全文获取类型
收费全文 | 125篇 |
免费 | 7篇 |
出版年
2022年 | 1篇 |
2021年 | 3篇 |
2020年 | 1篇 |
2019年 | 4篇 |
2018年 | 2篇 |
2017年 | 3篇 |
2016年 | 3篇 |
2015年 | 9篇 |
2014年 | 8篇 |
2013年 | 6篇 |
2012年 | 6篇 |
2011年 | 8篇 |
2010年 | 7篇 |
2009年 | 2篇 |
2008年 | 8篇 |
2007年 | 9篇 |
2006年 | 4篇 |
2005年 | 7篇 |
2004年 | 3篇 |
2003年 | 1篇 |
2002年 | 3篇 |
2001年 | 5篇 |
2000年 | 1篇 |
1999年 | 5篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1993年 | 2篇 |
1991年 | 1篇 |
1990年 | 3篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1966年 | 1篇 |
排序方式: 共有132条查询结果,搜索用时 15 毫秒
1.
Evolution of the 28S ribosomal RNA gene in anurans: regions of variability and their phylogenetic implications 总被引:1,自引:0,他引:1
Fifteen restriction sites were mapped to the 28S ribosomal RNA gene of
individuals representing 54 species of frogs, two species of salamanders, a
caecilian, and a lungfish. Eight of these sites were present in all species
examined, and two were found in all but one species. Alignment of these
conserved restriction sites revealed, among anuran 28S rRNA genes, five
regions of major length variation that correspond to four of 12 previously
identified divergent domains of this gene. One of the divergent domains
(DD8) consists of two regions of length variation separated by a short
segment that is conserved at least throughout tetrapods. Most of the
insertions, deletions, and restriction-site variations identified in the
28S gene will require sequence-level analysis for a detailed reconstruction
of their history. However, an insertion in DD9 that is coextensive with
frogs in the suborder Neobatrachia, a BstEII site that is limited to
representatives of two leptodactylid subfamilies, and a deletion in DD10
that is found only in three ranoid genera are probably synapomorphies.
相似文献
2.
Neurotransmitter, opiodergic system, steroid-hormone interaction and involvement in the replacement therapy of sexual disorders 总被引:1,自引:0,他引:1
G Frajese R Lazzari A Magnani C Moretti V Sforza D Nerozzi 《The Journal of steroid biochemistry and molecular biology》1990,37(3):411-419
Dopamine (DA) and serotonin (5-HT) are the neurotransmitters most directly involved in sexual activity. DA plays a stimulatory role while 5-HT has an inhibitory effect. The two monoaminergic systems modulate the secretion of many hormones (GnRH, LH, testosterone, prolactin and endorphins) involved in sexual functional capacity. Furthermore, hormones influence synthesis and storage of brain neurotransmitters. Impotence can often be associated to clinical depression and altered neurotransmitter function. Moreover, stress represents an unbalance between various neurotransmitter systems and can induce impotence especially when disorders of the endorphinic system are present. Replacement therapy is based upon the understanding of these basic concepts. Impotence due to an underlying depressive illness must be treated with dopaminergic antidepressant drugs; while in stressful conditions a good response to the naloxone test is the preliminary criterion to subsequent naltrexone treatment. When a hormonal deficiency has been proved, the hormone replacement therapy is of course highly effective (gonadotropins in hypogonadotropic syndromes, testosterone in aging, etc.). Finally, idiopathic impotence could be treated by DA agonist and/or 5-HT antagonist drugs either alone or better yet in association with psychotherapy. 相似文献
3.
Characterization of cross-bridge elasticity and kinetics of cross-bridge cycling during force development in single smooth muscle cells 总被引:4,自引:2,他引:2 下载免费PDF全文
Force development in smooth muscle, as in skeletal muscle, is believed to reflect recruitment of force-generating myosin cross-bridges. However, little is known about the events underlying cross-bridge recruitment as the muscle cell approaches peak isometric force and then enters a period of tension maintenance. In the present studies on single smooth muscle cells isolated from the toad (Bufo marinus) stomach muscularis, active muscle stiffness, calculated from the force response to small sinusoidal length changes (0.5% cell length, 250 Hz), was utilized to estimate the relative number of attached cross-bridges. By comparing stiffness during initial force development to stiffness during force redevelopment immediately after a quick release imposed at peak force, we propose that the instantaneous active stiffness of the cell reflects both a linearly elastic cross-bridge element having 1.5 times the compliance of the cross-bridge in frog skeletal muscle and a series elastic component having an exponential length-force relationship. At the onset of force development, the ratio of stiffness to force was 2.5 times greater than at peak isometric force. These data suggest that, upon activation, cross-bridges attach in at least two states (i.e., low-force-producing and high-force-producing) and redistribute to a steady state distribution at peak isometric force. The possibility that the cross-bridge cycling rate was modulated with time was also investigated by analyzing the time course of tension recovery to small, rapid step length changes (0.5% cell length in 2.5 ms) imposed during initial force development, at peak force, and after 15 s of tension maintenance. The rate of tension recovery slowed continuously throughout force development following activation and slowed further as force was maintained. Our results suggest that the kinetics of force production in smooth muscle may involve a redistribution of cross-bridge populations between two attached states and that the average cycling rate of these cross-bridges becomes slower with time during contraction. 相似文献
4.
Excess amino acid polymorphism in mitochondrial DNA: contrasts among genes from Drosophila, mice, and humans 总被引:13,自引:3,他引:10
Recent studies of mitochondrial DNA (mtDNA) variation in mammals and
Drosophila have shown an excess of amino acid variation within species
(replacement polymorphism) relative to the number of silent and replacement
differences fixed between species. To examine further this pattern of
nonneutral mtDNA evolution, we present sequence data for the ND3 and ND5
genes from 59 lines of Drosophila melanogaster and 29 lines of D. simulans.
Of interest are the frequency spectra of silent and replacement
polymorphisms, and potential variation among genes and taxa in the
departures from neutral expectations. The Drosophila ND3 and ND5 data show
no significant excess of replacement polymorphism using the
McDonald-Kreitman test. These data are in contrast to significant
departures from neutrality for the ND3 gene in mammals and other genes in
Drosophila mtDNA (cytochrome b and ATPase 6). Pooled across genes, however,
both Drosophila and human mtDNA show very significant excesses of amino
acid polymorphism. Silent polymorphisms at ND5 show a significantly higher
variance in frequency than replacement polymorphisms, and the latter show a
significant skew toward low frequencies (Tajima's D = -1.954). These
patterns are interpreted in light of the nearly neutral theory where mildly
deleterious amino acid haplotypes are observed as ephemeral variants within
species but do not contribute to divergence. The patterns of polymorphism
and divergence at charge-altering amino acid sites are presented for the
Drosophila ND5 gene to examine the evolution of functionally distinct
mutations. Excess charge-altering polymorphism is observed at the carboxyl
terminal and excess charge-altering divergence is detected at the amino
terminal. While the mildly deleterious model fits as a net effect in the
evolution of nonrecombining mitochondrial genomes, these data suggest that
opposing evolutionary pressures may act on different regions of
mitochondrial genes and genomes.
相似文献
5.
DM Woolley 《The Journal of cell biology》1977,75(3):851-865
Motile spermatozoa from the golden hamster have been arrested by rapid freezing and then fixed with glutaraldehyde at low temperature after substitution with ethylene glycol. As far as can be judged, the flagellar waveforms thus stabilized are similar to those seen in living sperm; in contrast, fixation in glutaraldehyde, without prior freezing, induces agonal changes in flagellar conformation. The characteristics waveform after freeze substitution contains three bends. Approx. half of these flagella are entirely planar. The rest are three dimensional, with the third bend displaced in a regular way from the plane containing the proximal two bends. From the geometry of these flagella, it is concluded that the plane of action of a given bending cycle undergoes a clockwise twist (from a forward viewpoint) as the cycle is succeeded by new bending cycles. This "twisted plane" undulation is quite different from helical movement. The twisting seems to occur abruptly, between cycles, as if each bending cycle has a preferred plane of action. The mechanism underlying the twisting is uncertain. However, on the basis of the angular displacements between the preferred planes, and the findings from electron microscopy, the following idea is presented as a working hypothesis: that, if the most proximal plane of bending is topographically determined by peripheral doublet 1, then successive distal planes of action are influenced predominantly by doublets 2, 3, etc., in clockwise sequence. The merits and weaknesses of this hypothesis are discussed. 相似文献
6.
Andrea Mapelli Renato Rodano Angelo Fiorentini Andrea Giustolisi Fernanda V. Sidequersky Chiarella Sforza 《Journal of electromyography and kinesiology》2013,23(5):1097-1105
Using an optoelectronic motion capture system, we quantitatively assessed the arrangement of body segments and the displacement of the horizontal projection of the center of mass (CM) in seven skaters performing off-ice back spins on a rotating device (spinner). The position of the CM at the beginning of the spins was not a determining factor, but its rapid stabilization towards the center of the spinner, together with the achievement of a stable arrangement of trunk and limbs, was crucial to get the dynamic equilibrium, necessary for a lasting performance. At full spinning, however, there was an indicative variety of individual body postures. A final deceleration, associable with the loss of body equilibrium, was detected in the last spin of most of skaters.In conclusion, the current investigation demonstrated that the off-ice execution of back spin, a critical movement of ice skating, can be measured in laboratory, thus providing quantitative information to both the skaters and the coaches. The analysis is not invasive, and it may be proposed also for longitudinal evaluations of skating and postural training. 相似文献
7.
Melania Manco Maria Rita Spreghini Rosa Luciano Cecilia Pensini Rita Wietrzycowska Sforza Carmela Rustico Marco Cappa Giuseppe Stefano Morino 《PloS one》2013,8(7)
Background
Insulin sensitivity decreases at puberty transition, but little information has been provided on its earlier time-course. Aim of the present study was to describe the time-course of insulin sensitivity in severely obese children at the transition from preschool to school age.Research design and methods
Retrospective study of a cohort of 47 severely obese [Body Mass Index (BMI) ≥99° percentile] preschoolers evaluated twice, once between 2 and 6 years of age, and once before age 8. Glucose tolerance, Whole Body Insulin Sensitivity Index (WBISI), Insulinogenic Index (IGI); β-cell demand index (BCDI) and Insulin Secretion-Sensitivity Index 2 (ISSI-2) were longitudinally estimated during the oral glucose tolerance test.Results
After a median follow-up of 2.23 (1–4.52) y, obese patients showed significant decrease in WBISI (p<0.0001), and increase in fasting (p = 0.005) and 2 h glucose (2HG, p = 0.001). One child in preschool age and 4 school age children presented with 2HG between 7.8–11.1 mmol/l. Best predictors of WBISI, 2HG and BCDI in the school age were changes in BMI z-score (R2 = 0.309; p = 0.002; β = −0.556), ISSI-2 (R2 = 0.465; p<0.0001; β = −0.682), and BMI z-score (R2 = 0.246; p = 0.008; 0.496), respectively.Conclusions
In morbidly obese children, insulin sensitivity seems to decline even before pubertal transition, but changes in total adiposity can only partially explain this variation. 相似文献8.
Ellyn V. Bitume Patrick J. Moran René F.H. Sforza 《Biocontrol Science and Technology》2019,29(7):615-625
In weed biocontrol, there is a need for pre-release efficacy assessments for potential agents. Genista monspessulana ((L.) L. A. S. Johnson (Fabaceae), French broom) is an invasive perennial shrub in the western U.S. The galling weevil Lepidapion argentatum Gerstaecker is a potential biocontrol agent. The impact of increasing weevil density on galling damage, plant height, width, leaf damage, and relative growth rate (RGR) was assessed in greenhouse experiments on two to three-month-old seedlings infested with either one or three weevils. Infestation by three female weevils caused 48% more galls producing 27% more larvae than did infestation with one female while causing only 1% leaf damage and no difference in total leaf area. Infestation with multiple weevils caused a 55% and 29% decrease in plant height and canopy width respectively, while single-weevil infestation decreased height by 32% and width to the same degree as for multiple weevils. The RGR of seedlings infested with three weevils was three times slower than the controls, while growth was reduced 2-fold by single-weevil infestation. Reductions in plant size and growth rate induced by weevil galling could reduce plant competitive survival to reproduction and also plant population dispersal as seedlings. Our results suggest that L. argentatum has the potential to cause impact to French broom seedlings if released in the invasive range. 相似文献
9.
Clément Daboné Ralph Buij Adama Oueda Jacques Boko Adjakpa Wendengoudi Guenda Peter DM Weesie 《Ostrich》2019,90(1):53-61
During the last decades, the critically endangered Hooded Vulture Necrosyrtes monachus has strongly declined across its African range. Although direct persecution has been suggested as a major cause of this decline, little is known about the impact of humans on reproductive output in West Africa. We studied the impact of human activities on the reproductive output of Hooded Vultures in the Garango area of Burkina Faso. Twenty and 56 nesting attempts were monitored, respectively, during the breeding season in 2013/14 and 2014/15, to determine reproductive success and identify causes of nest failure. Annual breeding success varied between 0.68 and 0.71 chicks fledged per breeding pair per year and productivity was assessed at 0.57 chicks fledged per territorial pair in 2014/15. The main threats imposed by humans were poaching of eggs, chicks and collection of nest materials, leading to 20% (13 out of 64 breeding attempts) of nest failures over the two years. An additional important reason for nest failure was the pruning and (partial) cutting of nest trees. Despite this high level of human interference, we found that Hooded Vulture nest success increased with proximity to human settlements, probably because breeding vultures benefit from protection by people against persecution and disturbance. 相似文献
10.
Giordano M Oefner PJ Underhill PA Cavalli Sforza LL Tosi R Richiardi PM 《Genomics》1999,56(3):247-253
Genetic association analysis of candidate regions where evidence of linkage has accumulated is becoming a key issue in the study of complex diseases. A high density of markers, at least one per centimorgan, is required to improve the chances of observing linkage disequilibrium with disease alleles. A recently available single nucleotide polymorphism (SNP) map designed to cover the whole genome provides an average density of one marker per 2 cM. In the present study we show that the number of markers can be approximately doubled in a selected region, thus reaching a density suitable for association studies, by applying a completely automated technique for polymorphism detection, denaturing high-performance liquid chromatography (DHPLC). A systematic search for SNPs was performed in the region 5ptel-q13, where weak but convergent evidence for linkage with multiple sclerosis has accumulated. Screening for polymorphisms was performed on 124 sequence tagged sites (STSs) in the 3'UTR ends of expressed sequence tags totaling about 30,000 bp. Thirty SNPs in 28 STSs were found with less than 10% overlap with the markers already detected in the same region. The data confirm the validity of the approach using DHPLC on expressed gene sequences tagged by a set of standard commercially available primers. 相似文献