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At least 343 records · Page 19Linked to original sources

PubMed2005

Aberrant cell plate formation in the Arabidopsis thaliana microtubule organization 1 mutant.

MICROTUBULE ORGANIZATION 1 encodes a microtubule-associated protein in Arabidopsis thaliana but different alleles have contradictory phenotypes. The original mutant mor1 alleles were reported to have disrupted cortical microtubules, swollen organs and normal cytokinesis, whereas other alleles, embryo-lethal gemini pollen 1 (gem1), have defective pollen cytokinesis. To determine whether MOR1 functions generally in cytokinesis, we examined the ultrastructure of cell division in roots of the original mor1-1 allele. Cell plates are misaligned, branched and meandering; the forming cell plates remain partly vesicular, with electron-dense or lamellar content. Phragmoplast microtubules are abundant but organized aberrantly. Thus, MOR1 functions in both phragmoplast and cortical arrays.

Arabidopsis↗

PubMed1979

Prophylactic revascularization of the gut.

Reconstitution of the mesenteric vascular circulation, in our experience, is advisable when advanced occlusive disease is noted on the preoperative arteriogram of patients selected for aortoiliofemoral, renal artery, or aortic aneurysm surgery. A lateral aortogram is mandatory, and the presence of an anastomotic meandering mesenteric artery on frontal arteriogram is especially valuable in signaling significant disease. This is the first report of prophylactic concomitant revascularization of compromised mesenteric vessels during aortic reconstructive procedures on selected patients. It is our opinion that such an approach can be a significant deterrent to subsequent catastrophic bowel infarction from mesenteric arterial occlusive disease.

Celiac Artery↗

PubMed2003

Stimulation of AChE activity in relation to changes in electronmicroscopic structure of adult rat cerebrocortical synaptosomes pretreated with 3-5-3'-triiodo-L-thyronine.

Triiodothyronine (T3) stimulated AChE activity in depolarization-induced intact synaptosomes (isolated from adult rat cerebral cortex) suspended in calcium-supplemented choline chloride buffer in a time-dependent manner maximally 45-60 s after T3 administration and in a dose-dependent manner with an optimum at 10-100 nM. T3 (100 nM) had no such effects on AChE activity in synaptosomes at non-depolarized conditions. There was no direct effect of T3 on AChE activity of lysed synaptosomal suspension in the physiological range (nM) of T3. The experiments suggest that T3 might have a role in the calcium-dependent release/co-release of acetylcholine from intact synaptosomes concomitant with the acceleration of choline uptake mechanisms that has been reported to accompany elevation of AChE activity. Additionally, electron microscopic structures showed condensation of the cytosolic content with increase in electron density, formation of intrasynaptosomal coarse vesicles and appearance of vesicular fusion like structures (meandering) at the periphery in depolarization-induced T3-treated (60 s) intact synaptosomes, indicating the occurrence of the release of neurotransmitters. The present investigation indicates a definite role of T3 on Ca2+-dependent cholinergic neurotransmission.

Acetylcholinesterase↗

PubMed1994

Probabilistic consideration of consequences of long-duration accidental releases based on complete weather data.

This paper presents a method based on chronological weather data for estimating how variations in weather conditions during a radioactive release affect the consequences of the release. The modifications of the method made in the computer-based applications owing to the limited calculation resources are also studied. Additionally, the length of the chronological weather data observation period needed to satisfactorily support the assessments was tested. The results of the different duration times for the normalized concentration--which is the concentration divided by the release rate, and called here the dispersion factor--are presented and compared with the commonly used sector average model and the generic wind meandering method. The new method can be used for a variety of release durations and it makes realistic estimates, managing to avoid the weaknesses of the methods compared. The necessary length of the weather data is on the order of 1 y to get statistically satisfactory results.

Accidents↗

PubMedDate not supplied

Spinal deformity and instability after multilevel cervical laminectomy.

Sixty-four patients who had undergone multilevel cervical laminectomy were studied for postoperative spinal deformity and instability. Special attention was given to patients with cervical spondylosis (CS), ossification of the posterior longitudinal ligament (OPLL), and spinal cord tumors. Twenty-three (36%) of 64 patients showed postoperative changes in curvature type and 9 (14%) had developed spinal deformity (kyphotic or meandering-type curvature). In two juvenile patients, the deformity developed soon after operation and spinal fusion was required to prevent neurologic complications. In the adult cases, contrary to the hitherto accepted concept, long-term follow-up revealed the tendency of the deformity to develop more frequently in OPLL cases than in CS cases. Mobility of the cervical spine was reduced considerably after laminectomy, both in CS and OPLL cases. There was no adult patient who required further operation for severe deformity or instability after laminectomy. Extensive laminectomy, even including the C2 lamina, seemed to have no adverse effect on the stability of the cervical spine.

Cervical Vertebrae↗

PubMed1996

Re-entrant activity and its control in a model of mammalian ventricular tissue.

We characterize the meander of re-entrant excitation in a model of a sheet of mammalian ventricular tissue, and its control by resonant drift under feedback driven stimulation. The Oxsoft equations for excitability in a guinea pig single ventricular cell were incorporated in a two dimensional reaction-diffusion system to model homogeneous, isotropic tissue with a plane wave conduction velocity of 0.35 m s-1. Re-entrant spiral wave solutions have a spatially extended transient motion (linear core) that settles down into rotation with an irregular period of 100-110 ms around an irregular, multi-lobed spiky core. In anisotropic tissue this would appear as a linear conduction block. The typical velocity of drift of the spiral wave induced by low amplitude resonant forcing is 0.4 cm s-1.

Animals↗

PubMed1998

Ventricular fibrillation: one spiral or many?

Ventricular fibrillation is the major cause of sudden cardiac death, the leading cause of death in the industrialized world; however, the mechanisms for its onset are not well understood. To further understand the dynamics of fibrillation at and near its onset, we compared spatial and temporal variability of mean interactivation intervals in a stable canine model for ventricular fibrillation. Temporal variability was very small, suggesting that the relevant physiological parameters remained constant during our experiments. Spatial variability was usually significantly larger and appeared incompatible with the dynamics of a single, meandering spiral wave. This confirmed recent results that a single spiral wave cannot generate ventricular fibrillation. Thus the onset of fibrillation is a multistage process, with spiral-wave breakdown providing a crucial step in the quasi-periodic route to fibrillation.

Analysis of Variance↗

PubMed1995

Initial studies of the equilibrium folding pathway of staphylococcal nuclease.

Spectroscopic methods were used to examine the sequential build up of structure in the denatured state of staphylococcal nuclease. The 'free energy distance' between the native and denatured states was manipulated by altering conditions in solution (for example altering urea or glycerol concentration) and by changing the amino acid sequences. Initial studies employed a fragment of nuclease, referred to as delta 131 delta, which lacks six structural residues from the amino terminus and one structural residue from the carboxy-terminus. Nuclear magnetic resonance analysis of this fragment in solution revealed a modest quantity of dynamic structure which is native-like in character. With the addition of urea, 12 new HN peaks appeared in the 1H-15N correlation spectrum, presumably as a result of the breakdown of residual structure involving the first three beta strands. With the addition of glycerol, there was a rapid increase in the quantity of beta sheet structure detected by circular dichroism spectroscopy. At very high glycerol concentrations, an increase in helical structure became apparent. These data in addition to previously published results suggest that: (i) a beta-meander (strands beta 1-beta 2-beta 3) and the second alpha helix (alpha 2) are among the most stable local structures; (ii) the five-strand beta-barrel forms in a reaction which does not require the presence of several other native substructures; and (iii) the last step on the equilibrium folding pathway may be the formation and packing of the carboxy terminal alpha helix (alpha 3) to give the native state.

Circular Dichroism↗

PubMed2002

Comparison of Artificial Neural Network (ANN) Model Development Methods for Prediction of Macroinvertebrate Communities in the Zwalm River Basin in Flanders, Belgium.

Modelling has become an interesting tool to support decision making in water management. River ecosystem modelling methods have improved substantially during recent years. New concepts, such as artificial neural networks, fuzzy logic, evolutionary algorithms, chaos and fractals, cellular automata, etc., are being more commonly used to analyse ecosystem databases and to make predictions for river management purposes. In this context, artificial neural networks were applied to predict macroinvertebrate communities in the Zwalm River basin (Flanders, Belgium). Structural characteristics (meandering, substrate type, flow velocity) and physical and chemical variables (dissolved oxygen, pH) were used as predictive variables to predict the presence or absence of macroinvertebrate taxa in the headwaters and brooks of the Zwalm River basin. Special interest was paid to the frequency of occurrence of the taxa as well as the selection of the predictors and variables to be predicted on the prediction reliability of the developed models. Sensitivity analyses allowed us to study the impact of the predictive variables on the prediction of presence or absence of macroinvertebrate taxa and to define which variables are the most influential in determining the neural network outputs.

Amphipoda↗

PubMed1999

Theory of spiral wave dynamics in weakly excitable media: asymptotic reduction to a kinematic model and applications.

In a weakly excitable medium, characterized by a large threshold stimulus, the free end of an isolated broken plane wave (wave tip) can either rotate (steadily or unsteadily) around a large excitable core, thereby producing a spiral pattern, or retract, causing the wave to vanish at boundaries. An asymptotic analysis of spiral motion and retraction is carried out in this weakly excitable large core regime starting from the free-boundary limit of the reaction-diffusion models, valid when the excited region is delimited by a thin interface. The wave description is shown to naturally split between the tip region and a far region that are smoothly matched on an intermediate scale. This separation allows us to rigorously derive an equation of motion for the wave tip, with the large scale motion of the spiral wave front slaved to the tip. This kinematic description provides both a physical picture and exact predictions for a wide range of wave behavior, including (i) steady rotation (frequency and core radius), (ii) exact treatment of the meandering instability in the free-boundary limit with the prediction that the frequency of unstable motion is half the primary steady frequency, (iii) drift under external actions (external field with application to axisymmetric scroll ring motion in three dimensions, and spatial- or/and time-dependent variation of excitability), and (iv) the dynamics of multiarmed spiral waves with the prediction that steadily rotating waves with two or more arms are linearly unstable. Numerical simulations of FitzHugh-Nagumo kinetics are used to test several aspects of our results. In addition, we discuss the semiquantitative extension of this theory to finite cores and pinpoint mathematical subtleties related to the thin interface limit of singly diffusive reaction-diffusion models.

Journal Article↗

PubMed1999

Interacting spiral waves in the Oregonator model of the light-sensitive Belousov-Zhabotinskii reaction.

We study the interaction of meandering spiral waves within the framework of a modified Oregonator model for the light-sensitive Belousov-Zhabotinskii medium. In this medium the local excitation threshold can be controlled by varying the intensity of incident light. At low as well as sufficiently high light intensity we find stable axis-symmetric bound states consisting of two counter-rotating spirals. At intensity values in between, spiral pairs undergo a symmetry-breaking instability, leading to one spiral suppressing and expelling the other. To avoid the instability, we consider a spiral wave interacting with its mirror image close to a plane boundary impermeable to diffusion. The drift velocity and the drift direction of those pseudobound states parallel to the boundary are strongly influenced by the light intensity.

Journal Article↗

PubMed2000

Spatiotemporal chaos and nonequilibrium transitions in a model excitable medium

We present a detailed study of the statistical steady states of a model for CO oxidation on Pt(110) proposed by Bar and co-workers. We show that the stability diagram of this model depends sensitively on the boundary conditions. We elucidate several novel properties of a state with meandering spirals (M) briefly mentioned by Bar and co-workers. (1) We show that, with periodic boundary conditions, M is the state MP, a binary mixture displaying a coexistence of quasiperiodically rotating spirals and chaotically moving pointlike defects. We show that the transition from MP to the turbulent state T1 is continuous; the transition line marks the locus where the two phases cease to be distinct. (2) With Neumann boundary conditions M is the state MN, a single quasiperiodically rotating spiral. We show that the MN-T1 transition is discontinuous or first order. We also characterize the transitions from MP and MN to the state S, which has quasiperiodically rotating spirals. We also propose qualitative mechanisms for these transitions.

Journal Article↗

PubMed2001

Forced two-dimensional turbulence in spectral and physical space.

Two-dimensional (2D) turbulence in the energy range exhibits nonuniversal features, manifested in the departure (at low k) from the k(-5/3) energy spectrum law, variable energy flux, and irregular, nonlocal transfers. To unravel the underlying mechanism we conducted a detailed study of the 2D turbulence in spectral and physical space. It revealed complex multiscale organization of vorticity field and dynamic processes, ranging from large-scale meandering jets to strong localized vortices. The latter bear prime responsibility for the nonuniversal behavior of 2D turbulence, and we examined their statistical features and the growth mechanism. Our results are based on the numeric simulation of 2D turbulence on the 512 grid under different forcing-dissipation conditions.

Journal Article↗

PubMed2002

Unusual spiral wave tip trajectories in a parametrically forced nonequilibrium system.

The effect of parametric modulations on spiral waves that form in a thin layer of liquid crystal under rotating magnetic field is studied in laboratory experiments and numerical simulations. Parametric forcings that are sinusoidal in time make a simply rotating spiral tip to meander, rendering a compound tip trajectory that is composed of a main circular orbit and a satellite orbit with an unusual crescent shape. No evidence of frequency locking is found. The underlying mechanism for the unusual shape of the satellite orbit is elucidated, and the dependence of the rotation frequency of the main circular orbit f(o) on the perturbation parameters are discussed.

Journal Article↗

PubMed2003

Spiral wave stability in cardiac tissue with biphasic restitution.

Human ventricular tissue as well as several animal ventricular preparations show a biphasic shape of the action potential duration restitution curve, with a local maximum at low diastolic intervals. We study numerically how the location and properties of this nonmonotonicity affect the stability of spiral waves. We find that, depending on the slopes of the ascending and of the descending parts of the restitution curve, we can have either stable rotation of the spiral wave or spiral breakup. We identify two types of spiral breakup: one due to a steep positive slope and another due to a steep negative slope in the restitution curve. We discuss the differences in their manifestation and possible implications. We also find that increasing the slope of the descending part of the restitution curve increases the meandering of the spiral wave, due to the repeated occurrence of conduction blocks near the spiral wave tip.

Action Potentials↗

PubMed2003

Pulling reptating polymers by one end: magnetophoresis in the Rubinstein-Duke model.

We consider the magnetophoresis problem within the Rubinstein-Duke model, i.e., a reptating polymer pulled by a constant field applied to a single end of a chain. Extensive density matrix renormalization calculations are presented of the drift velocity and the profile of the chain for various strengths of the driving field and chain lengths. We show that the velocities and the average densities of the stored length are well described by simple interpolating crossover formulas, derived under the assumption that the difference between the drift and curvilinear velocities vanishes for sufficiently long chains. The profiles, which describe the average shape of the reptating chain, also show such interesting features as some nonmonotonic behavior of the link densities for sufficiently strong pulling fields. We develop a description in which a distinction is made between links entering at the pulled head and at the unpulled tail. At weak fields the separation between the head zone and the tail zone meanders through the whole chain, while the probability of finding it close to the edges drops off. At strong fields the tail zone is confined to a small region close to the unpulled edge of the polymer.

Journal Article↗

PubMed2004

Fluctuations of an atomic ledge bordering a crystalline facet.

When a high symmetry facet joins the rounded part of a crystal, the step line density vanishes as square root of r with r denoting the distance from the facet edge. This means that the ledge bordering the facet has a lot of space to meander as caused by thermal activation. We investigate the statistical properties of the border ledge fluctuations. In the scaling regime they turn out to be non-Gaussian and related to the edge statistics of Gaussian unitary ensemble multimatrix models.

Journal Article↗

PubMed2004

Attraction of spiral waves by localized inhomogeneities with small-world connections in excitable media.

Trapping and untrapping of spiral tips in a two-dimensional homogeneous excitable medium with local small-world connections are studied by numerical simulation. In a homogeneous medium which can be simulated with a lattice of regular neighborhood connections, the spiral wave is in the meandering regime. When changing the topology of a small region from regular connections to small-world connections, the tip of the spiral waves is attracted by the small-world region, where the average path length declines with the introduction of long distant connections. The "trapped" phenomenon also occurs in regular lattices where the diffusion coefficient of the small region is increased. The above results can be explained by the eikonal equation, the Luther equation, and the relation between the core radius and the diffusion coefficient.

Journal Article↗