全文获取类型
收费全文 | 2939篇 |
免费 | 195篇 |
国内免费 | 108篇 |
专业分类
3242篇 |
出版年
2024年 | 7篇 |
2023年 | 46篇 |
2022年 | 60篇 |
2021年 | 94篇 |
2020年 | 99篇 |
2019年 | 101篇 |
2018年 | 118篇 |
2017年 | 84篇 |
2016年 | 89篇 |
2015年 | 127篇 |
2014年 | 147篇 |
2013年 | 186篇 |
2012年 | 115篇 |
2011年 | 155篇 |
2010年 | 139篇 |
2009年 | 155篇 |
2008年 | 169篇 |
2007年 | 141篇 |
2006年 | 123篇 |
2005年 | 104篇 |
2004年 | 91篇 |
2003年 | 84篇 |
2002年 | 85篇 |
2001年 | 61篇 |
2000年 | 58篇 |
1999年 | 57篇 |
1998年 | 38篇 |
1997年 | 49篇 |
1996年 | 54篇 |
1995年 | 35篇 |
1994年 | 33篇 |
1993年 | 28篇 |
1992年 | 31篇 |
1991年 | 38篇 |
1990年 | 31篇 |
1989年 | 19篇 |
1988年 | 19篇 |
1987年 | 15篇 |
1986年 | 16篇 |
1985年 | 19篇 |
1984年 | 8篇 |
1983年 | 10篇 |
1982年 | 13篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 19篇 |
1978年 | 13篇 |
1976年 | 13篇 |
1975年 | 5篇 |
1973年 | 12篇 |
排序方式: 共有3242条查询结果,搜索用时 0 毫秒
71.
Information about transmission opportunities triggers a life-history switch in a parasite 总被引:1,自引:0,他引:1
Poulin R 《Evolution; international journal of organic evolution》2003,57(12):2899-2903
Many microbial pathogens can switch to new hosts or adopt alternative transmission routes as environmental conditions change, displaying unexpected flexibility in their infection pathways and often causing emerging diseases. In contrast, parasitic worms that must develop through a fixed series of host species appear less likely to show phenotypic plasticity in their transmission pathways. Here, I demonstrate experimentally that a trematode parasite, Coitocaecum parvum, can accelerate its development and rapidly reach precocious maturity in its crustacean intermediate host in the absence of chemical cues emanating from its fish definitive host. Juvenile trematodes can also mature precociously when the mortality rate of their intermediate hosts is increased. Eggs produced by precocious adults hatch into viable larvae, capable of pursuing the parasite's life cycle. In the absence of chemical cues from fish hosts, the size of eggs released by precocious trematodes in their intermediate hosts becomes more variable, possibly indicating a bet-hedging strategy. These results illustrate that parasitic worms with complex life cycles have development and transmission strategies that are more plastic than commonly believed, allowing them to skip one host in their cycle when they perceive limited opportunities for transmission. 相似文献
72.
丙型肝炎病毒(HCV)可引起急性和慢性病毒性肝炎,可发展成肝纤维化、肝硬化,甚至肝细胞癌。HCV经典的传播途径为经血液或血液制品传播,但1992年后献血员HCV的筛检已使输血后肝炎大为减少。在发达国家,HCV传播途径正在发生改变,儿童非血液制品的丙肝日渐增多。母婴间宫内、分娩时及产后感染已成为当前及今后的重要研究课题。研究证实,HCV可经胎盘引起胎儿感染,宫内感染是HCV传播的一条重要途径。尽管人们对HCV母婴传播中所涉及的风险因素逐渐明确,但到目前为止对具体的传播机制和传播时机仍知之甚少。我们就丙型病毒性肝炎母婴传播的现状、进展及未来做简要综述。 相似文献
73.
Hideki Sezutsu Eiji Nitasaka Tsuneyuki Yamazaki 《Molecular & general genetics : MGG》1995,249(2):168-178
LINE-like retrotransposons, the so-called I elements, control the system of I-R (inducer-reactive) hybrid dysgenesis in Drosophila melanogaster. I elements are present in many Drosophila species. It has been suggested that active, complete I elements, located at different sites on the chromosomes, invaded natural populations of D. melanogaster recently (1920–1970). But old strains lacking active I elements have only defective I elements located in the chromocenter. We have cloned I elements from D. melanogaster and the melanogaster subgroup. In D. melanogaster, the nucleotide sequences of chromocentral I elements differed from those on chromosome arms by as much as 7%. All the I elements of D. mauritiana and D. sechellia are more closely related to the chromosomal I elements of D. melanogaster than to the chromocentral I elements in any species. No sequence difference was observed in the surveyed region between two chromosomal I elements isolated from D. melanogaster and one from D. simulans. These findings strongly support the idea that the defective chromocentral I elements of D. melanogaster originated before the species diverged and the chromosomal I elements were eliminated. The chromosomal I elements reinvaded natural populations of D. melanogaster recently, and were possibly introduced from D. simulans by horizontal transmission. 相似文献
74.
Four bulls and 56 heifers seronegative to Neospora caninum were used to determine the feasibility of venereal transmission in bovine neosporosis under natural conditions. Bulls were experimentally infected with 108 live N. caninum tachyzoites. Two of them with the Nc-1 isolate and the other two with the Nc-Spain-7 isolate. After 13 months of initial infection, each bull was re-infected with the same isolate and dose. The experiments were carried out from March to September during 2006 and 2007 where groups of cyclic heifers were naturally mated by the experimentally infected bulls. In year 2006, two bulls infected with different N. caninum isolate serviced 12 heifers each. In year 2007, the same bulls serviced the same heifers a second time (now primiparous) and six new heifers were also added to each group. In addition, the other two bulls serviced 10 additional heifers each. Experimental animals were monitored for 30 weeks and serum samples were collected weekly and fortnightly in years 2006 and 2007, respectively to evaluate the presence of specific antibodies to N. caninum. Experimentally infected bulls showed a significant increase of specific IgG antibodies from 13 (Nc-SP-7) and 21 (Nc-1) days post-infection. Serum IgG antibody responses of individual animals were similar in kinetics but slightly different in magnitude. Serum samples from heifers were all negative. Pregnant rates were 100% in heifers and 91% in primiparous animals. Calves did not show precolostral specific antibodies to N. caninum. Venereal transmission of bovine neosporosis under natural grazing conditions is unlikely to occur. 相似文献
75.
Pre-laying nutrition mediates maternal effects on offspring immune capacity and growth in the pied flycatcher 总被引:1,自引:1,他引:1
We have aimed at detecting prelaying maternal effects on nestling antibody defences and growth through experimental food supplementation of female pied flycatchers Ficedula hypoleuca and subsequent exchange of whole clutches with control nests. The levels of immunoglobulins and the mass and size of chicks at 12 days of age were ascertained. This is the first study controlling for maternal incubation effects by exchanging eggs rather than nestlings. Our prediction is that the females' availability of pre-laying nutritional resources affects offspring immune capacity and growth through maternal effects in the eggs when conditions during incubation and rearing are controlled for. Nestling immunoglobulin Y (IgY) levels and tarsus length were indeed positively associated with maternal food supplementation at laying. The only rearing environmental effect detected was that of mite infestation which affected both IgY levels and growth of nestlings. Nestlings that recruited to the population in the subsequent 2 years had higher IgY levels than those that did not. Maternal adaptations for allocating resources to eggs play an important role in moulding offspring phenotypes and may affect their survival prospects. 相似文献
76.
Pérez de la Mora M Jacobsen KX Crespo-Ramírez M Flores-Gracia C Fuxe K 《Neurochemical research》2008,33(8):1618-1633
The amygdala plays a key role in anxiety. Information from the environment reaches the amygdaloid basolateral nucleus and
after its processing is relayed to the amygdaloid central nucleus where a proper anxiogenic response is implemented. Experimental
evidence indicates that in this information transfer a GABAergic interface controls the trafficking of impulses between the
two nuclei. Recent work indicates that interneuronal communication can take place by classical synaptic transmission (wiring
transmission) and by volume transmission in which the neurotransmitter diffuses and flows through the extracellular space
from its site of release and binds to extrasynaptic receptors at various distances from the source. Based on evidence from
our laboratory the concept is introduced that neurotransmitters in the amygdala can modulate anxiety involving changes in
fear learning and memories by effects on receptor mosaics in the fear circuits through wiring and volume transmission modes
of communication.
Special issue article in honor of Dr. Ricardo Tapia. 相似文献
77.
ABBEY K. THOMPSON MICHAEL D. SAMUEL TIMOTHY R. VAN DEELEN 《The Journal of wildlife management》2008,72(2):416-421
ABSTRACT We conducted experimental feeding using 3 feeding methods (pile, spread, trough) and 2 quantities (rationed, ad libitum) of shelled corn to compare deer activity and behavior with control sites and evaluate potential direct and indirect transmission of infectious disease in white-tailed deer (Odocoileus virginianus) in central Wisconsin, USA. Deer use was higher at 2 of the feeding sites than at natural feeding areas (P ≤ 0.02). Deer spent a higher proportion of time (P < 0.01) feeding at pile (49%) and spread (61%) treatments than at natural feeding areas (36%). We found higher deer use for rationed than ad libitum feeding quantities and feeding intensity was greatest at rationed piles and lowest at ad libitum spreads. We also observed closer pairwise distances (≤0.3 m) among deer when corn was provided in a trough relative to spread (P=0.03). Supplemental feeding poses risks for both direct and indirect disease transmission due to higher deer concentration and more intensive use relative to control areas. Concentrated feeding and contact among deer at feeding sites can also increase risk for disease transmission. Our results indicated that restrictions on feeding quantity would not mitigate the potential for disease transmission. None of the feeding strategies we evaluated substantially reduced the potential risk for disease transmission and banning supplemental feeding to reduce transmission is warranted. 相似文献
78.
J.L. Shipp R. Buitenhuis L. Stobbs K. Wang W.S. Kim G. Ferguson 《The Annals of applied biology》2008,153(2):149-155
Pepino mosaic virus (PepMV) has become an important viral disease of greenhouse tomatoes worldwide. The ability of bumble‐bees (Bombus impatiens), used for pollination, to acquire and transmit PepMV was investigated, and the prevalence of PepMV in plants and bumble‐bees in commercial tomato greenhouses was determined. PepMV infection in plants was determined using enzyme‐linked immunosorbent assay, while in bumble‐bees direct real‐time PCR was used. In the first experiment, the bumble‐bees were exposed for 14 days to PepMV‐infected plants. After 14 days, almost all bumble‐bees were PepMV positive both in the hive (78.5 ± 17.5%) and in the flowers (96.3 ± 3.6%). In the second experiment, bumble‐bees were released into a greenhouse with both PepMV‐infected source plants and healthy non‐infected target plants for 14 days. At the end of the experiment, 61.0 ± 19.5% of the bees collected from the hive and 83.3 ± 16.7% of the bees sampled from the flowers were PepMV positive. Bumble‐bees transmitted PepMV from the infected to the healthy non‐infected tomato plants. Two weeks after bumble‐bee release, the virus was detected in leaf, fruit and flower samples of formerly healthy plants. After 6 weeks, the percentage of PepMV positive samples from the target plants increased to 52.8 ± 2.8% of the leaves and 80.6 ± 8.4% of the fruits. In the control greenhouse without bumble‐bees, the target plants did not become infected. Based on the infection levels in flowers, fruits and leaves, the PepMV infection occurred possibly first in the pollinated flowers, and then spread from the fruit that developed from the flowers to other parts of the plant. In commercial greenhouses where PepMV was present, 92–100% of the plants and 88–100% of the bumble‐bees were PepMV positive. No infected plant samples were found in the control commercial greenhouse, but a small number of bumble‐bees (10%) tested PepMV positive. 相似文献
79.
Leandro Bueno Bergantin Cláudio Fontes Souza Regiane Miranda Ferreira Soraya Soubhi Smaili Neide Hyppolito Jurkiewicz Afonso Caricati-Neto Aron Jurkiewicz 《Cell calcium》2013
It is well established that reduction of Ca2+ influx through L-type voltage-dependent Ca2+ channel (L-type VDCC), or increase of cytosolic cAMP concentration ([cAMP]c), inhibit contractile activity of smooth muscles in response to transmitters released from sympathetic nerves. Surprisingly, in this work we observed that simultaneous administration of L-type VDCC blocker (verapamil) and [cAMP]c enhancers (rolipram, IBMX and forskolin) potentiated purinergic contractions evoked by electrical field stimulation of rat vas deferens, instead of inhibiting them. These results, including its role in sympathetic transmission, can be considered as a “calcium paradox”. On the other hand, this potentiation was prevented by reduction of [cAMP]c by inhibition of adenylyl cyclase (SQ 22536) or depletion of Ca2+ storage of sarco-endoplasmic reticulum by blockade of Ca2+ reuptake (thapsigargin). In addition, cytosolic Ca2+ concentration ([Ca2+]c) evaluated by fluorescence microscopy in rat adrenal medullary slices was significantly reduced by verapamil or rolipram. In contrast, simultaneous incubation of adrenal slices with these compounds significantly increased [Ca2+]c. This effect was prevented by thapsigargin. Thus, a reduction of [Ca2+]c due to blockade of Ca2+ influx through L-type VDCC could stimulate adenylyl cyclase activity increasing [cAMP]c thereby stimulating Ca2+ release from endoplasmic reticulum, resulting in augmented transmitter release in sympathetic nerves and contraction. 相似文献
80.
Katherine E. Atkins Andrew F. Read Nicholas J. Savill Katrin G. Renz AFM Fakhrul Islam Stephen W. Walkden‐Brown Mark E. J. Woolhouse 《Evolution; international journal of organic evolution》2013,67(3):851-860
Marek’s disease virus (MDV), a commercially important disease of poultry, has become substantially more virulent over the last 60 years. This evolution was presumably a consequence of changes in virus ecology associated with the intensification of the poultry industry. Here, we assess whether vaccination or reduced host life span could have generated natural selection, which favored more virulent strains. Using previously published experimental data, we estimated viral fitness under a range of cohort durations and vaccine treatments on broiler farms. We found that viral fitness maximized at intermediate virulence, as a result of a trade‐off between virulence and transmission previously reported. Our results suggest that vaccination, acting on this trade‐off, could have led to the evolution of increased virulence. By keeping the host alive, vaccination prolongs infectious periods of virulent strains. Improvements in host genetics and nutrition, which reduced broiler life spans below 50 days, could have also increased the virulence of the circulating MDV strains because shortened cohort duration reduces the impact of host death on viral fitness. These results illustrate the dramatic impact anthropogenic change can potentially have on pathogen virulence. 相似文献