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1.
Anatol Rapoport 《Bulletin of mathematical biology》1952,14(1):73-83
The refractory periods of an aggregate of simple “counter” neurons are assumed distributed according to some probability frequency.
The output of the aggregate is computed for rectangular and triangular distributions. In particular, it is shown that the
maximum output of an aggregate with any triangular distribution cannot exceed the maximum output of its average neuron by
a factor greater than 2 ln 2. This puts an upper bound on the amount of departure from the behavior of the average neuron
which an aggregate characterized by a certain type of distribution can show.
Next, the aggregate is supposed to be subjected to regularly spaced stimuli. Under these conditions, a single neuron will
give a discontinuous output curve. If, however, the refractory periods are distributed according to some frequency, the output
curve may be “smoothed out.” A general condition on the distribution is derived which makes the output monotone increasing
with the input. The condition is applied to some special cases. 相似文献
2.
From an observation of efferent interspike intervals of a neuron, we consider how to decode the input temporal information. It is found that the integrate-and-fire model is blind in the temporal domain due to the fact that its efferent firing rate is independent of the input temporal frequency. The conclusion is then confirmed for the integrate-and-fire model with correlated inputs, with reversal potentials, with a nonlinear leakage and with a subthreshold oscillation. For the Hodgkin-Huxley model, however, in terms of efferent firing rates alone, it is possible to read out the input temporal information. 相似文献
3.
Matthieu Gilson Anthony N. Burkitt David B. Grayden Doreen A. Thomas J. Leo van Hemmen 《Biological cybernetics》2009,101(2):103-114
Spike-timing-dependent plasticity (STDP) is believed to structure neuronal networks by slowly changing the strengths (or weights)
of the synaptic connections between neurons depending upon their spiking activity, which in turn modifies the neuronal firing
dynamics. In this paper, we investigate the change in synaptic weights induced by STDP in a recurrently connected network
in which the input weights are plastic but the recurrent weights are fixed. The inputs are divided into two pools with identical
constant firing rates and equal within-pool spike-time correlations, but with no between-pool correlations. Our analysis uses
the Poisson neuron model in order to predict the evolution of the input synaptic weights and focuses on the asymptotic weight
distribution that emerges due to STDP. The learning dynamics induces a symmetry breaking for the individual neurons, namely
for sufficiently strong within-pool spike-time correlation each neuron specializes to one of the input pools. We show that
the presence of fixed excitatory recurrent connections between neurons induces a group symmetry-breaking effect, in which
neurons tend to specialize to the same input pool. Consequently STDP generates a functional structure on the input connections
of the network. 相似文献
4.
Seigen Tsukuda Masahito Sugiyama Yunosuke Harita Kazuo Nishimura 《Biogeochemistry》2006,77(1):117-138
Atmospheric bulk depositions of soluble reactive phosphorus (SRP), soluble unreactive phosphorus (SUP), particulate inorganic
phosphorus (PIP), particulate organic phosphorus (POP), total phosphorus (TP) and some other dissolved and particulate components
were monitored for 3 years in Ashiu, Central Japan. The mean bulk depositions of SRP, SUP, PIP, POP, TP, dissolved components
(Na, Mg, nss-Ca, K, V, Mo, nss-SO4) and particulate components (Al, Fe, Ti, Ca, Mg, Mn, Ba, Sr, Zn) were 175, 76, 136, 397, 783, 156,000, 10,900, 7450, 5470,
10.3, 1.52, 40,100, 13,200, 3590, 2630, 576, 624, 42.3, 30.2, 17.4, 8.2 μmol m−2 year−1, respectively. The value for TP deposition was in the lower range of previous literature. The low P deposition probably reflected
the method applied to reduce the contribution of local particles, including (1) placement of samplers off the ground surface,
(2) installation of multiple samplers, and (3) rejection of contaminated samples. Al data suggested that 15 ± 5% of TP was
brought by lithogenic dust from East Eurasia. Nss-SO4 and Mo data and air-mass backward trajectories suggested that 39 ± 4% of TP was derived from coal combustion in China. It
was speculated that the rest (47 ± 6%) of the TP deposition might be predominantly attributed to the contribution of local
biogenic particles. Net atmospheric TP input (lithogenic dust and fossil fuel combustion) was almost equal to the TP outflow
from Japanese forests on granitic soils. 相似文献
5.
Hajime Fujii Masatoshi Inoue Hiroyuki Okuno Yoshikazu Sano Sayaka Takemoto-Kimura Kazuo Kitamura Masanobu Kano Haruhiko Bito 《Cell reports》2013,3(4):978-987
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6.
Matthieu Gilson Anthony N. Burkitt David B. Grayden Doreen A. Thomas J. Leo van Hemmen 《Biological cybernetics》2009,101(2):81-102
Spike-timing-dependent plasticity (STDP) determines the evolution of the synaptic weights according to their pre- and post-synaptic
activity, which in turn changes the neuronal activity. In this paper, we extend previous studies of input selectivity induced
by (STDP) for single neurons to the biologically interesting case of a neuronal network with fixed recurrent connections and
plastic connections from external pools of input neurons. We use a theoretical framework based on the Poisson neuron model
to analytically describe the network dynamics (firing rates and spike-time correlations) and thus the evolution of the synaptic
weights. This framework incorporates the time course of the post-synaptic potentials and synaptic delays. Our analysis focuses
on the asymptotic states of a network stimulated by two homogeneous pools of “steady” inputs, namely Poisson spike trains
which have fixed firing rates and spike-time correlations. The (STDP) model extends rate-based learning in that it can implement,
at the same time, both a stabilization of the individual neuron firing rates and a slower weight specialization depending
on the input spike-time correlations. When one input pathway has stronger within-pool correlations, the resulting synaptic
dynamics induced by (STDP) are shown to be similar to those arising in the case of a purely feed-forward network: the weights
from the more correlated inputs are potentiated at the expense of the remaining input connections. 相似文献
7.
The aim of the present study was to simulate the input system and the motoneuron (MN) pool of the MN pool–muscle complex
(MNPMC). Input fibers, which can originate from command centers in the central nervous system or from sensory organs, activate
the MN pool. They generate sequences of action potentials, the frequency of which is proportional to a time-dependent activation
factor (which is an input to the model). Different connection patterns between the input fibers and motor units (MUs) are
allowed. For simplicity and since no precise experimental data are available, 70 input fibers and 4 boutons per fiber and
MN are simulated (this corresponds approximately to the monosynaptic group-Ia input of the cat medial gastrocnemius muscle).
Each bouton generates the same conductance change in the postsynaptic membrane. The MNs are modeled with a single compartment
and a homogenous membrane. According to experimental data, the membrane leakage conductance and capacitance are MU dependent.
Since the precise relation is unknown: (a) the computed relation between MU contraction force and the MN leakage conductance
was taken from a steady-state MNPMC model, and (b) the capacitance was assumed to be proportional to the leakage conductance.
The MN membrane includes time- and voltage-dependent ionic channels (fast and slow K+ and low- and high-threshold Ca2+ channels). The density and time constant of the slow K+ channels and the density of the Ca2+ channels were fitted to approximate afterhyperpolarization characteristics and frequency-injected current relations of type-identified
cat MNs. If the membrane reaches a voltage threshold the MNs generate action potentials, which were simulated by voltage pulses.
The activation of the MN pool of the human first dorsal interosseus muscle was simulated with injected and synaptic currents
in order to illustrate the size principle, synaptic noise, and other features of muscle activation. It is concluded that the
present model reproduces the main properties of the input–output relations of different MN types within a muscle. Together
with the simulation of the muscle force and the surface EMG, which will be published in subsequent papers, it will be a powerful
tool for reproducing experiments on the motor system and investigating functional mechanisms of motor control.
Received: 17 April 2001 / Accepted in revised form: 6 November 2001 相似文献
8.
S. Ramachandra Rao Usha Tripathi G.A. Ravishankar 《Biocatalysis and Biotransformation》2013,31(2):137-143
Cell suspension cultures of Capsicum frutescens accumulated digoxin, purpureaglycoside A and other unknown derivatives when digitoxin, a cardiac glycoside, was used as a precursor. The feeding of digitoxin complexed with g -cyclodextrin increased the accumulation of digoxin, purpureaglycoside A and other unknown derivatives. Control cultures (without digitoxin) did not produce any of these metabolites. The growth of cells was affected by both digitoxin as well as digitoxin- g -cyclodextrin. The accumulation of purpureaglycoside A and digoxin reached a maximum of 1241 and 374 w g 100 ml m 1 culture on the 6th and 2nd day, respectively, which was 3.9 and 4.5 fold higher than cultures treated with digitoxin alone (sampled on the 13th day). The other unknown derivatives formed in digitoxin- g -cyclodextrin fed cultures were 15 times higher than digitoxin alone fed C. frutescens cultures. The addition of glucose to digitoxin- g -cyclodextrin treated cultures increased the accumulation of purpureaglycoside A which reached a maximum of 3589 w g 100 ml m 1 culture after 12 h incubation, which was a 2.9 fold increase over cultures treated with digitoxin- g -cyclodextrin alone. 相似文献
9.
In experiments on non-anesthetized Wistar white rats there was studied reaction of kidney to an intramuscular injection of arginine vasotocin or arginine vasopressin at doses from 0.001 to 0.05 µg/100 g body mass on the background of a water load. Water (5 ml/100 g body mass) was administered through a catheter into stomach to suppress secretion of endogenous antidiuretic hormone (ADH). In experiments with water administration, diuresis increased due to a decrease of osmotic permeability of renal tubules and to excretion of osmotically free water, with the constant clearance of sodium ions. Injection of 0.05 µg arginine vasopressin led to a marked decrease of diuresis due to a rise of reabsorption of osmotically free water without elevation of excretion of osmotically active substances. Injection of the same dose of arginine vasotocin resulted in no increase of diuresis; however, reabsorption of osmotically free water and excretion of osmotically active substances including sodium ions were more pronounced. Hence, both vasotocin and vasopressin increased osmotic permeability of the tubular epithelium, but vasotocin, unlike vasopressin, promoted reduction of reabsorption of sodium ions and their loss with urine. A suggestion is made that one of the reasons for replacement in mammals of the molecular ADH forms (vasotocin by vasopressin) was the absence of the pronounced natriuretic effect in arginine vasopressin. This was of crucial significance to preserve sodium ions in the organism, to maintain water–salt balance in animals adapted to the terrestrial life, and to provide not only osmo-, but also volumoregulation. 相似文献
10.
Phylogenetic profiling is a well-established approach for predicting gene function based on patterns of gene presence and absence across species. Much of the recent developments have focused on methodological improvements, but relatively little is known about the effect of input data size on the quality of predictions. In this work, we ask: how many genomes and functional annotations need to be considered for phylogenetic profiling to be effective? Phylogenetic profiling generally benefits from an increased amount of input data. However, by decomposing this improvement in predictive accuracy in terms of the contribution of additional genomes and of additional annotations, we observed diminishing returns in adding more than ∼100 genomes, whereas increasing the number of annotations remained strongly beneficial throughout. We also observed that maximising phylogenetic diversity within a clade of interest improves predictive accuracy, but the effect is small compared to changes in the number of genomes under comparison. Finally, we show that these findings are supported in light of the Open World Assumption, which posits that functional annotation databases are inherently incomplete. All the tools and data used in this work are available for reuse from http://lab.dessimoz.org/14_phylprof. Scripts used to analyse the data are available on request from the authors. 相似文献
11.
Rahul Mittal Saroj Sharma Sanjay Chhibber Kusum Harjai 《World journal of microbiology & biotechnology》2006,22(9):915-919
Summary Tamm-Horsfall glycoprotein is the most abundant protein in human urine. The present investigation was planned to study the effect of Tamm-Horsfall protein (THP) on elaboration of virulence factors by biofilm cells of Pseudomonas aeruginosa. It was observed that with increase in concentration of THP from 10 to 50 μg/ml there was significant enhancement in elaboration of all the virulence factors by biofilm cells of P. aeruginosa. However, with further increase in concentration of THP from 50 to 70 μg/ml, significant decrease in elaboration of all the virulence traits was observed. Implications of these findings in relation to urinary tract infections caused by P. aeruginosa have been discussed. 相似文献
12.
GaoCunchen ZhangXinzheng LiuYongqing DepartmentofAutomaticControlEnineering SouthChinaUniversityofTechnology Guangzhou 《生物数学学报》1997,(4)
Abstract:Somenewconceptsofthestructureandinterconnectedstabilizationareestablishedforthemulti-groupmulti-delaysneutrallineartime-invariantinterconnectedcontrolsystemwiththetime-delayinputcontrolandperturbationparameters.Aposi-tivedefinitequadraticformV-functionviachoosingthesymmetricpositivedefinitesolu-tionmatrixofRiccatimatrixalgebraicequationismadeup,theinterconnectedstabiliza-tionofthelineartime-invariantcontrolsystemwithoutdelaysandPerturbationParame-tersimplytheinterconnectedstabilizationofthemulti-groupmulti-delaysneutrallineartime-invariantinterconnectedcontrolsystemwiththetime-delayinputcontrolandper-turbationparametersisobtainedbyusingtheLyapunovfunctionequivalencemethod.Atthesametime,someestimateformulaeoftheboundedforbothtime-delaysandper-turbationparametersaregiven. 相似文献
13.
Peter J. Hornsby Sandra E. Harris Kathy A. Aldern 《Chemico-biological interactions》1984,51(3):335-346
Previously, loss of 11β-hydroxylase activity when adrenocortical cells are incubated with the pseudosubstrate cortisol was found to be reduced when the concentration of oxygen was lowered, or when butylated hydroxyanisole (BHA) or dimethyl sulfoxide (Me2SO) were included in the medium. In the present experiments, we tested the hypothesis that Me2SO protects 11β-hydroxylase by scavenging OH? radicals. Substances known to react with OH? at high rates and non-toxic enough to be used at concentrations of 10–100 mM, including several alcohols, benzoate and radioprotectant thiols, did not prevent loss of activity of 11β-hydroxylase in the presence of 50 μM cortisol. Two of the alcohols, ethanol and glycerol, as well as Me2SO, were radioprotective in cultured bovine adrenocortical cells. Therefore free OH? radicals do not appear to be involved in loss of 11β-hydroxylase activity. When sulfoxides other than dimethyl sulfoxide were tested for their ability to protect 11β-hydroxylase in the presence of cortisol, several aryl sulfoxides, particularly dibenzyl sulfoxide, as well as dipropyl sulfoxide, were active at concentrations to of that required for Me2SO. Previously, we have demonstrated that 11β-hydroxylase inhibitors, particularly metyrapone, effectively protect against loss of 11β-hydroxylase activity in the presence of pseudosubstrates and therefore we examined whether sulfoxides may act by directly inhibiting 11β-hydroxylase. Me2SO showed an ED50 for inhibition of 11β-hydroxylase activity of > 1 M, in contrast to its ED50 for protection of 34 mM. For metyrapone, however, the ED50 for inhibition of the enzyme (250 nM) was close to that for protection of activity (270 nM). The other sulfoxides showed ED50-values for inhibition of 11β-hydroxylase that were substantially higher than the ED50-values for protection. Sulfoxides may have a mixed mode of action in protection of 11β-hydroxylase activity, as previously shown for phenols; they may protect by radical scavenging, but may also need to bind close to the active site of the enzyme where destructive radicals may be formed. 相似文献
14.
Els C. M. Brinkman-Van Der Linden Esther C. R. Van Ommen Willem Van Dijk 《Glycoconjugate journal》1996,13(1):27-31
The occurrence of differences in acute-phase response, with respect to concentration and glycosylation of 1-acid glycoprotein (AGP) was studied in the sera of patients surviving or not from septic shock. Crossed affino-immunoelectrophoresis was used with concanavalin A andAleuria aurantia lectin for the detection of the degree of branching and fucosylation, respectively, and the monoclonal CSLEX-1 for the detection of sialyl Lewisx (SLeX) groups on AGP. Septic shock apparently induced an acute-phase response as indicated by the increased serum levels and changed glycosylation of AGP. In the survivor group a transient increase in diantennary glycan content was accompanied by a gradually increasing fucosylation and SLeX expression, comparable to those observed in the early phase of an acute-inflammatory response. Remarkably, in the non-survivor group a modest increase in diantennary glycan content was accompanied by a strong elevation of the fucosylation of AGP and the expression of SLeX groups on AGP, typical for the late phase of an acute-phase response. Our results suggest that these changes in glycosylation of AGP can have a prognostic value for the outcome of septic shock.Abbreviations AAL
Aleuria aurantia lectin
- AGP
1-acid glycoprotein
- CAIE
crossed affinoimmunoelectrophoresis
- Con A
Concanavalin A
- HSPC
human serum protein calibrator
- IL-1
interleukin 1
- IL-6
interleukin 6
- LIF
leukaemia inhibitory factor
- LPS
lipopolysaccharide
- SLeX
sialyl Lewisx
- TNF
tumour necrosis factor 相似文献
15.
《生态学报》2007,27(11):F0003-F0003
China, as one of the most important Range States of orchids in the world, has about 1300 species withabundant varieties of orchids. However, owing to over, abuse harvest and illegal trades, the wild populations ofthese species have declined dramatically, even bringing some species to the edge of extinction. In response to such challenges, the Chinese government has put conservation of orchids as a priority agenda, taking a series of Innovative and effective measures to protect the orchids in the wild.[第一段] 相似文献
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Summary Histoenzymorphologic investigations on -D-glucuronidase in cortical granules in oocytes of some representative vertebrates revealed interesting differences in localisation of the enzyme in the two techniques employed using 8-hydroxyquinoline -D-glucosiduronic acid and naphthol AS-BI -D-glucosiduronic acid as substrates. In the former a nonspecific localisation in the entire cytoplasm of the oocytes excepting nucleus was evident whereas in the latter the cortical granules alone exhibited intense activity. To some extent this difference could be accounted for on the basis of yolk content of the eggs. -D-glucoronidase-rich lysosomal cortical granules were found in the amphibian, reptilian and avian oocytes whereas mammalian oocyte cortical granules were devoid of this enzyme. -D-glucuronidase in cortical granules was sensitive to low concentrations of Hg++, Cu++, glucuronic and mucic acids. Other histochemical evidence for the lysosomal nature of the cortical granules is presented and discussed. -D-glucuronidase seems to have a role in ovum-nutrition and activation which, thus, make a new addition to the existing knowledge of physiological role of -D-glucuronidase and lysosomes.Dr. A. T. Varute — Research Project guide. 相似文献
20.
《生态学报》2007,27(10):F0003-F0003
China,as one of the most important Range States of orchids in the world, has about 1300 species with abundant varieties of orchids. However, owing to over, abuse harvest and illegal trades, the wild populations of these species have declined dramatically, even bringing some species to the edge of extinction. In response to such challenges, the Chinese 相似文献