全文获取类型
收费全文 | 365篇 |
免费 | 33篇 |
专业分类
398篇 |
出版年
2021年 | 7篇 |
2020年 | 5篇 |
2018年 | 4篇 |
2017年 | 3篇 |
2016年 | 6篇 |
2015年 | 10篇 |
2014年 | 11篇 |
2013年 | 20篇 |
2012年 | 17篇 |
2011年 | 13篇 |
2010年 | 11篇 |
2008年 | 16篇 |
2007年 | 16篇 |
2006年 | 10篇 |
2005年 | 12篇 |
2004年 | 10篇 |
2003年 | 10篇 |
2002年 | 11篇 |
2001年 | 15篇 |
2000年 | 12篇 |
1999年 | 12篇 |
1998年 | 9篇 |
1997年 | 7篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1992年 | 9篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1989年 | 2篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 9篇 |
1985年 | 11篇 |
1984年 | 9篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1980年 | 3篇 |
1979年 | 8篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1974年 | 5篇 |
1973年 | 7篇 |
1972年 | 6篇 |
1971年 | 4篇 |
1969年 | 2篇 |
1968年 | 3篇 |
1967年 | 5篇 |
1966年 | 3篇 |
排序方式: 共有398条查询结果,搜索用时 0 毫秒
41.
42.
Humans running and hopping maintain similar center-of-mass motions, despite large changes in surface stiffness and damping. The goal of this study was to determine the contributions of anticipation and reaction when human hoppers encounter surprise, expected, and random changes from a soft elastic surface (27 kN/m) to a hard surface (411 kN/m). Subjects encountered the expected hard surface on every fourth hop and the random hard surface on an average of 25% of the hops in a trial. When hoppers on a soft surface were surprised by a hard surface, the ankle and knee joints were forced into greater flexion by passive interaction with the hard surface. Within 52 ms after subjects landed on the surprise hard surface, joint flexion increased, and the legs became less stiff than on the soft surface. These mechanical changes occurred before electromyography (EMG) first changed 68-188 ms after landing. Due to the fast mechanical reaction to the surprise hard surface, center-of-mass displacement and average leg stiffness were the same as on expected and random hard surfaces. This similarity is striking because subjects anticipated the expected and random hard surfaces by landing with their knees more flexed. Subjects also anticipated the expected hard surface by increasing the level of EMG by 24-76% during the 50 ms before landing. These results show that passive mechanisms alter leg stiffness for unexpected surface changes before muscle EMG changes and may be critical for adjustments to variable terrain encountered during locomotion in the natural world. 相似文献
43.
J M Farley C H Romero M G Spalding M L Avery D J Forrester 《Journal of wildlife diseases》2001,37(4):808-812
In order to understand the epidemiology of Newcastle disease (ND) outbreaks in double-crested cormorants (Phalacrocorax auritus), a study was conducted on wintering migratory cormorants (P. a. auritus) in Alabama and Mississippi (USA) and non-migratory cormorants (P. a. floridanus) that breed in Florida (USA). Antibodies against ND virus were detected by the hemagglutination-inhibition method in sera from 86 of 183 (47%) migratory cormorants over-wintering in eight roosting sites in Alabama and Mississippi between November, 1997 and April, 1999. Titers ranged from 5 to 40. Antibody prevalences in sera collected from females in early winter (November and December) (26%) and late winter (February and March) (56%) were significantly different (P = 0.0007). None of 45 serum samples from 1- to 7-wk-old nestlings from 11 colonies in Florida during the 1997-98 and 1998-99 breeding seasons was positive. However, antibodies were detected in yolk samples from 98 of 126 (78%) eggs collected in these same colonies. Titers ranged from 4 to 256. The prevalence of antibodies in eggs collected from fresh-water colonies (63% prevalence, n = 30) and salt-water colonies (82% prevalence, n = 96) was significantly different (P = 0.041). ND virus was not isolated from tissues of 18 cormorants and cloacal and tracheal swabs from 202 cormorants collected in Alabama and Mississippi; virus was also not isolated from cloacal and tracheal swabs from 51 nestlings from Florida. 相似文献
44.
Interannual variations in distribution, size, indices of feeding and condition of juvenile Bristol Bay sockeye salmon Oncorhynchus nerka collected in August to September (2000–2003) during Bering–Aleutian Salmon International Surveys were examined to test possible mechanisms influencing their early marine growth and survival. Juvenile sockeye salmon were mainly distributed within the southern region of the eastern Bering Sea, south of 57°0' N during 2000 and 2001 and farther offshore, south of 58°0' N during 2002 and 2003. In general, juvenile sockeye salmon were significantly larger ( P < 0·05) and had significantly higher indices of condition ( P < 0·05) during 2002 and 2003 than during 2000 and 2001. The feeding index was generally higher for age 1.0 year sockeye salmon than age 2.0 year during all years. Among-year comparisons suggested that Pacific sand lance Ammodytes hexapterus were important components of the juvenile sockeye salmon diet during 2000 and 2001 (20 to 50% of the mean wet mass) and age 0 year walleye pollock Theragra chalcogramma were important components during 2002 and 2003 (50 to 60% of the mean wet mass). Warmer sea temperatures during spring and summer of 2002 and 2003 probably increased productivity on the eastern Bering Sea shelf, enhancing juvenile sockeye salmon growth. 相似文献
45.
46.
Farley RA Elquza E Müller-Ehmsen J Kane DJ Nagy AK Kasho VN Faller LD 《Biochemistry》2001,40(21):6361-6370
We have proposed a model for part of the catalytic site of P-type pumps in which arginine in a signature sequence functions like lysine in P-loop-containing enzymes that catalyze adenosine 5'-triphosphate hydrolysis [Smirnova, I. N., Kasho, V. N., and Faller, L. D. (1998) FEBS Lett. 431, 309-314]. The model originated with evidence from site-directed mutagenesis that aspartic acid in the DPPR sequence of Na,K-ATPase binds Mg(2+) [Farley, R. A., et al. (1997) Biochemistry 36, 941-951]. It was developed by assuming that the catalytic domain of P-type pumps evolved from enzymes that catalyze phosphoryl group transfer. The functions of the positively charged amino group in P-loops are to bind substrate and to facilitate nucleophilic attack upon phosphorus by polarizing the gamma-phosphorus-oxygen bond. To test the prediction that the positively charged guanidinium group of R596 in human alpha(1) Na,K-ATPase participates in phosphoryl group transfer, the charge was progressively decreased by site-directed mutagenesis. Mutants R596K, -Q, -T, -M, -A, -G, and -E were expressed in yeast membranes, and their ability to catalyze phosphorylation with inorganic phosphate was evaluated by following (18)O exchange. R596K, in which the positive charge is retained, resembled the wild type. Substitution of a negative charge (R596E) resulted in complete loss of activity. The remaining mutants with uncharged side chains had both lowered affinity for inorganic phosphate and altered phosphate isotopomer distributions, consistent with increased phosphate-off rate constants compared to that of the wild type. Therefore, mutations of R596 strengthen our hypothesis that the oppositely charged side chains of the DPPR peptide in Na,K-ATPase form a quaternary complex with magnesium phosphate. 相似文献
47.
To obtain information about the chemical composition of pore structures in antennal sensilla, the antennae were exposed to lipid solvents, or they were prepared to show negative-contrast images in electron micrographs. A heavy-metal tracer, lanthanum nitrate, was also used to indicate the permeability of the receptors to water. The grooves of the large grooved peg open into tubular cavities containing electronopaque material, through which stimulatory molecules must pass to reach the sensory dendrites at the center of the sensillum. The material in these cavities was removed by chloroform or acetone, suggesting a lipid composition. Lanthanum penetrated this receptor only after it had been exposed to acetone or chloroform. Strands at pores of the thin-walled pegs were also removed by the lipid solvents, and the water-soluble tracer failed to penetrate these receptors unless they had been previously exposed to chloroform or acetone. The pore structures appear to be hydrophobic, allowing entry of lipid-soluble substances, while preventing passage of water. The differential action of the solvents on the various types of sensilla suggests that receptor discrimination among different classes of chemical stimuli may be partially determined by the chemical properties of structures at the sensillar pores. 相似文献
48.
Phospholipase D (PLD) is a phosphodiesterase that catalyses hydrolysis of phosphatidylcholine to produce phosphatidic acid and choline. In the presence of ethanol, PLD also catalyses the formation of phosphatidylethanol, which is a unique characteristic of this enzyme. Muscarinic receptor-induced changes in the activity of PLD were investigated in porcine tracheal smooth muscle by measuring the formation of [3H]phosphatidic acid ([3H]PA) and [3H]phosphatidylethanol ([3H]PEth) after labeling the muscle strips with [3H]palmitic acid. The cholinergic receptor agonist acetylcholine (Ach) significantly but transiently increased formation of both [3H]PA and [3H]PEth in a concentration-dependent manner (>105–400% vs. controls in the presence of 10–6 to 10–4
M Ach) when pretreated with 100 mM ethanol. The Ach receptor-mediated increase in PLD activity was inhibited by atropine (10–6
M), indicating that activation of PLD occurred via muscarinic receptors. Activation of protein kinase C (PKC) by phorbol-12-myristate-13-acetate (PMA) increased PLD activity that was effectively blocked by the PKC inhibitors calphostin C (10–8 to 10–6
M) and GFX (10–8 to 10–6
M). Ach-induced increases in PLD activity were also significantly, but incompletely, inhibited by both GFX and calphostin C. From the present data, we conclude that in tracheal smooth muscle, muscarinic acetylcholine receptor-induced PLD activation is transient in nature and coupled to these receptors via PKC. However, PKC activation is not solely responsible for Ach-induced activation of PLD in porcine tracheal smooth muscle. 相似文献
49.
50.