全文获取类型
收费全文 | 485篇 |
免费 | 49篇 |
出版年
2020年 | 4篇 |
2018年 | 5篇 |
2017年 | 5篇 |
2015年 | 7篇 |
2014年 | 4篇 |
2013年 | 13篇 |
2012年 | 18篇 |
2011年 | 22篇 |
2010年 | 24篇 |
2009年 | 10篇 |
2008年 | 32篇 |
2007年 | 24篇 |
2006年 | 24篇 |
2005年 | 24篇 |
2004年 | 19篇 |
2003年 | 23篇 |
2002年 | 21篇 |
2001年 | 7篇 |
1999年 | 6篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 7篇 |
1994年 | 5篇 |
1993年 | 5篇 |
1992年 | 7篇 |
1991年 | 8篇 |
1990年 | 8篇 |
1989年 | 5篇 |
1987年 | 5篇 |
1985年 | 4篇 |
1984年 | 7篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 7篇 |
1980年 | 4篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 5篇 |
1975年 | 7篇 |
1973年 | 12篇 |
1972年 | 4篇 |
1968年 | 5篇 |
1967年 | 4篇 |
1966年 | 5篇 |
1959年 | 4篇 |
1951年 | 3篇 |
1944年 | 3篇 |
1942年 | 3篇 |
1939年 | 3篇 |
排序方式: 共有534条查询结果,搜索用时 15 毫秒
1.
Antibodies against the chondroitin sulfate proteoglycan NG2 label a subpopulation of glial cells within the CNS, which have a small cell body and thin radiating processes. Physiological recordings from these small cells in acute brain slices have revealed that they possess unique properties, suggesting that they may comprise a class of glial cells distinct from astrocytes, oligodendrocytes, or microglia. NG2-expressing glial cells (abbreviated as “NG2 cells” here) have a moderate input resistance and are not dye- or tracer-coupled to adjacent cells. They express voltage-gated Na+, K+and Ca2+conductances, though they do not exhibit regenerative Na+or Ca2+action potentials due to the much larger K+conductances present. In addition to voltage-gated conductances, they express receptors for various neurotransmitters. In the hippocampus, AMPA and GABAAreceptors on these cells are activated by release of transmitter from neurons at defined synaptic junctions that are formed with CA3 pyramidal neurons and GABAergic interneurons. These rapid forms of neuron-glial communication may regulate the proliferation rate of NG2 cells or their development into mature oligodendrocytes. These depolarizing inputs may also trigger the release of neuroactive substances from NG2 cells, providing feedback regulation of signaling at neuronal synapses. Although the presence of Ca2+permeable AMPA receptors provides a pathway to link neuronal activity to Ca2+dependent processes within the NG2 cells, these receptors also put these cells at risk for glutamate-associated excitotoxicity. This vulnerability to the sustained elevation of glutamate may underlie ischemic induced damage to white matter tracts and contribute to cerebral palsy in premature infants. 相似文献
2.
Aileen F. G. Bongat Rina Saksena Roberto Adamo Yukari Fujimoto Zenyu Shiokawa Dwight C. Peterson Koichi Fukase Willie F. Vann Pavol Kováč 《Glycoconjugate journal》2010,27(1):69-77
Using recombinant tetanus toxin HC fragment (rTT-HC) as carrier, we prepared multimeric bivalent immunogens featuring the synthetic hexasaccharide fragment of O-PS of Vibrio cholerae O:1, serotype Ogawa, in combination with either the synthetic hexasaccharide fragment of O-PS of Vibrio cholerae O:1, serotype Inaba, or a synthetic disaccharide tetrapeptide peptidoglycan fragment as adjuvant. The conjugation reaction
was effected by squaric acid chemistry and monitored in virtually real time by SELDI-TOF MS. In this way, we could prepare
well-defined immunogens with predictable carbohydrate–carrier ratio, whose molecular mass and the amount of each saccharide attached could be independently determined. The ability to prepare such neoglycoconjugates opens unprecedented possibilities
for preparation of conjugate vaccines for bacterial diseases from synthetic carbohydrates. 相似文献
3.
Dwight D. Baker 《Physiologia plantarum》1987,70(2):245-248
Fifty strains of Frankia were tested for their ability to nodulate six species of actinorhizal plants. Pure cultured strains were used to inoculate seedlings of Alnus glutinosa (L.) Gaertn., Alnus rubra Bong., Casuarina equisetifolia L., Elaeagnus angustifolia L., Hippophaë rhamnoides L. and Myrica cerifera L. in nutrient solution culture. From the results of this study, host inoculation groups among the actinorhizal plants were defined. Although overlap between host inoculation groups appears to be common, the results from this study did not support the view that Frankia strains are promiscuous. All Frankia strains tested in this study could easily be classified into four major host-specificity groups. 相似文献
4.
Ciliary ganglion neurons and nonneurons can be separated from each other, based on the greater adhesivity of the nonneurons to untreated tissue culture plastic in the presence of serum. When the separation was carried out in the presence of Serum Plus (a commercially available supplemented serum), two populations of neurons were distinguished. Neurons in the first class (50–60% of total) adhered to plastic within 15 min, tended to aggregate into clumps, and were not well supported in long term culture by brain extracts. Neuronal adhesion to plastic was inhibited by heparin but not by chondroitin sulfate. Neurons in the second class did not attach to plastic for up to 90 min (and could thus be purified), were not as prone to aggregation, and were quantitatively supported for long periods (>2 weeks) by the neurotrophic factor(s) present in extracts of injured brain. Although no direct evidence is provided, these populations may correspond to the well characterized ciliary and choroid neurons.Special Issue dedicated to Dr. E. M. Shooter and Dr. S. Varon. 相似文献
5.
Arthur H. L. From Dwight S. Fullerton Khalil Ahmed 《Molecular and cellular biochemistry》1990,94(2):157-165
The structure-activity relationships of the genin moieties of digitalis glycosides are commonly elucidated by determining the inhibitory potency of a variety of genins toward the plasma membrane Na+, K+-ATPase; qualitatively these relationships appear to be fairly independent of the specific Na+, K+-ATPase preparation utilized for the analysis. To determine whether this is the case with regard to the sugar moieties of glycosides, the inhibitory effects of 12 monoglycosides of digitoxigenin toward four Na+, K+-ATPase preparations of different origin were measured. It was found that while recognition of the major structural determinants of sugar activity appeared to be independent of enzyme source, recognition of the minor structural determinants of activity showed some source dependence. It was also observed that the intrinsic sensitivity to sugar potentiation may be source dependent and unrelated to intrinsic sensitivity to inhibition by digitoxigenin. These observations are compatible with a model of the Na+, K+-ATPase sugar binding site(s) in which intrinsic sensitivity to sugar attachment as well as recognition characteristics (for sugar structural features) both determine the extent to which a sugar moiety may contribute to the activity of monoglycosides. Further, in these studies one of the Na+, K+-ATPase preparations employed was obtained from rat brain, a tissue known to contain a mixture of ouabain sensitive and insensitive isoforms. We have observed that the rigorous purification techniques employed appear to have selectively removed from or denatured the less ouabain sensitive al isoform found in this enzyme preparation. 相似文献
6.
Roberto Refinetti Dwight E. Nelson Michael Menaker 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1992,170(2):181-187
Summary The ability of social stimuli to act as entraining agents of circadian rhythms was investigated in golden hamsters (Mesocricetus auratus). In a first experiment, pairs of male hamsters (one of them enucleated and the other intact) were maintained under a light-dark (LD) cycle with a period of 23.3 h. Running-wheel activity was recorded to determine the effect of social interaction on the free-running circadian rhythm of activity. In several pairs, general activity and body temperature were also recorded. In all pairs the intact animals entrained to the LD cycle, whereas the activity rhythms of the enucleated animals free-ran with periods of approximately 24 h and showed no apparent sign of synchronization or relative coordination with the other member of the pair. In a second experiment, male hamsters maintained in constant darkness received pulses of social interaction, which have been reported to induce phase shifts of the activity rhythm. Consistent phase shifts in the running-wheel activity rhythm were not induced by the social pulses in our experiment. These results suggest strongly that social stimuli are not effective entraining agents of circadian rhythms in the golden hamster.Abbreviations
CT
circadian time
-
LD
light-dark 相似文献
7.
Current perspectives on plasmodesmata: structure and function 总被引:2,自引:0,他引:2
Recent studies on plasmodesmata have shown that these important intercellular passages for communication and transport are much more sophisticated in both structure and regulatory abilities than previously imagined. A complex, but not well understood, substructure has been revealed by a variety of increasingly reliable ultrastructural techniques. Proteinaceous particles are seen within the cytoplasmic sleeve surrounding the desmotubule. Dye-coupling studies have provided experimental evidence for the physical pathway of solute movement, supporting conclusions about substructural dimensions within plasmodesmata drawn from the ultrastructural studies. Calcium has been identified as a major factor in the regulation of intercellular communication via plasmodesmata. Evidence from studies on virus movement through plasmodesmata suggests a direct interaction between virallycoded movement proteins and plasmodesmata in the systemic spread of many viruses. There is increasing evidence, albeit indirect, that in some plant species phloem loading may involve transport of photoassimilate entirely within the symplast from mesophyll cells to the sieve element-companion cell complexes of minor veins. 相似文献
8.
Distribution of actinomycetes in near-shore tropical marine sediments 总被引:15,自引:0,他引:15
Actinomycetes were isolated from near-shore marine sediments collected at 15 island locations throughout the Bahamas. A total of 289 actinomycete colonies were observed, and all but 6 could be assigned to the suprageneric groups actinoplanetes and streptomycetes. A bimodal distribution in the actinomycete population in relation to depth was recorded, with the maximum numbers occurring in the shallow and deep sampling sites. This distribution can be accounted for by a rapid decrease in streptomycetes and an increase in actinoplanetes with increasing depth and does not conform to the theory that actinomycetes isolated from marine sources are of terrestrial origin. Sixty-three of the isolated actinomycetes were tested for the effects of seawater on growth. Streptomycete growth in nonsaline media was reduced by 39% compared with that in seawater. The actinoplanetes had a near obligate requirement of seawater for growth, and this is presented as evidence that actinomycetes can be physiologically active in the marine environment. Problems encountered with the enumeration of actinomycetes in marine sediments are also discussed. 相似文献
9.
The kinetics of asymbiotic nitrogenase activity in three strains of the actinomycete Frankia were studied. Decay rates for enzyme activity were determined by adding chloramphenicol to active acetylene-reducing cells and measuring the time required for all activity to cease. Synthesis rates were measured by bubbling oxygen through actively-reducing cells (which totally destroyed all activity) and then measuring the time required for activity to return to normal. Decay rates (t
1/2) for these three strains were approximately 30 to 40 min. Synthesis rates were slower and initial nitrogenase activities were recorded about 110 min (DDB 011610) or 210 min (DDB 020210 and WgCc1.17) after return to air-equilibrated cultures. Frankia strain WgCc1.17 showed a greater sensitivity to oxygen and nitrogenase activity was totally lost when cells were bubbled only with atmospheric concentrations of oxygen. The results presented here indicate that nitrogenase activity turnover time is relatively rapid, on the order of minutes rather than hours or days. However, regulation of nitrogenase activity will differ from one strain to another and asmmbiotic characterization will be useful for understanding nitrogenase regulation in the bacterial-plant symbiosis.Contribution no. 879 from the Battelle-Kettering Laboratory 相似文献
10.
The Identity of Hexokinase Activities from Mitochondrial and Cytoplasmic Fractions of Rat Brain Homogenates 总被引:4,自引:4,他引:0
Cytoplasmic hexokinase (ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1) was purified from the soluble fraction of a rat brain homogenate by a procedure that included a unique affinity elution of the enzyme from Blue Dextran-Sepharose. The purified enzyme was examined with respect to properties in which the impure cytoplasmic enzyme has been reported to differ from the solubilized mitochondrial enzyme. These included the ability to bind to mitochondria, inhibition by quercetin, effect of pH on activity, and kinetics. In all regards the purified mitochondrial and cytoplasmic enzymes appeared identical. In addition, comparative peptide maps after partial proteolysis showed no detectable differences. These results do not support the view that there exist distinct mitochondrial and cytoplasmic forms of hexokinase, the latter being permanently relegated to a cytoplasmic location and unable to participate in a dynamic equilibrium with the mitochondrially-bound enzyme. Alternatives are proposed to explain previous results that had been interpreted as indirect evidence for the existence of a distinct cytoplasmic hexokinase. 相似文献