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   2690篇
  免费   13篇
  国内免费   1篇
电工技术   8篇
化学工业   61篇
金属工艺   7篇
机械仪表   3篇
建筑科学   5篇
矿业工程   1篇
能源动力   12篇
轻工业   22篇
水利工程   1篇
石油天然气   2篇
无线电   77篇
一般工业技术   75篇
冶金工业   2355篇
原子能技术   8篇
自动化技术   67篇
  2023年   3篇
  2022年   9篇
  2021年   7篇
  2020年   5篇
  2019年   3篇
  2018年   7篇
  2017年   9篇
  2016年   11篇
  2015年   7篇
  2014年   9篇
  2013年   18篇
  2012年   16篇
  2011年   20篇
  2010年   17篇
  2009年   18篇
  2008年   22篇
  2007年   7篇
  2006年   13篇
  2005年   11篇
  2004年   13篇
  2003年   17篇
  2002年   10篇
  2001年   12篇
  2000年   12篇
  1999年   80篇
  1998年   769篇
  1997年   402篇
  1996年   282篇
  1995年   139篇
  1994年   119篇
  1993年   129篇
  1992年   27篇
  1991年   34篇
  1990年   22篇
  1989年   22篇
  1988年   21篇
  1987年   25篇
  1986年   19篇
  1985年   18篇
  1983年   8篇
  1982年   17篇
  1981年   13篇
  1980年   20篇
  1978年   3篇
  1977年   69篇
  1976年   169篇
  1975年   6篇
  1974年   3篇
  1964年   2篇
  1955年   2篇
排序方式: 共有2704条查询结果,搜索用时 46 毫秒
1.
Excessive energy intake may evoke complex biochemical processes characterized by inflammation, oxidative stress, and impairment of mitochondrial function that represent the main factors underlying noncommunicable diseases. Because cow milk is widely used for human nutrition and in food industry processing, the nutritional quality of milk is of special interest with respect to human health. In our study, we analyzed milk produced by dairy cows fed a diet characterized by a high forage:concentrate ratio (high forage milk, HFM). In view of the low n-6:n-3 ratio and high content of conjugated linoleic acid of HFM, we studied the effects of this milk on lipid metabolism, inflammation, mitochondrial function, and oxidative stress in a rat model. To this end, we supplemented for 4 wk the diet of male Wistar rats with HFM and with an isocaloric amount (82 kJ, 22 mL/d) of milk obtained from cows fed a diet with low forage:concentrate ratio, and analyzed the metabolic parameters of the animals. Our results indicate that HFM may positively affect lipid metabolism, leptin:adiponectin ratio, inflammation, mitochondrial function, and oxidative stress, providing the first evidence of the beneficial effects of HFM on rat metabolism.  相似文献   
2.
Phospholamban (PLN) was expressed in Escherichia coli as a protein fusion with glutathione S-transferase (GST). GST-PLN was mostly present in the insoluble protein fraction and accounted for approximately 50% of total insoluble protein. Attempts to suppress inclusion body formation or to use GST as an affinity-purification tag failed. A successful purification method is based on preparative SDS/PAGE and electrodialysis. From 1 g cells we typically purified 13.5 mg fusion protein with a PLN content of 2.8 mg. We genetically inserted an enterokinase (EK) protease site just in front of the PLN sequence and demonstrated the proteolytical liberation of PLN from the carrier protein. The approach described represents a substantial advancement in PLN expression and purification.  相似文献   
3.
BACKGROUND: The effect of topical corticosteroids after excimer laser photorefractive keratectomy (PRK) remains a matter of some controversy. Refractive effects may be different according to the amount of myopia and timing of instillation. METHODS: Two groups of patients were studied: Study A consisted of 215 eyes (128 patients) with PRK (mean baseline myopia, -6.53 +/- 2.22 D) that received no corticosteroids (No Corticosteroid Group) unless significant regression or corneal haze appeared (Delayed Corticosteroid Group), and in Study B, we randomly assigned eyes to the Initial Corticosteroid Group (mean baseline myopia, -6.39 +/- 1.84 D) or the No/delayed Corticosteroid Group (mean baseline myopia -5.78 +/- 2.02 D). Clinical results after PRK for low-to-moderate and high myopia were compared. RESULTS: In the first group, 70.9% (73 eyes) of moderately myopic eyes (mean, -4.56 +/- 1.10 D) belonged to the No Corticosteroid Group that had a mean refraction of -5.39 +/- 1.77 D. Delayed Corticosteroid Group eyes were more myopic (mean, -7.52 +/- 2.10 D), and showed more severe haze than those in the No Corticosteroid Group. In study B, only in high myopes with more than -6.00 D (mean, -7.76 +/- 1.15 D) did refraction and corneal haze outcomes show significant difference between the Initial Corticosteroid Group and the No/delayed Corticosteroid Group. CONCLUSIONS: The effects of topical corticosteroids after PRK were less in moderate myopes compared to high myopes. Delayed instillation of corticosteroids did not reverse the regression or haze whereas initial instillation showed a beneficial effect on high myopes but not on moderate myopes.  相似文献   
4.
5.
We previously reported that in Cushing's disease (CD) the ACTH- and cortisol (F)-releasing activity of Hexarelin (HEX), a GH secretagogue, is exaggerated with respect to that in normal subjects and is higher than that of human CRH (hCRH), but it is absent in Cushing's syndrome. Our aim was to extend the study about the effects of HEX (2.0 microg/kg, iv) on ACTH and F secretion in 21 patients with CD (3 men and 18 women, 16-68 yr old). Based on magnetic resonance imaging, 15 CD patients had pituitary microadenoma, and 6 had macroadenoma. The results in CD patients were compared with those in 27 normal age-matched controls (NS; 10 men and 17 women, 24-69 yr old). Basal ACTH and F levels in CD were similar in patients with microadenom (mean+/-SEM, 78.3+/-7.2 pg/mL and 237.1+/-23.6 microg/L, respectively) and macroadenoma (57.4+/-9.0 pg/mL and 196.9+/-20.1 microg/L, respectively) and were higher (P < 0.001) than those in NS (17.7+/-2.0 pg/mL and 115.3+/-6.7 microg/L, respectively). In microadenoma CD patients, HEX induced marked ACTH and F increases (delta peak, mean+/-SEM: 261.2+/-77.6 pg/mL and 226.1+/-87.2 microg/L, respectively), which were higher (P < 0.04) than those induced by hCRH (45.6+/-16.9 pg/mL and 84.6+/-25.7 microg/L, respectively). Moreover, in microadenoma CD patients, the ACTH and F responses to HEX were higher (P < 0.001) than those in NS (18.5+/-4.0 pg/mL and 36.1+/-6.8 microg/L, respectively). In macroadenoma CD patients, HEX induced a slight, but significant increase (P < 0.02) in ACTH and F levels (33.9+/-18.0 pg/mL and 89.6+/-34.3 microg/L, respectively), which was not significantly different from that elicited by hCRH (20.0+/-7.0 pg/mL and 54.8+/-21.3 microg/L, respectively). In macroadenoma CD patients, the ACTH and F responses to HEX and hCRH were, in turn, similar to those in NS. In conclusion, our findings demonstrate that the ACTH and F hyperresponsiveness to HEX is present in Cushing's disease with micro-, but not macro- ACTH-secreting pituitary adenoma. This finding agrees with other evidence pointing toward differences in the hormonal behavior between micro- and ACTH-secreting pituitary macroadenomas.  相似文献   
6.
7.
Larynx-associated lymphoid tissue (LALT) in young children   总被引:1,自引:0,他引:1  
BACKGROUND: Mucosa-associated lymphoid tissue (MALT) plays a central role in mucosal immunity. Whereas the characteristics and function of MALT in the intestine are well established, almost nothing is known about MALT in the larynx. METHODS: In this study we examined the morphology and the lymphocyte subset composition of MALT in the larynges of children who had died of sudden infant death or various defined traumatic or nontraumatic causes. RESULTS: Organized lymphoid tissue was found in the supraglottic parts of the larynx in nearly 80% of the children in both groups. This lymphoid tissue showed all morphological signs of MALT, such as typical lymphoid follicles with germinal centers, infiltration of the overlying epithelium by lymphocytes, and high endothelial venules (HEV). Thus we will use the term LALT (larynx-associated lymphoid tissue) to refer to this tissue. The lymphoid follicles of LALT contained mainly B lymphocytes with some CD4+ lymphocytes in the germinal centers. Remarkably, T lymphocytes of both subset types and B lymphocytes were observed in comparable numbers in the parafollicular area. CONCLUSIONS: We assume that LALT is a physiological structure of the larynx in young children. The morphology and the distribution of lymphocyte subsets are similar to those of MALT in the human gut. LALT may be a regular part of the mucosal immune system in young children with the role of respiratory inductive site for mucosal immunity.  相似文献   
8.
Recordings from the central branches of single identified dactyl sensory afferent (DSA) neurons in a crayfish in vitro preparation were performed to study modifications of the sensory message occurring before the first central synapse. These afferents comprised hairs and force-sensitive mechanoreceptors with phasic and phasotonic response characteristics in the terminal segment (dactyl) of the crayfish leg. More than one afferent spike size was often observed in intracellular recordings from these afferents, thus indicating the presence of electrical coupling between the central processes of DSA fibers. Additionally, in identified DSA fibers with large spike sizes, primary afferent depolarizations (PADs) of up to 15 mV were observed, which sometimes triggered antidromic spikes in the afferent. Nevertheless, PADs were clearly inhibitory, because they shunted the afferent spikes. They exhibited the following properties. First, each PAD was preceded by an afferent spike from a neighboring hair, indicating that the PADs had a sensory rather than central origin. Second, PADs could follow high frequencies of afferent discharges without failure, a property suggestive of monosynaptic connections, but because PAD latencies varied by +/-0.5 ms it is more likely that they were mediated by a disynaptic pathway. Third, although PADs were evoked in an extremely reliable manner, their amplitude varied in a quantal manner. Most unitary PADs were the result of the release of < 12 quanta, the mean quantal content lying between 4 and 5; quantal size was large, approximately 1 mV. Fourth, PADs showed facilitation in some fibers, whereas in others they became much smaller when occurring at brief intervals. We suggest that PADs may be an efficient and parsimonious way to limit sensory inflow in space and time, allowing the crayfish to identify precisely both weak and strong mechanical stimuli.  相似文献   
9.
The studies aimed to estimate an incidence of the low back pain (LBP) in the youngsters and teenagers and correlating it with risk factors. A groups of 2,346 secondary school pupils (1,704 girls and 642 boys) of a mean age 17 +/- 1 yrs, and 970 high-school students (532 women and 438 men) of a mean age 24 +/- 2 yrs have been examined. Low back pain has been seen in 1,416 out of 2,346 secondary school pupils (60%), and in 32% of the examined students. Statistical analysis with chi 2 test has confirmed a correlation between LBP and such risk factors as the incorrect sedentary position (p < .001 for pupils, and p < .02 for students), and smoking (p < .001 for students and p < .02 for pupils).  相似文献   
10.
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