全文获取类型
收费全文 | 2551篇 |
免费 | 248篇 |
国内免费 | 1篇 |
专业分类
2800篇 |
出版年
2021年 | 30篇 |
2019年 | 20篇 |
2018年 | 22篇 |
2017年 | 28篇 |
2016年 | 24篇 |
2015年 | 56篇 |
2014年 | 94篇 |
2013年 | 104篇 |
2012年 | 114篇 |
2011年 | 114篇 |
2010年 | 80篇 |
2009年 | 68篇 |
2008年 | 106篇 |
2007年 | 100篇 |
2006年 | 96篇 |
2005年 | 79篇 |
2004年 | 84篇 |
2003年 | 79篇 |
2002年 | 88篇 |
2001年 | 65篇 |
2000年 | 86篇 |
1999年 | 60篇 |
1998年 | 39篇 |
1997年 | 32篇 |
1996年 | 33篇 |
1995年 | 22篇 |
1994年 | 19篇 |
1993年 | 30篇 |
1992年 | 63篇 |
1991年 | 70篇 |
1990年 | 52篇 |
1989年 | 55篇 |
1988年 | 43篇 |
1987年 | 61篇 |
1986年 | 56篇 |
1985年 | 45篇 |
1984年 | 37篇 |
1983年 | 40篇 |
1982年 | 35篇 |
1981年 | 18篇 |
1980年 | 17篇 |
1979年 | 37篇 |
1978年 | 31篇 |
1977年 | 25篇 |
1976年 | 18篇 |
1974年 | 19篇 |
1973年 | 29篇 |
1970年 | 24篇 |
1969年 | 18篇 |
1968年 | 17篇 |
排序方式: 共有2800条查询结果,搜索用时 15 毫秒
991.
Objective: The present study compared surface electromyographic (sEMG) activity obtained from the submental muscle group for a tongue press and a head lift exercise as potential therapeutic exercises for dysphagic elderly. Materials and methods: Fifty‐three healthy volunteers with a mean age of 35.3 participated in this study. Subjects were required to perform an isometric task, pressing their tongue against the hard palate, and an isotonic task requiring sustained lingual force against the hard palate. Pressure sensors were used to measure the amount of lingual pressure against the hard palate. Submental sEMG data from these tasks were compared with those obtained from the isometric and isotonic aspects of a head lift exercise. Results: No sEMG differences were identified between the isometric tongue press task and head lift exercise. Isotonic tongue press exercises resulted in significantly higher maximum and mean sEMG values compared with the isotonic head lift exercise (p < 0.05). The submental sEMG activity from the tongue press exercise was equal (isometric) to, or greater (isotonic) than comparable muscle activation obtained during the head lift exercise. Conclusions: The tongue press exercise may be less strenuous than the head lift exercise while achieving the same therapeutic effect. 相似文献
992.
993.
Mild and moderate dyskaryosis: can women be selected for colposcopy on the basis of social criteria?
D. J. Anderson G. M. Flannelly H. C. Kitchener P. M. Fisher E. M. Mann M. K. Campbell A. Templeton 《BMJ (Clinical research ed.)》1992,305(6845):84
OBJECTIVE--To describe the distribution of cervical intraepithelial neoplasia grades among women with mild and moderate dyskaryosis after a single cervical smear and to determine whether social criteria could help identify women who are at increased risk of grade II or III disease. DESIGN--Cross sectional analysis within a randomised prospective study. Subjects had a repeat smear, a colposcopic examination, and an excision biopsy of the transformation zone. In addition, women were asked to complete a social questionnaire. SETTING--Colposcopy clinic, Aberdeen. SUBJECTS--228 women with a single smear test showing mild or moderate dyskaryosis. MAIN OUTCOME MEASURES--Histology, age, sexual and contraceptive history, cigarette smoking. RESULTS--159 (70%) women had cervical intraepithelial neoplasia grades II or III. Among current smokers the prevalence of grade II and III disease was higher in women who smoked greater than or equal to 20 cigarettes a day (84%) than among those who smoked less (66%; p less than 0.04). Women with more than one sexual partner also had a higher prevalence (75%) than women with only one partner (50%; p = 0.0028). Use of oral contraceptives and younger age were not significantly associated. The prevalence of grade II or III disease was up to 66% in the lower risk groups. CONCLUSIONS--Because of the high prevalence of cervical intraepithelial neoplasia grades II and III in both the high and the low risk groups social factors are not useful for selecting women with mild or moderate dyskaryosis for either early referral to colposcopy or cytological surveillance. 相似文献
994.
Human platelets are known to possess 5HT2 receptors which, when activated, amplify the aggregation response produced by other aggregating agents. Several 5HT2 receptor antagonists, including ketanserin and ritanserin, are known to antagonize serotonin-mediated aggregation of human platelets. In the present study, we document the ability of three ergoline 5HT2 receptor antagonists, LY53857, sergolexole, and LY237733, to antagonize the serotonergic component of the human platelet aggregation response. Potencies of the ergoline esters (LY53857 and sergolexole) and the ergoline amide (LY237733) to inhibit serotonin-amplified platelet aggregation responses were similar to the potencies of ketanserin and ritanserin under the conditions of our study. Furthermore, all five 5HT2 receptor antagonists were capable of fully inhibiting the serotonergic component of the platelet aggregation response. In contrast to these potent ergoline esters and amides, 1-isopropyl dihydrolysergic acid (up to 10(-5)M), a putative metabolite of the ergoline esters, was ineffective under these in vitro conditions. These data are consistent with the high potency of these ergolines as antagonists of 5HT2 receptors and further support the involvement of 5HT2 receptors on human platelets in the amplifying response to serotonin. 相似文献
995.
Prabhakaran J Arango V Majo VJ Simpson NR Kassir SA Underwood MD Polavarapu H Bruce JN Canoll P Mann JJ Kumar JS 《Bioorganic & medicinal chemistry letters》2012,22(15):5104-5107
Synthesis and in vitro evaluation of [(18)F](R)-N-(4-bromo-2-fluorophenyl)-7-((1-(2-fluoroethyl)piperidin-3-yl)methoxy)-6-methoxyquinazolin-4-amine ((R)-[(18)F]FEPAQ or [(18)F]1), a potential imaging agent for the VEGFR2, using phosphor image autoradiography are described. Synthesis of 2, the desfluoroethyl precursor for (R)-FEPAQ was achieved from t-butyl 3-(hydroxymethyl)piperidine-1-carboxylate (3) in five steps and in 50% yield. [(18)F]1 was synthesized by reaction of sodium salt of compound 2 with [(18)F]fluoroethyl tosylate in DMSO. The yield of [(18)F]1 was 20% (EOS based on [(18)F]F(-)) with >99% radiochemical purity and specific activity of 1-2 Ci/μmol (n=10). The total synthesis time was 75 min. The radiotracer selectively labeled VEGFR2 in slide-mounted sections of human brain and higher binding was found in surgically removed human glioblastoma sections as demonstrated by in vitro phosphor imager studies. These findings suggest [(18)F]1 may be a promising radiotracer for imaging VEGFR2 in brain using PET. 相似文献
996.
Fu C Sunkar R Zhou C Shen H Zhang JY Matts J Wolf J Mann DG Stewart CN Tang Y Wang ZY 《Plant biotechnology journal》2012,10(4):443-452
Switchgrass (Panicum virgatum L.) has been developed into a dedicated herbaceous bioenergy crop. Biomass yield is a major target trait for genetic improvement of switchgrass. microRNAs have emerged as a prominent class of gene regulatory factors that has the potential to improve complex traits such as biomass yield. A miR156b precursor was overexpressed in switchgrass. The effects of miR156 overexpression on SQUAMOSA PROMOTER BINDING PROTEIN LIKE (SPL) genes were revealed by microarray and quantitative RT-PCR analyses. Morphological alterations, biomass yield, saccharification efficiency and forage digestibility of the transgenic plants were characterized. miR156 controls apical dominance and floral transition in switchgrass by suppressing its target SPL genes. Relatively low levels of miR156 overexpression were sufficient to increase biomass yield while producing plants with normal flowering time. Moderate levels of miR156 led to improved biomass but the plants were non-flowering. These two groups of plants produced 58%-101% more biomass yield compared with the control. However, high miR156 levels resulted in severely stunted growth. The degree of morphological alterations of the transgenic switchgrass depends on miR156 level. Compared with floral transition, a lower miR156 level is required to disrupt apical dominance. The improvement in biomass yield was mainly because of the increase in tiller number. Targeted overexpression of miR156 also improved solubilized sugar yield and forage digestibility, and offered an effective approach for transgene containment. 相似文献
997.
J?rn Güldenhaupt Till Rudack Peter Bachler Daniel Mann Gemma Triola Herbert Waldmann Carsten K?tting Klaus Gerwert 《Biophysical journal》2012,103(7):1585-1593
Ras is a central regulator of cellular signaling pathways. It is mutated in 20–30% of human tumors. To perform its function, Ras has to be bound to a membrane by a posttranslationally attached lipid anchor. Surprisingly, we identified here dimerization of membrane anchored Ras by combining attenuated total reflectance Fourier transform infrared spectroscopy, biomolecular simulations, and Förster resonance energy transfer experiments. By analyzing x-ray structural models and molecular-dynamics simulations, we propose a dimerization interface between α-helices 4 and 5 and the loop between β2 and β3. This seems to explain why the residues D47, E49, R135, R161, and R164 of this interface are influencing Ras signaling in cellular physiological experiments, although they are not positioned in the catalytic site. Dimerization could catalyze nanoclustering, which is well accepted for membrane-bound Ras. The interface could provide a new target for a seemingly novel type of small molecule interfering with signal transduction in oncogenic Ras mutants. 相似文献
998.
Jim Hardie Alastair J. Hick Carsten Hller Judy Mann Lynda Merritt Stephen F. Nottingham Wilf Powell Lester J. Wadhams Jan Witthinrich Ann F. Wright 《Entomologia Experimentalis et Applicata》1994,71(2):95-99
In autumn 1991, aphid parasitoids of the genusPraon (Hymenoptera; Braconidae) were caught in water traps with lures containing synthetic aphid sex pheromone components at three
sites in England and one in Germany. At two of the English sites and at the German site, the traps were placed in winter cereal
fields whilst the third English site was in woodland. Three species were caught,P. volucre, P. dorsale andP. abjectum. Those caught in cereal fields were almost entirelyP. volucre, whilstP. dorsale dominated at the woodland site. Of the known aphid sex pheromone components, the most effective lure was the (+)-(4aS, 7S,
7aR)-nepetalactone. Nepetalactone traps placed at the woodland site in spring and summer caught fewPraon females, and attraction may be confined to the autumn, when sexual female aphids are present in the field. Male parasitoids
did not respond to the aphid pheromones at any time, although they were caught in suction traps operated at the woodland site
during the autumn. At the cereal sites height had a significant influence on the efficiency of the pheromone traps, those
placed just above the crop canopy being most effective. There was no evidence that any other genus of parasitoid responded
to aphid sex pheromones at these sites. 相似文献
999.
The Synechocystis sp. PCC 6803 genome encodes four putative homologues of the AAA protease FtsH, two of which (slr0228 and sll1463) have been subjected to insertional mutagenesis in this study. Disruption of sll1463 had no discernible effect but disruption of slr0228 caused a 60% reduction in the abundance of functional photosystem I, without affecting the cellular content of photosystem II or phycobilisomes. Fluorescence and immunoblotting analyses show reductions in PS I polypeptides and possible structural alterations in the residual PS I, indicating an important role for slr0228 in PS I biogenesis. 相似文献
1000.
Jacqueline F. Webb Ryan M. Walsh Brandon M. Casper David A. Mann Natasha Kelly Nicole Cicchino 《Environmental Biology of Fishes》2012,95(2):275-290
The ontogeny of the ear, swim bladder and laterophysic connection was investigated in the spotfin butterflyfish, Chaetodon ocellatus in order to determine how the development of the laterophysic connection (a Chaetodon synapomorphy) is correlated with ontogenetic changes in the hearing capabilities in these abundant and ecologically important coral reef fishes. Histological and cleared and stained material revealed that the medial opening in the lateral line canal in the supracleithrum (which defines the laterophysic connection), an inflated physoclistous swim bladder, and the three otolithic organs are already present in the smallest individuals examined (7?C15?mm SL). The medial opening in the supracleithrum increases in size and the cylindrical swim bladder horns form after the loss of the head plates characteristic of the tholichthys stage, in individuals ??29?mm SL. The three sensory maculae of the ear increase in size, and the shape of the sacculus changes most dramatically with fish growth; hair cell density is highest in the utriculus. Physiological analysis of the reponse to sound pressure showed that larval and juvenile C. ocellatus had a hearing sensitivity peak at 100?C200?Hz, which was ~30?C40?dB more sensitive than that measured in larval coral reef fishes (e.g., damselfishes) that lack swim bladder horns. C. ocellatus did not show any ontogenetic changes in sensitivity to sound pressure, which may be explained by the fact that the growth of the swim bladder horns maintains the small distance between the swim bladder and ear that was established earlier during the larval stage. The timing of the development of the swim bladder horns suggests that if the laterophysic connection has a sensory acoustic function, its presence in individuals >29?mm SL suggests that its role is limited to post-settlement, reef-based behaviors. 相似文献