全文获取类型
收费全文 | 11627篇 |
免费 | 45篇 |
国内免费 | 72篇 |
专业分类
系统科学 | 61篇 |
丛书文集 | 29篇 |
教育与普及 | 21篇 |
理论与方法论 | 32篇 |
现状及发展 | 5433篇 |
研究方法 | 586篇 |
综合类 | 5454篇 |
自然研究 | 128篇 |
出版年
2012年 | 205篇 |
2011年 | 300篇 |
2010年 | 76篇 |
2009年 | 74篇 |
2008年 | 201篇 |
2007年 | 242篇 |
2006年 | 246篇 |
2005年 | 233篇 |
2004年 | 202篇 |
2003年 | 195篇 |
2002年 | 247篇 |
2001年 | 435篇 |
2000年 | 427篇 |
1999年 | 288篇 |
1992年 | 236篇 |
1991年 | 194篇 |
1990年 | 215篇 |
1989年 | 191篇 |
1988年 | 201篇 |
1987年 | 219篇 |
1986年 | 190篇 |
1985年 | 252篇 |
1984年 | 226篇 |
1983年 | 171篇 |
1982年 | 176篇 |
1981年 | 180篇 |
1980年 | 170篇 |
1979年 | 403篇 |
1978年 | 340篇 |
1977年 | 250篇 |
1976年 | 290篇 |
1975年 | 260篇 |
1974年 | 272篇 |
1973年 | 224篇 |
1972年 | 244篇 |
1971年 | 308篇 |
1970年 | 374篇 |
1969年 | 254篇 |
1968年 | 307篇 |
1967年 | 292篇 |
1966年 | 249篇 |
1965年 | 178篇 |
1964年 | 99篇 |
1959年 | 88篇 |
1958年 | 162篇 |
1957年 | 100篇 |
1956年 | 91篇 |
1955年 | 85篇 |
1954年 | 77篇 |
1948年 | 64篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
991.
K Steinmeyer R Klocke C Ortland M Gronemeier H Jockusch S Gründer T J Jentsch 《Nature》1991,354(6351):304-308
MYOTONIA (stiffness and impaired relaxation of skeletal muscle) is a symptom of several diseases caused by repetitive firing of action potentials in muscle membranes. Purely myotonic human diseases are dominant myotonia congenita (Thomsen) and recessive generalized myotonia (Becker), whereas myotonic dystrophy is a systemic disease. Muscle hyperexcitability was attributed to defects in sodium channels and/or to a decrease in chloride conductance (in Becker's myotonia and in genetic animal models). Experimental blockage of Cl- conductance (normally 70-85% of resting conductance in muscle) in fact elicits myotonia. ADR mice are a realistic animal model for recessive autosomal myotonia. In addition to Cl- conductance, many other parameters are changed in muscles of homozygous animals. We have now cloned the major mammalian skeletal muscle chloride channel (ClC-1). Here we report that in ADR mice a transposon of the ETn family has inserted into the corresponding gene, destroying its coding potential for several membrane-spanning domains. Together with the lack of recombination between the Clc-1 gene and the adr locus, this strongly suggests a lack of functional chloride channels as the primary cause of mouse myotonia. 相似文献
992.
Summary The double refraction of oxide films on Al conditioned by an electrolytic treatment of the metal in oxalic acid, sulphuric acid and so on (Eloxal-layers) is due only to the disperse structure of the film (Formdoppel-brechung), since the substances the film consists of are optically isotropic.The optical properties depend, excepting the formation conditions, in a very characteristic manner on the crystallographic orientation of the metallic surface. 相似文献
993.
Bleaching of rhodopsin markedly desensitizes the vertebrate visual system during a subsequent period of dark adaptation. Previous studies have indicated an origin of bleaching desensitization in the visual pigment itself, but have not identified the mechanism of action. A candidate for the site at which densensitization is initially expressed is the activation of transducin (formation of T*) on the rod disk membranes; this reaction directly involves rhodopsin in its photoactivated (R*) form and mediates initial amplification of the visual signal (reviewed in refs 7-9). We have analysed the effect of bleaching on the sensitivity of a flash-induced light-scattering signal known to monitor the disk-based amplifier, and which has been established as specifically monitoring transducin activation. We have recorded this signal from functioning retinal rods in situ ('ATR' signal) and find that bleaches inducing a pronounced, sustained loss in rod electrophysiological sensitivity do not alter the sensitivity of the ATR response after correction for reduced quantum catch. Our results indicate that the biochemical gain of the R*----T* transduction stage remains unchanged in the presence of bleached pigment and implicate a subsequent reaction as the first to show a sustained, bleaching-dependent gain reduction. 相似文献
994.
Localization of muscle gene products in nuclear domains 总被引:26,自引:0,他引:26
The localization of gene products is central to the development of cell polarity and pattern specification during embryogenesis. To monitor the distribution of gene products encoded by different nuclei in the same cell in tissue culture, we fused cells of different species to form multinucleated non-dividing heterokaryons. In previous fusion studies, cell-surface antigens and organelles contributed by disparate cell types intermixed within minutes. Using heterokaryons produced with differentiated muscle cells, we demonstrate here that a muscle membrane component, the Golgi apparatus mediating its transport, and a sarcomeric myosin heavy chain are localized in the vicinity of the nuclei responsible for their synthesis. These results provide direct evidence that products (organelle, membrane and structural proteins) derived from individual nuclei can remain localized in myotubes, a finding with implications both for neuromuscular synapse formation and for the carrier state of Duchenne muscular dystrophy. 相似文献
995.
DNA mutagenesis and recombination 总被引:10,自引:0,他引:10
The polymerase chain reaction is used for site-specific mutagenesis and for DNA recombination without any enzymatic reaction in vitro, apart from DNA amplification. 相似文献
996.
Among mycobacteria, Mycobacterium leprae is unique in its ability to oxidize a variety of diphenols to quinones in vitro. What physiologic role o-diphenoloxidase has in the organism remained unknown. Reducing substrates like NADPH, NADH and ascorbic acid reacted with the quinone formed from dopa (3,4-dihydroxyphenylalanine); the substrates were oxidized and the quinone was reduced back to diphenol in the process. Since the quinone undergoes reversible oxidation-reduction, diphenoloxidase might serve as an alternative respiratory mechanism in M. leprae for the utilization of other substrates, as has been reported in plants. 相似文献
997.
Inverse relationship of the durations of adjacent open and shut intervals for C1 and K channels 总被引:3,自引:0,他引:3
Ion channels in cell membranes, whether voltage-dependent or activated by ligands, make repeated transitions among open and shut states during activity. Information about the number of states and the transitional pathways between them can be obtained from the durations of open and shut intervals, as transitions to states of different lifetimes result in intervals of different mean durations. If there is only one open conformation, or state, then the durations of open intervals would be independent of the durations of adjacent shut intervals. On the other hand, if a channel has two or more open states with different mean lifetimes, and if each open state is entered directly from a different shut state with a different mean lifetime, then the open intervals should be related to the adjacent shut intervals. We now report that the durations of adjacent open and shut intervals for both a C1 channel and a large conductance Ca-activated K channel in skeletal muscle are inversely related; shorter open intervals are adjacent to longer shut intervals. These findings indicate that two or more shut states make direct transitions to two or more open states, and suggest that the lifetimes of adjacent open and shut states are inversely related. 相似文献
998.
999.
Colcemid effects on homologue pairing and crossing over during fetal mouse oogenesis 总被引:1,自引:0,他引:1
G. Jagiello W. K. Sung J. -S. Fang M. B. Ducayen 《Cellular and molecular life sciences : CMLS》1989,45(5):482-484
Summary Colcemid was administered to gestational day 13 female mice to test effects on homologue pairing, synapsis and recombination of fetal oogenesis. Pairing abnormalities were detected in pachytene oocytes by light and electron microscopy examination of bivalents and synaptonemal complexes. Reduction of total chiasmata per treated diplotene oocyte (22.74) compared to controls (31.07) was found. 相似文献
1000.
N K Gupta 《上海交通大学学报》2004,38(Z2)
Axial compression experiments on as received aluminum tubes and tubes with wall thickness eccentricity incorporated by off centre machining showed that the eccentricity caused transition of their collapse mode from axisymmetric to diamond. The numerical simulation of the collapse phenomenon was undertaken using a static non-linear Finite Element Analysis in ANSYS, and results are found to compare well with the experimental load compression and energy absorption responses for axisymmetric concertina and non-axisymmetric diamond modes. Having validated the numerical model with experiments, it has been used to undertake a systematic study of the load - compression characteristics and collapse mode transitions of the tubes with varying eccentricities in wall thickness and boundary conditions, as well as the material properties. 相似文献