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   198篇
  免费   26篇
  224篇
  2023年   3篇
  2022年   2篇
  2020年   5篇
  2019年   2篇
  2018年   7篇
  2017年   7篇
  2016年   6篇
  2015年   4篇
  2014年   15篇
  2013年   11篇
  2012年   20篇
  2011年   23篇
  2010年   8篇
  2009年   6篇
  2008年   16篇
  2007年   9篇
  2006年   10篇
  2005年   12篇
  2004年   6篇
  2003年   5篇
  2002年   8篇
  2001年   5篇
  2000年   4篇
  1999年   5篇
  1998年   1篇
  1997年   2篇
  1995年   2篇
  1994年   3篇
  1990年   3篇
  1988年   1篇
  1987年   2篇
  1986年   1篇
  1984年   1篇
  1982年   1篇
  1981年   1篇
  1980年   2篇
  1978年   1篇
  1976年   2篇
  1975年   1篇
  1974年   1篇
排序方式: 共有224条查询结果,搜索用时 31 毫秒
41.
The usefulness of features of leaf hairs in distinguishing subgenera and sections is well documented in bothRhododendron andSolanum. In this analysis of the taxa ofSolanum sect.Basarthrum (23 species), and of a sample of closely related taxa from sect.Petota (22 species), it is shown that such features serve to delineate subsectional groups such as series and some species as well. SectionBasarthrum has an unexpected diversity of hair types. Although this group has been characterized by 2-celled bayonet hairs, more than one half of the taxa in the section bear multicellular finger hairs, and 3 species also possess branched hairs. Thus, major rearrangements of the species previously assigned to sect.Basarthrum are indicated or supported by pubescence features. The taxa studied from seriesEtuberosa andJuglandifolia (both of sect.Petota) show hair types that a) are relatively primitive for the section, and b) show linkage between sects.Petota andBasarthrum.  相似文献   
42.
Recent studies indicate that phosphatidylinositide-3OH kinase (PI3K)-induced S6 kinase (S6K1) activation is mediated by protein kinase B (PKB). Support for this hypothesis has largely relied on results obtained with highly active, constitutively membrane-localized alleles of wild-type PKB, whose activity is independent of PI3K. Here we set out to examine the importance of PKB signaling in S6K1 activation. In parallel, glycogen synthase kinase 3beta (GSK-3beta) inactivation and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1) phosphorylation were monitored as markers of the rapamycin-insensitive and -sensitive branches of the PI3K signaling pathway, respectively. The results demonstrate that two activated PKBalpha mutants, whose basal activity is equivalent to that of insulin-induced wild-type PKB, inhibit GSK-3beta to the same extent as a highly active, constitutively membrane-targeted wild-type PKB allele. However, of these two mutants, only the constitutively membrane-targeted allele of PKB induces S6K1 activation. Furthermore, an interfering mutant of PKB, which blocks insulin-induced PKB activation and GSK-3beta inactivation, has no effect on S6K1 activation. Surprisingly, all the activated PKB mutants, regardless of constitutive membrane localization, induce 4E-BP1 phosphorylation and the interfering PKB mutant blocks insulin-induced 4E-BP1 phosphorylation. The results demonstrate that PKB mediates S6K1 activation only as a function of constitutive membrane localization, whereas the activation of PKB appears both necessary and sufficient to induce 4E-BP1 phosphorylation independently of its intracellular location.  相似文献   
43.
44.
In this study we investigated the arrangement of synapses on local axon collaterals of Golgi-stained pyramidal neurons in the mouse cerebral cortex. As synaptic markers we considered axonal swellings visible at high magnification under the light microscope. Such axonal swellings coincide with synaptic boutons, as has been demonstrated in a number of combined light and electron microscopic studies. These studies also indicated that, in most cases, one bouton corresponds precisely to one synapse. Golgi-impregnated axonal trees of 20 neocortical pyramidal neurons were drawn with a camera lucida. Axonal swellings were marked on the drawings. Most swellings were ‘en passant’; occasionally, they were situated at the tip of short, spine-like processes. On axon collaterals, the average interval between swellings was 4.5 μm. On the axonal main stem, the swellings were always less densely packed than on the collaterals. Statistical analysis of the spatial distribution of the swellings did not reveal any special patterns. Instead, the arrangement of swellings on individual collaterals follows a Poisson distribution. Moreover, the same holds to a large extent for the entire collection of pyramidal cell collaterals. This suggests that a single Poisson process, characterized by only one rate parameter (number of synapses per unit length), describes most of the spatial distribution of synapses along pyramidal cell collaterals. These findings do not speak in favour of a pronounced target specificity of pyramidal neurons at the synaptic level. Instead, our results support a probabilistic model of cortical connectivity. Received: 6 June 1993/Accepted in revised form: 22 December 1993  相似文献   
45.
First-principles density functional theory calculations have been used to study the relative stability and analyse the chemical bonding of novel cross-linked carborane polymers. Atomic charges with several population analysis methods based on fully relaxed structures were calculated to interpret the chemical binding energy shifts of XPS spectra of these boron carbide polymers. The results indicate that a base structure with one aromatic linking unit with carborane is energetically favoured. The linear relationship between experimental core-level photoemission binding energies and computational partial atomic charges from four population analysis methods (Mulliken, Hirshfeld, atoms-in-molecules (AIM) and natural bond order (NBO)) were analysed and the results indicate that cross-linking occurs at icosahedral B sites non-adjacent to icosahedral carbon sites, in agreement with recently reported experimental results. The role of basis set size in determining partial atomic charges was found to vary with population analysis method. Best linear correlations were identified with the more robust population analysis methods (Hirshfeld, AIM and NBO) with the AIM methods noted as being particularly sensitive to basis set size.  相似文献   
46.
47.
BackgroundGabapentinoids are increasingly prescribed to manage chronic noncancer pain (CNCP) in older adults. When used concurrently with opioids, gabapentinoids may potentiate central nervous system (CNS) depression and increase the risks for fall. We aimed to investigate whether concurrent use of gabapentinoids with opioids compared with use of opioids alone is associated with an increased risk of fall-related injury among older adults with CNCP.Methods and findingsWe conducted a population-based cohort study using a 5% national sample of Medicare beneficiaries in the United States between 2011 and 2018. Study sample consisted of fee-for-service (FFS) beneficiaries aged ≥65 years with CNCP diagnosis who initiated opioids. We identified concurrent users with gabapentinoids and opioids days’ supply overlapping for ≥1 day and designated first day of concurrency as the index date. We created 2 cohorts based on whether concurrent users initiated gabapentinoids on the day of opioid initiation (Cohort 1) or after opioid initiation (Cohort 2). Each concurrent user was matched to up to 4 opioid-only users on opioid initiation date and index date using risk set sampling. We followed patients from index date to first fall-related injury event ascertained using a validated claims-based algorithm, treatment discontinuation or switching, death, Medicare disenrollment, hospitalization or nursing home admission, or end of study, whichever occurred first. In each cohort, we used propensity score (PS) weighted Cox models to estimate the adjusted hazard ratios (aHRs) with 95% confidence intervals (CIs) of fall-related injury, adjusting for year of the index date, sociodemographics, types of chronic pain, comorbidities, frailty, polypharmacy, healthcare utilization, use of nonopioid medications, and opioid use on and before the index date. We identified 6,733 concurrent users and 27,092 matched opioid-only users in Cohort 1 and 5,709 concurrent users and 22,388 matched opioid-only users in Cohort 2. The incidence rate of fall-related injury was 24.5 per 100 person-years during follow-up (median, 9 days; interquartile range [IQR], 5 to 18 days) in Cohort 1 and was 18.0 per 100 person-years during follow-up (median, 9 days; IQR, 4 to 22 days) in Cohort 2. Concurrent users had similar risk of fall-related injury as opioid-only users in Cohort 1(aHR = 0.97, 95% CI 0.71 to 1.34, p = 0.874), but had higher risk for fall-related injury than opioid-only users in Cohort 2 (aHR = 1.69, 95% CI 1.17 to 2.44, p = 0.005). Limitations of this study included confounding due to unmeasured factors, unavailable information on gabapentinoids’ indication, potential misclassification, and limited generalizability beyond older adults insured by Medicare FFS program.ConclusionsIn this sample of older Medicare beneficiaries with CNCP, initiating gabapentinoids and opioids simultaneously compared with initiating opioids only was not significantly associated with risk for fall-related injury. However, addition of gabapentinoids to an existing opioid regimen was associated with increased risks for fall. Mechanisms for the observed excess risk, whether pharmacological or because of channeling of combination therapy to high-risk patients, require further investigation. Clinicians should consider the risk–benefit of combination therapy when prescribing gabapentinoids concurrently with opioids.

In a cohort study, Cheng Chen and colleagues investigate associations between concurrent use of gabapentinoids and opioids and risk of fall-related injury, compared with use of opioids alone, among adults aged 65 years or older with chronic noncancer pain in the United States.  相似文献   
48.
Xenopus cadherin-11 (Xcadherin-11) is an exceptional cadherin family member, which is predominantly expressed in cranial neural crest cells (NCCs). Apart from mediating cell–cell adhesion it promotes cranial NCC migration by initiating filopodia and lamellipodia formation. Here, we demonstrate an unexpected function of Xcadherin-11 in NCC specification by interfering with canonical Wnt/β-catenin signaling. Loss-of-function experiments, using a specific antisense morpholino oligonucleotide against Xcadherin-11, display a nuclear β-catenin localization in cranial NCCs and a broader expression domain of the proto-oncogene cyclin D1 which proceeds c-myc up-regulation. Additionally, we observe an enhanced NCC proliferation and an expansion of specific NCC genes like AP2 and Sox10. Thereby, we could allocate NCC proliferation and specification to different gene functions. To clarify which domain in Xcadherin-11 is required for early NCC development we tested different deletion mutants for their rescue ability in Xcadherin-11 morphants. We identified the cytoplasmic tail, specifically the β-catenin binding domain, to be necessary for proper NCC development. We propose that Xcadherin-11 is necessary for controlled NCC proliferation and early NCC specification in tuning the expression of the canonical Wnt/β-catenin target genes cyclin D1 and c-myc by regulating the concentration of the nuclear pool of β-catenin.  相似文献   
49.
Pollen stainability is often equated with pollen viability or fertility and is used by botanists to evaluate degree of hybridity among individuals of a plant population. Under certain conditions, however, the effects of meiotic irregularities resulting from hybridization may be masked by those of the environment. In the genus Aster high percentages of pollen stainability were obtained in most samples from plants flowering in the greenhouse, including hybrids of distantly related species. When grown outdoors, plants of the same clones frequently suffered significant reduction in the amounts of normal pollen produced. During the period of flower development, two climatic parameters were assessed, relative humidity and maximum/minimum temperatures. Fluctuations in relative humidity were considerable but of nearly equal amplitude both in the greenhouse and in the open field. Diurnal temperature fluctuations, however, were much less drastic in the greenhouse than outdoors. The differential percentages of stainable pollen observed are probably attributable to the sum of environmental factors, including hard rains and strong winds, rather than any single parameter. In the genus Aster high amounts of aborted pollen grains appear to be largely a reflection of sensitivity to unfavorable microclimatic conditions during the post-meiotic period of flower development, and data on pollen stainability cannot be employed as a measure of fertility or degree of hybridity.  相似文献   
50.
A survey of over 30 species of Aster and 61 species in other genera of Astereae indicates that the somewhat flattened style branches of perfect disk florets characteristically unfold and spread in a pronate, rather than a supinate manner. While not exhibited in all genera (Baccharis and Conyza are among the exceptions), this phenomenon appears to be restricted to and, therefore, diagnostic for the tribe. The slender style branches of pistillate ray florets display the supinate expansion which is regularly observed in other tribes of Compositae.  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