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   2292篇
  免费   220篇
  国内免费   162篇
  2674篇
  2024年   3篇
  2023年   24篇
  2022年   70篇
  2021年   108篇
  2020年   81篇
  2019年   89篇
  2018年   102篇
  2017年   74篇
  2016年   129篇
  2015年   163篇
  2014年   152篇
  2013年   176篇
  2012年   209篇
  2011年   182篇
  2010年   109篇
  2009年   80篇
  2008年   118篇
  2007年   109篇
  2006年   83篇
  2005年   86篇
  2004年   64篇
  2003年   65篇
  2002年   48篇
  2001年   44篇
  2000年   39篇
  1999年   35篇
  1998年   29篇
  1997年   25篇
  1996年   30篇
  1995年   22篇
  1994年   17篇
  1993年   12篇
  1992年   13篇
  1991年   9篇
  1990年   14篇
  1989年   5篇
  1988年   7篇
  1987年   13篇
  1986年   6篇
  1985年   7篇
  1984年   4篇
  1983年   3篇
  1978年   1篇
  1977年   2篇
  1975年   3篇
  1974年   2篇
  1971年   2篇
  1969年   1篇
  1968年   1篇
  1965年   1篇
排序方式: 共有2674条查询结果,搜索用时 0 毫秒
31.
Lysosomal disturbances may be a contributing factor to Alzheimer's disease. We used novel compounds to test if suppression of the lysosomal protease cathepsin D blocks production of known precursors to neurofibrillary tangles. Partial lysosomal dysfunction was induced in cultured hippocampal slices with a selective inhibitor of cathepsins B and L. This led within 48 h to hyperphosphorylated tau protein fragments recognized by antibodies against human tangles. Potent nonpeptidic cathepsin D inhibitors developed using combinatorial chemistry and structure-based design blocked production of the fragments in a dose-dependent fashion. Threshold was in the submicromolar range, with higher concentrations producing complete suppression. The effects were selective and not accompanied by pathophysiology. Comparable results were obtained with three structurally distinct inhibitors. These results support the hypothesis that cathepsin D links lysosomal dysfunction to the etiology of Alzheimer's disease and suggest a new approach to treating the disease.  相似文献   
32.
天敌对麦长管蚜和麦二叉蚜种群数量影响程度的分析   总被引:6,自引:0,他引:6  
通过对1993~1996年两种麦蚜种群数量及其天敌数量的系统调查,并采用灰色关联分析法,研究各种天敌对两种麦蚜种群数量的影响程度,得出对麦长管蚜种群数量影响最大的是龟纹瓢虫、蚜茧蜂和食蚜蝇;对麦二叉蚜种群数量影响最大的是草间小黑蛛,其次是龟纹瓢虫.  相似文献   
33.
利用造血细胞体外琼脂培养技术,比较了狗的不同来源的GM—CFC增殖、分化性能和辐射敏感性。结果表明,在正常生理条件下循环血中GM—CFC集落产率约为骨髓的1/60;细胞集落开始形成时间较骨髓晚1天,细胞集落随培养时间(前3—5天)增加而增加,其增加速率约为骨髓的20%;辐射敏感性D_0值为0.34Gy,明显低于骨髓中GM—CFC的D_0值(0.82Gy)。造血干细胞动员剂动员后血中GM—CFC数量明显增加,细胞集落增加速率约为骨髓的59%,D_0值为0.72Gy。从而为循环血干细胞移植疗效提供了实验依据。  相似文献   
34.
With intensive training, human can achieve impressive behavioral improvement on various perceptual tasks. This phenomenon, termed perceptual learning, has long been considered as a hallmark of the plasticity of sensory neural system. Not surprisingly, high-level vision, such as object perception, can also be improved by perceptual learning. Here we review recent psychophysical, electrophysiological, and neuroimaging studies investigating the effects of training on object selective cortex, such as monkey inferior temporal cortex and human lateral occipital area. Evidences show that learning leads to an increase in object selectivity at the single neuron level and/or the neuronal population level. These findings indicate that high-level visual cortex in humans is highly plastic and visual experience can strongly shape neural functions of these areas. At the end of the review, we discuss several important future directions in this area.  相似文献   
35.
Tang XQ  Fang HR  Li YJ  Zhou CF  Ren YK  Chen RQ  Wang CY  Hu B 《Neurochemical research》2011,36(11):2176-2185
Asymmetric dimethylarginine (ADMA), an endogenous nitric oxide synthase (NOS) inhibitor, is profoundly protective against 1-methy-4-phenylpyridinium ion (MPP+)-induced neurotoxicity. Reactive oxygen species (ROS) overproduction contributes to the neurotoxicity of MPP+; while hydrogen sulfide (H2S) is a pivotal endogenous antioxidant. This study is to assess the potential role of endogenous H2S in the neuroprotection of ADMA against MPP+-induced toxicity in PC12 cells. We showed that ADMA prevented MPP+-induced inhibition of endogenous H2S generation through inhibiting the down-regulation of cystathionine-β-synthetase (CBS, the major enzyme responsible for endogenous H2S generation in PC12 cells) expression and activity elicited by MPP+. ADMA obviously attenuated MPP+-triggered accumulation of intracellular ROS, dissipation of mitochondrial membrane potential (MMP), release of cytochrome c (Cyt-c), and downregulation of Bcl-2 protein expression in PC12 cells. Inhibition of CBS activity by amino-oxyacetate and CBS silencing with a short hairpin RNA vector targeting rat CBS gene reversed the protective action of ADMA against MPP+-caused cytotoxicity, ROS overproduction, and MMP loss in PC12 cells. These results indicate that the protection of ADMA against MPP+-mediated neurotoxicity involves the melioration of MPP+-induced inhibition of endogenous H2S generation. Our findings suggest that modulation of H2S production provide new therapeutic targets for the treatment of neurodegenerative disease, such as Parkinson’s disease.  相似文献   
36.
Bemisia tabaci has caused significant crop losses in China during the last decade. Recent research has shown that two potentially invasive variants, biotypes B and Q, have been found in several regions of China. Our objective was to determine the biotype status and the distribution of B. tabaci in Shandong province, an important agricultural region of China. Based on mitochondrial DNA markers, both biotypes B and Q were detected, with B being the predominant biotype. The results indicate that the more recently introduced biotype Q has not only been located in China but also has established and spread in some regions.  相似文献   
37.
Self-healing phenomenon was found in the periarterial elastase-induced abdominal aortic aneurysm (AAA) in rabbit. This kind of aneurysm model does not progress and heals spontaneously in the long term, which is quite different from the performance of AAA disease in human. In order to better mimic human AAA and overcome this shortcoming of traditional AAA model in rabbit, we studied the pathogenesis of cerebral aneurysm (CA) model in small animal, which shows an excellent long-term patency and progressive enlargement. We found that hemodynamic conditions, such as turbulence flow, high blood flow, and shear stress, play an important role in the formation and progression of CA. So, we hypothesize that hemodynamic change may also play an essential role in the initiation and progression of rabbit AAA, and self-healing will be overcome if hemodynamic condition changes by coarctation of infra-renal aorta after elastase incubation.  相似文献   
38.
几个生态因子对黄盖鲽的影响   总被引:1,自引:0,他引:1  
毕庶万  郑振虎 《生态学杂志》1989,8(5):19-21,F004
黄盖鲽(Psuedopleuronectes yokohameGünther)是鲽科中的冷温性鱼类,广泛分布在日本、朝鲜及我国的东海、黄海和渤海。地方种群多,洄游范尉小,适应能力强,既是底栖生物食性,又可耐较低的温度,虽非名贵鱼种,却具有一定的经济价值,在开展近海增殖时,不存在越冬和效益外流问题。因此,早在60年代,日本山口县和大分县的学者们,便开始着手该品种的人工孵化和增殖放流试验。80年代初已达年放流2—3cm的稚鱼苗7.0×10~6尾的水平。为了在渤海开展增殖工作,建立综  相似文献   
39.
Patients with age related macular degeneration (AMD) will develop vision loss in the center of the visual field. Reactive oxygen species (ROS)-mediated retinal pigment epithelium (RPE) cell apoptosis is an important contributor of AMD. In this study, we explored the pro-survival effect of α-melanocyte stimulating hormone (α-MSH) on oxidative stressed RPE cells. We found that α-MSH receptor melanocortin 1 receptor (MC1R) was functionally expressed in primary and transformed RPE cells. RPE cells were response to α-MSH stimulation. α-MSH activated Akt/mammalian target of rapamycin (mTOR) and Erk1/2 signalings in RPE cells, which were inhibited by MC1R siRNA knockdown. α-MSH protected RPE cells from hydrogen peroxide (H2O2)-induced apoptosis, an effect that was almost abolished when MC1R was depleted by siRNA. α-MSH-mediated S6K1 activation and pro-survival effect against H2O2 was inhibited by Akt inhibitors (perifosine, MK-2206 and LY294002). Further, mTOR inhibition by rapamycin, or by mTOR siRNA knockdown, diminished α-MSH’s pro-survival effect in RPE cells. Thus, Akt and its downstream mTOR signaling mediates α-MSH-induced survival in RPE cells. In summary, we have identified a new α-MSH–MC1R physiologic pathway that reduces H2O2-induced RPE cell damage, and might minimize the risk of developing AMD.  相似文献   
40.
为探讨细胞因子基因(人IL-2、IL-6)转导对于肿瘤细胞膜MHC抗原及细胞膜糖蛋白表达调控的影响,本文利用脂质体介导的方法,将含人IL-6、IL-2基因的逆转录病毒载体分别导入人乳腺癌细胞系MCF-7细胞中,采用间接免疫荧光染色流式细胞仪测定法,对基因转导的瘤细胞细胞膜糖蛋白及MHC抗原表达进行测定。结果表明经两种基因修饰的MCF-7细胞MHCⅠ型抗原表达均获得增强,此外,基因转导细胞可程度不同地表现出细胞膜多种糖蛋白表达的变化。提示肿瘤细胞膜抗原及糖蛋白表达的改变可能是细胞因子基因转导影响肿瘤细胞免疫原性的重要结构基础。  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