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   936篇
  免费   26篇
  2015年   9篇
  2014年   8篇
  2013年   21篇
  2012年   11篇
  2011年   10篇
  2010年   28篇
  2009年   22篇
  2008年   29篇
  2007年   45篇
  2006年   17篇
  2005年   23篇
  2004年   15篇
  2002年   13篇
  1999年   13篇
  1998年   9篇
  1997年   17篇
  1996年   13篇
  1995年   14篇
  1994年   12篇
  1993年   17篇
  1992年   10篇
  1991年   19篇
  1990年   19篇
  1989年   12篇
  1988年   10篇
  1987年   19篇
  1986年   13篇
  1985年   15篇
  1984年   17篇
  1982年   15篇
  1980年   15篇
  1979年   13篇
  1976年   13篇
  1975年   10篇
  1974年   17篇
  1973年   21篇
  1972年   16篇
  1971年   11篇
  1970年   11篇
  1969年   9篇
  1968年   12篇
  1965年   36篇
  1964年   19篇
  1963年   32篇
  1962年   23篇
  1961年   34篇
  1960年   21篇
  1959年   27篇
  1958年   27篇
  1957年   12篇
排序方式: 共有962条查询结果,搜索用时 31 毫秒
71.
Endothelial nitric oxide synthase (eNOS) is an attractive target for cardiovascular gene therapy. Marrow stromal cells (MSCs), also known as mesenchymal stem cells, hold great promise for use in adult stem cell-based cell and gene therapy. To determine the feasibility of adenoviral-mediated eNOS gene transfer into ex vivo expanded MSCs, rat MSCs (rMSCs) were isolated, expanded ex vivo, and transduced with Ad5RSVeNOS, an adenoviral vector containing the eNOS gene under the control of the Rous sarcoma virus promoter. The presence of eNOS protein in Ad5RSVeNOS-transduced rMSCs was confirmed by immunohistochemical and Western blot analysis. Transduction efficiency was dose dependent, and eNOS transgene expression in rMSCs persisted for =" BORDER="0">21 days in culture. The rMSCs retained multipotential differentiation capability after adenoviral-mediated eNOS gene transfer. Furthermore, intracavernosal injection of Ad5RSVeNOS-transduced rMSCs increased the expression of eNOS in the corpus cavernosum, and stem cells were identified within corporal sinusoids. These findings demonstrate that replication-deficient recombinant adenovirus can be used to engineer ex vivo expanded rMSCs and that high-level eNOS transgene expression can be achieved, pointing out the clinical potential of using this novel adult stem cell-based gene therapy method for the treatment of cardiovascular diseases. adenoviral vector; nitric oxide; gene expression; differentiation; gene therapy  相似文献   
72.
73.

Objective

Persons with visual impairment (VI) are at greater risk for falls due to irreparable damage to visual sensory input contributing to balance. Targeted training may significantly improve postural stability by strengthening the remaining sensory systems. Here, we evaluate the Ashtanga-based Yoga Therapy (AYT) program as a multi-sensory behavioral intervention to develop postural stability in VI.

Design

A randomized, waitlist-controlled, single-blind clinical trial

Methods

The trial was conducted between October 2012 and December 2013. Twenty-one legally blind participants were randomized to an 8-week AYT program (n = 11, mean (SD) age = 55(17)) or waitlist control (n=10, mean (SD) age = 55(10)). AYT subjects convened for one group session at a local yoga studio with an instructor and two individual home-based practice sessions per week for a total of 8 weeks. Subjects completed outcome measures at baseline and post-8 weeks of AYT. The primary outcome, absolute Center of Pressure (COP), was derived from the Wii Balance Board (WBB), a standalone posturography device, in 4 sensory conditions: firm surface, eyes open (EO); firm surface, eyes closed (EC); foam surface, EO; and foam surface, EC. Stabilization Indices (SI) were computed from COP measures to determine the relative visual (SIfirm, SIfoam), somatosensory (SIEO, SIEC) and vestibular (SIV, i.e., FoamEC vs. FirmEO) contributions to balance. This study was not powered to detect between group differences, so significance of pre-post changes was assessed by paired samples t-tests within each group.

Results

Groups were equivalent at baseline (all p > 0.05). In the AYT group, absolute COP significantly increased in the FoamEO (t(8) = -3.66, p = 0.01) and FoamEC (t(8) = -3.90, p = 0.01) conditions. Relative somatosensory SIEO (t(8) = -2.42, p = 0.04) and SIEC (t(8) = -3.96, p = 0.01), and vestibular SIV (t(8) = -2.47, p = 0.04) contributions to balance increased significantly. As expected, no significant changes from EO to EC conditions were found indicating an absence of visual dependency in VI. No significant pre-post changes were observed in the control group (all p > 0.05).

Conclusions

These preliminary results establish the potential for AYT training to develop the remaining somatosensory and vestibular responses used to optimize postural stability in a VI population.

Trial Registration

www.ClinicalTrials.gov NCT01366677  相似文献   
74.
75.
ABSTRACT Management of Pacific Flyway Canada geese (Branta canadensis) requires information on winter distribution of different populations. Recoveries of tarsus bands from Vancouver Canada geese (B. canadensis fulva) marked in southeast Alaska, USA, ≥4 decades ago suggested that ≥83% of the population was non-migratory and that annual adult survival was high (Ŝ = 0.836). However, recovery distribution of tarsus bands was potentially biased due to geographic differences in harvest intensity in the Pacific Flyway. Also, winter distribution of Vancouver Canada geese could have shifted since the 1960s, as has occurred for some other populations of Canada geese. Because winter distribution and annual survival of this population had not recently been evaluated, we surgically implanted very high frequency radiotransmitters in 166 adult female Canada geese in southeast Alaska. We captured Vancouver Canada geese during molt at 2 sites where adults with goslings were present (breeding areas) and 2 sites where we observed nonbreeding birds only. During winter radiotracking flights in southeast Alaska, we detected 98% of 85 females marked at breeding areas and 83% of 70 females marked at nonbreeding sites, excluding 11 females that died prior to the onset of winter radiotracking. We detected no radiomarked females in coastal British Columbia, or western Washington and Oregon, USA. Most (70%) females moved ≤30 km between November and March. Our model-averaged estimate of annual survival (Ŝ = 0.844, SE = 0.050) was similar to the estimate of annual survival of geese marked from 1956 to 1960. Likely <2% of Vancouver Canada geese that nest in southeast Alaska migrate to winter areas in Oregon or Washington where they could intermix with Canada geese from other populations in the Pacific Flyway. Because annual survival of adult Vancouver Canada geese was high and showed evidence of long-term consistency, managers should examine how reproductive success and recruitment may affect the population.  相似文献   
76.
1. Rates of whole‐system metabolism (production and respiration) are fundamental indicators of ecosystem structure and function. Although first‐order, proximal controls are well understood, assessments of the interactions between proximal controls and distal controls, such as land use and geographic region, are lacking. Thus, the influence of land use on stream metabolism across geographic regions is unknown. Further, there is limited understanding of how land use may alter variability in ecosystem metabolism across regions. 2. Stream metabolism was measured in nine streams in each of eight regions (n = 72) across the United States and Puerto Rico. In each region, three streams were selected from a range of three land uses: agriculturally influenced, urban‐influenced, and reference streams. Stream metabolism was estimated from diel changes in dissolved oxygen concentrations in each stream reach with correction for reaeration and groundwater input. 3. Gross primary production (GPP) was highest in regions with little riparian vegetation (sagebrush steppe in Wyoming, desert shrub in Arizona/New Mexico) and lowest in forested regions (North Carolina, Oregon). In contrast, ecosystem respiration (ER) varied both within and among regions. Reference streams had significantly lower rates of GPP than urban or agriculturally influenced streams. 4. GPP was positively correlated with photosynthetically active radiation and autotrophic biomass. Multiple regression models compared using Akaike’s information criterion (AIC) indicated GPP increased with water column ammonium and the fraction of the catchment in urban and reference land‐use categories. Multiple regression models also identified velocity, temperature, nitrate, ammonium, dissolved organic carbon, GPP, coarse benthic organic matter, fine benthic organic matter and the fraction of all land‐use categories in the catchment as regulators of ER. 5. Structural equation modelling indicated significant distal as well as proximal control pathways including a direct effect of land‐use on GPP as well as SRP, DIN, and PAR effects on GPP; GPP effects on autotrophic biomass, organic matter, and ER; and organic matter effects on ER. 6. Overall, consideration of the data separated by land‐use categories showed reduced inter‐regional variability in rates of metabolism, indicating that the influence of agricultural and urban land use can obscure regional differences in stream metabolism.  相似文献   
77.

Introduction  

Rheumatoid arthritis (RA) frequently involves the loss of tolerance to citrullinated antigens, which may play a role in pathogenicity. Citrullinated fibrinogen is commonly found in inflamed synovial tissue and is a frequent target of autoantibodies in RA patients. To obtain insight into the B-cell response to citrullinated fibrinogen in RA, its autoepitopes were systematically mapped using a new methodology.  相似文献   
78.
79.
Research into dolphin swimming historically was guided by false assumptions pertaining to maximum speed. Accurate measurements on swimming speed and duration of effort of free-ranging dolphins are rare. To examine the variance of maximum swimming speeds, nearly 2,000 speed measurements were obtained for both captive and free-ranging dolphins, including Tursiops truncatus, Pseudorca crassidens, Delphinus capensis , and Delphinus delpbis . Measurements were made from videotapes of dolphins trained to swim fast around a large pool or jumping to a maximum height, videotapes of captured wild dolphins immediately after release, and sequential aerial photographs of a school of free-ranging dolphins startled by a passing airplane. Maximum horizontal speeds for trained animals were 8.2 m/sec for T. truncatus , 8.0 m/sec for D. delphis , and 8.0 m/sec for P. crassidens . Maximum speeds for T. truncatus swimming upwards, prior to vertical leaps ranged from 8.2 to 11.2 m/sec. Wild T. truncatus demonstrated a maximum speed of 5.7 m/sec. Maximum swimming speed of free-ranging D. capensis responding to multiple passes by a low flying airplane was 6.7 m/sec. There was no evidence that the freeranging dolphins have superior swimming capabilities to captive animals. The results of this study imply that realistic maximum swimming speeds for dolphins are lower than previous reports which were based on sparse data and imprecise measurement techniques.  相似文献   
80.
ABSTRACT Introduced disease is a major mortality factor in some populations of bighorn sheep (Ovis canadensis). Epizootics of infectious keratoconjunctivitis (IKC) and contagious ecthyma occurred in bighorn sheep in the Silver Bell Mountains of south-central Arizona, USA, from 1 December 2003 to 31 March 2004. Our objectives were to 1) investigate the influence of the epizootic on abundance and demographics and 2) examine how IKC affected the mortality, behavior, and movements of clinically affected animals. Morbidity was 39%, and all sex and age classes were affected. The population declined 23%, with most mortality in the adult female (1 M, 11 F) segment of the population. Of the diseased animals that were marked (n = 27), 44% recovered and 44% died. Predation (50%) and starvation (33%) were the primary causes of mortality of diseased bighorn sheep. Bighorn sheep that were infected spent less time feeding and moved less than noninfected animals during the epizootic. Managers might be able to minimize losses of infected animals through predator control. To minimize losses to starvation, managers should refrain from any activity that disturbs infected animals (including treatment) because disturbances increase energy expenditures and expose infected animals to injury.  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