全文获取类型
收费全文 | 66779篇 |
免费 | 4361篇 |
国内免费 | 7篇 |
出版年
2023年 | 482篇 |
2022年 | 446篇 |
2021年 | 965篇 |
2020年 | 864篇 |
2019年 | 886篇 |
2018年 | 2052篇 |
2017年 | 1926篇 |
2016年 | 2547篇 |
2015年 | 3367篇 |
2014年 | 3329篇 |
2013年 | 4498篇 |
2012年 | 5378篇 |
2011年 | 4753篇 |
2010年 | 3005篇 |
2009年 | 2341篇 |
2008年 | 3710篇 |
2007年 | 3524篇 |
2006年 | 3323篇 |
2005年 | 2830篇 |
2004年 | 2749篇 |
2003年 | 2445篇 |
2002年 | 2277篇 |
2001年 | 1451篇 |
2000年 | 1416篇 |
1999年 | 1097篇 |
1998年 | 501篇 |
1997年 | 364篇 |
1996年 | 363篇 |
1995年 | 349篇 |
1994年 | 280篇 |
1993年 | 256篇 |
1992年 | 556篇 |
1991年 | 477篇 |
1990年 | 414篇 |
1989年 | 417篇 |
1988年 | 418篇 |
1987年 | 357篇 |
1986年 | 349篇 |
1985年 | 346篇 |
1984年 | 348篇 |
1983年 | 245篇 |
1982年 | 225篇 |
1981年 | 172篇 |
1980年 | 177篇 |
1979年 | 219篇 |
1978年 | 212篇 |
1975年 | 198篇 |
1974年 | 218篇 |
1973年 | 220篇 |
1972年 | 181篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
The majority of mechanisms proposed to explain epileptic discharge suggest an excessive synaptic input into the cell or possible changes in cellular excitability which result in a decreased firing threshold and in the presence of self-sustained activity. It is likely that these changes are caused by modifications in the membrane receptor sensitivity to a specific neurotransmitter. In view of the above, the purpose of the present study has been to evaluate the sensitivity of the postsynaptic receptor by means of the microiontophoretic applications of substances whose pharmacological effect is known, thus determining its possible involvement in the epileptic process. Changes in cortical excitability were induced by electric stimuli in the sensorimotor cortex of rats anesthetized with urethane (1 g/kg intraperitoneally), immobilized with pancuronium bromide and kept alive with mechanical respiration. The electric stimuli consisted of trains of biphasic pulses, each lasting one millisecond, with a frequency of 100 pps and with a train duration of 1 second. The response of the neuron to acetylcholine was evaluated before and after the kindling had been established. The dosage was measured in nanoamperes of microiontophoretic ejecting current. Extracellular field potentials were recorded with the central barrel of 4-barrel micropipettes. Peripheral barrels were used for iontophoretic applications of Acetylcholine (Ach .1, 1M), Atropine (25mM). One of these barrels containing NaCl (2M) was used for the automatic passage of balancing current.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
992.
In an earlier study we reported on the fertilization of in vitro-matured bovine oocytes by ram spermatozoa. The present results extend those observations and demonstrate the penetration of bovine and ovine oocytes matured in culture by goat spermatozoa. Freshly ejaculated and in vitro capacitated goat spermatozoa penetrated 37.1% of bovine and 48.1% of ovine intact oocytes. Monospermic fertilization was detected in about 90% of the oocytes of both species, and the development of pronuclei followed a developmental pattern similar to that of homologous fertilization. 相似文献
993.
The object of this study was to adapt the triple stain technique to diluted and incubated boar spermatozoa. Freshly ejaculated semen was resuspended in MR-A diluent to contain 3x10(7) cells/ml (diluted spermatozoa) and was subsequently capacitated (incubated spermatozoa). Experiments were conducted to show the conditions required for optimal staining quality and validation of triple stain technique. The most satisfactory staining solutions for diluted spermatozoa were 2% Trypan blue at 37 degrees C for 15 minutes, 0.8% Bismarck brown in 30% ethyl alcohol (pH 2.8) at 40 degrees C for 10 minutes and 0.8% rose Bengal in 0.1 M of Tris (pH 4.3) at 21 degrees C for 20 minutes. Satisfactory results were obtained for incubated spermatozoa with rose Bengal when the staining time was 10 minutes. Triple stain technique seemed to be a useful method for the simultaneous assessment of sperm vitality and acrosomal status; consequently, it should be valuable tool, for use in porcine in vitro fertilization systems. 相似文献
994.
Summary Explants from petioles, folioles or hypocotyls ofOnonis natrix have been used for calli initiation. Hypocotyls inoculated on MS medium supplemented with 2% sucrose and 0.5 mg.1–1 2,4-D / 1 mg.1–1 Kin showed to be the best primary explant. Cell suspension cultures were established in MS basal medium supplemented with 2% sucrose, 0.5 mg.1–1 NAA or 2,4-D and 1 mg.1–1 Kin. Different subculturing periods, inoculum density, hormonal supplementation and sucrose concentration were assayed in order to obtain the best culture growth conditions. The optimal conditions were achieved with cultures initiated with 40 g.1–1 of initial inoculum, growing in MS basal medium supplemented with 4% sucrose, 0.5 mg.1–1 NAA and 1 mg.1–1 Kin subcultured every twelve days. Under these experimental conditions, the cultures showed a doubling time of 36.3 hours. 相似文献
995.
C Navarro-Ranninger P A Ochoa J M Pérez J H Rodríguez J R Masaguer C Alonso 《Journal of inorganic biochemistry》1992,48(3):163-171
Four platinum(II) aminobenzamidine complexes have been prepared and characterized by IR and 1H and 13C NMR spectroscopy, and tested for their ability to interact with the nicked and closed circular forms of the pUC8 plasmid DNA. The results show that the complexes of formula [Pt(LH)2Cl2]2X have a cis- geometry with an amino-Pt bonding, where L is either p- or m-aminobenzamidine and where 2X is 2Cl- or PtCl4(2-). It was observed that these complexes significantly alter the electrophoretic mobility of nicked and closed circular forms of DNA and that the alteration in electrophoretic mobility due to Pt(II)-p-aminobenzamidine binding is higher than that due to Pt(II)-m-aminobenzamidine. No difference in mobility was observed whether the DNA interacted with complexes having as counteranion Cl- or PtCl4(2-). The synthesized compounds were, in addition, assayed for antitumor activity in vitro against colon (CX-1), lung (LX-1), and mammary (MX-1) human tumor cells. The results show that these complexes inhibited the multiplication of the tumor cells and that they show higher specificity for lung cells. 相似文献
996.
Pablo Collazo Lluís Montoliu Pere Puigdomènech Joan Rigau 《Plant molecular biology》1992,20(5):857-867
The isolation and characterization of cDNA and homologous genomic clones encoding the lignin O-methyltransferase (OMT) from maize is reported. The cDNA clone has been isolated by differential screening of maize root cDNA library. Southern analysis indicates that a single gene codes for this protein. The genomic sequence contains a single 916 bp intron. The deduced protein sequence from DNA shares significant homology with the recently reported lignin-bispecific caffeic acid/5-hydroxyferulic OMTs from alfalfa and aspen. It also shares homology with OMTs from bovine pineal glands and a purple non-sulfur photosynthetic bacterium. The mRNA of this gene is present at different levels in distinct organs of the plant with the highest accumulation detected in the elongation zone of roots. Bacterial extracts from clones containing the maize OMT cDNA show an activity in methylation of caffeic acid to ferulic acid comparable to that existing in the plant extracts. These results indicate that the described gene encodes the caffeic acid 3-O-methyltransferase (COMT) involved in the lignin biosynthesis of maize. 相似文献
997.
998.
Mario Díaz Antonio Lorenzo 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1992,162(2):189-196
Summary Bioelectrical parameters and unidirectional sodium and chloride fluxes were measured under voltageclamp conditions in groups of lizards submitted to single or chronic aldosterone treatment. Both acute (AT) and chronic (CT) treatment induced significant increases in the short-circuit current (I
sc), as well as in the mucosa-to-serosa (J
m-s
Na
) and net sodium flux (J
net
Na
). In AT tissues, aldosterone did not change net chloride flux (J
net
Cl
) but did so in CT tissues. Amiloride reduced the aldosterone-increased I
sc in AT and CT tissues, inhibited J
net
Na
in AT tissues and abolished it in CT colons. J
net
Cl
was also reduced by the diuretic in the group of AT colons, whereas no changes were observed in the CT tissues. Addition of luminal DIDS reduced Na+ absorption and totally inhibited Cl- absorption in the AT tissues, but did not change I
sc. However, in CT tissues neither Na+ nor Cl- transport were affected by DIDS. A good relationship between I
sc and J
m-s
Na
was apparent after DIDS treatment in AT tissues. In this group, simultaneous addition of DIDS and amiloride totally abolished J
net
Na
and reduced I
sc to untreated control values. Addition of serosal ouabain abolished I
sc and Na+ absorption in AT and CT colons, but Cl- absorption was only altered in AT tissues. These results support the hypothesis that aldosterone induces an electrogenic, amiloride-sensitive sodium absorption, and in a dose-dependent fashion suppresses electroneutral NaCl absorption in the lizard colon.Abbreviations AT
acutely treated
- CT
chronically treated animals
- DIDS
4-4-diisothiocyanatostibene-2-2-disulfonic acid
- DMSO
dimethylsulphoxide
-
G
t
tissue conductance
-
I
sc
short circuit current
- PD
transepithelial potential difference
- SITS
4-acetamido-4-isothiocyanatostilbene-2-2-disulfonic acid
- UC
untreated controls
Preliminary results of this paper were presented at the X
th meeting of the European Intestinal Transport Group (EITG), Askov Hojskole, Denmark, 16–19 September 1990 相似文献
999.
Miguel A. Pérez-Pinzón Myron Rosenthal Peter L. Lutz Thomas J. Sick 《Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology》1992,162(1):68-73
Intact turtle brain provides a useful model for the study of anoxia and potential survival strategies, since this tissue maintains transmembrane ion gradients and ATP levels during prolonged anoxia and recovers functional activity afterwards. Since isolated tissues offer experimental advantages, the present study sought to determine effects of anoxia on the isolated turtle cerebellum and to define relationships between anoxia survival and glucose supply. In normoxia, the extracellular potassium ([K+]o) activity and evoked potentials were maintained with 5 mM glucose, but 20 mM glucose was required to maintain adenosine triphosphate (ATP) levels and prevent significant increases in [K+]o during anoxia. Inhibition of glycolysis by iodoacetic acid (IAA) during anoxia provoked large increases in [K+]o at all glucose levels. These results demonstrate the usefulness of the isolated turtle cerebellum for studies of anoxic survival since this tissue can maintain ATP levels and [K+]o during prolonged anoxia with 20 mM glucose in the artificial cerebrospinal fluid medium. They also suggest the presence of a Pasteur effect at least during the transition to a hypometabolic state. 相似文献
1000.
Summary The white-rot fungus Ganoderma australis selectively degrades lignin in the ecosystem palo podrido. Using conditions that simulate those of palo podrido in the laboratory, it was found that low nitrogen content and low O2 tension stimulate the productio of manganese peroxidase and lignin degradation, and depress cellulose degradation and cellulase production. The inverse is found at high nitrogen concentration and high O2 tension. This agrees with previous results indicating that low O2 tension and low nitrogen stimulate selective lignin degradation by this fungus.
Correspondence to: J. Eyzaguirre 相似文献