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At least 1,225 records · Page 68Linked to original sources
PubMed1998

Interactions of erythropoietin, granulocyte colony-stimulating factor, stem cell factor, and interleukin-11 on murine hematopoiesis during simultaneous administration.

We investigated how in vivo effects of single hematopoietic cytokines change if given in combination for a prolonged time. Mice were treated with every combination of recombinant human (rh) erythropoietin (EPO), rh granulocyte colony-stimulating factor (G-CSF), recombinant rat (rr) stem cell factor (SCF), and rh interleukin (IL)-11 by continuous infusion over 7 days (full factorial design with three dose levels for each cytokine). Burst-forming unit-erythroid (BFU-E), colony-forming unit-erythroid (CFU-E), and colony-forming unit-granulocyte-macrophage (CFU-GM) were determined in bone marrow and spleen, reticulocytes, hematocrit, granulocytes, and thrombocytes in the peripheral blood. An analysis of variance (ANOVA) and multiple comparison of means was used to evaluate the data. For several cell types, cytokine effects superimposed in an additive way if combined. However, in a large number of circumstances, nonadditive pairwise interactions were found. They differed in type and magnitude involving high-dose saturation, high-dose antagonistic effects, and even effect reversals (qualitative interactions). Hence, in general, it was not possible to foresee the combination effects on the basis of existing knowledge of single effects. On the other hand, the cytokine network was robust and no system hazards were observed under multiple cytokine combinations. The results illustrate that the cytokine network has nonlinear dynamic properties in vivo with dose-response characteristics of one cytokine being continuously modified by other cytokines.

Animals↗
PubMedDate not supplied

[Self-organization nature of forming speech rhythm (the voice source model].

The rhythmic structure of the temporal organization for acoustical speech signal is outlined on the base of self-organization. Suggested physiologically plausible nonlinear dynamics for process in discrete time has agreement with the experiment and explains the nature of the speech rhythm. Empirical speech rhythm has a good agreement with the model prediction.

Humans↗
PubMed1997

An elbow joint movement control model with visual feedback.

A motor program generator control model is proposed to simulate neuromuscular control. Three muscles (Biceps, Triceps, Brachialis) driving elbow joint flexion in a plane are simulated by integrating their nonlinear dynamic property and spinal neural circuitry. The motor descending commands are described by a visual feedback signal from the joint and an excitation signal for the motor neuron pool. The visual feedback signal mimics the gamma command whereas the excitation signal mimics another descending co-activation command. The gamma command is expressed as the output of a PID controller with the visual feedback error signal as the input. The gamma command and the motoneuron pool background activity are the inputs to the motoneuron pool model coupled with the Renshaw cell recurrent inhibitions. The output of the motoneuron pool model mimics the alpha command feeding directly to the muscle dynamics. A movement is produced by reducing the error signal between goal position and actual position and altering excitation signal properly. The simulation results show that a burst pattern of excitation signal and a PID controller can accurately trace the terminal goal and generate a smooth movement with a bell shaped velocity profile. The muscle activation signals have the characteristic similar to the smoothed EMG. Changing different parameters of the PID can cause the same effects as the stimulus pulse intensity or duration modulation.

Elbow Joint↗
PubMed1997

Heart rate variability during exercise in the horse.

High heart rate variability (HRV) has been associated with more efficient autonomic control, allowing more responsivity and sensitivity to changing environmental demands. Previous results from spectral analysis of interbeat intervals of equine heart rate (HR) indicated increased HRV, reflecting more effective vagal control of the heart. This study focuses on characterizing equine HR and HRV during increasing then decreasing physical demand. A non-invasive ambulatory monitoring system continuously records HR and HRV as horses exercise on a high speed equine treadmill at increasing then decreasing workloads. After a 5 minute baseline, the horses walk, then trot, then canter or trot fast, return to the slower trot, then walk, and then remain quiet for a 5 minute recovery period. Results indicate a decrease of HRV and large individual differences in parasympathetic activity during exercise. Resulting patterns of HR fluctuation indicate a nonlinear dynamical approach may describe the task responses more completely than the more traditional models.

Animals↗
PubMed2006

Nonlinear ionic conductivity of lithium silicate glass studied via molecular dynamics simulations.

Nonlinear ionic conductivity effects are studied via molecular dynamics simulations. We present numerical results for the field dependence of the current density as well as the second moment of the ion displacements. Furthermore we check whether structural changes occur upon application of strong electric fields. We obtain a complex picture of the field-dependent ion dynamics of a disordered system that is beyond simple model descriptions.

Journal Article↗
PubMed2006

Instability and dynamics of two nonlinearly coupled laser beams in a two-temperature electron plasma.

We consider nonlinear interactions between two colliding laser beams in an electron plasma, accounting for the relativistic electron mass increase in the laser fields and radiation pressure driven electron-acoustic (EA) perturbations that are supported by hot and cold electrons. By using the hydrodynamic and Maxwell equations, we obtain the relevant equations for nonlinearly coupled laser beams and EA perturbations. The coupled equations are then Fourier analyzed to obtain a nonlinear dispersion relation. The latter is numerically solved to show the existence of new classes of the parametric instabilities in the presence of two colliding laser beams in a two-electron plasma. The dynamics of nonlinearly coupled laser beams in our electron plasma is also investigated. The results should be useful in understanding the nonlinear propagation characteristics of multiple electromagnetic beams in laser-produced plasmas as well as in space plasmas.

Journal Article↗
PubMed1995

Nonlinear forecasting and the dynamics of cardiac rhythm.

Since the initial development of the electrocardiogram, cardiologists have made dramatic advances in the description and understanding of cardiac arrhythmias. Despite these successes, the analysis of cardiac rhythm has remained largely descriptive. Recently, the principles of nonlinear dynamics, or chaos theory, have been applied to the quantitative analysis of cardiac rhythm in a variety of diverse situations. In chaos theory, three types of signals can be defined: periodic signals, which repeat themselves over some finite time interval, chaotic signals, which, while deterministic, demonstrate complex behavior and do not repeat themselves, and random signals, which are unpredictable and nondeterministic. The technique of nonlinear forecasting defines trajectories in a suitably defined phase space and uses the future evolution of trajectories that are close to each other over short distances to make predictions for times further into the future. The ability to reliably predict the future evolution of the trajectories derived from any signal is an important characteristic of the underlying dynamics of the signal and can therefore used to determine those dynamics. The foundation of nonlinear forecasting is reviewed, and an algorithm is described that can be used to determine the underlying dynamics of a signal and has been applied to the analysis of R-R interval data.

Algorithms↗
PubMed1992

Fundamental issues in nonlinear urban population dynamic models: theory and a synthesis.

"The main purpose of this paper is to examine certain basic similarities between three nonlinear models of urban dynamics which appeared in [1980], and seem to be driven by somewhat different mechanisms.... By looking at their implicit assumptions we set the stage for addressing certain fundamental premises underlying the process of urban growth or decline. Searching for these fundamental assumptions permits the construction of a comprehensive-unifying theory of urban population dynamics placing urban evolution squarely within the domain of evolutionary theory."

Demography↗
PubMedDate not supplied

[Nonlinear elasticity and dynamics of globular proteins].

A molecule of native protein exhibits a high degree of mechanical nonlinearity, which gives rise to a peculiar dynamics behavior of globular proteins. Intramolecular motions strongly depend on pressure. The reaction of specific volume to the action of electric field (electrostriction) inside the molecule has a sign opposite to that observed in liquids. The probability that a small ion would penetrate inside the globule from the solvent depends nonmonotonously on hydrostatic pressure. The nonmonotonicity should manifest itself in all processes related to the transfer of ions inside the globule. The relationship between the mechanical properties of the protein and the kinetics of hydrogen exchange at normal and high pressure is discussed.

Elasticity↗
PubMedDate not supplied

[Dynamic structures and nonlinear spectral transformations based on structure interaction reactions].

A notion of dynamic excitation structure (DES) is introduced. Such structure is a mobile picture which adequately reflects external nonstationary excitation spectra. The stimulated structure transitions are possibly caused by DES generation if the substrate structure has space-time polimorphism. The stimulated associative transitions can be a basis of structure-holographic information processing in biological systems. Possibility of associative transformations of nonstationary processes is discussed and elements of associative structure logic are given. The model of semantic organization of nonequilibrium cooperative systems is discussed. This model may be useful for explaining the gnosiological activity of biological objects.

Kinetics↗
PubMed2006

Bistability of cell-matrix adhesions resulting from nonlinear receptor-ligand dynamics.

Bistability is a major mechanism for cellular decision making and usually results from positive feedback in biochemical control systems. Here we show theoretically that bistability between unbound and bound states of adhesion clusters results from positive feedback mediated by structural rather than biochemical processes, namely by receptor-ligand dissociation and association dynamics that depend nonlinearly on mechanical force and receptor-ligand separation. For small cell-matrix adhesions, we find rapid switching between unbound and bound states, which in the initial stages of adhesion allows the cell to explore its environment through many transient adhesions.

Cell-Matrix Junctions↗
PubMed2000

Dynamics of coupled gap solitons in diatomic lattices with cubic and quartic nonlinearities

The dynamics of coupled gap solitons in diatomic lattices with cubic and quartic nonlinearities is considered analytically based on an extended quasidiscreteness approach. For various mass differences (and thus different gap widths of the phonon spectrum), the coupled gap solitons are shown to display very rich dynamical behavior and their properties are strongly dependent on the force-constant ratio K(2)(3)/(K2K4), where K(j)(j=1,2,3) are the force constants for the quadratic, cubic, and quartic parts of the intersite interaction potential, respectively. Several previous theoretical approaches for studying gap soliton dynamics in diatomic lattices are recovered in our scheme, and the relations between these methods are elucidated in a systematic way.

Journal Article↗
PubMed2002

Dynamics of the nonlinear dielectric properties of a nematogenic compound dissolved in a nonpolar medium.

The nonlinear dielectric spectra of 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene (C6H13CyHx Ph NCS, 6CHBT) in benzene solutions were recorded in the whole concentration range up to an appearance of the two-phase isotropic and nematic region (0.82 mole fraction of 6CHBT, at 25 degrees C). Two electric fields: (i) the field of high strength (110 kV/m) and low frequency (85 Hz) causing the dielectric nonlinearity, and (ii) the probing field of low strength and high frequency (100 kHz-3 GHz), were used. At about of 0.2 mole fraction of 6CHBT the nonlinear dielectric increment becomes positive. With increasing of 6CHBT concentration the dynamics of the increment shows a critical-like behavior, which can be interpreted in terms of the reorientation of the growing pseudonematic domains in the solutions studied.

Journal Article↗
PubMed2002

Rotating unstable Langevin-type dynamics: linear and nonlinear mean passage time distributions.

To characterize the decay process of linear rotating unstable Langevin-type dynamics in the presence of constant external force, through the mean passage time distribution, two theoretical descriptions are proposed: one is called the Quasideterministic (QD) approach described in the limit of long times, and the other approach is formulated for not so long times. Both theories are matrix based and formulated in two x and y dynamical representations, y being the transformed space of coordinates by means of a time-dependent rotation matrix. In the y dynamical representation the noise as well as the external force are rotational. The QD approach is studied when the dynamics is not influenced by the external force and when it is influenced by it. In the absence of this force, the theory is given for n variables and leads to the same results as those obtained in the characterization of nonrotating unstable systems; a fact that is better understood in the space of coordinates y. In the presence of the external force, the characterization is given for two variables and it is only valid for weak amplitude forces. For large amplitudes, the dynamics is almost dominated by the deterministic rotational evolution; then the QD approach is no longer valid and therefore the other approach is required. The theory in this case is general and verified for systems of two and three variables. In the case of two variables we study a laser system and use the experimental data of this system to compare with both theoretical and simulation results. In the case of three variables, the theory foresees application in other fields, for instance, in plasma physics. We also study the time characterization of the nonlinear rotating unstable systems and show in general that the nonlinear correction to the linear case is a quantity evaluated in the deterministic limit. The same laser system studied in the linear case is used as a prototype model.

Journal Article↗
PubMed2003

Influence of system non-uniformity on dynamic phenomena in arrays of coupled nonlinear networks.

In this paper we investigate the influence of system non-uniformity on the existence and stability of synchronous motion in an array of bi-directionally coupled electronic circuits. In computer simulations we find the level of non-uniformity for which synchronous behavior is sustained. We also present several examples of attractors, which appear when the synchronous motions is no longer stable.

Neural Networks, Computer↗
PubMed2002

Modeling of nonlinear physiological systems with fast and slow dynamics. I. Methodology.

Effective modeling of nonlinear dynamic systems can be achieved by employing Laguerre expansions and feedforward artificial neural networks in the form of the Laguerre-Volterra network (LVN). This paper presents a different formulation of the LVN that can be employed to model nonlinear systems displaying complex dynamics effectively. This is achieved by using two different filter banks, instead of one as in the original definition of the LVN, in the input stage and selecting their structural parameters in an appropriate way. Results from simulated systems show that this method can yield accurate nonlinear models of Volterra systems, even when considerable noise is present, separating at the same time the fast from the slow components of these systems effectively.

Algorithms↗