首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   21018篇
  免费   1774篇
  国内免费   5篇
  2023年   77篇
  2022年   212篇
  2021年   441篇
  2020年   239篇
  2019年   310篇
  2018年   439篇
  2017年   360篇
  2016年   646篇
  2015年   1046篇
  2014年   1203篇
  2013年   1466篇
  2012年   1802篇
  2011年   1692篇
  2010年   1097篇
  2009年   968篇
  2008年   1296篇
  2007年   1329篇
  2006年   1135篇
  2005年   1095篇
  2004年   1065篇
  2003年   983篇
  2002年   918篇
  2001年   237篇
  2000年   171篇
  1999年   181篇
  1998年   254篇
  1997年   164篇
  1996年   156篇
  1995年   139篇
  1994年   124篇
  1993年   116篇
  1992年   99篇
  1991年   91篇
  1990年   87篇
  1989年   84篇
  1988年   65篇
  1987年   78篇
  1986年   61篇
  1985年   61篇
  1984年   58篇
  1983年   78篇
  1982年   46篇
  1981年   41篇
  1980年   42篇
  1979年   41篇
  1978年   57篇
  1977年   48篇
  1976年   47篇
  1975年   39篇
  1973年   38篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
121.
It has been well established that “normal” pregnancy gives rise to much anxiety whose source is variable. When not adequately dealt with, the anxiety may masquerade in the guise of physical symptoms such as fatigue, dizziness, nausea and vomiting, or, more often, as disquieting emotional counterparts, like irritability and depression.A study was undertaken in the outpatient obstetrical department at U.C.L.A. utilizing a group psychotherapeutic approach. The results helped the patients and offered training to staff in dealing with emotional problems of pregnancy. Patients were seen in groups of seven, twice a month for one-hour sessions. Participating in each group were an obstetrical resident, a psychiatric resident and a nurse. The subject material was not selected beforehand. Groups were similar in that the expected time of delivery of the patients was approximately the same. Results of the study suggested that the much needed emotional support may be supplied in this way with little to no additional time expenditure on the part of the physician or nurse.  相似文献   
122.
Summary The effect of temperatures below 0° C on different stages of the two-spotted spider mite, and in particular on active adult females, has been investigated.Survival of active females at-5° was higher at approximately 100% relative humidity than at a lower degree of humidity. Higher rates of survival were also caused by acclimation of active females and larvae for one day at +5° before storage at-15°. No differences in cold-hardiness were found in active females from an organophosphorous resistant strain and a susceptible strain, and the ability to survive depended on the time-temperature exposures (see Fig. 3).Eggs were killed very rapidly at-15° C, while larvae survived longer exposures than active females. Compared at the LT50 level females in diapause survived three times the exposure endured by acclimated active females.Supercooling points measured at a rate of cooling of 2° C per minute were highest in diapausing females and lowest in eggs. Acclimation had no effect on the supercooling points of active females. All stages were killed by freezing. It seems likely that the difference in survival between acclimated and not acclimated active females at-15° is caused by different ability to live in a supercooled state.
Zusammenfassung Es wurde die Wirkung von Temperaturen unter 0° auf verschiedene Stadien der Bohnenspinnmilbe, insbesondere auf aktive erwachsene Weibchen untersucht.Die Überlebensrate aktiver Weibchen war bei nahezu hundertprozentiger relativer Luftfeuchtigkeit höher als bei niederer Feuchtigkeit. Höhere Überlebensraten wurden auch durch Akklimatisation aktiver Weibchen und Larven für einen Tag bei 5° vor der Behandlung mit-15° erreicht. Es wurden keine Unterschiede in der Kältewiderstandfähigkeit aktiver Weibchen eines phosphorsäureesterresistenten und eines anfälligen Stammes gefunden. Die Überlebensfähigkeit hängt von Dauer und Temperatur der Einwirkung ab.Eier wurden von-15° sehr schnell abgetötet, während Larven längere Einwirkungszeiten überlebten als aktive Weibchen. Bei einem Vergleich der LT50 überlebten diapausierende Weibchen dreimal längere Einwirkungszeiten, als sie von akklimatisierten aktiven Weibchen ertragen wurden.Unterkühlungspunkte—gemessen bei einer Abkühlungsrate von 2° pro Minute—lagen bei diapausierenden Weibchen am höchsten und bei Eiern am niedrigsten.Akklimatisierung hatte keinen Einfluß auf die Unterkühlungspunkte aktiver Weibchen. Durch Gefrieren wurden alle Stadien getötet. Es erscheint möglich, daß der Unterschied im Überleben akklimatisierter und nichtakklimatisierter Weibchen bei-15° durch die verschiedene Fähigkeit bedingt ist, in einem unterkühlten Zustande zu leben.
  相似文献   
123.
Electron Microscope Study of the Human Neuromuscular Junction   总被引:5,自引:4,他引:1       下载免费PDF全文
A preliminary electron microscope study of human neuromuscular junction is presented. The biopsy material was taken from the palmarus longus, and fixed routinely in osmium tetroxide and embedded in methacrylate. The structure of the motor endings and the relationship of the synaptic vesicles to the axolemmal membrane are described. The synaptic clefts are filled with an homogeneous material in continuity with the basement membrane covering the muscle fiber. The subneural apparatus is described, and special attention is paid to a vesicular component present in the sarcoplasm of the junctional area, which differs from synaptic vesicles and is presumed to be a derivate of the sarcoplasmic reticulum.  相似文献   
124.
(Bellevalia ciliata was recorded in north-east Bulgaria south of the Dobrudsha, within field and steppe vegetation. Vegetation records and a distribution map are presented. Based on taxonomic studies it is proposed to combineB. ciliata, B. sarmatica (Pall.) Wor. andB. speciosa Wor. under the oldest nameB. ciliata (Cyr.) Nees.  相似文献   
125.
Arginine can be metabolized by inflammatory cells through at least two pathways. One is an oxidative l-arginine deiminase (OAD) that results in the formation of citrulline and reactive nitrogen intermediates. The other is arginase, which determines the production of ornithine and urea. The temporal expression of these pathways in an experimental wound model (s.c. implanted polyvinyl alcohol sponges in the rat) was investigated by examining the concentrations of amino acids and of nitrite in fluids obtained from the sponges 6 h to 15 day after implantation. These analyses revealed two distinct periods during which the arginine concentration in the fluids was markedly below plasma levels. During the early period (less than 3 days after sponge implantation) wound fluid contained more citrulline and nitrite than at any other time, suggesting OAD activity. In contrast, ornithine accumulated in the fluids during the late decrease in arginine concentration that extended beyond day 3, during which time the wound fluid also contained a high arginase activity. This time-dependent expression of different pathways of arginine metabolism in wounds was confirmed in sponge cultures containing [guanido-14C]-l-arginine. Cells contained in sponges harvested less than 48 h after implantation metabolized labeled arginine mainly to labeled citrulline, whereas labeled urea was produced during culture of sponges harvested after this time. The low arginine content of wound fluid did not appear to be rate limiting for the expression of OAD in late sponges because no OAD activity was evidenced when 4 mM arginine was added to the cultures. These results indicate that the OAD pathway is expressed in this model predominantly during the early, polymorphonuclear leukocyte-predominant, phase of repair. At this time, the reactive nitrogen intermediates resulting from the metabolism of arginine may mediate some of the events characteristic of early inflammation, including microbiostasis, vasodilation, and inhibition/reversal of platelet aggregation. In turn, the late suppression of this pathway and the catabolism of arginine through arginase may promote macrophage function within wounds.  相似文献   
126.
The possible activation of protein kinase C (PKC) during total cerebral ischemia was investigated in the rat. Translocation of PKC activity from the soluble to the particulate fraction was used as an index of PKC activation. There was a drop in the proportion of particulate PKC activity from 30% for controls to 20% by 30 min of ischemia (p less than 0.01). By 20 min of cardiac arrest, there was a 40% decline of the total cellular PKC activity (p less than 0.01). This was not accompanied by an increase in activator-independent activity, a finding indicating PKC was not being converted to protein kinase M. These data suggest that PKC was not activated during ischemia, but rather that ischemia causes a reduction in cellular PKC activity. Translocation of PKC activity to the particulate fraction was not observed in the cerebral cortex or hippocampus of reperfused brain for up to 6 h of recovery following 11-13 min of total cerebral ischemia. The level of total, soluble, and particulate PKC activity in the cerebral cortex was reduced (p less than 0.05), corresponding to the decrease observed by 15 min of ischemia without reflow. A similar decline in activity was also observed in the hippocampus. No increase in activator-independent activity was observed. These data suggest that PKC was inhibited during cerebral ischemia and that this reduced level of PKC activity was maintained throughout 6 h of recovery. We conclude that pathological activation of PKC was not responsible for the evolution of ischemic brain damage.  相似文献   
127.
The alpha 1-adrenergic receptor has been shown to mediate the release of arachidonic acid in FRTL5 thyroid cells and MDCK kidney cells. In primary cultures of spinal cord cells, norepinephrine stimulated release of arachidonic acid (from neurons only) and turnover of inositol phospholipids (from neurons and glia) via alpha 1-adrenergic receptors. These two responses were dissociated by treatment with phorbol ester and pertussis toxin, which inhibited production of inositol phosphates with no appreciable effect on release of arachidonic acid. Extracellular calcium was required for release of arachidonic acid, but not for production of inositol phosphates. The calcium channel blockers nifedipine and verapamil inhibited release of arachidonic acid only. However, 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8), a compound that blocks intracellular calcium release, diminished production of inositol phosphates, but had little effect on release of arachidonic acid. These results suggest that alpha 1-adrenergic receptors couple to release of arachidonic acid in primary cultures of spinal cord cells by a mechanism independent of activation of phospholipase C, possibly via the activation of phospholipase A2.  相似文献   
128.
Summary We have measured Ca2+ uptake and Ca2+ release in isolated permeabilized pancreatic acinar cells and in isolated membrane vesicles of endoplasmic reticulum prepared from these cells. Ca2+ uptake into cells was monitored with a Ca2+ electrode, whereas Ca2+ uptake into membrane vesicles was measured with45Ca2+. Using inhibitors of known action, such as the H+ ATPase inhibitors NBD-Cl and NEM, the Ca2+ ATPase inhibitor vanadate as well as the second messenger inositol 1,4,5-trisphosphate (IP3) and its analog inositol 1,4,5-trisphosphorothioate (IPS3), we could functionally differentiate two non-mitochondrial Ca2+ pools. Ca2+ uptake into the IP3-sensitive Ca2+ pool (IsCaP) occurs by a MgATP-dependent Ca2+ uptake mechanism that exchanges Ca2+ for H+ ions. In the absence of ATP Ca2+ uptake can occur to some extent at the expense of an H+ gradient that is established by a vacuolar-type MgATP-dependent H+ pump present in the same organelle. The other Ca2+ pool takes up Ca2+ by a vanadate-sensitive Ca2+ ATPase and is insensitive to IP3 (IisCaP). The IsCaP is filled at higher Ca2+ concentrations (10–6 mol/liter) which may occur during stimulation. The low steady-state [Ca2+] of 10–7 mol/liter is adjusted by the IisCaP.It is speculated that both Ca2+ pools can communicate with each other, the possible mechanism of which, however, is at present unknown.  相似文献   
129.
We have studied the cytoskeletal nature of a brain subcellular fraction previously shown to contain polyribosomes. We have identified the major proteins of this fraction by electrophoretic comparison to a standard cytoskeletal fraction and by immunodetection. These methods have shown the presence of actin, glial fibrillary acidic protein, and neurofilament triplet proteins. We have also studied the effect of various ions and nonionic detergents on the stability of this structure. It was stable in presence of Triton X-100 up to 2% but disrupted by 200 mM K+ acetate.Abbreviations CMT cytomatrix - CSK cytoskeleton - DOC sodium deoxycholate - DTT dithiothreitol - EGTA ethylenglycolbis (-Ether)-N,N-N-N-Tetraacetic Acid - GFAP glial fibrillary acidic protein - PR polyribosome - PRCMC polyribosomes-cytomatrix complex  相似文献   
130.
Summary Competition for light among species in a mixed canopy can be assessed quantitatively by a simulation model which evaluates the importance of different morphological and photosynthetic characteristics of each species. A model was developed that simulates how the foliage of all species attenuate radiation in the canopy and how much radiation is received by foliage of each species. The model can account for different kinds of foliage (leaf blades, stems, etc.) for each species. The photosynthesis and transpiration for sunlit and shaded foliage of each species is also computed for different layers in the canopy. The model is an extension of previously described single-species canopy photosynthesis simulation models. Model predictions of the fraction of foliage sunlit and interception of light by sunlit and shaded foliage for monoculture and mixed canopies of wheat (Triticum aestivum) and wild oat (Avena fatua) in the field compared very well with measured values. The model was used to calculate light interception and canopy photosynthesis for both species of wheat/wild oat mixtures grown under normal solar and enhanced ultraviolet-B (290–320 nm) radiation (UV-B) in a glasshouse experiment with no root competition. In these experiments, measurements showed that the mixtures receiving enhanced UV-B radiation had a greater proportion of the total foliage area composed of wheat compared to mixtures in the control treatments. The difference in species foliage area and its position in the canopy resulted in a calculated increase in the portion of total canopy radiation interception and photosynthesis by wheat. This, in turn, is consistent with greater canopy biomass of wheat reported in canopies irradiated with supplemental UV-B.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号