全文获取类型
收费全文 | 2507篇 |
免费 | 262篇 |
国内免费 | 1篇 |
出版年
2021年 | 45篇 |
2020年 | 30篇 |
2019年 | 44篇 |
2018年 | 48篇 |
2017年 | 31篇 |
2016年 | 63篇 |
2015年 | 103篇 |
2014年 | 115篇 |
2013年 | 161篇 |
2012年 | 188篇 |
2011年 | 170篇 |
2010年 | 106篇 |
2009年 | 118篇 |
2008年 | 137篇 |
2007年 | 119篇 |
2006年 | 130篇 |
2005年 | 103篇 |
2004年 | 88篇 |
2003年 | 74篇 |
2002年 | 87篇 |
2001年 | 47篇 |
2000年 | 37篇 |
1999年 | 35篇 |
1998年 | 22篇 |
1997年 | 13篇 |
1996年 | 18篇 |
1995年 | 14篇 |
1994年 | 20篇 |
1993年 | 20篇 |
1992年 | 44篇 |
1991年 | 32篇 |
1990年 | 27篇 |
1989年 | 27篇 |
1988年 | 31篇 |
1987年 | 17篇 |
1986年 | 24篇 |
1985年 | 23篇 |
1983年 | 18篇 |
1982年 | 12篇 |
1981年 | 13篇 |
1980年 | 14篇 |
1979年 | 13篇 |
1976年 | 11篇 |
1973年 | 15篇 |
1972年 | 25篇 |
1971年 | 18篇 |
1970年 | 10篇 |
1969年 | 12篇 |
1967年 | 14篇 |
1966年 | 16篇 |
排序方式: 共有2770条查询结果,搜索用时 31 毫秒
1.
2.
Niki Rensen Lindsay M. H. Steur Noa Wijnen Eus J. W. van Someren Gertjan J. L. Kaspers Raphaële R. L. van Litsenburg 《Chronobiology international》2020,37(5):660-672
ABSTRACT Sleep and the sleep-wake rhythm are essential for children’s health and well-being, yet reference values are lacking. This study therefore aimed to assess actigraphic estimates of sleep and the 24-h sleep-wake rhythm, as well as 6-sulfatoxymelatonin (aMT6s) levels in healthy children of different age groups. Additionally, relationships between the outcomes and sex, highest parental educational level (as an indication of socioeconomic status (SES)), and body-mass-index (BMI) were explored. In this cross-sectional study, healthy Dutch children (2–18 years) wore an actigraph (GT3x) for 7 consecutive days, collected first-morning void urine and completed a sleep log and sociodemographic questionnaire. Actigraphically estimated sleep variables were sleep onset latency (SOL), sleep efficiency (SE), total sleep time (TST), and wake after sleep onset (WASO). Non-parametric sleep-wake rhythm variables were intradaily variability (IV); interdaily stability (IS); the activity counts and timing of the least active 5-h period (L5counts and midpoint) and of the most active 10-h period (M10 counts and midpoint); and the relative amplitude (RA), i.e. the ratio of the difference and the sum of M10 and L5 counts. Finally, creatinine-corrected aMT6s levels were obtained by isotope dilution mass spectrometry. Effects of age group (preschool 2–5 years/school-aged 6–12 years/teenager 13–18 years), sex, highest parental educational level and BMI (Z-scores) were explored. Ninety-four children participated, equally divided across age groups (53% boys). Teenagers slept less, but more efficiently, than younger children, while their 24 h sleep-wake rhythm was the least stable and most fragmented (likely due to fragmentation of daytime activity). Additionally, aMT6s levels significantly declined over the age groups. Children from highly educated parents had lower sleep efficiency, but a more stable sleep-wake rhythm. Finally, sex or increase in BMI was not associated with any of the outcomes in this study. In conclusion, this study provides reference values of healthy children across different age groups and different sociodemographic factors. In the future, this information may help to better interpret outcomes in clinical populations. 相似文献
3.
4.
5.
H Menkes D Lindsay G Gamsu L Wood A Muir P T Macklem 《Journal of applied physiology》1973,35(6):917-921
6.
7.
The effect of dipicolinic acid (DPA) or its calcium chelate (CaDPA) on the spectral characteristics of nucleic acids was examined. Dipicolinic acid was found to displace ethidium gromide from DNA; this indicates that it may bind by intercalation. On interaction with DNA, the ultraviolet absorption spectrum revealed downfield shifts and caused progressive diminution in both DNA and dipicolinate chromophores. The strength and type of interaction may be ion-specific but not discriminatory to any type of base pairing. Spectral analysis also indicated that both dipicolinate and calcium dipicolinate bound to different RNA species, although the mechanism of binding was not elucidated. We conclude that the interaction of dipicolinate/ calcium dipicolinate with nucleic acids is a mechanism whereby water can be removed from spore polynucleotides, increasing their stability to denaturation. 相似文献
8.
Monica L. Gavala Lindsay M. Hill Lisa Y. Lenertz Maya R. Karta Paul J. Bertics 《The Journal of biological chemistry》2010,285(44):34288-34298
Activation of the ionotropic P2RX7 nucleotide receptor by extracellular ATP has been implicated in modulating inflammatory disease progression. Continuous exposure of P2RX7 to ligand can result in apoptosis in many cell types, including monocytic cells, whereas transient activation of P2RX7 is linked to inflammatory mediator production and the promotion of cell growth. Given the rapid hydrolysis of ATP in the circulation and interstitial space, transient activation of P2RX7 appears critically important for its action, yet its effects on gene expression are unclear. The present study demonstrates that short-term stimulation of human and mouse monocytic cells as well as mouse osteoblasts with P2RX7 agonists substantially induces the expression of several activating protein-1 (AP-1) members, particularly FosB. The potent activation of FosB after P2RX7 stimulation is especially noteworthy considering that little is known concerning the role of FosB in immunological regulation. Interestingly, the magnitude of FosB activation induced by P2RX7 stimulation appears greater than that observed with other known inducers of FosB expression. In addition, we have identified a previously unrecognized role for FosB in osteoblasts with respect to nucleotide-induced expression of cyclooxygenase-2 (COX-2), which is the rate-limiting enzyme in prostaglandin biosynthesis from arachidonic acid and is critical for osteoblastic differentiation and immune behavior. The present studies are the first to link P2RX7 action to FosB/AP-1 regulation in multiple cell types, including a role in nucleotide-induced COX-2 expression, and support a role for FosB in the control of immune and osteogenic function by P2RX7. 相似文献
9.
Genetic, functional and sequence analysis of the xylR and xylS regulatory genes of the TOL plasmid pWW0 总被引:14,自引:0,他引:14
Mutant derivatives of a plasmid, pCF20, which carries the XhoI-D fragment of the TOL plasmid pWW0 have been isolated using Tn5 transposon mutagenesis. Insertion mutations of the xylR and xylS regulatory genes of the catabolic pathway have been isolated and characterized and their ability to induce catechol 2,3-oxygenase activity determined. Analysis of the insertion mutants and also segments of the XhoI-D fragment cloned into plasmid pUC8 in maxicells has identified a 68 kDa polypeptide product encoded by the xylR gene. No clear candidate for the xylS polypeptide was observed. The nucleotide sequence of the xylS region, the intergenic region and part of the xylR region has been determined and open reading frames (ORFs) assigned for both genes. The ORF designated xylS appears capable of encoding a polypeptide of approximately 37 kDa. 相似文献
10.
The technique of E.S.R. spectroscopy, when employed in conjunction with a continuous flow system, provides direct evidence for the nature of free radicals formed from organic substrates in the presence of FeII and H2O2 in aqueous solution. It is shown, both via the identification of hydroxyl-radical adducts to alkenes and via the observed site-selectivity of radical attack, that the hydroxyl radical is formed as the reactive intermediate in the presence of various chelators (e.g. EDTA, DTPA). This approach also allows the rate constants for the FeII-H2O2 reaction in the presence of the different chelates to be determined; values obtained are in reasonable agreement with most of those measured by other methods. Examples of radical oxidation (by FeIII) and reduction (by FeII) are revealed. 相似文献