全文获取类型
收费全文 | 1098篇 |
免费 | 158篇 |
国内免费 | 177篇 |
出版年
2024年 | 9篇 |
2023年 | 46篇 |
2022年 | 40篇 |
2021年 | 56篇 |
2020年 | 54篇 |
2019年 | 59篇 |
2018年 | 41篇 |
2017年 | 41篇 |
2016年 | 38篇 |
2015年 | 44篇 |
2014年 | 60篇 |
2013年 | 72篇 |
2012年 | 46篇 |
2011年 | 42篇 |
2010年 | 37篇 |
2009年 | 52篇 |
2008年 | 51篇 |
2007年 | 43篇 |
2006年 | 51篇 |
2005年 | 48篇 |
2004年 | 53篇 |
2003年 | 41篇 |
2002年 | 37篇 |
2001年 | 41篇 |
2000年 | 34篇 |
1999年 | 33篇 |
1998年 | 24篇 |
1997年 | 21篇 |
1996年 | 30篇 |
1995年 | 20篇 |
1994年 | 10篇 |
1993年 | 17篇 |
1992年 | 15篇 |
1991年 | 13篇 |
1990年 | 13篇 |
1989年 | 9篇 |
1988年 | 5篇 |
1987年 | 9篇 |
1986年 | 7篇 |
1985年 | 14篇 |
1984年 | 8篇 |
1983年 | 9篇 |
1982年 | 10篇 |
1981年 | 4篇 |
1980年 | 5篇 |
1979年 | 5篇 |
1978年 | 6篇 |
1976年 | 2篇 |
1973年 | 3篇 |
1971年 | 2篇 |
排序方式: 共有1433条查询结果,搜索用时 31 毫秒
1.
Abstract: The enzymatic hydrolysis of UDP-galactose in rat and calf brain was studied. The hydrolysis occurs in two steps: The first is the conversion of UDP-galactose to galactose-1-phosphate catalyzed by nucleotide pyrophosphatase (EC 3.6.1.9), and the second is the conversion of the latter to free galactose by alkaline phosphatase (EC 3.1.3.1). The overall conversion has a pH optimum of 9.0, but there is considerable activity at pH 7.4, which is the optimum for UDP-galactose:ceramide galactosyltransferase in the synthesis of cerebrosides. Preparations from cytosol from calf brain cerebellum or stem that were enriched in UDP-galactose hydrolytic activity inhibit cerebroside synthesis under conditions optimal for the synthesis. Microsome-rich and nuclear debris fractions contain the highest apparent specific activity among the subcellular fractions studied. Hydrolysis of UDP-galactose occurs in all areas of brain, brainstem having the highest activity. The apparent specific activity in jimpy mouse brain homogenate is nearly twice as high as in the control brain homogenate. 相似文献
2.
3.
A number of methods of construction of partially balanced incomplete block designs with nested rows and columns are developed and new balanced incomplete block designs with nested rows and columns are obtained as a by-product. 相似文献
4.
A particle-induced X-ray emission (PIXE) analysis method is presented, which allows measurement of eight elements (i.e., K,
Ca, Mn, Fe, Cu, Zn, Se, and Rb) in human brain samples of only a few mg dry weight. The precision and accuracy of the method
were investigated by analyzing animal brain matter with both PIXE and instrumental neutron activation analysis (INAA). The
method was applied to measure the 8 elements in 46 different regions of 3 human brains. The sections analyzed originated from
either the left or the right cerebral hemisphere, brain stem, and cerebellum. For one of the brains, sections were also analyzed
from 26 corresponding regions of both hemispheres. For all elements, similar concentrations were found in the corresponding
areas of the left and right sides of the brain. The concentrations (in μg/g dry weight) of the elements K, Fe, Cu, Zn, Se,
and Rb were consistently higher in cortical structures than in white matter. Deep nuclei and brain stem, which have a mixed
composition, showed intermediate values for K, Zn, Se, and Rb. A hierarchical cluster analysis indicated that the various
brain regions clustered into two large groups, one comprising gray and mixed matter regions and the other, white and mixed
matter brain areas. 相似文献
5.
Regional environments, life-history patterns, and habitat use of spirostreptid millipedes in arid regions 总被引:1,自引:0,他引:1
C. S. CRAWFORD K. BERCOVITZ M. R. WARBURG 《Zoological Journal of the Linnean Society》1987,89(1):63-88
Among diplopods with desert populations, only three species of Spirostreptida have been studied in an ecological context. The present review compares regional environments, life-history patterns, and uses of habitat by Orlhoporus ornatus (Girard) from southwestern North America, Archispirostreptus tumuliporus judaicus (Attems) from the eastern Mediterranean seaboard, and Harpagophora nigra (Attems) from southwestern Africa. Published and unpublished studies are used to explore evidence for convergence among these species, as opposed to traits adapting them to physical aspects of given regions or habitats. Unlike A. t. judaicus, O. ornatus and H. nigra are relatively restricted to arid habitats, although populations of all three species experience a variety of rainfall regimes and regional topographies. Where studied, O. ornatus and H. nigra hibernate during the long, often cool or cold dry season; they forage following warm-season rains. A. t. judaicus , in contrast, forages during its long, warm dry season and hibernates in the cool, wet winter. Populations from the Judaean and Negev deserts differ from those inhabiting a mesic habitat (Megiddo) closer to the coast in regard to rates of development, seasonal activity and seasonal water balance. Convergence in the form of well-developed desiccation resistance characterizes the two strictly desert species. All three species, together with other subtropical millipedes exposed to long dry seasons, are convergent with respect to patterns of diel surface activity and use of shelter. However species- and habitat-specific life-history features such as the seasonal timing of dormancy and emergence tend to mask convergence at the habitat level. Hence, the independent evolution of the three species with desert populations has resulted in life histories and habitat use that combine a moderate amount of convergence with considerable opportunistic adaptation to regional and local conditions. 相似文献
6.
心房钠尿因子对麻醉家兔局部血流的影响 总被引:3,自引:2,他引:1
在42只麻醉家兔,观察了静脉注射心房肽Ⅱ(AtriopeptinⅡ,APⅡ)对局部血流量以及动脉内注射 AP Ⅱ 对局部血管阻力的影响。结果如下:(1)静脉注射 APⅡ(30μg/kg)5min后,平均动脉压(MAP)降低11.0±1.5mmHg(n=8,M±SE,下同),与溶剂对照组相比有明显差异(P相似文献
7.
The effects of a single does of LiCl (2.5 or 10 mEq/kg) on brain inositol and inositol-1-phosphate (Ins1P), intermediates of brain phosphoinositude (PI) turnover, were determinated in male Han: Wistar rats. There was a remarkable, 36–58 fold elevation of brain Li+ as the single does of LiCl was increased 4-fold. Moreover, the accumulation of brain lithium was slow during repeated administration of LiCl. Brain lithium did not correlate with changes in brain PI turnover either after a single or repeated doses. Thus, after a single does of LiCl the increases in brain Ins1P were much less than the decreases in brain inositol. Also, brain inositol was significantly decreased only with the high dose of LiCl whereas brain Ins1P accumulation was more prominent with the lower dose. Moreover, repeated daily doses of LiCl only transiently increased brain Ins1P at 1 and 7 d whereas inositol remained at control levels throughout the 14 d observation period. Lithium probably caused the transient decrease in brain inositol by inhibiting several enzymes, in addition to the inhibition of myo-inositol mono-phosphates, in the PI cycle. Moreover, a slow dampening down of PI turnover by lithium, possible via an inhibitory action on G-protein-coupling, may also explain the present findings. 相似文献
8.
The effect of naloxone-precipitated withdrawal after acute morphine was studied on the concentrations of noradrenaline (NA), 4-hydroxy-3-methoxyphenylethyleneglycol (MHPG), dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and on the metabolite/parent amine ratios MHPG/NA, DOPAC/DA and HVA/DA, in eight regions of the guineapig brain. Guinea-pigs were treated with a single dose of morphine sulphate (15 mg/kg s.c.) or saline (control) and 2h later with naloxone hydrochloride (15 mg/kg s.c.) to precipitate withdrawal. The animals were decapitated at 0.5 h or 1 h after naloxone injections and their brains analysed for monoamine concentrations by HPLC-ECD. At 0.5 h after naloxone-precipitated withdrawal NA and MHPG levels, and the MHPG/NA ratio, were increased in the hypothalamus, and the NA levels were increased in the hypothalamus, medulla/pons and cortex 1 h after naloxone. Naloxoneprecipitated withdrawal also produced increased DA metabolism in the cortex, midbrain and medulla 0.5 h later, and in the cortex, hypothalamus and striatum 1 h later. Hence naloxone-precipitated withdrawal from acute morphine treatment produced a complex pattern of increased synthesis and metabolism of NA and DA which varied over time and with the brain region examined. 相似文献
9.
Summary The gap junction morphology was quantified in freeze-fracture replicas prepared from rat auricles that had been either quickly frozen at 6 K or chemically fixed by glutaraldehyde, in a state of normal cell-to-cell conduction or in a state of electrical uncoupling. The general appearance of the gap junctions was similar after both preparative procedures. A quantitative analysis of three gap junctional dimensions provided the following measurements in the quickly frozen conducting auricles (mean±sd): (a) P-face particles' diameter 8.27±0.74 nm (n =5709), (b) P-face particles' center-to-center distance 10.78±2.12 nm (n=4800), and (c) E-face pits' distance 9.99±2.19 nm (n=1600). Corresponding values obtained from chemically fixed tissues were decreased by about 3% for the particle's diameter and about 5% for the particles' and pits' distances. Electrical uncoupling by the action of either 1 mM 2–4-dinitrophenol (DNP), or 3.5 mMn-Heptan-1-ol (heptanol), induced a decrease of the particle's diameter, which amounted to –0.69±0.01 nm (mean ±se) in the quickly frozen preparations and –0.71±0.01 nm in the chemically fixed ones. The particles' distance was decreased by –0.96±0.04 nm in the quickly frozen samples and by –0.90 ±0.03 nm in the chemically fixed ones and the E-face pits' distance was similarly reduced. All differences were statistically significant (P<0.001 for all dimensions). Electrical recoupling after the heptanol effect promoted a return of these gap junctional dimensions towards normal values, which was about 50% complete within 20 min. It is concluded that very similar morphological alterations of the gap junctional structure are induced in the mammalian heart by different treatments promoting electrical uncoupling and that these conformational changes appear independently of the preparative procedure. The suggestion that the observed decrease of the particles' diameter is genuinely related to the closing mechanism of the unit cell-to-cell channel set in thei centers is thus confirmed. 相似文献
10.
Human Brain Phenol Sulfotransferase: Biochemical Properties and Regional Localization 总被引:7,自引:5,他引:2
William F. Young Jr. Haruo Okazaki† Edward R. Laws Jr. ‡ Richard M. Weinshilboum 《Journal of neurochemistry》1984,43(3):706-715
Phenol sulfotransferase (PST) catalyzes the sulfate conjugation of catecholamines and phenol and catechol drugs. The human blood platelet contains a thermolabile (TL) form of PST that catalyzes the sulfate conjugation of dopamine and other monoamines and a thermostable (TS) form that catalyzes the sulfate conjugation of micromolar concentrations of phenol and p-nitrophenol. Experiments were performed to determine whether the brain contains forms of PST analogous to the TL and TS forms found in the human platelet, and to determine whether there are regional variations in human brain PST activity. We found that the human brain contains at least two forms of PST, forms that are similar to the platelet TS and TL forms of the enzyme with respect to substrate specificity, apparent Km constants, thermal stability, and sensitivity to inhibitors. Optimal conditions were determined for the measurement of these two activities in brain homogenates. The stability of PST activities in the human brain after death was determined in five samples of cerebral cortex that were obtained during clinically indicated neurosurgical procedures. An average of 76 +/- 8% and 80 +/- 9% (mean +/- SEM) of the basal TL and TS PST activities, respectively, remained in these five samples of cerebral cortex after 8 h of storage under simulated post-mortem conditions. Six human brains were then obtained less that 8 h after death from patients who had no neurological disease prior to death. The mean activities of the TL and TS forms of PST were measured in 17 different regions of the six brains. If the pituitary was excluded from consideration, TL and TS PST activities both varied approximately fivefold among these regions, and both activities were highest in cerebral cortex. However, the average TS activity in the anterior pituitary, a tissue of non-neural origin embryologically, was 6.5-fold greater than the highest average TS PST activity found in cerebral cortex. 相似文献