全文获取类型
收费全文 | 1870篇 |
免费 | 121篇 |
国内免费 | 74篇 |
出版年
2023年 | 21篇 |
2022年 | 20篇 |
2021年 | 26篇 |
2020年 | 49篇 |
2019年 | 56篇 |
2018年 | 52篇 |
2017年 | 43篇 |
2016年 | 56篇 |
2015年 | 44篇 |
2014年 | 98篇 |
2013年 | 164篇 |
2012年 | 58篇 |
2011年 | 60篇 |
2010年 | 53篇 |
2009年 | 64篇 |
2008年 | 74篇 |
2007年 | 72篇 |
2006年 | 69篇 |
2005年 | 78篇 |
2004年 | 45篇 |
2003年 | 66篇 |
2002年 | 67篇 |
2001年 | 49篇 |
2000年 | 41篇 |
1999年 | 37篇 |
1998年 | 70篇 |
1997年 | 50篇 |
1996年 | 43篇 |
1995年 | 43篇 |
1994年 | 44篇 |
1993年 | 48篇 |
1992年 | 34篇 |
1991年 | 25篇 |
1990年 | 17篇 |
1989年 | 26篇 |
1988年 | 30篇 |
1987年 | 22篇 |
1986年 | 27篇 |
1985年 | 19篇 |
1984年 | 15篇 |
1983年 | 21篇 |
1982年 | 26篇 |
1981年 | 22篇 |
1980年 | 7篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有2065条查询结果,搜索用时 109 毫秒
41.
42.
Pulsed field gel electrophoresis using OFAGE, TAFE, and CHEF systems has been used to more fully characterize karyotypic variation within the two closely related fungal species of Ophiostoma ulmi sensu lato. Twelve wild-type and laboratory strains, representing the less agressive species O. ulmi and both of the biotypes of the more aggressive species O. novo-ulmi were studied and their karyotypes determined. Depending on the strain, a minimum of four to a minimum of eight chromosomal DNA bands were present that fall into three distinct size classes, with one exception. Strain CESSI6K (O. novo-ulmi, North American aggressive subgroup) contains a unique chromosomal DNA band which comigrated near a Saccharomyces cerevisiae chromosome of 0.95 Mb. This unique band was the smallest O. ulmi s. l. chromosomal DNA observed. Seven of the twelve strains shared a common chromosomal DNA banding pattern, whereas each of the other five had a unique karyotype. There was no correlation between chromosome profile and species, as some O. novo-ulmi and O. ulmi strains shared common electrophoretic karyotypes. 相似文献
43.
B. Rasnow C. Assad J. M. Bower 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1993,172(4):481-491
Summary The electric organ discharge (EOD) potential was mapped on the skin and midplane of several Apteronotus leptorhynchus. The frequency components of the EOD on the surface of the fish have extremely stable amplitude and phase. However, the waveform varies considerably with different positions on the body surface. Peaks and zero crossings of the potential propagate along the fish's body, and there is no point where the potential is always zero. The EOD differs significantly from a sinusoid over at least one third of the body and tail. A qualitative comparison between fish showed that each individual had a unique spatiotemporal pattern of the EOD potential on its body.The potential waveforms have been assembled into high temporal and spatial resolution maps which show the dynamics of the EOD. Animation sequences and Macintosh software are available by anonymous ftp (mordor.cns.caltech.edu; cd/pub/ElectricFish).We interpret the EOD maps in terms of ramifications on electric organ control and electroreception. The electrocytes comprising the electric organ do not all fire in unison, indicating that the command pathway is not synchronized overall. The maps suggest that electroreceptors in different regions fulfill different computational roles in electroreception. Receptor mechanisms may exist to make use of the phase information or harmonic content of the EOD, so that both spatial and temporal patterns could contribute information useful for electrolocation and communication.Abbreviations EOD
electric organ discharge
- EO
electric organ
- CV
coefficient of variance 相似文献
44.
M. Kavaliers L. A. Eckel K.-P. Ossenkopp 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》1993,173(2):241-248
1. | This study examined spatial learning and memory in breeding adult male and female meadow voles, Microtus pennsylvanicus, and the effects of brief exposure to weak (0.1 mT or 1.0 Gauss rms) 60 Hz magnetic fields on their spatial performance. |
2. | There were significant sex differences in the spatial performance of the polygynous meadow voles. Reproductive male voles displayed significantly better spatial learning and retention than reproductive females in a Morris water maze task, whereby individual voles had to learn and remember the location of a submerged hidden platform using extramaze visual cues. Males showed over 9 days of daily blocks of 4 trials both a faster rate and greater overall level of acquisition (shorter latency to find hidden platform) of the spatial task than did the females. |
3. | Brief (maximum 5 min) exposure to 60 Hz magnetic fields during acquisition of the water maze task significantly enhanced the spatial performance of both the male and female meadow voles. Females showed a relatively greater facilitation of spatial acquisition and retention than did the males, resulting by day 9 in the elimination of sex differences in water maze performance. |
4. | The time-varying magnetic fields may be : (i) functioning as orientation cues and providing directional cues, and/or (ii) affecting neuromodulatory systems that are involved in the mediation of spatial learning and memory. Exposure to weak time-varying magnetic fields has been shown to attenuate the activity of endogenous opioid systems, enhance protein kinase C (PKC) activity, and alter calcium ion flux. These effects have been shown to affect spatial learning and memory and are consistent with an enhancement of water maze performance. |
45.
《Current biology : CB》2023,33(14):3041-3047.e4
46.
Manfred J. Sippl 《Proteins》1993,17(4):355-362
A major problem in the determination of the three-dimensional structure of proteins concerns the quality of the structural models obtained from the interpretation of experimental data. New developments in X-ray crystallography and nuclear magnetic resonance spectroscopy have acceleratedd the process of structure determination and the biological community is confronted with a steadily increasing number of experimentally determined protein folds. However, in the recent past several experimentally determined protein structures have been proven to contain major errors, indicating that in some cases the interpretation of experimental data is difficult and may yield incorrect models. Such problems can be avoided when computational methods are employed which complement experimental structure determinations. A prerequisite of such computational tools is that they are independent of the parameters obtained from a particular experiment. In addition such techniques are able to support and accelerate experimental structure determinations. Here we present techniques based on knowledge based mean fields which can be used to judge the quality of protein folds. The methods can be used to identify misfolded structures as well as faulty parts of structural models. The techniques are even applicable in cases where only the Cα trace of a protein conformation is available. The capabilities of the technique are demonstrated using correct and incorrect protein folds. © 1993 Wiley-Liss, Inc. 相似文献
47.
用细胞分析成像光盘记录系统测量了3T3细胞在某些ELF磁场和温度条件下生长周期的变化.实验结果表明,只有某些频率的ELF磁场才对细胞生长产生影响,磁场对细胞生长的影响还与培养细胞的生化环境有关.温度和ELF磁场都能影响细胞的生长.但二者的机理是不一样的,当撤除ELF磁场后,细胞在短时间内(2天以上)继续保持着ELF场对其生长的影响.而细胞生长周期能在短时间内(2天以内)随着温度的变化而变化.温度引起的细胞生长的变化可能与细胞内的各种生长因子、生物离子的活性有关.ELF磁场引起的细胞生长的变化可能与ELF磁场对细胞膜的影响有关,与细胞内细胞生长必不可少的生物离子(如Ca~(2+))的浓度有关. 相似文献
48.
49.
This paper is the first detailed analysis of situation-specific temporal patterning of electric organ discharges (EODs) in a strong electric fish. Using a resident-intruder paradigm EODs were recorded during interactions between dyads composed of Malapterurus electricus (Gmelin) and four different types of fish: (1) conspecifics; (2) large prey-type mid-water fish, goldfish ( Carassius auratus , Linnaeus 1758) and tilapia ( Oreochromis melanotheron , Rüppel, 1852); (3) a sympatric competitor, Polypterus palmas (Ayres 1850) and (4) a larger, threatening catfish, Clarias sp.
An analysis of the EODs emitted showed that in the presence of conspecifics the average EOD volley consisted of a single long-duration, low frequency train of EODs. The presence of the midwater fish (goldfish and Tilapia) elicited volleys consisting of two short trains, and P. palmas elicited long duration volleys with two trains and long inter-train intervals. Finally, an attacking Clarias resulted on average in volleys consisting of two high-frequency trains of EODs. With nonconspecific partner species resident electric catfish emitted volleys with more pulses, more trains that were longer in duration and higher in frequency than the EODs in volleys emitted by intruder electric catfish with the same species stimulus fish. 相似文献
An analysis of the EODs emitted showed that in the presence of conspecifics the average EOD volley consisted of a single long-duration, low frequency train of EODs. The presence of the midwater fish (goldfish and Tilapia) elicited volleys consisting of two short trains, and P. palmas elicited long duration volleys with two trains and long inter-train intervals. Finally, an attacking Clarias resulted on average in volleys consisting of two high-frequency trains of EODs. With nonconspecific partner species resident electric catfish emitted volleys with more pulses, more trains that were longer in duration and higher in frequency than the EODs in volleys emitted by intruder electric catfish with the same species stimulus fish. 相似文献
50.
本文在青蛙胚胎诸生长期的核驰豫研究及电磁场对淡水鱼胚刺激后能促进其生长发育及机理研究的基础上,利用华东师范大学物理系研制的ACHV-2型脉冲电场刺激仪对美国青蛙的早期及神经期胚胎进行适当强度的刺激发现,它能使胚胎的孵化率提高20%以上,存活率提高30%左右,并能促进其后斯的生长发育速率达30%,研究结果进一步证明作者提出的电场作用机理的合理。 相似文献