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   3440篇
  免费   245篇
  3685篇
  2023年   18篇
  2022年   66篇
  2021年   101篇
  2020年   46篇
  2019年   61篇
  2018年   93篇
  2017年   74篇
  2016年   121篇
  2015年   203篇
  2014年   214篇
  2013年   269篇
  2012年   311篇
  2011年   301篇
  2010年   195篇
  2009年   158篇
  2008年   229篇
  2007年   221篇
  2006年   184篇
  2005年   171篇
  2004年   158篇
  2003年   158篇
  2002年   131篇
  2001年   20篇
  2000年   14篇
  1999年   12篇
  1998年   26篇
  1997年   16篇
  1996年   18篇
  1995年   6篇
  1994年   12篇
  1993年   9篇
  1992年   7篇
  1991年   4篇
  1990年   11篇
  1989年   6篇
  1988年   3篇
  1987年   4篇
  1986年   3篇
  1985年   2篇
  1984年   6篇
  1982年   5篇
  1981年   4篇
  1980年   6篇
  1977年   1篇
  1976年   1篇
  1975年   1篇
  1974年   1篇
  1973年   1篇
  1966年   1篇
  1964年   1篇
排序方式: 共有3685条查询结果,搜索用时 15 毫秒
81.
82.
Abstract

A. Kerner is correctly the author of the name Iris cengialti previously ascribed to Ambrosi, and consequently the type of the species is here selected, as lectotype, among Kerner's original material kept in WU.  相似文献   
83.
Cadmium (Cd) is an environmental contaminant, highly toxic to humans. This biologically non-essential element accumulates in the body, especially in the kidney, liver, lung and brain and can induce several toxic effects, depending on the concentration and the exposure time. Cd has been linked to Alzheimer’s disease (AD) as a probable risk factor, as it shows higher concentrations in brain tissues of AD patients than in healthy people, its implication in the formation of neurofibrillary tangles and in the aggregation process of amyloid beta peptides (AβPs). AβPs seem to have toxic properties, particularly in their aggregated state; insoluble AβP forms, such as small and large aggregates, protofibrils and fibrils, appear to be implicated in the pathogenesis of AD. In our study, we have evaluated the effect of Cd, at different concentrations, both on the AβP1–42 ion channel incorporated in a planar lipid membrane made up of phosphatidylcholine containing 30 % cholesterol and on the secondary structure of AβP1–42 in aqueous environment. Cadmium is able to interact with the AβP1–42 peptide by acting on the channel incorporated into the membrane as well as on the peptide in solution, both decreasing AβP1–42 channel frequency and in solution forming large and amorphous aggregates prone to precipitate. These experimental observations suggesting a toxic role for Cd strengthen the hypothesis that Cd may interact directly with AβPs and may be a risk factor in AD.  相似文献   
84.
The present study was designed to investigate the brain functional architecture that subserves visuo-spatial and motor processing in highly skilled individuals. By using functional magnetic resonance imaging (fMRI), we measured brain activity while eleven Formula racing-car drivers and eleven ‘naïve’ volunteers performed a motor reaction and a visuo-spatial task. Tasks were set at a relatively low level of difficulty such to ensure a similar performance in the two groups and thus avoid any potential confounding effects on brain activity due to discrepancies in task execution. The brain functional organization was analyzed in terms of regional brain response, inter-regional interactions and blood oxygen level dependent (BOLD) signal variability. While performance levels were equal in the two groups, as compared to naïve drivers, professional drivers showed a smaller volume recruitment of task-related regions, stronger connections among task-related areas, and an increased information integration as reflected by a higher signal temporal variability. In conclusion, our results demonstrate that, as compared to naïve subjects, the brain functional architecture sustaining visuo-motor processing in professional racing-car drivers, trained to perform at the highest levels under extremely demanding conditions, undergoes both ‘quantitative’ and ‘qualitative’ modifications that are evident even when the brain is engaged in relatively simple, non-demanding tasks. These results provide novel evidence in favor of an increased ‘neural efficiency’ in the brain of highly skilled individuals.  相似文献   
85.
Despite being the main insect pest on soybean crops in the Americas, very few studies have approached the general biology of the lepidopteran Anticarsia gemmatalis and there is a paucity of studies with embryo formation and yolk mobilization in this species. In the present work, we identified an acid phosphatase activity in the eggs of A. gemmatalis (agAP) that we further characterized by means of biochemistry and cell biology experiments. By testing several candidate substrates, this enzyme proved chiefly active with phosphotyrosine; in vitro assays suggested a link between agAP activity and dephosphorylation of egg yolk phosphotyrosine. We also detected strong activity with endogenous and exogenous short chain polyphosphates (PolyP), which are polymers of phosphate residues involved in a number of physiological processes. Both agAP activity and PolyP were shown to initially concentrate in small vesicles clearly distinct from typically larger yolk granules, suggesting subcellular compartmentalization. As PolyP has been implicated in inhibition of yolk proteases, we performed in vitro enzymatic assays with a cysteine protease to test whether it would be inhibited by PolyP. This cysteine protease is prominent in Anticarsia egg homogenates. Accordingly, short chain PolyP was a potent inhibitor of cysteine protease. We thereby suggest that PolyP hydrolysis by agAP is a triggering mechanism of yolk mobilization in A. gemmatalis.  相似文献   
86.
Synaptic transmission and plasticity mediated by NMDA receptors (NMDARs) could modulate the severity of multiple sclerosis (MS). Here the role of NMDARs in MS was first explored in 691 subjects carrying specific allelic variants of the NR1 subunit gene or of the NR2B subunit gene of this glutamate receptor. The analysis was replicated for significant SNPs in an independent sample of 1548 MS subjects. The C allele of rs4880213 was found to be associated with reduced NMDAR-mediated cortical excitability, and with increased probability of having more disability than the CT/TT MS subjects. MS severity was higher in the CC group among relapsing-remitting MS (RR-MS) patients, while primary progressive MS (PP-MS) subjects homozygous for the T allele had more pronounced clinical worsening. Mean time to first relapse, but not to an active MRI scan, was lower in the CC group of RR-MS patients, and the number of subjects with two or more clinical relapses in the first two years of the disease was higher in CC compared to CT/TT group. Furthermore, the percentage of relapses associated with residual disability was lower in subjects carrying the T allele. Lesion load at the MRI was conversely unaffected by the C or T allele of this SNP in RR-MS patients. Axonal and neuronal degeneration at the optical coherence tomography was more severe in the TT group of PP-MS patients, while reduced retinal nerve fiber thickness had less consequences on visual acuity in RR-MS patients bearing the T allele. Finally, the T allele was associated with preserved cognitive abilities at the Rao’s brief repeatable neuropsychological battery in RR-MS. Signaling through glutamate NMDARs enhances both compensatory synaptic plasticity and excitotoxic neurodegeneration, impacting in opposite ways on RR-MS and PP-MS pathophysiological mechanisms.  相似文献   
87.
The present study aimed to ascertain the influence of crude protein (CP) digestibility in the rumen on the quantity and quality of milk production and reproductive performance, blood (BU) and milk (MU) urea, haematological profile and vaginal mucus urea, ammonia and potassium of buffalo cows. Lactating buffaloes (n = 84), 60 days in milk, were randomly subdivided into Group C (control, n = 42) and Group T (fed a diet supplemented with Aspergillus oryzae, n = 42). In three fistulated buffalo, the diet supplemented with Aspergillus oryzae showed a decrease (P < 0.01) in protein digestibility in the rumen (79.3 vs. 45.9%). No differences were registered in productive performance. Nine buffaloes not in oestrus during the dietary treatment (Groups T1 and C1), 30 days in milk, were used to study the haematological profile and to determine milk urea and ammonia in the vaginal mucus. The animals in Group T1 had higher ammonia values in the blood (P < 0.01) but not in the vaginal mucus than Group C1. A relationship was found between MU and BU. MU was influenced by CP intake and dry matter intake. No differences between the treatments were observed in reproductive performance and the conception rate and calving interval were 37.9% and 41.4% (90 trial-day) and 449 and 419 days respectively in Groups T and C. Reproductive performance was not influenced by high levels of BU nor by blood ammonia levels, although the latter were higher in the group fed the diet supplemented with Aspergillus oryzae.  相似文献   
88.
89.
90.
The present study aims to characterize and compare the diet of bigeye and yellowfin tunas caught on aggregated schools in the western equatorial Atlantic Ocean. The samples were collected from January 2011 to June 2016. The tunas were measured on board and the stomachs were removed after evisceration. The stomachs were analyzed regarding their Index of Fullness and the importance of each prey in the diet was estimated by the Index of Relative Importance (IRI). The diet overlap was assessed by the Morisita‐Horn's Index, Non‐Metric Multidimensional Scale (NMDS), and Analysis of Similarity (ANOSIM). The feeding strategy was determined by the Costello's Diagram. The 195 bigeye and 212 yellowfin tunas ranged in fork length from 51 to 137 cm and 43 to 174 cm, respectively. The diet of bigeye tuna was composed of 10 families of fish, three cephalopod families, and four crustacean orders. The diet of yellowfin tuna was composed of 11 families of fish, three cephalopod families, and three crustacean orders. The yellowfin tuna seems to feed upon more abundant prey species near the surface like flying fish, which have the concentration enhanced by the light attractors on the boat, and occasionally on other prey from deeper habitats like lanternfish, squids, and pomfret. Bigeye tuna feed mainly at prey that commonly occurs in deeper habitats like squids, drift fish, lanternfish, and pomfret.  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