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排序方式: 共有135条查询结果,搜索用时 140 毫秒
21.
Michele Giugliano Giancarlo La Camera Stefano Fusi Walter Senn 《Biological cybernetics》2008,99(4-5):303-318
The response of a population of neurons to time-varying synaptic inputs can show a rich phenomenology, hardly predictable from the dynamical properties of the membrane’s inherent time constants. For example, a network of neurons in a state of spontaneous activity can respond significantly more rapidly than each single neuron taken individually. Under the assumption that the statistics of the synaptic input is the same for a population of similarly behaving neurons (mean field approximation), it is possible to greatly simplify the study of neural circuits, both in the case in which the statistics of the input are stationary (reviewed in La Camera et al. in Biol Cybern, 2008) and in the case in which they are time varying and unevenly distributed over the dendritic tree. Here, we review theoretical and experimental results on the single-neuron properties that are relevant for the dynamical collective behavior of a population of neurons. We focus on the response of integrate-and-fire neurons and real cortical neurons to long-lasting, noisy, in vivo-like stationary inputs and show how the theory can predict the observed rhythmic activity of cultures of neurons. We then show how cortical neurons adapt on multiple time scales in response to input with stationary statistics in vitro. Next, we review how it is possible to study the general response properties of a neural circuit to time-varying inputs by estimating the response of single neurons to noisy sinusoidal currents. Finally, we address the dendrite–soma interactions in cortical neurons leading to gain modulation and spike bursts, and show how these effects can be captured by a two-compartment integrate-and-fire neuron. Most of the experimental results reviewed in this article have been successfully reproduced by simple integrate-and-fire model neurons. 相似文献
22.
23.
Usdin K; Chevret P; Catzeflis FM; Verona R; Furano AV 《Molecular biology and evolution》1995,12(1):73-82
The single most difficult problem in phylogenetic analysis is deciding
whether a shared taxonomic character is due to common ancestry or one that
appeared independently due to convergence, parallelism, or reversion to an
ancestral state. Mammalian L1 retrotransposons undergo periodic
amplifications in which multiple copies of the elements are interspersed in
the genome. Because these elements apparently are transmitted only by
inheritance and are retained in the genome, a shared L1 amplification event
can only be an inherited ancestral character. We propose that L1
amplification events can be an excellent tool for analyzing mammalian
evolution and demonstrate here how we addressed several refractory problems
in rodent systematics using L1 DNA as a taxonomic character.
相似文献
24.
Sso7d is a basic 7-kDa DNA-binding protein from Sulfolobus solfataricus, also endowed with ribonuclease activity. The protein consists of a double-stranded antiparallel beta-sheet, onto which an orthogonal triple-stranded antiparallel beta-sheet is packed, and of a small helical stretch at the C-terminus. Furthermore, the two beta-sheets enclose an aromatic cluster displaying a fishbone geometry. We previously cloned the Sso7d-encoding gene, expressed it in Escherichia coli, and produced several single-point mutants, either of residues located in the hydrophobic core or of Trp23, which is exposed to the solvent and plays a major role in DNA binding. The mutation F31A was dramatically destabilizing, with a loss in thermo- and piezostabilities by at least 27 K and 10 kbar, respectively. Here, we report the solution structure of the F31A mutant, which was determined by NMR spectroscopy using 744 distance constraints obtained from analysis of multidimensional spectra in conjunction with simulated annealing protocols. The most remarkable finding is the change in orientation of the Trp23 side chain, which in the wild type is completely exposed to the solvent, whereas in the mutant is largely buried in the aromatic cluster. This prevents the formation of a cavity in the hydrophobic core of the mutant, which would arise in the absence of structural rearrangements. We found additional changes produced by the mutation, notably a strong distortion in the beta-sheets with loss in several hydrogen bonds, increased flexibility of some stretches of the backbone, and some local strains. On one hand, these features may justify the dramatic destabilization provoked by the mutation; on the other hand, they highlight the crucial role of the hydrophobic core in protein stability. To the best of our knowledge, no similar rearrangement has been so far described as a result of a single-point mutation. 相似文献
25.
26.
CTS Silva V. Wanderley-Teixeira FM Cunha JV Oliveira KA Dutra DMA Ferraz Navarro 《Biotechnic & histochemistry》2018,93(1):36-48
The armyworm, Spodoptera frugiperda, is the principal pest of corn in Brazil. Control is achieved primarily by synthetic insecticides, which cause problems for the agro-ecosystem. Alternative methods of control are under investigation and citronella (Cymbopogon winterianus) essential oil appears to be a promising agent. We investigated the effects of citronella oil using histological, histochemical and immunohistochemical methods. The midgut of larvae treated with citronella exhibited altered epithelium including cytoplasmic protrusions, columnar cell extrusion, pyknotic nuclei, and increased periodic acid-Schiff positive granules. Regenerative cells in the epithelium of the midgut increased in number, which facilitated subsequent regeneration of this tissue. After exposure to citronella, trophocytes, the principal cell type of the fat body, possessed enlarged vacuoles and mitotic bodies, and contained reduced amounts of glycogen, lipid, and protein. Citronella oil caused morphological changes of the midgut and reduction of stored resources in the fat body, which may adversely affect insect reproduction and survival. 相似文献
27.
Phylogenetic relationships of artiodactyls and cetaceans as deduced from the comparison of cytochrome b and 12S rRNA mitochondrial sequences 总被引:8,自引:0,他引:8
A data set of complete mitochondrial cytochrome b and 12S rDNA sequences is
presented here for 17 representatives of Artiodactyla and Cetacea, together
with potential outgroups (two Perissodactyla, two Carnivora, two
Tethytheria, four Rodentia, and two Marsupialia). We include seven
sequences not previously published from Hippopotamidae (Ancodonta) and
Camelidae (Tylopoda), yielding a total of nearly 2.1 kb for both genes
combined. Distance and parsimony analyses of each gene indicate that 11
clades are well supported, including the artiodactyl taxa Pecora,
Ruminantia (with low 12S rRNA support), Tylopoda, Suina, and Ancodonta, as
well as Cetacea, Perissodactyla, Carnivora, Tethytheria, Muridae, and
Caviomorpha. Neither the cytochrome b nor the 12S rDNA genes resolve the
relationships between these major clades. The combined analysis of the two
genes suggests a monophyletic Cetacea +Artiodactyla clade (defined as
"Cetartiodactyla"), whereas Perissodactyla, Carnivora, and Tethytheria fall
outside this clade. Perissodactyla could represent the sister taxon of
Cetartiodactyla, as deduced from resampling studies among outgroup
lineages. Cetartiodactyla includes five major lineages: Ruminantia,
Tylopoda, Suina, Ancodonta, and Cetacea, among which the phylogenetic
relationships are not resolved. Thus, Suiformes do not appear to be
monophyletic, justifying their split into the Suina and Ancodonta
infraorders. An association between Cetacea and Hippopotamidae is supported
by the cytochrome b gene but not by the 12S rRNA gene. Calculation of
divergence dates suggests that the Cetartiodactyla could have diverged from
other Ferungulata about 60 MYA.
相似文献
28.
Gunnel Hellgren Björn Andersson Andreas FM Nierop Jovanna Dahlgren Ze'ev Hochberg Kerstin Albertsson-Wikland 《Proteome science》2008,6(1):35
Background
The broad range in growth observed in short prepubertal children receiving the same growth hormone (GH) dose is due to individual variation in GH responsiveness. This study used a pharmaco-proteomic approach in order to identify novel biomarkers that discriminate between short non-GH-deficient (GHD) children who show a good or poor growth response to GH treatment. 相似文献29.
Learning flexible sensori-motor mappings in a complex network 总被引:1,自引:1,他引:0
Given the complex structure of the brain, how can synaptic plasticity explain the learning and forgetting of associations
when these are continuously changing? We address this question by studying different reinforcement learning rules in a multilayer
network in order to reproduce monkey behavior in a visuomotor association task. Our model can only reproduce the learning
performance of the monkey if the synaptic modifications depend on the pre- and postsynaptic activity, and if the intrinsic
level of stochasticity is low. This favored learning rule is based on reward modulated Hebbian synaptic plasticity and shows
the interesting feature that the learning performance does not substantially degrade when adding layers to the network, even
for a complex problem. 相似文献
30.
Björn Andersson Gunnel Hellgren Andreas FM Nierop Ze'ev Hochberg Kerstin Albertsson-Wikland 《Proteome science》2009,7(1):40-12