全文获取类型
收费全文 | 18336篇 |
免费 | 1555篇 |
国内免费 | 4篇 |
出版年
2023年 | 65篇 |
2022年 | 151篇 |
2021年 | 317篇 |
2020年 | 174篇 |
2019年 | 230篇 |
2018年 | 319篇 |
2017年 | 280篇 |
2016年 | 402篇 |
2015年 | 790篇 |
2014年 | 844篇 |
2013年 | 1063篇 |
2012年 | 1482篇 |
2011年 | 1452篇 |
2010年 | 876篇 |
2009年 | 865篇 |
2008年 | 1195篇 |
2007年 | 1211篇 |
2006年 | 1102篇 |
2005年 | 1118篇 |
2004年 | 985篇 |
2003年 | 925篇 |
2002年 | 900篇 |
2001年 | 188篇 |
2000年 | 138篇 |
1999年 | 185篇 |
1998年 | 271篇 |
1997年 | 159篇 |
1996年 | 144篇 |
1995年 | 142篇 |
1994年 | 124篇 |
1993年 | 115篇 |
1992年 | 96篇 |
1991年 | 82篇 |
1990年 | 91篇 |
1989年 | 87篇 |
1988年 | 77篇 |
1987年 | 55篇 |
1986年 | 74篇 |
1985年 | 76篇 |
1984年 | 97篇 |
1983年 | 75篇 |
1982年 | 86篇 |
1981年 | 104篇 |
1980年 | 88篇 |
1979年 | 56篇 |
1978年 | 66篇 |
1977年 | 53篇 |
1976年 | 38篇 |
1974年 | 56篇 |
1973年 | 35篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
Dr. Brian H. Peters 《Cell and tissue research》1987,249(2):341-350
Summary An electron-microscopical study of locust thoracic ganglia reveals that synapses in the neuropily are morphologically heterogeneous. In addition to the conventional dyadic type described frequently in the literature, there is a second type with a complex arrangement of presynaptic dense material and a non-dyadic postsynaptic configuration. Serial-section analysis of these synapses suggests that the presynaptic structures include irregular or curved bars, and small projections.Although the proportion of non-dyadic synapses in the neuropile as a whole is small, a substantial number have been found on the branches of an identified flight motor neurone, labelled intracellularly with metal ions in conjunction with silver intensification. Samples from the arborization of this neurone give some indications of the distribution of non-dyadic synapses on it.The results are discussed in the context of distribution of synapses on other identified locust neurones, and the functional morphology of synapses in other phyla. 相似文献
52.
GABA-mediated synaptic inhibition of projection neurons in the antennal lobes of the sphinx moth,Manduca sexta 总被引:3,自引:0,他引:3
Brian Waldrop Thomas A. Christensen John G. Hildebrand 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1987,161(1):23-32
Responses of neurons in the antennal lobe (AL) of the moth Manduca sexta to stimulation of the ipsilateral antenna by odors consist of excitatory and inhibitory synaptic potentials. Stimulation of primary afferent fibers by electrical shock of the antennal nerve causes a characteristic IPSP-EPSP synaptic response in AL projection neurons. The IPSP in projection neurons reverses below the resting potential, is sensitive to changes in external and internal chloride concentration, and thus is apparently mediated by an increase in chloride conductance. The IPSP is reversibly blocked by 100 microM picrotoxin or bicuculline. Many AL neurons respond to application of GABA with a strong hyperpolarization and an inhibition of spontaneous spiking activity. GABA responses are associated with an increase in neuronal input conductance and a reversal potential below the resting potential. Application of GABA blocks inhibitory synaptic inputs and reduces or blocks excitatory inputs. EPSPs can be protected from depression by application of GABA. Muscimol, a GABA analog that mimics GABA responses at GABAA receptors but not at GABAB receptors in the vertebrate CNS, inhibits many AL neurons in the moth. 相似文献
53.
Scott Brian Lew James Clandinin M. T. Cinader B. 《Cellular and molecular neurobiology》1989,9(1):105-113
1. SJL/J mice were maintained on semipurified diets which differed in the ratio of polyunsaturated/saturated fatty acid content (P/S). Exposure was from conception and was maintained for periods ranging from 6 to 34 weeks. 2. Neural cell cultures were prepared from dorsal root ganglia (DRG). After 6 and 20 days of culture, neuronal electric membrane properties were determined quantitatively by intracellular recording. 3. A number of significant differences were observed for the two dietary conditions. DRG from mice on the low-P/s diet had an increase in the rate of fall of both phases of repolarization which, in conjunction with the reduced action potential overshoot, led to a reduced action potential duration. This shift to shorter-duration action potentials was accompanied by a shift to more monophasic falling phases. The low-P/S neurons also exhibited a decreased afterhyperpolarization, decreased specific membrane resistance, and decreased membrane electrical time constant compared to high-P/S neurons. 4. It was concluded that the P/S ratio in the diet can have a significant effect on the electric properties of neurons. The high-P/S neurons tended to have action potentials with biphasic repolarizations and longer durations. In contrast, the low-P/S neurons tended to have action potentials with monophasic repolarizations and shorter durations. Moreover, the known ionic dependence of these two types of action potentials suggested that the low-P/S diet resulted in action potentials with a more exclusive Na dependence, while the high-P/S diet resulted in action potentials with both Na and Ca dependence. 相似文献
54.
55.
Brian M. Boom 《Brittonia》1989,41(2):129-130
Pagamea aracaënsis Boom, endemic to Serra Aracá, Brazil is described and illustrated, and its relationship toP. anisophylla Standley & Steyerm, is discussed. 相似文献
56.
Ronald J. Hauptman Brian A. Perry David G. Capco 《Development, growth & differentiation》1989,31(2):157-164
Proteins of the detergent-resistant cytoskeleton fraction and the detergent-soluble fraction from Xenopus oocytes and embryos are examined using a procedure which allows rapid and uniform extraction of tissues and large, single cells. SDS-polyacrylamide gels reveal only a few prominent cytoskeletal proteins in the early embryo, however qualitatively different proteins begin to appear after gastrulation. Incorporation of [35 S]-methionine into newly synthesized proteins indicates that there is synthesis and assembly of proteins into the cytoskeleton, but the amount remains low until after gastrulation. The use of nucleic acid probes for alpha-tubulin and actin mRNA indicates that about 80% of these mRNAs in the oocyte and meiotically mature egg are bound to the detergent-resistant cytoskeleton. 相似文献
57.
58.
Pierre J. Charest Jiro Hattori Janice DeMoor V. N. Iyer Brian L. Miki 《Plant cell reports》1990,8(11):643-646
Summary Genes coding for the enzyme acetohydroxyacid synthase, often referred to as acetolactate synthase (AHAS, ALS; EC 4.1.3.18), from wild type Arabidopsis thaliana and a sulfonylurea-resistant mutant line GH50 (csrl-1; Haughn et al. 1988) were introduced in Nicotiana tabacum. Both genes were expressed at high levels with the 35S promoter. The csrl-1 gene conferred high levels of resistance to chlorsulfuron whereas the wild type gene did not. As selectable markers, chimaeric AHAS genes yielded transgenic plants on chlorsulfuron but at much lower efficiencies than with a chimaeric neomycin phosphotransferase gene on kanamycin (Sanders et al. 1987). Shoot differentiation from leaf discs was delayed on chlorsulfuron by 4–6 weeks. This study indicated a role for mutant AHAS genes in the genetic manipulation of herbicide resistance in transgenic plants but as selectable markers for plant cells undergoing differentiation no advantage over other genes was perceived. 相似文献
59.
60.
Summary For numerical solution of the reaction-mass transfer equations for immobilised biocatalysts it may be better to start integration at the particle surface and proceed inwards: calculations are targetted on the region to which practically interesting changes are often confined (because concentrations are effectively zero in the interior); and during iterative solution wrong initial estimates may be rejected after detecting anomalies early in the integration.Symbols Cb
substrate concentration in bulk (mol m–3)
- c
dimensionless substrate concentration (C/Cb) (-)
- De
effective diffusion coefficient (m2s–1)
- Da
Damkohler number (V.ro
2/De.Ks) (-)
- Ks
substrate concentration kinetic coefficient (mol m–3)
- ke
external mass transfer coefficient (ms–1)
- ro
bead radius (m)
- Sh
Sherwood number (ke.ro/De) (-)
- V
maximum rate per unit volume in beads (mol m–3s–1)
- x
dimensionless distance from bead centre (r/ro) (-)
-
dimensionless kinetic coefficient (Ks/Cb) (-)
- o
effectiveness factor (-) 相似文献