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Results for “Coupled model”

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At least 865 records · Page 48Linked to original sources

PubMed2001

Noise strength effects on the relaxation properties of weakly coupled Ginzburg-Landau models.

We search for new effects and extend previous investigations on the relaxation properties of weakly coupled stochastic Ginzburg-Landau models under noise with different intensity. We calculate the one-particle mass up to second order in the Ginzburg-Landau potential coupling constant and show that, as previously predicted, it is in fact less sensitive to the noise strength than the two-particle bound state. For d=1 and 2 and a negative quartic term in the potential, we show that the difference between these masses becomes smaller as we increase the noise strength, which indicates the possibility of a crossover for large noise (i.e., the two-particle bound state mass may become smaller than the one-particle mass) or of a phase transition (the masses are going to zero). For d=1 and in the ladder approximation (first order in the coupling constant), we show the absence of resonances in the spectrum close to the two-particle threshold and the existence of one antibound state for a positive quartic term in the potential, which indicates the possibility of some bound state due to changes in the system (e.g., in the noise intensity).

Journal Article↗

PubMed2005

A population-based Bayesian approach to the minimal model of glucose and insulin homeostasis.

The minimal model was proposed in the late 1970s by Bergman et al. (Am. J. Physiol. 1979; 236(6):E667) as a powerful model consisting of three differential equations describing the glucose and insulin kinetics of a single individual. Considering the glucose and insulin simultaneously, the minimal model is a highly ill-posed estimation problem, where the reconstruction most often has been done by non-linear least squares techniques separately for each entity. The minimal model was originally specified for a single individual and does not combine several individuals with the advantage of estimating the metabolic portrait for a whole population. Traditionally it has been analysed in a deterministic set-up with only error terms on the measurements. In this work we adopt a Bayesian graphical model to describe the coupled minimal model that accounts for both measurement and process variability, and the model is extended to a population-based model. The estimation of the parameters are efficiently implemented in a Bayesian approach where posterior inference is made through the use of Markov chain Monte Carlo techniques. Hereby we obtain a powerful and flexible modelling framework for regularizing the ill-posed estimation problem often inherited in coupled stochastic differential equations. We demonstrate the method on experimental data from intravenous glucose tolerance tests performed on 19 normal glucose-tolerant subjects.

Adult↗

PubMed1986

An analysis of a dendritic neuron model with an active membrane site.

We formulate and analyze a mathematical model that couples an idealized dendrite to an active boundary site to investigate the nonlinear interaction between these passive and active membrane patches. The active site is represented mathematically as a nonlinear boundary condition to a passive cable equation in the form of a space-clamped FitzHugh-Nagumo (FHN) equation. We perform a bifurcation analysis for both steady and periodic perturbation at the active site. We first investigate the uncoupled space-clamped FHN equation alone and find that for periodic perturbation a transition from phase locked (periodic) to phase pulling (quasiperiodic) solutions exist. For the model coupling a passive cable with a FHN active site at the boundary, we show for steady perturbation that the interval for repetitive firing is a subset of the interval for the space-clamped case and shrinks to zero for strong coupling. The firing rate at the active site decreases as the coupling strength increases. For periodic perturbation we show that the transition from phase locked to phase pulling solutions is also dependent on the coupling strength.

Animals↗

PubMed2006

Statistical associating fluid theory coupled with restrictive primitive model extended to bivalent ions. SAFT2: 1. Single salt + water solutions.

Statistical associating fluid theory coupled with the restricted primitive model is extended to multivalent ions by relaxing the range of the square-well width parameter, which leads to a new dispersion term approximation and calls for a new set of salt and ion parameters. This new approximation, referred to as SAFT2, requires a single set of parameters derived from the salt (mean ionic) activity coefficients and liquid densities of single-salt solutions for five cations (Li(+), Na(+), K(+), Ca(2+), Mg(2+)), six anions (Cl(-), Br(-), I(-), NO(3)(-), SO(4)(-2), HCO(3)(-)), and 24 salts. These parameters, in turn, are shown to predict the osmotic coefficients for single salt + water solutions.

Journal Article↗

PubMed1972

Excitatory and inhibitory interactions in localized populations of model neurons.

Coupled nonlinear differential equations are derived for the dynamics of spatially localized populations containing both excitatory and inhibitory model neurons. Phase plane methods and numerical solutions are then used to investigate population responses to various types of stimuli. The results obtained show simple and multiple hysteresis phenomena and limit cycle activity. The latter is particularly interesting since the frequency of the limit cycle oscillation is found to be a monotonic function of stimulus intensity. Finally, it is proved that the existence of limit cycle dynamics in response to one class of stimuli implies the existence of multiple stable states and hysteresis in response to a different class of stimuli. The relation between these findings and a number of experiments is discussed.

Evoked Potentials↗

PubMed2006

Traveling waves in coupled reaction-diffusion models with degenerate sources.

We consider a general system of coupled nonlinear diffusion equations that are characterized by having degenerate source terms and thereby not having isolated rest states. Using a general form of physically relevant source terms, we derive conditions that are required to trigger traveling waves when a stable uniform steady-state solution is perturbed by a highly localized disturbance. We show that the degeneracy in the source terms implies that traveling waves have a number of surprising properties that are not present for systems with nondegenerate source terms. We also show that such systems can lead to a pair of waves that initially propagate outwards from the disturbance, slow down, and reverse direction before ultimately colliding and annihilating each other.

Action Potentials↗

PubMed1994

Hidden Markov Models of the G-protein-coupled receptor family.

Hidden Markov Model techniques are used to derive a new model of the G-protein-coupled receptor family. The transition and emission parameters of the model are adjusted using a training set comprising 142 sequences. The resulting model is shown to perform well on a number of tasks, including multiple alignments, discrimination, large data base searches, classification, and fragment detection. General analytical results on the expectation and standard deviation of the likelihood of random sequences are also presented.

Algorithms↗

PubMed1982

Cell cycle models for the aberrant coupling of growth arrest and differentiation in hyperplasia, metaplasia, and neoplasia.

The control of cell proliferation can be regulated by the coupling of growth arrest and differentiation. Since this process has been demonstrated both in vivo and in vitro, it is thought to be of considerable physiologic significance. The mechanisms that serve to couple growth arrest and differentiation were, however, poorly defined prior to our recent studies. We established that the coupling of growth arrest and differentiation of proadipocytes occurs at a distinct state in the G1 phase of the cell cycle, GD, and that it consists of at least five phases. These include: 1) growth arrest at GD; 2) nonterminal differentiation; 3) terminal differentiation; 4) loss of the differentiated phenotype, and 5) reinitiation of cell proliferation. On the basis of these observations we developed a cell cycle model to explain the biologic mechanisms of the coupling process. This model is now used to predict where defects in the coupling of growth arrest and differentiation may occur in hyperplastic, metaplastic, and neoplastic cells.

Adipose Tissue↗

PubMed1995

Computer simulation of the electrotonic modulation of pacemaker activity in the sinoatrial node by atrial muscle.

Electrotonic interaction between the sinoatrial (SA) node and surrounding atrial muscle was investigated in a computer simulation using a modified Oxsoft HEART model (Oxsoft, Oxford, UK). When an SA node cell model was coupled to a passive atrial membrane model (RC circuit) with various coupling conductances (Gc), there was a Gc-dependent prolongation of spontaneous cycle length (SCL). At a sufficiently high value of Gc, the spontaneous activity was finally stopped. A nonlinear relationship between Gc and SCL was obtained, similar to that observed in experiments on rabbit SA node cells. When the muscarinic potassium current (iK,ACh) was activated in the SA node cell model, the coupling-induced inhibition of pacemaker activity was potentiated. Although coupling current and iK,ACh were additive, their effects on SCL were more than additive because of the nonlinear dependence of SCL on net current. A decrease in the input resistance of the atrial membrane model to stimulate the activation of iK,ACh in atrial muscle was also shown to potentiate the coupling-induced inhibition of SA node spontaneous activity.

Action Potentials↗

PubMed2005

Phase twisted modes and current reversals in a lattice model of waveguide arrays with nonlinear coupling.

We consider a lattice model for waveguide arrays embedded in nonlinear Kerr media. Inclusion of nonlinear coupling results in many phenomena involving complex, phase-twisted, stationary modes. The norm (Poynting power) current of stable plane-wave solutions can be controlled in magnitude and direction, and may be reversed without symmetry-breaking perturbations. Also stable localized phase-twisted modes with zero current exist, which for particular parameter values may be compact and expressed analytically. The model also describes coupled Bose-Einstein condensates.

Journal Article↗

PubMed2006

Model for proton transport coupled to protein conformational change: application to proton pumping in the bacteriorhodopsin photocycle.

A modeling method is presented for protein systems in which proton transport is coupled to conformational change, as in proton pumps and in motors driven by the proton-motive force. Previously developed methods for calculating pKa values in proteins using a macroscopic dielectric model are extended beyond the equilibrium case to a master-equation model for the time evolution of the system through states defined by ionization microstate and a discrete set of conformers. The macroscopic dielectric model supplies free energy changes for changes of protonation microstate, while the method for obtaining the energetics of conformational change and the relaxation rates, the other ingredients needed for the master equation, are system dependent. The method is applied to the photoactivated proton pump, bacteriorhodopsin, using conformational free energy differences from experiment and treating relaxation rates through three adjustable parameters. The model is found to pump protons with an efficiency relatively insensitive to parameter choice over a wide range of parameter values, and most of the main features of the known photocycle from very early M to the return to the resting state are reproduced. The boundaries of these parameter ranges are such that short-range proton transfers are faster than longer-range ones, which in turn are faster than conformational changes. No relaxation rates depend on conformation. The results suggest that an "accessibility switch", while not ruled out, is not required and that vectorial proton transport can be achieved through the coupling of the energetics of ionization and conformational states.

Bacteriorhodopsins↗

PubMed2001

Acoustic and electroacoustic spectroscopy for characterizing concentrated dispersions and emulsions.

We describe two different techniques (acoustics and electroacoustics), both of which employ ultrasound instead of light for extracting information about the properties of liquid-based dispersions. Ultrasound can propagate through samples that are not transparent for light, which open up many new applications not possible with classical light scattering methods. Acoustic and electroacoustic techniques offer a unique opportunity to characterize concentrated dispersion, emulsions and microemulsions in their natural states. Elimination of a dilution step required for most other techniques (light scattering, sedimentation, electrophoresis) is crucial for an adequate characterization of liquid dispersions, especially when the high concentration leads to structured systems. As with any macroscopic method, ultrasonic techniques characterize the sample in two steps. The first step is to measure some macroscopic property. The second step involves some theoretical treatment of the measured raw data which yields the desired information. Acoustic spectroscopy deals with measuring the attenuation of ultrasound within a certain frequency range. Electroacoustic spectroscopy has two implementations depending on the driving force. We emphasize here on the so-called Colloid Vibration Current (CVI) which is generated by the sound wave as it passes through the dispersion. A review of the theoretical basis of acoustics and electroacoustics is given, with emphasis on models that have been applied to concentrated systems. Recently, new theories have been developed for both acoustics and electroacoustics using a 'coupled phase model' and 'cell model concept'. The coupled phase model is widely used for describing a relative motion of the particles and liquid in the sound wave. The cell model approach opens the way to include both particle-particle interactions and polydispersity into the theoretical model. Experimental evidence is presented that shows that this new approach is successful in concentrated systems up to 45% vol. A short review of the possible applications of acoustics and electroacoustics measurements to a range of systems is presented including: ceramics, mixed dispersed systems, chemical-mechanical polishing abrasives, emulsions, microemulsions and latex materials.

Acoustics↗

PubMed2006

Typological approaches to violence in couples: a critique and alternative conceptual approach.

Typological approaches have become highly influential in the research on violence in couples, and yet issues related to such approaches have not been well addressed. We review the utility of batterer typologies, both for clinical applications and for understanding violence in couples. The principal types of batterer typologies are discussed, along with a number of issues that might limit their utility for explaining the etiology and developmental course of partner violence in couples. We propose a dyadic model of couples' aggression, and we explain ways that such a model provides better conceptualization of the development of the couples' violence over time, including issues of persistence and desistance of violence, and that can help inform prevention and treatment.

Adolescent↗

PubMed1986

A systemic approach to couple therapy.

This article presents a model of couple therapy based on the reciprocal double bind. Two cases are presented to illustrate the model, and different modes of intervention are described. This paper stresses that therapy has more to do with the intersection of the maps of the world of members of a system than it has with a search for some individual or systemic truth.

Behavior Therapy↗

PubMed1998

Intersegmental coordination in invertebrates and vertebrates.

How does the CNS coordinate muscle contractions between different body segments during normal locomotion? Work on several preparations has shown that this coordination relies on excitability gradients and on differences between ascending and descending intersegmental coupling. Abstract models involving chains of coupled oscillators have defined properties of coordinating circuits that would permit them to establish a constant intersegmental phase in the face of changing periods. Analyses that combine computational and experimental strategies have led to new insights into the cellular organization of intersegmental coordinating circuits and the neural control of swimming in lamprey, tadpole, crayfish and leech.

Animals↗

PubMed1987

Hysteresis phenomena between periodic and stationary solutions in a model of pacemaker and nonpacemaker coupled cardiac cells.

We were interested in investigating the behaviour of a cardiac electrophysiological model including coupled pacemaker (PM) and nonpacemaker (NPM) cells. To this aim, a modified version of the model of Van Capelle and Durrer was used. First, few discrete values were assigned to coupling resistance (CR) and respective cell sizes and numerical simulations versus time showed three possible kinds of response pattern: sustained rhythmic activity, subthreshold oscillations, and complete inhibition. Then, after setting a fixed value to PM cell size, we undertake a thorough study of the system by using bifurcation-continuation techniques and CR was chosen as the continuation parameter. On the maximum action potential--CR plane representation, we could describe five behavioural zones: complete inhibition, coexistence of complete inhibition and NPM large oscillations, NPM large oscillations, coexistence of NPM large oscillations and subthreshold oscillations, subthreshold oscillations. Within the zones of qualitatively different coexisting solutions, a detailed exploration clearly demonstrated the presence of hysteresis cycles. Indeed, the status of the system depended on its immediate previous story within narrow ranges of CR values. Such a coexistence of stable solutions for identical values of CR may suggest an explanation of the intermittant activity elicited from abnormal ectopic foci observed in certain ventricular rhythm disturbances. In addition, a Hopf bifurcation point, from which emerged stationary and periodic solutions, was followed on the PM cell size--CR plane and from this representation we could deduce that the smaller the PM cell, the higher the CR must be for the PM cell to escape from the NPM cell inhibition.

Cell Membrane↗

PubMed2005

Silicon-nitrogen bonding in silatranes: assignment of photoelectron spectra.

Silicon-nitrogen bonding and the photoelectron spectra of hydro-silatrane and methyl-silatrane, XSi[OCH2CH2]3N (X = H and Me), were studied with ab initio electron propagator theory, many-body methods, and density functional models. A linear vibronic coupling (LVC) model was employed to estimate vibrational widths of the ionization bands and to study the dependence of the ionization energies on the molecular geometry. Particular attention was given to coordinates that change the Si-N distance and the strength of the donor-acceptor interaction between these two atoms. The ionization energy of the highest occupied molecular orbital has a very strong geometrical dependence which leads to an unusually large vibrational width in the corresponding photoelectron band. The assignment of this band in methyl-silatrane, which was controversial for a long time, is resolved by the present study. The calculated photoelectron spectra allow for clear assignment of at least three more bands in the observed spectra. The present results demonstrate the important role of electrostatic interactions in Si <-- N bonding and in the outer-valence ionization energies of the silatranes.

Journal Article↗

PubMed2000

A tetramethyl rhodamine (Tamra) phosphoramidite facilitates solid-phase-supported synthesis of 5'-Tamra DNA.

5-Carboxy Tamra 1 was conjugated to 4-hydroxypiperidine with BOP and N-methylmorpholine, and the resulting 5-(N-pipyridyl-4-hydroxy)-Tamra carboxamide 2 was treated with 2-cyanoethyl tetraisopropylphosphorodiamidite to give 5-[N-pipyridyl-4-O-(2-cyanoethyl diisopropylphosphoramidite)]-Tamra carboxamide 3. Solutions of 3 were coupled onto the 5'-hydroxyl of solid-phase-supported DNA fragments with standard amidite coupling techniques. Cleavage and deprotection with aqueous tert-butylamine cocktail gave 5-Tamra-functionalized DNA as well as an additional compound without the Tamra chromophore. A mass spectrum of this product showed the incorporation of tert-butylamine. The extra product was completely suppressed by including a 5 min acetylation step after coupling. A model study of 3 coupled onto thymidine-functionalized CPG showed similar results. NMR and mass spectra of cleaved products confirmed the addition of tert-butylamine to the minor product. Coupling a Tamra active ester onto T CPG which was previously coupled with N-(4-methoxytrityl)piperidyl-4-O-(2-cyanoethyl diisopropylphosphoramidite) 4 produced the same major Tamra-bearing product, which coeluted on reverse phase HPLC with the major product generated with 3.

Chromatography, High Pressure Liquid↗