全文获取类型
收费全文 | 4326篇 |
免费 | 460篇 |
国内免费 | 3篇 |
出版年
2021年 | 56篇 |
2020年 | 28篇 |
2019年 | 44篇 |
2018年 | 60篇 |
2017年 | 42篇 |
2016年 | 100篇 |
2015年 | 135篇 |
2014年 | 159篇 |
2013年 | 236篇 |
2012年 | 247篇 |
2011年 | 241篇 |
2010年 | 156篇 |
2009年 | 146篇 |
2008年 | 230篇 |
2007年 | 202篇 |
2006年 | 221篇 |
2005年 | 201篇 |
2004年 | 185篇 |
2003年 | 169篇 |
2002年 | 167篇 |
2001年 | 70篇 |
2000年 | 57篇 |
1999年 | 72篇 |
1998年 | 59篇 |
1997年 | 40篇 |
1996年 | 38篇 |
1995年 | 62篇 |
1994年 | 71篇 |
1993年 | 65篇 |
1992年 | 76篇 |
1991年 | 63篇 |
1990年 | 55篇 |
1989年 | 53篇 |
1988年 | 39篇 |
1987年 | 50篇 |
1986年 | 63篇 |
1985年 | 64篇 |
1984年 | 55篇 |
1983年 | 41篇 |
1982年 | 33篇 |
1981年 | 44篇 |
1980年 | 35篇 |
1979年 | 47篇 |
1978年 | 34篇 |
1977年 | 30篇 |
1976年 | 30篇 |
1975年 | 23篇 |
1974年 | 31篇 |
1973年 | 31篇 |
1970年 | 35篇 |
排序方式: 共有4789条查询结果,搜索用时 455 毫秒
11.
Herman van der Kooij Ron Jacobs Bart Koopman Henk Grootenboer 《Biological cybernetics》1999,80(5):299-308
A model is presented to study and quantify the contribution of all available sensory information to human standing based
on optimal estimation theory. In the model, delayed sensory information is integrated in such a way that a best estimate of
body orientation is obtained. The model approach agrees with the present theory of the goal of human balance control. The
model is not based on purely inverted pendulum body dynamics, but rather on a three-link segment model of a standing human
on a movable support base. In addition, the model is non-linear and explicitly addresses the problem of multisensory integration
and neural time delays. A predictive element is included in the controller to compensate for time delays, necessary to maintain
erect body orientation. Model results of sensory perturbations on total body sway closely resemble experimental results. Despite
internal and external perturbations, the controller is able to stabilise the model of an inherently unstable standing human
with neural time delays of 100 ms. It is concluded, that the model is capable of studying and quantifying multisensory integration
in human stance control. We aim to apply the model in (1) the design and development of prostheses and orthoses and (2) the
diagnosis of neurological balance disorders.
Received: 25 August 1997 / Accepted in revised form: 8 December 1998 相似文献
12.
Two different Aspergillus nidulans recombinant strains producing either the Aspergillus nidulans a-L-arabinofuranosidase A or a Candida molischiana b-glucosidase have been constructed. Depending on the growing conditions, the modified strains produce up to 4 or 18 times more b-glucosidase or a-L-arabinofuranosidase activity levels, respectively, than the wild type strain. 相似文献
13.
Ochratoxin A, a nephrotoxin produced as a secondary metabolite by A. ochraceus, is a potent inhibitor of renal PEPCK activity, in vivo. When fed orally to rats for 2 days, renal PEPCK activity is reduced 50% by a total dose of 0.3-0.5 mg toxin. Renal gluconeogenic capacity is reduced only after PEPCK activity is inhibited by 50%. Hepatic PEPCK activity is unaffected up to 1.5-2.0 mg ochratoxin A, which were the highest doses tested. Other enzymes located in proximal convoluted tubules, including phosphatedependent glutaminase, γ-glutamyl transpeptidase, pyruvate carboxylase, and Na,K-ATPase, are not affected. Renal protein synthesis from [3H]phenylalanine or [3H]leucine is inhibited 30–40% by ochratoxin A in vivo. By covalently coupling the toxin to albumin with carbodiimide or mixed anhydride, the inhibitory effect on renal PEPCK activity is retained, but protein synthesis is not affected and cytological evidence of nephrotoxicity is lost. Injection of the ochratoxin A-albumin carbodiimide complex results in a decrease of hepatic PEPCK activity as well. Removal of the phenylalanine group from the toxin prevents the in vivo inhibition of PEPCK activity, as well as protein synthesis. We conclude that the decrease in renal PEPCK activity, in vivo, requires the phenylalanine group of ochratoxin A, and occurs by a mechanism independent of the known nephrotoxicity effects. 相似文献
14.
Quintana Xavier D.; Comin Francisco A.; Moreno-Amich Ramon 《Journal of plankton research》2002,24(11):1149-1161
The distribution of biomass among phytoplankton and free-livinginvertebrates was analysed in a shallow Mediterranean salt marshsubmitted to fluctuating water level. Among phytoplankton, biomassaccumulated in sizes dominated by mixotrophic species, indicatinga competitive advantage for these species, which also prey onother smaller primary producers. Among invertebrates, biomassaccumulated in the larger sizes, corresponding to species whichpartially exploit other nearby systems, such as the aerial environment(insects), or to those able to exploit particulated organicmatter in marsh-bed sediment (amphipods). Biomass distributionmodels developed for pelagic systems are discussed in relationto fluctuating temporary waters. The integrated spectrum approximated(r2 = 0.96) a Pareto distribution with a slope of c = 1.38.Intense disturbances caused a decrease in r2 and an increasein c. Under stable conditions, two different tendencies wereobserved, depending on the degree of eutrophy of the basins:higher values of c were measured in the more eutrophic basins,and lower values in the less eutrophic ones. We hypothesizethat highly irregular nutrient input could explain these differences. 相似文献
15.
16.
Stanley A. Vinores Mary M. Herman Lucien J. Rubinstein 《The Histochemical journal》1987,19(8):439-448
Summary Neuron-specific () enolase, a glycolytic enzyme used as a relatively specific marker for normal neurons and neuroendocrine cells, has recently been found in a variety of neoplastic cells and in reactive astrocytes. Its localization was investigated by immunohisto- and electron-immunocyto-chemistry, in the proliferating supportive Schwann cells of a peripheral ganglioneuroblastoma and in the neoplastic Schwann cells of four acoustic tumours. By light microscopy, the neoplastic Schwann cells showed moderate uneven diffuse immunopositivity for enolase. By electron-immunocy-tochemistry, both types of Schwann cells demonstrated immunopositivity discretely limited to their cell surface membranes. The neoplastic ganglion cells and axons of the ganglioneuroblastoma and the normal neurons and axons included in the schwannomas were, as expected, intensely immunopositive. The visualization of enolase on the cell surface membranes of both neoplastic and non-neoplastic proliferating Schwann cells suggests that increased glycolytic activity may occur on the surface of these proliferating cells irrespective of the nature of the proliferation. 相似文献
17.
18.
Characterization of the internal initiation of translation on the vesicular stomatitis virus phosphoprotein mRNA 总被引:1,自引:0,他引:1
R C Herman 《Biochemistry》1987,26(25):8346-8350
Internal initiation of translation on the vesicular stomatitis virus (VSV) phosphoprotein (P) mRNA leads to the synthesis of a second protein [Herman, R. C. (1986) J. Virol. 58, 797-804]. Characterization of this phenomenon shows that initiation at the 5'-proximal and internal AUG codons has different optima for mono- and divalent cations in the reticulocyte lysate. Whereas 5' initiation is stimulated by increasing concentration of K+ over the endogenous level, internal initiation is inhibited. Internal initiation is much less sensitive to the effects of the cap analogue 7mGpppG in both the reticulocyte lysate and the wheat-germ extract under conditions that reduce 5'-proximal initiation to only about 4-5% of the control level. These results imply that 5'-proximal and internal initiations are distinct biochemical processes. 相似文献
19.
Catherine Ronin Herman van Halbeek Johannah GM Mutsaers Johannes F G Vliegenthart 《Glycoconjugate journal》1987,4(3):247-254
The lipid-linked precursor ofN-type glycoprotein oligosaccharides was isolated from porcine thyroid microsomes after in cubation with UDP[3H] Glucose. The carbohydrate was released from dolichol pyrophosphate by mild acid hydrolysis, purified by gel filtration and characterized by 500-MHz1H-NMR spectroscopy in combination with enzymatic degradation. The parent oligosaccharide was found to be Glc3Man9Glc-NAc2. The three glucose residues are present in the linear sequence Glcα1-2Glα1-3 Glc, the latter being α(1-3)-linked to one of the mannose residues. In order to establish the branch location of the triglucosyl unit, the parent compound was digested with jack-bean α-mannosidase. The oligosaccharide product was purified by gel filtration, and identified by1H-NMR as Glc3Man5GlcNAc2 lacking the mannose residues A, D2, B and D3. Therefore, the structure of the precursor oligosaccharide is as follows: $$\begin{gathered} c b a D_1 C 4 \hfill \\ Glc\alpha 1 - 2Glc\alpha 1 - 3Glc\alpha 1 - 3Man\alpha 1 - 2Man\alpha 1 - 2Man\alpha 1 \hfill \\ 3 \swarrow 3 2 1 \hfill \\ Man\alpha 1 - 2Man\alpha 1 Man\beta 1 - 4GlcNAc\beta 1 - 4GlcNAc \hfill \\ D_{2 } A 3 6 \hfill \\ Man\alpha 1 \hfill \\ 6 \hfill \\ Man\alpha 1 - 2Man\alpha 1 \nwarrow 4 \hfill \\ D_3 B \hfill \\ \end{gathered} $$ 相似文献
20.
Human elongation factor 1α: a polymorphic and conserved multigene family with multiple chromosomal localizations 总被引:1,自引:0,他引:1