全文获取类型
收费全文 | 80256篇 |
免费 | 7090篇 |
国内免费 | 42篇 |
专业分类
87388篇 |
出版年
2022年 | 509篇 |
2021年 | 988篇 |
2020年 | 590篇 |
2019年 | 756篇 |
2018年 | 965篇 |
2017年 | 897篇 |
2016年 | 1497篇 |
2015年 | 2485篇 |
2014年 | 2834篇 |
2013年 | 3873篇 |
2012年 | 4753篇 |
2011年 | 4908篇 |
2010年 | 3244篇 |
2009年 | 2904篇 |
2008年 | 4327篇 |
2007年 | 4442篇 |
2006年 | 4233篇 |
2005年 | 4202篇 |
2004年 | 4296篇 |
2003年 | 3873篇 |
2002年 | 3904篇 |
2001年 | 985篇 |
2000年 | 718篇 |
1999年 | 982篇 |
1998年 | 1226篇 |
1997年 | 893篇 |
1996年 | 830篇 |
1995年 | 800篇 |
1994年 | 781篇 |
1993年 | 726篇 |
1992年 | 707篇 |
1991年 | 689篇 |
1990年 | 655篇 |
1989年 | 693篇 |
1988年 | 622篇 |
1987年 | 583篇 |
1986年 | 525篇 |
1985年 | 647篇 |
1984年 | 794篇 |
1983年 | 685篇 |
1982年 | 795篇 |
1981年 | 845篇 |
1980年 | 761篇 |
1979年 | 560篇 |
1978年 | 604篇 |
1977年 | 569篇 |
1976年 | 564篇 |
1975年 | 438篇 |
1974年 | 558篇 |
1973年 | 512篇 |
排序方式: 共有10000条查询结果,搜索用时 24 毫秒
181.
John G. New 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1990,167(2):285-294
1. Ampullary electroreceptors in elasmobranchs are innervated by fibers of the ALLN, which projects to the dorsal octavolateralis nucleus (DON). The purpose of this study is to examine the response characteristics of ALLN fibers and DON neurons to weak D.C. and sinusoidal electric field stimuli presented as local dipole fields. 2. ALLN fibers respond to presentation of D.C. fields with a phasic burst, followed by a more slowly adapting period of firing. Ascending efferent neurons (AENs) in the DON respond to stimuli with a similar initial burst, which adapts more quickly. 3. Type 1, 2, and 3 neurons are possible local interneurons or commissural DON neurons. Type 1 neurons demonstrate response properties similar to those of AENs. Type 2 cells demonstrated slowly adapting responses to excitatory stimuli, the duration of the response increased with the amplitude of the stimulus. Type 3 neurons demonstrated an increased rate of firing, but the response lacked any specific temporal characteristics. 4. ALLN fibers typically have receptive fields consisting of a single ampulla. The receptive field sizes of DON neurons exhibited varying degrees of convergence for different cell types. 5. Responses of ALLN fibers and DON neurons to weak sinusoidal stimuli demonstrated very similar frequency response characteristics for all cell types. The peak sensitivity of electrosensory neurons was between 5-10 Hz. 相似文献
182.
Fos-Jun and the primary genomic response in the nervous system 总被引:3,自引:0,他引:3
183.
184.
Kinetic analysis of the cerebral creatine kinase reaction under hypoxic and hypoglycaemic conditions in vitro. A 31P-n.m.r. study. 下载免费PDF全文
1. The tissue concentration of phosphocreatine (PCr) and the pseudo-first-order rate constant of creatine kinase (kf) were monitored in superfused guinea-pig brain slices in vitro by using 31P-n.m.r. techniques. 2. Superfusion of slices in low oxygen partial pressure (pO2 approx. 16 kPa) decreased tissue PCr concentrations by 48% but ATP concentrations were unchanged. Regression analysis revealed a significant negative correlation between the PCr concentration in hypoxic tissue and the increase in the rate constant, kf. Nevertheless the forward flux through the enzyme (Jf = kf.[PCr]) declined under these conditions. 3. Lowering the glucose concentration to 0.2 or 0.1 mM decreased PCr concentrations by 29% and 48% respectively; here ATP concentrations as well as PCr concentrations also decreased. Only in the presence of the lower glucose concentration (0.1 mM) was kf increased. However, unlike the situation in hypoxic tissue, Jf was maintained at control rates. 4. In spectra obtained in the presence of low oxygen or low glucose concentrations, a resonance attributable to tissue inorganic phosphate became dectectable. This observation is discussed in terms of known changes in tissue phosphate concentrations and possible alterations in cytoplasmic pH. 相似文献
185.
John M. Fairbrother Réal Lallier Linda Leblanc Mario Jacques Serge Larivière 《FEMS microbiology letters》1988,56(3):247-252
Abstract The production of fimbrial antigen F165 by Escherichia coli strains was found to be dependent on the composition of the culture medium and was repressed in the presence of alanine or high levels of glucose, in anaerobic conditions or at growth temperatures of lower than 37°C. Optimal F165 production was found on a minimal medium containing 1% (w/v) casamino acids (MD-1). F165 antigen was isolated from bacteria by mechanical shearing, precipitated with ammonium sulfate, and purified by deoxycholate treatment and gel filtration on Superose 12. The purified fimbriae retained their native morphology as observed by electron microscopy and consisted of two separate protein subunits with apparent molecular weights of 17 500 and 19 000 on sodium dodecyl sulfate-polyacrylamide gels. 相似文献
186.
Molecular organisation of the quinic acid utilization (QUT) gene cluster in Aspergillus nidulans 总被引:9,自引:0,他引:9
Alastair R. Hawkins Heather K. Lamb Melanie Smith John W. Keyte Clive F. Roberts 《Molecular & general genetics : MGG》1988,214(2):224-231
Summary The functional integrity of the QUTB gene (encoding quinate dehydrogenase) has been confirmed by transformation of a qutB mutant strain. The DNA sequence of the contiguous genes QUTD (quinate permease), QUTB and QUTG (function unknown) has been determined and analysed, together with that of QUTE (catabolic 3-dehydroquinase). The QUTB sequence shows significant homology with the shikimate dehydrogenase function of the complex AROM locus of Aspergillus nidulans, and with the QA-3 quinate dehydrogenase and QA-1S (repressor) genes of Neurospora crassa. The QUTD gene shows strong homology with the N. crassa QA-Y gene and QUTG with the QA-X gene. QUTD, QUTB, and QUTG, QUTE form two pairs of divergently transcribed genes, and conserved sequence motifs identified in the two common 5 non-coding regions show significant homology with UAS
GAL
and UAS
QA
sequences of the Saccharomyces cerevisiae and N. crassa Gal and QA systems. In addition, conserved 5 sequences homologous to the mammalian CAAT box are noted and a previously unreported conserved 22 nucleotide motif is presented. 相似文献
187.
Marion J. Couch Bendicht U. Pauli Ronald S. Weinstein John S. Coon 《Journal of cellular biochemistry》1988,37(2):213-223
Murine squamous carcinoma cells (KLN205) grown in a medium supplemented with the retinoid, 13-cis retinoic acid (RA), had dose-dependent, selective increases in the expression of certain lectin receptors, which correlated with a dramatic decrease in the ability to form pulmonary colonies (P ?.0003) (Couch MJ, Pauli BU, Weinstein RS, Coon JS: JNCI, 78:971 ?977, 1987). These findings suggest a possible relationship between the RA-induced glycoconjugate alterations and the decreased experimental metastatic behavior. We further define the mechanism of RA's action. The finding that RA treatment (5 × 10?6 M, 5 × 10?7 M) did not perturb the cell cycle of KLN205 cells provides further proof that the decreased metastatic behavior is not attributable to any inhibition in the rate of growth or to alterations in the cell cycle. Furthermore, since stable subpopulations with variable lectin binding could not be detected, the mechanism of RA's action does not appear to be due to selection of variant tumor-cell subpopulations. Finally, in a scries of experiments designed to determine the reversibility of the RA treatment, the RA-induced decrease in metastatic behavior reverted back to a more metastatic state in the same time frame (3 days) as the reversion of the RA-induced changes in cell-surface glycoconjugate expression. This reversion provides further evidence for a close relationship between the RA-induced modulation of tumor cell-surface glycoconjugate expression and the decreased metastatic behavior; it suggests that transient, reversible modulation of the tumor cell surface may play a role in determining metastatic behavior. 相似文献
188.
189.
190.