diagnosability, using the Sylvia cantillans species complex as a model, which compares an identification based on phenotype with that based on genotype. Individual warblers sampled during their migration in central Italy were first identified to the (sub)species level using putatively diagnostic plumage traits. Nucleotide sequences of a (598-bp) fragment of the mitochondrial (mt)DNA cytochrome b were then used to assign each individual to distinct phylogenetic clades, as determined by reference haplotypes that had been sequenced in breeding individuals. This resulted in the construction of clearly distinct clades corresponding to known taxa of the complex. The new haplotypes were assigned to one of the previously identified groups (corresponding to three different taxa); no sample was assigned outside of them. In contrast, when plumage traits were used to assign the birds into distinct phylogenetic clades, 11 of 58 birds were classified as ‘uncertain/intermediate’ among two taxa, while five were classified differently with the two methods. A perfect agreement between the two methods was found for only for one taxon (Sylvia subalpina, syn. S. moltonii). For the other two taxa of the complex, diagnosability is therefore not guaranteed, and their field identification by hand should be carefully addressed. We provide here an example of an explicit test for establishing the diagnosability of taxa in which two or more ‘markers’ can be used for determining discordant identification and/or unambiguous diagnosability. Our results outline the importance of considering different features for taxa diagnosis and illustrate the weakness of visual appearance-based identification (currently widely used for taxa determination) in our study complex.  相似文献   
118.
Veillonella parvula (Veillon and Zuber 1898) Prévot 1933 is the type species of the genus Veillonella in the family Veillonellaceae within the order Clostridiales. The species V. parvula is of interest because it is frequently isolated from dental plaque in the human oral cavity and can cause opportunistic infections. The species is strictly anaerobic and grows as small cocci which usually occur in pairs. Veillonellae are characterized by their unusual metabolism which is centered on the activity of the enzyme methylmalonyl-CoA decarboxylase. Strain Te3(T), the type strain of the species, was isolated from the human intestinal tract. Here we describe the features of this organism, together with the complete genome sequence, and annotation. This is the first complete genome sequence of a member of the large clostridial family Veillonellaceae, and the 2,132,142 bp long single replicon genome with its 1,859 protein-coding and 61 RNA genes is part of the Genomic Encyclopedia of Bacteria and Archaea project.  相似文献   
119.
Desulfarculus baarsii (Widdel 1981) Kuever et al. 2006 is the type and only species of the genus Desulfarculus, which represents the family Desulfarculaceae and the order Desulfarculales. This species is a mesophilic sulfate-reducing bacterium with the capability to oxidize acetate and fatty acids of up to 18 carbon atoms completely to CO(2). The acetyl-CoA/CODH (Wood-Ljungdahl) pathway is used by this species for the complete oxidation of carbon sources and autotrophic growth on formate. The type strain 2st14(T) was isolated from a ditch sediment collected near the University of Konstanz, Germany. This is the first completed genome sequence of a member of the order Desulfarculales. The 3,655,731 bp long single replicon genome with its 3,303 protein-coding and 52 RNA genes is a part of the Genomic Encyclopedia of Bacteria and Archaea project.  相似文献   
120.

Background

Many situations involving animal communication are dominated by recurring, stereotyped signals. How do receivers optimally distinguish between frequently recurring signals and novel ones? Cortical auditory systems are known to be pre-attentively sensitive to short-term delivery statistics of artificial stimuli, but it is unknown if this phenomenon extends to the level of behaviorally relevant delivery patterns, such as those used during communication.

Methodology/Principal Findings

We recorded and analyzed complete auditory scenes of spontaneously communicating zebra finch (Taeniopygia guttata) pairs over a week-long period, and show that they can produce tens of thousands of short-range contact calls per day. Individual calls recur at time scales (median interval 1.5 s) matching those at which mammalian sensory systems are sensitive to recent stimulus history. Next, we presented to anesthetized birds sequences of frequently recurring calls interspersed with rare ones, and recorded, in parallel, action and local field potential responses in the medio-caudal auditory forebrain at 32 unique sites. Variation in call recurrence rate over natural ranges leads to widespread and significant modulation in strength of neural responses. Such modulation is highly call-specific in secondary auditory areas, but not in the main thalamo-recipient, primary auditory area.

Conclusions/Significance

Our results support the hypothesis that pre-attentive neural sensitivity to short-term stimulus recurrence is involved in the analysis of auditory scenes at the level of delivery patterns of meaningful sounds. This may enable birds to efficiently and automatically distinguish frequently recurring vocalizations from other events in their auditory scene.  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