全文获取类型
收费全文 | 474篇 |
免费 | 39篇 |
专业分类
513篇 |
出版年
2023年 | 1篇 |
2022年 | 13篇 |
2021年 | 17篇 |
2020年 | 10篇 |
2019年 | 11篇 |
2018年 | 10篇 |
2017年 | 16篇 |
2016年 | 11篇 |
2015年 | 22篇 |
2014年 | 34篇 |
2013年 | 35篇 |
2012年 | 47篇 |
2011年 | 41篇 |
2010年 | 21篇 |
2009年 | 23篇 |
2008年 | 23篇 |
2007年 | 33篇 |
2006年 | 22篇 |
2005年 | 26篇 |
2004年 | 18篇 |
2003年 | 14篇 |
2002年 | 13篇 |
2001年 | 9篇 |
2000年 | 9篇 |
1999年 | 6篇 |
1998年 | 1篇 |
1997年 | 3篇 |
1996年 | 1篇 |
1995年 | 2篇 |
1994年 | 2篇 |
1993年 | 1篇 |
1991年 | 1篇 |
1989年 | 2篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1973年 | 1篇 |
1972年 | 1篇 |
排序方式: 共有513条查询结果,搜索用时 15 毫秒
61.
62.
K. E. van de Wolfshaar H. Middelkoop E. Addink H. V. Winter L. A. J. Nagelkerke 《Ecosystems》2011,14(6):920-934
River-floodplain systems are amongst the most productive—but often severely impacted—aquatic systems worldwide. We explored
the ecological response of fish to flow regime in a large river-floodplain system by studying the relationships between (1)
discharge and inundated floodplain area, with a focus on spatial and temporal patterns in floodplain lake connectivity, and
(2) flood volume and fisheries catch. Our results demonstrate a non-linear relationship between discharge and floodplain inundation
with considerable hysteresis due to differences in inundation and drainage rate. Inundation extent was mostly determined by
flood volume, not peak discharge. We found that the more isolated lakes (that is, lakes with a shorter connection duration
to the river) are located at higher local elevation and at larger hydrological distance from the main rivers: geographical
distance to the river appears a poor predictor of lake isolation. Although year-to-year fish catches in the floodplain were
significantly larger with larger flood volumes in the floodplain, they were not in the main river, suggesting that mechanisms
that increase catch, such as increased floodplain access or increased somatic growth, are stimulated by flooding in the floodplain,
but not in the river. Fish species that profit from flooding belong to different feeding guilds, suggesting that all trophic
levels may benefit from flooding. We found indications that the ecological functioning of floodplains is not limited to its
temporary availability as habitat. Refugia can be present within the floodplain itself, which should be considered in the
management of large rivers and their floodplain. 相似文献
63.
Similar cation channels mediate protection from cerebellar exitotoxicity by exercise and inheritance
Ben-Ari S Ofek K Barbash S Meiri H Kovalev E Greenberg DS Soreq H Shoham S 《Journal of cellular and molecular medicine》2012,16(3):555-568
Exercise and inherited factors both affect recovery from stroke and head injury, but the underlying mechanisms and interconnections between them are yet unknown. Here, we report that similar cation channels mediate the protective effect of exercise and specific genetic background in a kainate injection model of cerebellar stroke. Microinjection to the cerebellum of the glutamatergic agonist, kainate, creates glutamatergic excito\xE2\x80\x90toxicity characteristic of focal stroke, head injury or alcoholism. Inherited protection and prior exercise were both accompanied by higher cerebellar expression levels of the Kir6.1 ATP-dependent potassium channel in adjacent Bergmann glia, and voltage-gated KVbeta2 and cyclic nucleotide-gated cation HCN1 channels in basket cells. Sedentary FVB/N and exercised C57BL/6 mice both expressed higher levels of these cation channels compared to sedentary C57BL/6 mice, and were both found to be less sensitive to glutamate toxicity. Moreover, blocking ATP-dependent potassium channels with Glibenclamide enhanced kainate-induced cell death in cerebellar slices from the resilient sedentary FVB/N mice. Furthermore, exercise increased the number of acetylcholinesterase-positive fibres in the molecular layer, reduced cerebellar cytokine levels and suppressed serum acetylcholinesterase activity, suggesting anti-inflammatory protection by enhanced cholinergic signalling. Our findings demonstrate for the first time that routine exercise and specific genetic backgrounds confer protection from cerebellar glutamatergic damages by similar molecular mechanisms, including elevated expression of cation channels. In addition, our findings highlight the involvement of the cholinergic anti-inflammatory pathway in insult-inducible cerebellar processes. These mechanisms are likely to play similar roles in other brain regions and injuries as well, opening new venues for targeted research efforts. 相似文献
64.
65.
Monitoring of three-dimensional floodplain vegetation structure is essential for ecological studies, as well as for hydrodynamic
modelling of rivers. Height and density of submerged vegetation and density of emergent vegetation are the key characteristics
from which roughness parameters in hydraulic models are derived. Airborne laser scanning is a technique with broad applications
in vegetation structure mapping, which therefore may be a promising tool in monitoring floodplain vegetation for river management
applications. This paper first provides an introduction to the laser scanning technique, and reviews previous studies on the
extraction of vegetation height and density of forests, low vegetation and meadows or unvegetated areas. Reliable predictions
using laser scan data have been reported for forest height (R2=0.64–0.98), parameters related to forest density, such as stem number, stem diameter, biomass, timber volume or basal area
(R2=0.42–0.93), and herbaceous vegetation height (summer condition; R2=0.75–0.89). No empirical relations have been reported on density of herbaceous vegetation. Laser data of meadows and unvegetated
areas show too much noise to predict vegetation structure correctly. In a case study for the lower Rhine river, the potential
of laser scan mapping of vegetation structure was further explored for winter conditions. Three laser-derived metrics that
are often reported in the literature have been applied to characterize local vertical distributions of laser reflections.
The laser data clearly show the large structural differences both between and within vegetation units that currently are the
basis of floodplain vegetation and roughness mapping. The results indicate that airborne laser scanning is a promising technique
for extraction of 3D-structure of floodplain vegetation in winter, except for meadows and unvegetated areas. 相似文献
66.
The exquisite immunological specificity of cytotoxic T lymphocytes-target cell (CTL-TC) conjugation and lysis is overridden in the presence of certain plant lectins. The role of concanavalin A (Con A) in lectin-dependent, CTL-mediated cytolysis (LDCC) has been investigated. Papain-treated TC are refractory to LDCC, but regain susceptibility following a 3-hr incubation without the enzyme. Papain-treated TC allowed to recover in the presence of tunicamycin (TM; an inhibitor of N-linked glycosylation), are totally refractory to LDCC. Refractoriness of TM-treated TC to LDCC is not due to an overall resistance to lysis or to lack of Con A binding, as these cells can be lysed by specifically sensitized CTL or by H-2 antibody and complement and display a sufficiently high Con A-binding capacity, indistinguishable from intact TC, probably through O-linked, cell-surface glycosyl residues. The finding that TC (TM-treated) capable of binding normal Con A quantities cannot, however, engage in lectin-dependent CTL-TC conjugation and lysis indicates that Con A must react selectively with a specific TC-surface component(s), thereby rendering the TC recognizable by effector CTL, rather than by simply bridging ("glueing") CTL and TC. Affinity absorption and elution from Sepharose-Con A beads as well as specific immunoprecipitations by antibodies against cell surface determinants, have shown effective Con A binding to TC surface components of molecular weights corresponding to 45-kDa product of the H-2K and D MHC genes and, possibly, to a 30-kDa component. Antibodies against MHC proteins but not against non-MHC surface proteins of the TC have produced effective inhibition of LDCC. This and previous investigations show that in nonspecific LDCC as in specific CTL-mediated lysis, TC-MHC determinants are involved in signaling TC recognition and lysis. 相似文献
67.
Xiaokai Li Siyuan Feng Yi Luo Keren Long Zhenghao Lin Jideng Ma Anan Jiang Long Jin Qianzi Tang Mingzhou Li Xun Wang 《In vitro cellular & developmental biology. Animal》2018,54(2):99-110
Macrophage-derived foam cells were one of the hallmarks of atherosclerosis, and microRNAs played an important role in the formation of foam cells. In order to explore the roles of miRNA in the formation of foam cells, we investigated miRNA expression profiles in foam cells through high-throughput sequencing technology. A total of 84 miRNAs were differentially expressed between RAW 264.7 macrophages and foam cells induced by ox-LDL. Thirty miRNAs were upregulated and 54 miRNAs were downregulated. GO terms and KEGG pathways analysis revealed that the target genes of most of DE miRNAs were mainly enriched in “cell differentiation,” “endocytosis,” “MAPK signaling pathway,” and “FoxO signaling pathway.” The target genes of some DE miRNAs were enriched in “Insulin signaling pathway,” “Hippo signaling pathway,” “TNF signaling pathway,” “NF-kappa B signaling pathway,” and “cell death.” Using bioinformatics analyses and dual-luciferase reporter assays, we found that miR-28a-5p and miR-30c-1-3p directly inhibited LRAD3 and LOX-1 mRNA expression through targeting the 3’UTR of LRAD3 and LOX-1 mRNA, respectively. Our study indicates that miRNAs are extensively involved in the formation of foam cells, and provides a valuable resource for further study the role of miRNAs in atherosclerosis. 相似文献
68.
Photosynthesis Research - The acclimation of cyanobacterial photosynthetic apparatus to iron deficiency is crucial for their performance under limiting conditions. In many cyanobacterial species,... 相似文献
69.
Psb29, a conserved 22-kD protein, functions in the biogenesis of Photosystem II complexes in Synechocystis and Arabidopsis 下载免费PDF全文
Photosystem II (PSII), the enzyme responsible for photosynthetic oxygen evolution, is a rapidly turned over membrane protein complex. However, the factors that regulate biogenesis of PSII are poorly defined. Previous proteomic analysis of the PSII preparations from the cyanobacterium Synechocystis sp PCC 6803 detected a novel protein, Psb29 (Sll1414), homologs of which are found in all cyanobacteria and vascular plants with sequenced genomes. Deletion of psb29 in Synechocystis 6803 results in slower growth rates under high light intensities, increased light sensitivity, and lower PSII efficiency, without affecting the PSII core electron transfer activities. A T-DNA insertion line in the PSB29 gene in Arabidopsis thaliana displays a phenotype similar to that of the Synechocystis mutant. This plant mutant grows slowly and exhibits variegated leaves, and its PSII activity is light sensitive. Low temperature fluorescence emission spectroscopy of both cyanobacterial and plant mutants shows an increase in the proportion of uncoupled proximal antennae in PSII as a function of increasing growth light intensities. The similar phenotypes observed in both plant and cyanobacterial mutants demonstrate that the function of Psb29 has been conserved throughout the evolution of oxygenic photosynthetic organisms and suggest a role for the Psb29 protein in the biogenesis of PSII. 相似文献
70.
Karin van der Hiele Dennis van Gorp Rob Ruimschotel No?lle Kamminga Leo Visser Huub Middelkoop 《PloS one》2015,10(6)
The majority of patients with Multiple Sclerosis (MS) are unable to retain employment within 10 years from disease onset. Executive abilities, such as planning, working memory, attention, problem solving, inhibition and mental flexibility may have a direct impact on the ability to maintain a job. This study investigated differences in subjective and objective executive abilities between relapsing-remitting MS patients with and without a paid job. We included 55 relapsing-remitting MS patients from a community-based sample (47 females; mean age: 47 years; 36% employed). Patients underwent neurological, cognitive and psychological assessments at their homes, including an extensive executive test battery. We found that unemployed patients had a longer disease duration (t(53)=2.76, p=0.008) and reported more organising and planning problems (χ2(1)=6.3, p=0.012), higher distractibility (Kendall’s tau-b= -0.24, p=0.03) and more cognitive fatigue (U=205.0, p=0.028, r=-0.30) than employed patients. Unemployed patients completed slightly less categories on the Wisconsin Card Sorting Test (U=243.5, p=0.042, r=-0.28). Possible influential factors such as age, educational level, physical functioning, depression and anxiety did not differ between groups. In conclusion, while relapsing-remitting MS patients without a paid job reported more executive problems and cognitive fatigue than patients with a paid job, little differences were found in objective executive abilities. Further research is needed to examine possible causal relations. 相似文献