全文获取类型
收费全文 | 2088篇 |
免费 | 205篇 |
出版年
2023年 | 16篇 |
2022年 | 17篇 |
2021年 | 41篇 |
2020年 | 36篇 |
2019年 | 28篇 |
2018年 | 28篇 |
2017年 | 33篇 |
2016年 | 53篇 |
2015年 | 89篇 |
2014年 | 96篇 |
2013年 | 106篇 |
2012年 | 130篇 |
2011年 | 94篇 |
2010年 | 54篇 |
2009年 | 68篇 |
2008年 | 88篇 |
2007年 | 99篇 |
2006年 | 107篇 |
2005年 | 79篇 |
2004年 | 94篇 |
2003年 | 77篇 |
2002年 | 75篇 |
2001年 | 53篇 |
2000年 | 58篇 |
1999年 | 48篇 |
1998年 | 21篇 |
1997年 | 15篇 |
1996年 | 26篇 |
1995年 | 33篇 |
1994年 | 18篇 |
1993年 | 29篇 |
1992年 | 34篇 |
1991年 | 43篇 |
1990年 | 40篇 |
1989年 | 30篇 |
1988年 | 19篇 |
1987年 | 24篇 |
1986年 | 27篇 |
1985年 | 34篇 |
1984年 | 27篇 |
1983年 | 20篇 |
1982年 | 14篇 |
1981年 | 13篇 |
1980年 | 12篇 |
1979年 | 13篇 |
1974年 | 10篇 |
1973年 | 14篇 |
1972年 | 15篇 |
1969年 | 12篇 |
1967年 | 9篇 |
排序方式: 共有2293条查询结果,搜索用时 15 毫秒
71.
We developed a new method to monitor fetal medullary respiratory neurons utilizing a two-stage approach. At 129-133 days of gestation, sheep were anesthetized, and a window was placed over the area of the fourth ventricle. After a recovery period of 3-5 days, the fetus was exteriorized into a saline bath under maternal spinal anesthesia, and the head was connected rigidly to a stereotaxic frame. Microelectrodes were inserted into the area of the nucleus tractus solitarius during rapid-eye-movement sleep, and extracellular recordings of 223 respiratory neurons were analyzed: 76% were inspiratory, 9% expiratory, and 15% phase spanning, as classified by visual and computer correlation to diaphragmatic activity. More detailed analysis of 100 neurons was done to assess the respiratory component (eta 2) by use of a modification of the method developed by Orem and Dick (J. Neurophysiol, 50: 1098-1107, 1983). With use of cohorts of 25 breaths, fetal respiratory neurons were found to frequently change their phase relationship to diaphragmatic activity. The eta 2 statistic of fetal respiratory neurons was not a stable characteristic but changed over time. This could be a reflection of an immature central respiratory system before birth or the lack of major sensory inputs. 相似文献
72.
Interleukin-4. A regulatory protein 总被引:2,自引:0,他引:2
Since its discovery in 1982, numerous biological activities of interleukin-4 (IL-4) have been described. Like other cytokines, IL-4 is highly pleiotropic, both with respect to the number of different target cells that are responsive to it and with respect to the number of different biological responses it elicits. Interleukin-4 was initially described as a costimulant for the proliferation of B lymphocytes stimulated with anti-IgM antibody. Synonyms for this cytokine are B cell growth factor-1 (BCGF-1) and B cell stimulatory factor-1 (BSF-1). After cloning of both the murine and human IL-4, the use of recombinant IL-4 enabled detailed studies of its biological functions. Many cell types, mainly of hematological origin, express receptors for IL-4. Accordingly, effects of IL-4 have been described on B lymphocytes, T lymphocytes, NK cells, mononuclear phagocytes, mast cells, fibroblasts and hematopoietic progenitor cells. Currently, there are three major areas in which IL-4 appears to play an important role: 1) regulation of B cell growth and of antibody isotype expression. In this context, a possible role for IL-4 in allergic reactions is of special interest. 2) Stimulation of T cell growth and the generation of cytotoxic T lymphocytes. In addition to the suppressive effects on the induction of non HLA-restricted cellular cytotoxicity by natural killer- (NK) and lymphokine-activated killer (LAK) cells, this suggests a role for IL-4 in the regulation of cellular immune responses. 3) Regulation of the growth and differentiation of hematopoietic bone marrow stem cells. IL-4 itself does not induce proliferation of hematological progenitor cells but it can modulate the growth-factor dependent proliferation of these cells. In this review the biological functions of IL-4, reported until present, are discussed. 相似文献
73.
H. Stam H. Jansen W.C. Hülsmann 《Biochemical and biophysical research communications》1980,96(2):899-906
Chylomicron degradation by hearts from fed and fasted rats was studied using a perfusion technique, which allows the separate collection of coronary (Qrv) and interstitial effluent (Qi). Upon perfusion with [3H]-cholesterol-containing chylomicrons the tissue recovery of label was highest in the fasted state, while label recovered in Qi was highest in the fed state. Density gradient centrifugation of Qi indicated that the label was recovered in lipoproteins with higher densities: low density lipoproteins (1.019<d<1.050), high density lipoproteins (1.050<d<1.21) and a fraction of d>1.21. These particles probably represent chylomicron degradation products (remnants and “surface fragments”). Our results indicate that tissue cholesterol uptake during chylomicron degradation may be inhibited in the fed state. Furthermore, the role of the myocyte (or interstitial) lipoprotein lipase in chylomicron degradation is discussed. 相似文献
74.
75.
Summary This paper discusses the analogy between phenomena in populations of coupled biological oscillators and the behaviour of systems of synchronized mathematical oscillators. Frequency entrainment in a set of coupled relaxation oscillators is investigated with perturbation methods. This analysis leads to quantitative results for entrainment and explains phenomena such as travelling waves in systems of spatially distributed oscillators. 相似文献
76.
W. F. Jansen H. van Loveren R. A. Woutersen R. A. de Weger 《Histochemistry and cell biology》1976,48(4):293-306
Summary In the saccus dorsalis of the rainbow trout, Salmo gairdneri Richardson, the activity of various enzymes (transferase, lyases, oxidoreductases, hydrolases) have been studied in detail.The results of this enzyme-cytochemical study firmly demonstrate that the organ is metabolically highly active. The epithelial cells have a strong energy metabolism. Energy production can take place under aerobic as well as under anaerobic conditions. Evidence is presented that glucose from blood is directly utilized for energy demands. The epithelial cells show also high synthetic activities. The moderate amino acid metabolism may participate in the synthesis of an acid mucopolysaccharide-protein complex, especially in the so-called dark cells. Lipid metabolism appears to be restricted to the mitochondria, indicating a high turnover of lipid moieties in the membranes. In contrast to the normal looking mitochondria, the macromitochondria — besides shape and localization — have an extremely high lipid and monoamine metabolism, which may point to a special function in the cellular economy. The high activity of enzymes involved in the degradation of monoamines and in the hydration of CO2 is of particular physiological interest. The significance of the observations is discussed in relation to formerly obtained indications on the involvement of the saccus dorsalis in fluid secretion, extrusion of organic substances of low molecular weight into the ventricular system and uptake of organic substances from the cerebrospinal fluid.The hypothesis of the saccus dorsalis being an analogue of the choroid plexus is supported by several relevant data. 相似文献
77.
H. Jansen H. Stam C. Kalkman W.C. Hülsmann 《Biochemical and biophysical research communications》1980,92(2):411-416
Rat hearts were perfused using a modified technique, allowing the separate collection of coronary- and interstitial effluents. When heparin was added to the perfusion medium lipoprotein lipase was found in the coronary, as well as in the interstitial effluents. The relative amounts of lipase activity in both effluents varied with the feeding conditions of the animals, being high in the coronary effluent during fasting and high in the interstitial effluent during feeding. When glucagon (2.10?7 M) was included in the perfusion medium, no differences between fasted and fed animals were obtained. The apparent Km of the interstitial lipase was lower than that of the lipase found in the coronary effluent. The results are discussed in the light of the localization of lipoprotein lipase in rat hearts . 相似文献
78.
An individual-based model forDrosophila is formulated, based on competition amongst larvae consuming the same batch of food. The predictions of the model are supported
by data for single speciesDrosophila populations reared in the laboratory. The model is used to build a simple discrete model for the dynamics ofDrosophila populations that are kept over a number of generations. The dynamics of a single species is shown to give either a stable
equilibrium or fluctuations which can be periodic or chaotic. When the dynamics of a species in the absence of the other is
periodic or chaotic, we found coexistence or two alternative states, on neither of which the species can coexist. 相似文献
79.
Bijvoet G. P. Teske A. J. Chamuleau S. A. J. Hart E. A. Jansen R. Schaap J. 《Netherlands heart journal》2020,28(2):63-72
Netherlands Heart Journal - The optimal treatment strategy for asymptomatic patients with severe mitral valve regurgitation (MR) and preserved left ventricular (LV) function is challenging. This... 相似文献
80.
Bethany S. Nichols Gerhard Leubner‐Metzger Vincent A. A. Jansen 《Ecology letters》2020,23(9):1370-1379
Environmental variability can lead to dispersal: why stay put if it is better elsewhere? Without clues about local conditions, the optimal strategy is often to disperse a set fraction of offspring. Many habitats contain environmentally differing sub‐habitats. Is it adaptive for individuals to sense in which sub‐habitat they find themselves, using environmental clues, and respond plastically by altering the dispersal rates? This appears to be done by some plants which produce dimorphic seeds with differential dispersal properties in response to ambient temperature. Here we develop a mathematical model to show, that in highly variable environments, not only does sensing promote plasticity of dispersal morph ratio, individuals who can sense their sub‐habitat and respond in this way have an adaptive advantage over those who cannot. With a rise in environmental variability due to climate change, our understanding of how natural populations persist and respond to changes has become crucially important. 相似文献