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   817篇
  免费   78篇
  国内免费   4篇
  899篇
  2022年   17篇
  2021年   17篇
  2020年   16篇
  2019年   18篇
  2018年   15篇
  2017年   14篇
  2016年   13篇
  2015年   31篇
  2014年   36篇
  2013年   48篇
  2012年   48篇
  2011年   39篇
  2010年   25篇
  2009年   20篇
  2008年   33篇
  2007年   32篇
  2006年   25篇
  2005年   32篇
  2004年   30篇
  2003年   28篇
  2002年   20篇
  2001年   9篇
  2000年   25篇
  1999年   10篇
  1998年   8篇
  1997年   9篇
  1996年   13篇
  1993年   8篇
  1992年   32篇
  1991年   23篇
  1990年   19篇
  1989年   10篇
  1988年   18篇
  1987年   9篇
  1986年   10篇
  1985年   12篇
  1984年   6篇
  1983年   16篇
  1982年   7篇
  1981年   6篇
  1979年   7篇
  1978年   6篇
  1976年   5篇
  1973年   4篇
  1972年   5篇
  1971年   6篇
  1970年   7篇
  1969年   8篇
  1968年   5篇
  1954年   4篇
排序方式: 共有899条查询结果,搜索用时 13 毫秒
131.
Little is known about the evolutionary history of most complex multi‐trophic insect communities. Widespread species from different trophic levels might evolve in parallel, showing similar spatial patterns and either congruent temporal patterns (Contemporary Host‐tracking) or later divergence in higher trophic levels (Delayed Host‐tracking). Alternatively, host shifts by natural enemies among communities centred on different host resources could disrupt any common community phylogeographic pattern. We examined these alternative models using two Megastigmus parasitoid morphospecies associated with oak cynipid galls sampled throughout their Western Palaearctic distributions. Based on existing host cynipid data, a parallel evolution model predicts that eastern regions of the Western Palaearctic should contain ancestral populations with range expansions across Europe about 1.6 million years ago and deeper species‐level divergence at both 8–9 and 4–5 million years ago. Sequence data from mitochondrial cytochrome b and multiple nuclear genes showed similar phylogenetic patterns and revealed cryptic genetic species within both morphospecies, indicating greater diversity in these communities than previously thought. Phylogeographic divergence was apparent in most cryptic species between relatively stable, diverse, putatively ancestral populations in Asia Minor and the Middle East, and genetically depauperate, rapidly expanding populations in Europe, paralleling patterns in host gallwasp species. Mitochondrial and nuclear data also suggested that Europe may have been colonized multiple times from eastern source populations since the late Miocene. Temporal patterns of lineage divergence were congruent within and across trophic levels, supporting the Contemporary Host‐tracking Hypothesis for community evolution.  相似文献   
132.
Physical and chemical adsorption of CO2 on ZnO surfaces were studied by means of two different implementations of periodic density functional theory. Adsorption energies were computed and compared to values in the literature. In particular, it was found that the calculated equilibrium structure and internuclear distances are in agreement with previous work. CO2 adsorption was analyzed by inspection of the density of states and electron localization function. Valence bands, band gap and final states of adsorbed CO2 were investigated and the effect of atomic displacements analyzed. The partial density of states (PDOS) of chemical adsorption of CO2 on the ZnO(0001) surface show that the p orbitals of CO2 were mixed with the ZnO valence band state appearing at the top of the valence band and in regions of low-energy conduction band.
Figure
ELF analysis of bidentate and tridentate chemical adsorptions  相似文献   
133.
Theoretical techniques have been developed and/or improved to predict the molecular structure of lanthanide complexes which were used to calculate their electronic properties, in particular, their electronic spectra and energy levels necessary to calculate the rates of energy transfer from the ligands to the metal ion. The molecular structure has been obtained by the SMLC/AM1 (Sparkle Model for the Calculation of Lanthanide Complexes – Austin Model 1) model where the lanthanide ion is simulated by a sparkle implemented into the AM1 Hamiltonian used to perform a HF-SCF (Hartree-Fock Self-Consistent Field) calculation. The previous implementation of the SMLC/AM1 model (sparkle/1) involving only two parameters has been generalized to be consistent with the AM1 Hamiltonian and the new model (sparkle/2) significantly improved the prediction of molecular structures of Eu(III) complexes. For the electronic spectra and energy level calculations of the lanthanide complexes the model replaces the metal ion by a point charge with the ligands held in their positions as determined by the SMLC/AM1 model, and uses a INDO/S-CI (intermediate neglect of differential overlap/spectroscopic-configuration interaction) model. A preliminary study of the solvent effects on the absorption spectra of the free ligand is also presented. For the ligand-lanthanide ion energy transfer Fermi's golden rule is used with the multipolar and exchange mechanisms being implemented and tested for several complexes. These theoretical techniques have been applied to several complexes yielding very good results when compared to experimental data as well as predictions for the molecular and electronic structures and the relative contributions of the mechanisms for the energy transfer rates. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
134.
As the critical gatekeeper for birth, prepartum remodeling of the cervix is associated with increased resident macrophages (Mφ), proinflammatory processes, and extracellular matrix degradation. This study tested the hypothesis that expression of genes unique to Mφs characterizes the prepartum from unremodeled nonpregnant cervix. Perfused cervix from prepartum day 21 postbreeding (D21) or nonpregnant (NP) rats, with or without Mφs, had RNA extracted and whole genome microarray analysis performed. By subtractive analyses, expression of 194 and 120 genes related to Mφs in the cervix from D21 rats were increased and decreased, respectively. In both D21 and NP groups, 158 and 57 Mφ genes were also more or less up- or down-regulated, respectively. Mφ gene expression patterns were most strongly correlated within groups and in 5 major clustering patterns. In the cervix from D21 rats, functional categories and canonical pathways of increased expression by Mφ gene related to extracellular matrix, cell proliferation, differentiation, as well as cell signaling. Pathways were characteristic of inflammation and wound healing, e.g., CD163, CD206, and CCR2. Signatures of only inflammation pathways, e.g., CSF1R, EMR1, and MMP12 were common to both D21 and NP groups. Thus, a novel and complex balance of Mφ genes and clusters differentiated the degraded extracellular matrix and cellular genomic activities in the cervix before birth from the unremodeled state. Predicted Mφ activities, pathways, and networks raise the possibility that expression patterns of specific genes characterize and promote prepartum remodeling of the cervix for parturition at term and with preterm labor.  相似文献   
135.
Previous work indicating that nerve growth factor (NGF) protein loops 2 and 4 interact with TrkA receptors raise the possibility that small molecule mimetics corresponding to TrkA-interacting domains that have NGF agonist activity can be developed. We applied our previously developed strategy of dimeric peptidomimetics to address the hypothesis that loop 4 small molecule dimeric mimetics would activate TrkA-related signal transduction and mimic NGF neurotrophic effects in a structure-specific manner. A loop 4 cyclized peptide dimer demonstrated NGF-like neurotrophic activity, whereas peptides with scrambled sequence, added or substituted residues, or cyclized in monomeric form were inactive. Activity was blocked by the TrkA inhibitors K252a and AG879 but not by NGF p75 receptor blocking antibody. Dimeric, but not monomeric, peptides partially blocked NGF activity. This profile was consistent with that of a NGF partial agonist. ERK and AKT phosphorylation was stimulated only by biologically active peptides and was blocked by K252a. The ERK inhibitor U0126 blocked the neurite- but not the survival-promoting activity of both NGF and active peptide. These studies support the proof of concept that small molecule NGF loop 4 mimetics can activate NGF signaling pathways and can mimic death-preventing and neurite-promoting effects of NGF. This finding will guide the rational design of NGF single-domain mimetics and contribute to elucidating NGF signal transduction mechanisms.  相似文献   
136.
The perinuclear theca is a novel cytoskeletal consisting of a densely layered lamina that surrounds the nucleus of mammalian sperm. Using antibodies specific for the multiple band polypeptides present in the perinuclear theca of bull sperm, we show that a heterogeneous group of immunological related proteins are present in the sperm heads of other mammals with greatly different morphologies, including guinea pig, hamster, rat, and mouse. In none of the species were identical groups of immunoreactive polypeptides found, although immunoreactive proteins of molecular weights 65,000 to 80,000 were present in the sperm heads of all species examined. Immunoreactive proteins less than Mr 55,000 were prominent in rat sperm heads and mouse sperm: guinea pig, hamster, and rat sperm heads and mouse sperm had one band in common at approximately Mr 50,000. Different immunoreactive proteins were present in isolated sperm tails. The perinuclear theca first appeared in the subacrosomal space of round to elongating spermatids. Later, with the caudal movement of the manchette, the postacrosomal segment of the perinuclear theca was deposited in a cephalad to caudal direction along the sperm nucleus. Concomitantly, the cytoplasmic space between the nuclear envelope and the plasma membrane narrowed such that only the theca occupied this portion of the sperm head. Immunoreactivity accompanied the ultrastructural appearance of the subacrosomal layer and the postacrosomal segment. The periods of spermiogenesis, in which sub- and post-acrosomal components of the perinuclear theca are formed and the morphogenesis of sperm organelles with which these elements are associated, suggest that components of this cytoskeletal structure function to join the acrosome and the postacrosomal plasma membrane to the nucleus.  相似文献   
137.
138.
139.
There has been much investigation into the use of music as an ergogenic aid to facilitate physical performance. However, previous studies have primarily focused on predetermined music and aerobic exercise. The purpose of this study was to investigate the effects of self-selected music (SSM) vs. those of no music (NM) on the mood and performance of the athletes performing bench press and squat jump. Twenty resistance trained collegiate men completed 2 experimental conditions, one while listening to SSM and the other with NM. The subjects reported their profile of mood states (POMS) and rating of perceived exertion (RPE) before and after performing 3 sets to failure of the bench press at 75% 1 repetition maximum (1RM) and 3 reps of the squat jump at 30% 1RM. Statistical analyses revealed no differences in squat jump height or relative ground reaction force, but the takeoff velocity (SSM-2.06 ± 0.17 m·s(-1); NM-1.99 ± 0.18 m·s(-1)), rate of velocity development (SSM-5.92 ± 1.46 m·s(-2); NM-5.63 ± 1.70 m·s(-2)), and rate of force development (SSM-3175.61 ± 1792.37 N·s(-1); NM-2519.12 ± 1470.32 N·s(-1)) were greater with SSM, whereas RPE (SSM-5.71 ± 1.37; NM-6.36 ± 1.61) was greater with NM. Bench press reps to failure and RPE were not different between conditions. The POMS scores of vigor (SSM-20.15 ± 5.58; NM-17.45 ± 5.84), tension (SSM-8.40 ± 3.99; NM-6.07 ± 3.26), and fatigue (SSM-8.65 ± 4.49; NM-7.40 ± 4.38) were greater with SSM. This study demonstrated increased performance during an explosive exercise and an altered mood state when listening to SSM. Therefore, listening to SSM might be beneficial for acute power performance.  相似文献   
140.
A retrospective review of 16 cases of midline (dermoid) cysts of the floor of the mouth is presented, evaluating the different surgical approaches. Sixteen cases of patients with a diagnosis of midline cyst of the floor of the mouth, treated at the Maxillofacial Surgery Department of the School of Medicine and Surgery of the "Federico II" University of Naples (Naples, Italy), were observed over a 10-year period, between 1988 and 1998; age, sex, localization, diagnostic technique, and type of treatment were evaluated. Male patients were more frequently affected, with a male-to-female ratio of 3:1 (12:4 cases). Patients ranged in age from 5 to 51 years (average age, 27.8 years). The preoperative assessment was made using ultrasonography in all cases but one, computed tomography in eight cases, and magnetic resonance imaging in three cases. Regarding surgical techniques used, a transcutaneous approach was adopted for median geniohyoid cysts, an extended median glossotomy technique was used for very large median genioglossal cysts, a median glossotomy technique was used for median genioglossal cysts, and a midline incision of the oral mucosa along the lingual frenulum was used for sublingual cysts. During the postoperative course, there were no complications except for modest edema in three cases. Follow-up ranged between 24 months and 12 years; no relapses or malignant changes were observed. In the authors' experience, the intraoral approach was also effective for the treatment of large lesions and led to very good cosmetic and functional results, whereas the extraoral incision was necessary only when the cysts were under the geniohyoid muscle.  相似文献   
[首页] « 上一页 [9] [10] [11] [12] [13] 14 [15] [16] [17] [18] [19] 下一页 » 末  页»
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