首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4790篇
  免费   380篇
  国内免费   2篇
  5172篇
  2023年   34篇
  2022年   31篇
  2021年   81篇
  2020年   42篇
  2019年   41篇
  2018年   99篇
  2017年   102篇
  2016年   124篇
  2015年   182篇
  2014年   221篇
  2013年   289篇
  2012年   355篇
  2011年   331篇
  2010年   222篇
  2009年   195篇
  2008年   282篇
  2007年   291篇
  2006年   244篇
  2005年   245篇
  2004年   250篇
  2003年   232篇
  2002年   222篇
  2001年   103篇
  2000年   109篇
  1999年   83篇
  1998年   64篇
  1997年   36篇
  1996年   54篇
  1995年   29篇
  1994年   38篇
  1993年   21篇
  1992年   46篇
  1991年   45篇
  1990年   37篇
  1989年   33篇
  1988年   31篇
  1987年   17篇
  1986年   25篇
  1985年   26篇
  1984年   21篇
  1983年   20篇
  1982年   12篇
  1981年   18篇
  1980年   23篇
  1979年   27篇
  1978年   23篇
  1977年   20篇
  1976年   23篇
  1975年   8篇
  1973年   9篇
排序方式: 共有5172条查询结果,搜索用时 9 毫秒
131.

Background

Mental disorders may be reducible to sets of symptoms, connected through systems of causal relations. A clinical staging model predicts that in earlier stages of illness, symptom expression is both non-specific and diffuse. With illness progression, more specific syndromes emerge. This paper addressed the hypothesis that connection strength and connection variability between mental states differ in the hypothesized direction across different stages of psychopathology.

Methods

In a general population sample of female siblings (mostly twins), the Experience Sampling Method was used to collect repeated measures of three momentary mental states (positive affect, negative affect and paranoia). Staging was operationalized across four levels of increasing severity of psychopathology, based on the total score of the Symptom Check List. Multilevel random regression was used to calculate inter- and intra-mental state connection strength and connection variability over time by modelling each momentary mental state at t as a function of the three momentary states at t-1, and by examining moderation by SCL-severity.

Results

Mental states impacted dynamically on each other over time, in interaction with SCL-severity groups. Thus, SCL-90 severity groups were characterized by progressively greater inter- and intra-mental state connection strength, and greater inter- and intra-mental state connection variability.

Conclusion

Diagnosis in psychiatry can be described as stages of growing dynamic causal impact of mental states over time. This system achieves a mode of psychiatric diagnosis that combines nomothetic (group-based classification across stages) and idiographic (individual-specific psychopathological profiles) components of psychopathology at the level of momentary mental states impacting on each other over time.  相似文献   
132.
OBJECTIVE: To review the cytologic features and potential pitfalls of pheochromocytoma and retroperitoneal paraganglioma and to evaluate complications of the aspiration procedure and the diagnostic utility of immunocytochemistry. STUDY DESIGN: We reviewed 15 cytologic specimens from 12 patients with 13 tumors (1 bilateral case). Ten were adrenal (pheochromocytomas) and 3 extraadrenal paragangliomas. Eleven specimens were from fine needle aspiration (FNA) procedures that were performed in collaboration with radiologists using 23-25-gauge needles. In 3 patients the cytologic material was obtained during intraoperative diagnosis. Immunocytochemistry was performed on alcohol-fixed smears. RESULTS: Two aspirates were hypocellular, while the remainder were cellular. Cells were distributed singly or formed discohesive groups. When present, cytoplasm was abundant and ill defined. Most cells had an eccentric nucleus and plasmacytoid morphology. Nuclear pleomorphism, binucleation and multinucletaion, naked nuclei and intranuclear preudoinclusions were common findings. In 2 cases a lipid background was seen focally. Evident cytoplasmic immunoexpression of synaptophysin or chromogranin was detected in the 10 cases analyzed. One patient developed a hypertensive episode during the FNA procedure. It was controlled medically without complications. CONCLUSION: When adequate cytologic material is present, the recognition of pheochromocytoma and extraadrenal paraganglioma is possible. Together with morphology, immunocytochemical studies allow a specific preoperative diagnosis. Scarce material can be a source of diagnostic errors. FNA of pheochromocytomas is not necessarily contraindicated. When analytic data are not diagnostic, FNA may follow. Aspiration must be performed in an area equipped with the therapeutic tools necessary to control a pheochromocytoma crisis.  相似文献   
133.
Using a sequential approach, we described efficient blastospore production in a stirred tank bioreactor (3?L capacity). We used the response surface methodology to optimise the media ingredients and fermentation parameters to obtain the maximum production of blastospores by a locally collected isolate of Metarhizium acridum (Ascomycota: Hypocreales). The results showed that a liquid culture medium supplemented with monopotassium phosphate (15.17?g/L), corn steep liquor (69.25?g/L), and casamino acids (80.68?g/L) in a stirred tank bioreactor under operating conditions constant at 635?rpm, a temperature of 26°C, and pH 3.3 produced 1.25?×?108?blastospores (bls)/ml, with 93% viability after 120?h of fermentation. This bioreactor yield compares favourably with the yields obtained by shake flask production and confirms the suitability of the media and production parameters for the potential scale-up fermentation production of M. acridum.  相似文献   
134.
Re-orientation of Arabidopsis seedlings induces a rapid, asymmetric release of the growth regulator auxin from gravity-sensing columella cells at the root apex. The resulting lateral auxin gradient is hypothesized to drive differential cell expansion in elongation-zone tissues. We mapped those root tissues that function to transport or respond to auxin during a gravitropic response. Targeted expression of the auxin influx facilitator AUX1 demonstrated that root gravitropism requires auxin to be transported via the lateral root cap to all elongating epidermal cells. A three-dimensional model of the root elongation zone predicted that AUX1 causes the majority of auxin to accumulate in the epidermis. Selectively disrupting the auxin responsiveness of expanding epidermal cells by expressing a mutant form of the AUX/IAA17 protein, axr3-1, abolished root gravitropism. We conclude that gravitropic curvature in Arabidopsis roots is primarily driven by the differential expansion of epidermal cells in response to an influx-carrier-dependent auxin gradient.  相似文献   
135.

Folates (vitamin B9) are essential micronutrients which function as cofactors in one-carbon transfer reactions involved in the synthesis of nucleotides and amino acids. Folate deficiency is associated with important diseases such as cancer, anemia, cardiovascular diseases, or neural tube defects. Epidemiological data show that folate deficiency is still highly prevalent in many populations. Hence, food fortification with synthetic folic acid (i.e., folic acid supplementation) has become mandatory in many developed countries. However, folate biofortification of staple crops and dairy products as well as folate bioproduction using metabolically engineered microorganisms are promising alternatives to folic acid supplementation. Here, we review the current strategies aimed at overproducing folates in microorganisms, in view to implement an economic feasible process for the biotechnological production of the vitamin.

  相似文献   
136.
ObjectiveTo compare the use and effect of a computer based information system for cancer patients that is personalised using each patient''s medical record with a system providing only general information and with information provided in booklets.DesignRandomised trial with three groups. Data collected at start of radiotherapy, one week later (when information provided), three weeks later, and three months later.Participants525 patients started radical radiotherapy; 438 completed follow up.InterventionsTwo groups were offered information via computer (personalised or general information, or both) with open access to computer thereafter; the third group was offered a selection of information booklets.OutcomesPatients'' views and preferences, use of computer and information, and psychological status; doctors'' perceptions; cost of interventions.ResultsMore patients offered the personalised information said that they had learnt something new, thought the information was relevant, used the computer again, and showed their computer printouts to others. There were no major differences in doctors'' perceptions of patients. More of the general computer group were anxious at three months. With an electronic patient record system, in the long run the personalised information system would cost no more than the general system. Full access to booklets cost twice as much as the general system.ConclusionsPatients preferred computer systems that provided information from their medical records to systems that just provided general information. This has implications for the design and implementation of electronic patient record systems and reliance on general sources of patient information.  相似文献   
137.
Chromosome 7E from Lophopyrum ponticum carries a valuable leaf rust resistant gene designated Lr19. This gene has not been widely used in common wheat breeding because of linkage with the yellow pigment gene Y. This gene tints flour yellow, reducing its appeal in bread making. However, a high level of yellow pigment is desirable in durum wheat breeding. We produced 97 recombinant chromosomes between L. ponticum transfer 7D.7E#1 and its wheat homoeologues, using the ph1b mutation that promotes homoeologous pairing. We characterized a subset of 37 of these lines with 11 molecular markers and evaluated their resistance to leaf rust and the abundance of yellow pigment. The Lr19 gene was mapped between loci Xwg420 and Xmwg2062, whereas Y was mapped distal to Xpsr687, the most distal marker on the long arm of chromosome 7. A short terminal 7EL segment translocated to 7A, including Lr19 and Y (line 1-23), has been transferred to durum wheat by backcrossing. The presence of this alien segment significantly increased the abundance of yellow pigment. The Lr19 also conferred resistance to a new durum leaf rust race from California and Mexico that is virulent on most durum wheat cultivars. The new durum lines with the recombinant 7E segment will be useful parents to increase yellow pigment and leaf rust resistance in durum wheat breeding programs. For the common wheat breeding programs, we selected the recombinant line 1-96, which has an interstitial 7E segment carrying Lr19 but not Y. This recombinant line can be used to improve leaf rust resistance without affecting flour color. The 7EL/7DL 1-96 recombinant chromosome did not show the meiotic self-elimination previously reported for a 7EL/7BL translocation.  相似文献   
138.
The establishment of rhizobia as nitrogen-fixing endosymbionts within legume root nodules requires the disruption of the plant cell wall to breach the host barrier at strategic infection sites in the root hair tip and at points of bacterial release from infection threads (IT) within the root cortex. We previously found that Rhizobium leguminosarum bv. trifolii uses its chromosomally encoded CelC2 cellulase to erode the noncrystalline wall at the apex of root hairs, thereby creating the primary portal of its entry into white clover roots. Here, we show that a recombinant derivative of R. leguminosarum bv. trifolii ANU843 that constitutively overproduces the CelC2 enzyme has increased competitiveness in occupying aberrant nodule-like root structures on clover that are inefficient in nitrogen fixation. This aberrant symbiotic phenotype involves an extensive uncontrolled degradation of the host cell walls restricted to the expected infection sites at tips of deformed root hairs and significantly enlarged infection droplets at termini of wider IT within the nodule infection zone. Furthermore, signs of elevated plant host defense as indicated by reactive oxygen species production in root tissues were more evident during infection by the recombinant strain than its wild-type parent. Our data further support the role of the rhizobial CelC2 cell wall-degrading enzyme in primary infection, and show evidence of its importance in secondary symbiotic infection and tight regulation of its production to establish an effective nitrogen-fixing root nodule symbiosis.  相似文献   
139.
Several large population-based or clinical trial studies have suggested that certain dihydropyridine (DHP) L-type calcium channel blockers (CCBs) used for the treatment of hypertension may confer protection against the development of Alzheimer disease (AD). However, other studies with drugs of the same class have shown no beneficial clinical effects. To determine whether certain DHPs are able to impact underlying disease processes in AD (specifically the accumulation of the Alzheimer Aβ peptide), we investigated the effect of several antihypertensive DHPs and non-DHP CCBs on Aβ production. Among the antihypertensive DHPs tested, a few, including nilvadipine, nitrendipine and amlodipine inhibited Aβ production in vitro, whereas others had no effect or raised Aβ levels. In vivo, nilvadipine and nitrendipine acutely reduced brain Aβ levels in a transgenic mouse model of AD (Tg PS1/APPsw) and improved Aβ clearance across the blood-brain barrier (BBB), whereas amlodipine and nifedipine were ineffective showing that the Aβ-lowering activity of the DHPs is independent of their antihypertensive activity. Chronic oral treatment with nilvadipine decreased Aβ burden in the brains of Tg APPsw (Tg2576) and Tg PS1/APPsw mice, and also improved learning abilities and spatial memory. Our data suggest that the clinical benefit conferred by certain antihypertensive DHPs against AD is unrelated to their antihypertensive activity, but rely on their ability to lower brain Aβ accumulation by affecting both Aβ production and Aβ clearance across the BBB.  相似文献   
140.
In the present study, we demonstrated that Ang II provokes a transitory enhancement of focal adhesion kinase (FAK) and paxillin phosphorylation in human umbilical endothelial cells (HUVEC). Moreover, Ang II induces a time- and dose-dependent augmentation in cell migration, but does not affect HUVEC proliferation. The effect of Ang II on FAK and paxillin phosphorylation was markedly attenuated in cells pretreated with wortmannin and LY294002, indicating that phosphoinositide 3-kinase (PI3K) plays an important role in regulating FAK activation. Similar results were observed when HUVEC were pretreated with genistein, a non-selective tyrosine kinases inhibitor, or with the specific inhibitor PP2 for Src family kinases, demonstrating the involvement of protein tyrosine kinases, and particularly Src family of tyrosine kinases, in the downstream signalling pathway of Ang II receptors. Furthermore, FAK and paxillin phosphorylation was markedly blocked after treatment of HUVEC with AG1478, a selective inhibitor of epidermal growth factor receptor (EGFR) phosphorylation. Pretreatment of cells with inhibitors of PI3K, Src family tyrosine kinases, and EGFR also decreased HUVEC migration. In conclusion, these results suggest that Ang II mediates an increase in FAK and paxillin phosphorylation and induces HUVEC migration through signal transduction pathways dependent on PI3K and Src tyrosine kinase activation and EGFR transactivation.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号