全文获取类型
收费全文 | 2321篇 |
免费 | 225篇 |
国内免费 | 1篇 |
专业分类
2547篇 |
出版年
2021年 | 31篇 |
2020年 | 24篇 |
2019年 | 29篇 |
2018年 | 32篇 |
2017年 | 16篇 |
2016年 | 43篇 |
2015年 | 87篇 |
2014年 | 106篇 |
2013年 | 134篇 |
2012年 | 115篇 |
2011年 | 128篇 |
2010年 | 86篇 |
2009年 | 65篇 |
2008年 | 108篇 |
2007年 | 115篇 |
2006年 | 100篇 |
2005年 | 116篇 |
2004年 | 102篇 |
2003年 | 85篇 |
2002年 | 83篇 |
2001年 | 42篇 |
2000年 | 36篇 |
1999年 | 42篇 |
1998年 | 32篇 |
1997年 | 18篇 |
1996年 | 24篇 |
1995年 | 20篇 |
1994年 | 17篇 |
1993年 | 27篇 |
1992年 | 37篇 |
1991年 | 22篇 |
1990年 | 18篇 |
1989年 | 26篇 |
1988年 | 23篇 |
1987年 | 15篇 |
1985年 | 21篇 |
1984年 | 19篇 |
1983年 | 23篇 |
1982年 | 28篇 |
1981年 | 24篇 |
1980年 | 28篇 |
1979年 | 22篇 |
1978年 | 24篇 |
1977年 | 20篇 |
1976年 | 15篇 |
1975年 | 15篇 |
1974年 | 18篇 |
1973年 | 18篇 |
1971年 | 25篇 |
1970年 | 15篇 |
排序方式: 共有2547条查询结果,搜索用时 15 毫秒
61.
Leon M. Rosenson 《Primates; journal of primatology》1977,18(3):579-588
Six lemur mothers of three different species and oneGalago crassicaudatus mother were observed in the presence of their own anesthetized infants. Two of the lemur mothers spent only very brief periods
sitting near their infants and seldom groomed them; the other four spent over half of the infant immobility period in close
proximity to their infants and groomed them frequently. Four lemur mothers groomed the ano-genital region of their infant
at least once. None of the lemur mothers picked up or carried her immobilized infant, as has been reported for some higher
primate mothers, although one lemur mother used her hands to pull the infant toward her ventrum while sitting on the floor.
Five lemur mothers rejected their infants when the infants displayed disoriented behavior while emerging from anesthesia.
The galago mother retrieved her anesthetized infant in her jaws but dropped the infant several times while attempting to groom
it. These results suggest very tentatively that prosimian mothers lack the ability shown by mothers of some higher primate
species to improvise protective ways of behaving toward helpless infants. 相似文献
62.
CARRIER MEDIATED BLOOD-BRAIN BARRIER TRANSPORT OF CHOLINE AND CERTAIN CHOLINE ANALOGS 总被引:24,自引:19,他引:5
Blood-brain barrier (BBB) transport of choline and certain choline analogs was studied in adult and suckling rats, and additionally compared in the paleocortex and neocortex of adult rats. Saturable uptake was characterized by a single kinetic system in all cases examined, and in adult rat forebrains we determined a Km= 442 ± 60 μM and Vmax= 10.0 ± 0.6 nmol min-1 g-1. In 14–15-day-old suckling forebrains a similar Km (= 404 ± 88 μM) but higher Vmax (= 12.5 ± 1.5 nmol min-1 g-1) was determined. When choline uptake was compared in two regions of the forebrain, similar Michaelis-Menten constants were determined but a higher uptake velocity was found in the neocortex (i.e. neocortex Km= 310 ± 103 μM and Vmax= 12.6 ± 2.8 nmol min-1g-1; paleocortex Km= 217 ± 76 μM and Vmax= 7.2 ± 1.5 nmol min-1 g-1). Administration of radiolabelled choline at low (5 μM) and high (100 μM) concentrations, followed by microwave fixation 60 s later and chloroform-methanol-water separations of the homogenized brain did not suggest a relationship between concentration and the appearance of label in lipid or aqueous fractions as observed in another in-vitro study elaborating two-component kinetics of choline uptake. It was observed that 60s after carotid injection 12–14% of the radiolabel in the ipsilateral cortex was found in the chloroform-soluble fraction. Hemicholinium-3 (Ki= 111 μM), dimethylaminoethanol (Ki= 42 μM), tetraethyl ammonium chloride, tetramethyl ammonium chloride, 2-hydroxyethyl triethylammonium iodide, carnitine, normal rat serum, and to a lesser extent lithium and spermidine all inhibited choline uptake in the BBB. Unsubstituted ammonium chloride and imipramine did not inhibit choline uptake. No difference was observed in blood-brain barrier choline uptake of unanesthetised, carotid artery-catheterized animals, and comparable sodium pentobarbital-anesthetized controls. 相似文献
63.
The present study was done to determine if a progesterone receptor is present in rat pituitary. Cytosol was labeled with 3H-progesterone (3HP) or 3H-RS020 (3HR) and subjected to sucrose-glycerol density-gradient centrifugation. Serum progesterone was measured for correlation with progesterone receptor levels. Two 3HP-binding peaks (4S + 6S) were evident in uterine and pituitary cytosols. The 4S peak was eliminated by competition with unlabeled cortisol leaving a single 6S peak (progesterone receptor). Estradiol (E) priming of the male or female rat increased progesterone receptor levels in pituitary cytosol as demonstrated using 3HP and 3HR, and pituitary progesterone receptor bound 3HR with a higher affinity than 3HP. Following adrenalectomy of gonadectomized rats, progesterone receptor levels were increased in pituitary and uterine cytosol of both E-primed and unprimed groups. An inverse relationship was established between serum progesterone and progesterone receptor levels in the uterus and pituitary suggesting that stressinduced adrenal progesterone secretion significantly influences progesterone receptor levels in the rat. These results demonstrate an estrogen-inducible progesterone receptor in the rat pituitary with properties similar to those of the uterine progesterone receptor. 相似文献
64.
65.
66.
67.
68.
Michael Leon 《Developmental neurobiology》1992,23(10):1557-1573
Young mammals come to approach the odor of their mother, a response that facilitates their survival during early life. Young rats induce a cascade of events in their mother to induce the emission of her odor. The pups increase circulating prolactin levels, which increases food intake and the emission of large quantities of cecotrophe containing the maternal odor. This odor is synthesized by the action of cecal microorganisms and changes with maternal diet. The diet-dependence of the odor requires the pups to acquire their attraction to the odor postnatally. The acquisition of this preference occurs when an odor is paired with the tactile stimulation that pups receive during maternal care. The action of the tactile stimulation appears to be mediated by noradrenaline. The development of this type of olfactory attraction is accompanied by changes in the regions of the olfactory bulb that are responsive to the attractive odor. Metabolic, anatomical, and neurophysiological changes in response to the attractive odor emerge in such regions of the bulb after early olfactory preference training. © 1992 John Wiley & Sons, Inc. 相似文献
69.
Leon Milewich Terry S. Hendricks Bettie Sue Masters Rene A. Frenkel Paul C. Mac Donald 《Archives of biochemistry and biophysics》1981,211(2):530-536
The estrogen synthetase present in human placental microsomes appears to be dependent on the cooperative interaction of the reduced cofactors NADPH and NADH for optimal activity. Using steady-state concentrations of either cofactor, it was found that while the estrogen synthetase activity followed hyperbolic saturation kinetics with NADPH (Kmapp = 14 μM), the enzyme followed sigmoidal saturation kinetics when the cofactor was NADH, with the half-maximum velocity attained at a cofactor concentration of 1.1 mm. The maximum velocity obtained with NADPH as the cofactor was greater than with corresponding concentrations of NADH. Estrogen synthetase activity in the presence of NADH was not due to NADPH contamination. NADH, in the presence of small concentrations of NADPH (0.5 to 5 μm), stimulated significantly the rate of estrogen formation from androstenedione by placental microsomes and, in addition, the enzyme saturation kinetics changed from sigmoidal to hyperbolic, thus mimicking the effect of NADPH. Estrogen synthetase activity, measured in the presence of 1 mm NADH, was stimulated in a dose-dependent manner by NADPH (Kmapp = 0.4 μM NADPH) and, when the enzyme was measured in the presence of 5 μm NADPH, the activity was stimulated in a dose-dependent manner by NADH (Kmapp = 45 μM NADH). Estrogen synthetase activity measured in the presence of NADH, without and with NADPH (1 μm) remained linear both with time of incubation for approximately 15 min and with microsomal protein concentration up to 3 mg/ml. The apparent Km of estrogen synthetase for androstenedione, when measured in the presence of NADH, was 1 μm. The synergistic interaction between NADH and NADPH in stimulating placental estrogen synthetase activity observed in vitro may, conceivably, take place in vivo in the intact placenta. 相似文献
70.
Leon N. Kapp 《Biochimica et Biophysica Acta (BBA) - Gene Structure and Expression》1982,696(2):226-227
DNA fork displacement rates were measured in three lines of Bloom's syndrome cells and in a normal diploid fibroblast line. Fork displacement rates in Bloom's cells were approx. 55–65% of the rate in normal fibroblasts. 相似文献