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   217篇
  免费   5篇
耳鼻咽喉   1篇
儿科学   6篇
妇产科学   6篇
基础医学   27篇
临床医学   11篇
内科学   55篇
皮肤病学   3篇
神经病学   23篇
特种医学   8篇
外科学   6篇
预防医学   23篇
眼科学   4篇
药学   32篇
肿瘤学   17篇
  2024年   2篇
  2023年   3篇
  2022年   29篇
  2021年   32篇
  2020年   7篇
  2019年   11篇
  2018年   11篇
  2017年   10篇
  2016年   7篇
  2015年   8篇
  2014年   2篇
  2013年   13篇
  2012年   7篇
  2011年   22篇
  2010年   3篇
  2009年   5篇
  2008年   8篇
  2007年   8篇
  2006年   11篇
  2005年   5篇
  2004年   6篇
  2003年   8篇
  2002年   3篇
  2001年   1篇
排序方式: 共有222条查询结果,搜索用时 25 毫秒
41.
Recovery of platinum group metals (PGM) from complex aqueous solutions generated as a result of leaching of various spent materials (e.g., spent automotive converters) is a vital issue in the context of the circular economy. In this study pyridinium derivatives containing an imidoamide or imine moiety (i.e., 3-[1-(2-ethylhexyloxyimine)methane]-1-propylpyridinium chloride, 3-[1-(decyloxyimine)methane]-1-propylpyridinium chloride, 3-[1-(decyloxyimine)ethane]-1-propylpyridinium chloride and 4-[1-amine(2-ethylhexyloxyimine)]-1-propylpyridinium chloride) are proposed as novel extractants for recovery of palladium(II) and platinum(IV) from model chloride aqueous solutions. The results of liquid-liquid extraction from one-component solutions of palladium(II) or platinum(IV) showed that quaternary pyridinium salts can be used as effective extractants for platinum metal ions. Moreover, PGM extraction from a two-component mixture proved no evident selectivity in the transfer of one of the metal ions to the organic phase. As the best extractant among the investigated ones, D3EI-PrCl (with straight alkyl chain at substituent) can be pointed out, however, problems with effective stripping or phase disengagement after stripping should be indicated as a drawback of the organic phases used. Further investigation should focus on the improvement of the organic phase properties (e.g., increase in hydrophobicity of the extractants and addition of an organic phase modifier) towards stripping efficiency.  相似文献   
42.
43.
Object-based attention describes the brain’s capacity to prioritize one set of stimuli while ignoring others. Human research suggests that the binding of diverse stimuli into one attended percept requires phase-locked oscillatory activity in the brain. Even insects display oscillatory brain activity during visual attention tasks, but it is unclear if neural oscillations in insects are selectively correlated to different features of attended objects. We addressed this question by recording local field potentials in the Drosophila central complex, a brain structure involved in visual navigation and decision making. We found that attention selectively increased the neural gain of visual features associated with attended objects and that attention could be redirected to unattended objects by activation of a reward circuit. Attention was associated with increased beta (20- to 30-Hz) oscillations that selectively locked onto temporal features of the attended visual objects. Our results suggest a conserved function for the beta frequency range in regulating selective attention to salient visual features.

Selective attention refers to the brain’s capacity to focus on a subset of stimuli while ignoring others (1). While subjectively intuitive in humans (2), selective attention has also been documented in a wide variety of animals, such as other primates (3), birds (4), and even insects (5). What is attended to depends on stimulus salience (e.g., loudness or brightness), as well as on the perceived value of a stimulus and the motivational state of the animal (1, 6). What is attended to also depends on what is perceived as a singular object. Object-based attention (7, 8) refers to the capacity to direct attention to a conjunction of different features linked as part of the same object. Attending to one feature of a given object would thus enhance not only the neural representation of that particular feature, but also other features that are associated with the object (9). How this form of generalization works is not entirely understood (10) but seems to require some form of feature binding (11) to first determine which stimuli belong together as a unified object (6, 12, 13) and then to link the object to some inherent value, or valence (14). Hence, feature binding appears to be essential for object-based attention (1517), as neural gain has to be allocated to specific features first in order to perceive an object as a whole. At the same time, distinct stimulus features can become unbound from an attended object if they are selectively ignored (18).In the mammalian brain, feature binding and object-based attention have been proposed to be associated mechanisms (19), both of which seem to be facilitated by synchronized activity of neuronal assemblies, which can be detected as phase-locked neural oscillations (16, 2023). In particular, oscillations in the range of 13 to 30 Hz (beta) and 30 to 80 Hz (gamma) seem to reflect this form of binding based on their strong synchronization at various time points following visual or auditory stimulation, with distinct oscillatory processes potentially reflecting different levels of perception. For example, early (<100-ms) stimulus-evoked synchronization in the gamma range has been suggested to represent rapid integration of unconscious sensory processes, whereas later (200- to 400-ms) synchronization in both the beta and gamma range is hypothesized to reflect feature binding and conscious perception (20, 22, 24, 25). Stimulus-evoked beta and gamma oscillations would thus represent a phase reset of ongoing neuronal activity associated with enhancing attentional gain for specific features, by facilitating information transfer or binding among different brain regions (26).While there is neural evidence for object-based attention in nonhuman primates (27), it is unknown if the smallest animal brains, such as those of insects, combine diverse sensory stimuli into unified percepts, or if they even have a subjective awareness (28). Behavioral studies in honeybees suggest that some insects can detect illusory contours as single objects (29) and can group distinct stimuli into abstract concepts such as “sameness” or “difference” (30), which could indicate a form of categorization through object-based attention. Similarly, visual learning paradigms for Drosophila melanogaster have uncovered a capacity for context generalization, where flies perceived visual objects as the same despite changes in color (31, 32), suggesting they were attending to the object shape feature and ignoring color cues. There is growing evidence for attention-like processes in insects, such as during visual fixation, decision making, and novelty detection in Drosophila flies (3336), as well as multiple object tracking in dragonflies (37). The latter electrophysiological study uncovered motion-detecting neurons in dragonflies that selectively lock onto the timing or phase of salient objects, which was shown by “tagging” competing objects with distinct flicker frequencies (37). However, it is unknown how such selective neural processes are controlled in the insect brain or whether these neural measures are relevant to behavioral decision making.It is possible that the insect brain, like the mammalian brain, employs oscillatory activity and stimulus-evoked phase locking to prioritize and bind stimulus features, and to enhance attentional gain. Indeed, earlier electrophysiological studies revealed oscillatory activity in the 20- to 30-Hz range in the Drosophila brain that was associated with detecting salience effects, such as visual novelty (3840), suggesting that these endogenously generated oscillations might be more broadly involved in regulating attention-like processes in the fly brain (41). However, it remained unclear which neurons might be generating these oscillations. One likely neuropil is the central complex (CX), a heterogeneous structure in the central brain that has been associated with visual pattern learning (4244). Recent studies in behaving Drosophila also identified the CX as a key brain region for visual navigation (45, 46). This suggests a broader role for the CX in directing attention-like processes (5), which could also reflect ring attractor dynamics within CX circuits (47, 48). While its role in visual perception is increasingly evident, whether the CX produces neural oscillations relevant to visual attention and feature binding is unknown. To address this question requires not only measuring electrical activity in the CX of behaving flies, but also correlating any endogenous brain activity to distinct neural signatures associated with competing visual stimuli or stimulus features.In tethered virtual reality experiments, flies tend to fixate on large objects and avoid small objects, whether they are flying (49) or walking (33). We exploited this innate visual dichotomy to examine mechanisms underlying visual selective attention in Drosophila. To disambiguate between the attractive and aversive stimuli in the fly brain, and to relate neural activity to ongoing behavioral choices, we recorded local field potentials (LFPs) from the CX and made the competing visual stimuli flicker at distinct frequencies, thereby evoking steady-state visually evoked potentials (SSVEPs) in the fly brain. We first showed that the SSVEPs varied in amplitude depending on the visual objects being fixated upon, allowing us to then investigate how attention guided the binding of different visual features, such as object size, brightness, and flicker frequency. By calculating phase-locking strength between the distinct SSVEPs and endogenous brain activity, we then examined how oscillations in the CX interacted with one another. We found frequency-specific phase locking between endogenous oscillations in the 20- to 30-Hz frequency range and the object features that the fly paid attention to, suggesting that beta-like oscillations could be employed for object-based attention in the insect brain.  相似文献   
44.
Copy number gains and high-level amplifications of the short arm of chromosome 5 are frequently observed in soft tissue sarcomas. To identify genes from this region possibly involved in tumor progression, we analyzed 34 soft tissue sarcomas (10 pleomorphic and 8 dedifferentiated liposarcomas, 6 malignant fibrous histiocytomas, and 10 malignant peripheral nerve sheath tumors (MPNST)) using a DNA microarray including 418 BAC clones representing 99% of chromosome arm 5p. In seven tumors, distinct high-level amplifications were identified affecting four different subregions. From these regions, genes TERT, TRIO, SKP2, FBXO32, NKD2, SLC6A3, IRX2, POLS, FYB, PTGER4, and FGF10 were selected for detailed quantitative expression analysis (RQ-PCR) based on their potential tumorigenic function. Of these, TRIO, coding for a guanidine nucleotide exchange factor, was consistently overexpressed in all cases, while IRX2 and NKD2, both involved in the regulation of developmental processes via the WNT pathway, showed a characteristic expression only in MPNSTs. Detailed nonparametric multidimensional scaling analysis further showed that the expression of TRIO, IRX2, and NKD2 strongly correlated with the gene copy number. In conclusion, we found TRIO, IRX2, and NKD2 frequently affected by high-level amplifications as well as up-regulated in a gene-dosage dependent manner. Thus, these genes represent candidate targets of 5p amplifications in soft tissue sarcomas and might play a crucial role during the progression of this disease.  相似文献   
45.
46.
The CD28:B7 signaling pathway is very important for the activity of mature peripheral T lymphocytes and thymocyte development. The proper development of thymocytes into mature single positive CD4+and CD8+ T cells is crucial for almost all immune functions. In naturally occurring conditions, T cells maturation in the thymus is influenced by environmental agents. The expression of CD28 and the distribution of CD28low/high thymocytes have been examined at various stages of thymocyte development in BALB/c mice exposed to air-suspended particulate matter (ASM). Acute exposure to ASM resulted in the decrease of CD28 expression in the total thymocyte population. The increase of the percentage of CD28low and the decrease of CD28high thymocytes were observed, which may account for the acceleration of thymocyte development under the conditions of elevated risk resulting from the exposure of animals to environmental xenobiotics. ASM exposure resulted in the increase of the level of proliferation of lymph node T cells induced by anti-CD3 and anti-CD28 monoclonal antibodies activation despite normal expression of CD28 molecule. In contrast, the level of proliferation of spleen T cells was lowered or normal dependently of the concentration of stimuli used for activation. Results of these studies demonstrate that acute exposure of mice to ASM can result in the progression of two contrasting processes in the immune system: upregulation of thymocyte development, which contributes to the maintenance of peripheral T cell pool, and over-activation of lymph node lymphocytes, which may lead to uncontrolled immunostimulation.  相似文献   
47.
Here, we report that mesoporous hollow carbon spheres (HCS) can be simultaneously functionalized: (i) endohedrally by iron oxide nanoparticle and (ii) egzohedrally by manganese oxide nanorods (FexOy/MnO2/HCS). Detailed analysis reveals a high degree of graphitization of HCS structures. The mesoporous nature of carbon is further confirmed by N2 sorption/desorption and transmission electron microscopy (TEM) studies. The fabricated molecular heterostructure was tested as the anode material of a lithium-ion battery (LIB). For both metal oxides under study, their mixture stored in HCS yielded a significant increase in electrochemical performance. Its electrochemical response was compared to the HCS decorated with a single component of the respective metal oxide applied as a LIB electrode. The discharge capacity of FexOy/MnO2/HCS is 1091 mAhg−1 at 5 Ag–1, and the corresponding coulombic efficiency (CE) is as high as 98%. Therefore, the addition of MnO2 in the form of nanorods allows for boosting the nanocomposite electrochemical performance with respect to the spherical nanoparticles due to better reversible capacity and cycling performance. Thus, the structure has great potential application in the LIB field.  相似文献   
48.
49.
Exclusive enteral nutrition (EEN) is a first-line treatment in active, mild to moderate Crohn’s disease (CD) in children. The Crohn’s disease exclusion diet (CDED), which avoids products known to have a pro-inflammatory effect on the intestinal mucosa, presents similar effectiveness to EEN for inducing remission in the paediatric population. The aim of the study was to evaluate the effectiveness of the CDED in inducing remission in adult patients. Between March 2020 and May 2021, 32 patients in a gastroenterology outpatient centre were treated according to the assumptions of the CDED. The patients were seen at baseline, at week 6, and at week 12 of the study. During the visits, anthropometric measurements and laboratory tests were performed, Crohn’s disease activity index (CDAI) was calculated, and the Inflammatory Bowel Disease Questionnaire (IBDQ) was completed. The study included a total of 32 participants, 18 women (56.3%) and 14 men (43.7%). Clinical remission was obtained in 76.7% patients after 6 weeks and in 82.1% after 12 weeks of therapy. Calprotectin levels were significantly lower in the second follow-up compared with baseline (p = 0.021). The CDED is an effective therapy for inducing remission in the adult CD population.  相似文献   
50.
The rapid progress of genetic engineering furthermore opens up new prospects in the therapy of this difficult‐to‐treat disease. IL‐23 inhibitors, phosphodiesterase 4 (PDE4) inhibitors, and Janus kinase (JAK) inhibitors are currently encouraging further research. Two drugs which are IL‐23 inhibitors are now in phase III of clinical trials. The aim of the action of both drugs is selective IL‐23 inhibition by targeting the p19 subunit. Guselkumab is a fully human monoclonal antibody. Tildrakizumab is a humanized monoclonal antibody, which also belongs to IgG class and is targeted to subunit p19 of interleukin 23 (IL‐23). Phosphodiesterase inhibitors exert an anti‐inflammatory action and their most common group is the PDE4 family. PDE4 inhibits cAMP, which reduces the inflammatory response of the pathway of Th helper lymphocytes, Th17, and type 1 interferon which modulates the production of anti‐inflammatory cytokines such as IL‐10 interleukins. The Janus kinase (JAK) signaling pathway plays an important role in the immunopathogenesis of psoriasis. Tofacitinib suppresses the expression of IL‐23, IL‐17A, IL‐17F, and IL‐22 receptors during the stimulation of lymphocytes. Ruxolitinib is a selective inhibitor of JAK1 and JAK2 kinases and the JAK‐STAT signaling pathway. This article is a review of the aforementioned drugs as described in the latest available literature.  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