全文获取类型
收费全文 | 21793篇 |
免费 | 1821篇 |
国内免费 | 2506篇 |
专业分类
26120篇 |
出版年
2024年 | 66篇 |
2023年 | 354篇 |
2022年 | 760篇 |
2021年 | 1212篇 |
2020年 | 960篇 |
2019年 | 1101篇 |
2018年 | 1083篇 |
2017年 | 832篇 |
2016年 | 1055篇 |
2015年 | 1391篇 |
2014年 | 1675篇 |
2013年 | 1736篇 |
2012年 | 2031篇 |
2011年 | 1866篇 |
2010年 | 1244篇 |
2009年 | 1020篇 |
2008年 | 1123篇 |
2007年 | 1008篇 |
2006年 | 825篇 |
2005年 | 734篇 |
2004年 | 569篇 |
2003年 | 504篇 |
2002年 | 467篇 |
2001年 | 347篇 |
2000年 | 291篇 |
1999年 | 240篇 |
1998年 | 165篇 |
1997年 | 177篇 |
1996年 | 155篇 |
1995年 | 131篇 |
1994年 | 119篇 |
1993年 | 84篇 |
1992年 | 113篇 |
1991年 | 83篇 |
1990年 | 89篇 |
1989年 | 81篇 |
1988年 | 53篇 |
1987年 | 35篇 |
1986年 | 40篇 |
1985年 | 43篇 |
1984年 | 32篇 |
1983年 | 29篇 |
1982年 | 27篇 |
1981年 | 12篇 |
1980年 | 15篇 |
1979年 | 20篇 |
1978年 | 20篇 |
1977年 | 13篇 |
1976年 | 19篇 |
1975年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
42.
43.
A mathematical model of spike train transmission by identified molluscan neurons was obtained by Wiener analysis. Poisson-distributed sequences of near-threshold stimuli were used as input trains for model construction and testing. Assuming that the error of describing responses of the synapse-neuron system purely by mean outflow frequency is 100%, addition of a linear component to the equation of the model reduces this error to 25%, and addition of a term allowing for nonlinear properties of the system reduces it to 16%. Comparison of the standard error of predicted responses of the model to testing stimulus trains and of responses of a real neuron to these same trains showed that the prediction error with allowance for nonlinear properties does not exceed 21%. Choice of adequate criteria for comparing model and experimental results is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 49–54, January–February, 1984. 相似文献
44.
45.
Chicken brain choline acetyltransferase was purified to homogeneity using ammonium sulfate fractionation, followed by chromatography on DEAE-Sephadex (A-25), hydroxyapatite, Sephadex G-150, immunoabsorption and Sepharose-CoA columns. A purification of 3500-fold was achieved and the final preparation had a specific activity of 2:32 μmol acetylcholine formed per minute per milligram protein. The purified chicken choline acetyltransferase migrated as a single band on polyacrylamide gel electrophoresis in the presence and absence of sodium deodecyl sulfate. The native enzyme, with a molecular weight of 67,000 daltons, consists of two subunits of identical molecular weight. Chicken choline acetyltransferase has a sharp pH optimum of 7.4. It is activated by sodium chloride and potassium chloride but inhibited by cupric ion and N-ethylmaleimide. 相似文献
46.
Changes in the characteristics of activity of sodium, calcium, and potassium channels in the surface membrane during variation of the calcium ion concentration in the extracellular and intracellular medium were investigated by the voltage clamp method during intracellular dialysis of isolated neurons of the mollusksLimnea stagnalis andHelix pomatia. Besides their direct role in passage of the current through the membrane, calcium ions were shown to have two actions, differing in their mechanism, on the functional properties of this membrane. The first was caused by the electrostatic action of calcium ions on the outer surface of the membrane and was manifested as a shift of the potential-dependent characteristics of the ion transport channels along the potential axis; the second is determined by closer interaction of calcium ions with the specific structures of the channels. During the action of calcium-chelating agents EGTA and EDTA on the inner side of the membrane the conductivity of the potassium channels is substantially reduced. With an increase in the intracellular free calcium concentration the conductivity is partially restored. The action of EGTA and EDTA on the outer side of the membrane causes a substantial decrease in the ion selectivity of the calcium channels and changes the kinetics of the portal mechanism. These changes are easily abolished by rinsing off the chelating agents or by returning calcium ions to the external medium. A specific blocking action of an increase in the intracellular free calcium concentration on conductivity of the calcium channels was found.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 9, No. 1, pp. 69–77, January–February, 1977. 相似文献
47.
Chromatographic separation, of extracellular acid phosphatase of tobacco cells cultured under Pi-supplied and omitted conditions 总被引:1,自引:0,他引:1
An extracellular acid phosphatase preparation of tobacco XD-6cells cultured in suspension was resolved into three fractionsby sequential chromatography. Two of these were neutral pyrophosphatasewith diesterase activity, having optimum pH at 6.8. The otheris a nonspecific acid phosphatase having optimum pH at 5.8.The latter was concluded to be involved in the increase in extracellularactivity upon Pi-depletion. (Received August 31, 1976; ) 相似文献
48.
A. Ya. Supin 《Neurophysiology》1978,10(1):9-16
Receptive fields of neurons of the rabbit visual cortex selective for stimulus orientation were investigated. These receptive fields were less well differentiated than those of the analogous neurons of the cat visual cortex (large in size and circular in shape). Two mechanisms of selectivity for stimulus orientation were observed: inhibition between on and off zones of the receptive field (sample type) and oriented lateral inhibition within the same zone of the receptive field (complex type). Lateral inhibition within the same zone of the receptive field also took place in unselective neurons; "complex" selective neurons differed from them in the orientation of this inhibition. A combination of both mechanisms was possible in the receptive field of the same neuron. It is suggested that both simple and complex receptive fields are derivatives of unselective receptive fields and that "complex" neurons are not the basis for a higher level of analysis of visual information than in "simple" neurons.A. N. Severtsov Institute of Evolutionary Morphology and Ecology of Animals, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 10, No. 1, pp. 13–21, January–February, 1978. 相似文献
49.
本文对引自日本的一株粗糙化学型变异株明尼苏达沙门氏菌Re595(J)和引自美国的一株Re595(A)对小鼠异源性G~-杆菌主动和被动保护作用进行了比较。结果表明,两株菌对异源G~-杆菌的大肠杆菌和变形杆菌攻击均有良好的保护作用,但Re595(J)对抗肺炎克雷伯氏杆菌攻击的保护作用明显优于Re595(A),而Re595(A)抗绿脓杆菌攻击的保护作用则明显优于Ke595(J)。表明两株Re595的免疫原存在着差异。 相似文献
50.
本文通过多年多点的田间接种试验,分析了大豆根瘤菌C_(33)(系VSDA_(110)的突变株)的增产效应.在合丰25号大豆接种C_(33)取得明显的增产效果,两年平均比CK增产28.55%,比61A76增产24.5%;C_(33)接种在其他品种以及在不同类型和肥力的土壤上增产效果也均高于CK和61A76,说明大豆根瘤菌C_(33)比当前生产上应用的61A76更具有广谱性和高效性. 相似文献