SEARCH · PubMed Health

Results for “Meanders”

Explore indexed PubMed citations for clinical trials, systematic reviews and public health research. Read source abstracts and follow each citation to its original PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2Linked to original sources

PubMed1997

Induction of meandering functional reentrant wave front in isolated human atrial tissues.

BACKGROUND: The purpose of this study was to test the hypothesis that a single meandering functional reentrant wave front can result in rapid and irregular electrogram activity in human atrial tissues. METHODS AND RESULTS: The study used the explanted hearts of five human cardiac transplant recipients. Three right and two left atrial tissue samples, 3.4+/-0.3 mm thick, were excised and trimmed to 3.5x3.0 cm. The isolated atrium was placed endocardial surface down in a chamber with a 477 bipolar recording electrode array built into the bottom of the tissue bath. The interelectrode distance was 1.6 mm. The tissue was constantly superfused with 36.5 degrees C oxygenated Tyrode's solution at a rate of 10 mL/min. After eight baseline stimuli (S1) delivered at 400- or 600-ms cycle length from the edge of the tissue, a single premature stimulus (S2) was given at the center of the tissue to induce reentry. A total of nine episodes of reentry were induced with S1-S2 coupling intervals of 232+/-29 ms (range, 190 to 290 ms) and an S2 strength of 10+/-3 mA (range, 5 to 15 mA). In all samples, a single meandering reentrant wave front was induced, causing irregular and rapid bipolar electrogram activity. These wave fronts had a mean cycle length of 229+/-45 ms (160 to 290 ms) and persisted for 1.1+/-0.3 seconds (0.6 seconds to 2.5 seconds), or 5.2+/-1.4 (3 to 9) cycles, before spontaneous termination. CONCLUSIONS: A single meandering functional reentrant wave front can be induced in human atrial tissues and produce rapid and irregular electrical activity.

Adult↗

PubMed1993

Developmental analysis of the external granular layer in the meander tail mutant mouse: do cerebellar microneurons have independent progenitors?

The cerebellum of the meander tail mutant mouse (mea/mea) is characterized by an apparently normal cytoarchitecture posteriorly with an abrupt transition to an abnormal anterior region. Anteriorly, there is abnormal foliation, a drastic reduction in the granule cells (GC) population, disorganization of the Purkinje cells (PC), and a virtual absence of Bergmann glial processes. In this paper we analyze the prenatal and postnatal development of the cerebellum in the mea/mea and attempt to determine the phenotypic onset of the mutation in the anterior region. Hematoxylin and eosin stained sections reveal a morphological difference in the cerebellum of the mea/mea as early as embryonic day 16 characterized by a reduction in the external granule cell layer (EGL). The reduction in the EGL becomes increasingly apparent as development proceeds. This deficit in the EGL most probably results in the absence of GC, but it is unclear at this point whether reduced migration, proliferation, and/or increased cell death is the major factor. Interestingly, immunohistochemical staining with a monoclonal antibody against parvalbumin reveals that the basket and stellate cells, which are also thought to arise from the EGL, are present in the anterior region of the mea/mea cerebellum. These results suggest that the lack of GC in the meander tail is due to an early expressed abnormality of the EGL. However, the presence of the basket and/or stellate cells raises some interesting questions concerning the lineage of the cerebellar microneurons.

Animals↗

PubMed1995

Characteristics of atrial re-entry and meander computed from a model of a rabbit single atrial cell.

A two-dimensional excitable medium model of atrial tissue has been constructed by incorporating the Earm-Hilgemann-Noble excitation equations for membrane voltage-dependent ionic currents and pump exchanger currents, time-dependent changes in intracellular and extracellular ionic concentrations, and storage and release of Ca2+, for an atrial cell in an homogeneous partial differential equation. A diffusion coefficient of 1.25 cm2 sec-1 gives the conduction velocity of a solitary planar wave as 0.6 m sec-1. A spiral wave with a period of 78 msec develops from a broken wavefront, and initially rotates around a circular core of 2 mm diameter. After 1.5 sec of rotation, a biperiodic meander, with 6 epicycles/cycle, develops. This simple meander pattern is reflected in a period-6 modulation of the intervals between successive wavefronts.

Action Potentials↗

PubMed1999

Meandering right pulmonary vein: a case of scimitar variant.

We report a healthy, asymptomatic 15-year-old girl with a meandering right pulmonary vein draining to the left atrium. A meandering pulmonary vein may or may not be associated with the scimitar syndrome. The differential diagnosis with the scimitar syndrome and other forms of scimitar variants is discussed.

Adolescent↗

PubMed1991

Genetic mapping of meander tail, a mouse mutation affecting cerebellar development.

The meander tail mouse harbors a recessive mutation on chromosome 4 that affects the anterior lobes of the cerebellum and the caudal vertebrae. Examination of the mea/mea cerebellum reveals that the complete disorganization of all cell types seen in the anterior lobes is separated by a sharp and consistent boundary from the normal cytoarchitecture of the posterior lobes. In the absence of any biochemical information regarding the affected gene product, attempts to clone the gene must rely on the strategy of reverse genetics. As an initial step in this process we have constructed a genetic linkage map spanning 68 cM of chromosome 4 using an intersubspecific phenotypic backcross. The loci included in this analysis are Calb, Ggtb, Lv, b, Ifa, mea, D4Rp1, Glut-1, Lck, Lmyc-1, and Eno-1. This analysis positions the mea phenotypic locus in the interval between Ifa and Glut1. These results also further define regions of homology between mouse chromosome 4 and human chromosomes 8, 1, and 9. This linkage map provides the means to evaluate candidate genes, and to identify tightly linked markers useful for cloning the meander tail locus.

Animals↗

PubMed2006

Longitudinal mixing in meandering channels: new experimental data set and verification of a predictive technique.

Evaluation of longitudinal mixing processes in open channel flows is important in environmental management, requiring the quantification of mixing coefficients. Estimates of these coefficients sufficiently accurate for environmental impact assessments cannot be achieved using current theoretical or semi-empirical methods for natural channels. This inaccuracy is caused by a limited understanding and quantification of the interaction of the dominant mechanisms resulting from natural channel features, such as plan form curvature and changes in cross-sectional shape. Experimental results are presented here from studies conducted in three self-formed channels, developed by known discharges. Longitudinal mixing was investigated at various flow rates within each of the channels by monitoring the development of tracer plumes during transit through the channels. Using an optimisation procedure, coefficients required for solution of the one-dimensional advection dispersion equation (1D-ADE) were found in the range 0.02-0.2m(2)/s. The coefficients were found to vary as functions of longitudinal meander location, channel form and discharge. Predictions of these longitudinal mixing coefficients were made using a mathematical technique requiring only channel form properties and flow rate as inputs. Predicted values were typically within 20% of the measured values, although deviation of up to 50% was found for the lowest discharge in each channel. This large error is likely to have been caused by increased dead zone effects associated with channel bathymetry at low discharges that are not captured by the method. The method was shown to be capable of capturing the variation in the longitudinal mixing coefficient with longitudinal meander location, with channel form and with discharge.

Diffusion↗

PubMed2003

Genotyping and site-directed mutagenesis of a cytochrome P450 meander Pro-X-Arg motif critical to CYP4B1 catalysis.

CYP4B1 isoforms from rodents and other common laboratory animals are involved in the bioactivation of a range of protoxins, including 2-aminofluorene, 4-ipomeanol, and valproic acid. However, an earlier study provided evidence for a human allele encoding a nonfunctional CYP4B1 enzyme due to a Pro427Ser transversion in the meander region of the protein. In the present study, the CYP4B1 gene from several racial groups, Caucasians, African-Americans, and Hispanics, and from six nonhuman primate species was genotyped using a PCR-Hinf1 restriction enzyme fragment length polymorphism assay or by direct sequencing. All human populations examined were found to possess only the Ser allele at codon 427 ((1279)TCT) and all of the nonhuman primate species possessed only the Pro (CCT) allele. Therefore, an inactivating (1279)C-->T mutation in the human CYP4B1 gene likely arose following divergence of the Homo and Pan clades. Amino acid sequence alignments revealed further that this key Pro residue is located two amino acid residues N-terminal to the distal Arg of a Glu-Arg-Arg triad thought to participate in heme binding and/or redox partner interactions. Mutation of the corresponding Arg424 residue in rabbit CYP4B1 to Leu, but not His, resulted in a loss of lauric acid hydroxylase activity and ability to generate a reduced-CO binding spectrum. These data provide additional evidence for the importance of this meander region Pro-X-Arg motif in CYP4B1 heme binding and catalytic function.

Amino Acid Motifs↗

PubMed2006

Chaotic mixing and transport in a meandering jet flow.

Mixing and transport of passive particles are studied in a simple kinematic model of a meandering jet flow motivated by the problem of lateral mixing and transport in the Gulf Stream. We briefly discuss a model stream function, Hamiltonian advection equations, stationary points, and bifurcations. The phase portrait of the chosen model flow in the moving reference frame consists of a central eastward jet, chains of northern and southern circulations, and peripheral westward currents. Under a periodic perturbation of the meander's amplitude, the topology of the phase space is complicated by the presence of chaotic layers and chains of oscillatory and ballistic islands with sticky boundaries immersed into a stochastic sea. Typical chaotic trajectories of advected particles are shown to demonstrate a complicated behavior with long flights in both the directions of motion intermittent with trapping in the circulation cells being stuck to the boundaries of vortex cores and resonant islands. Transport is asymmetric in the sense that mixing between the circulations and the peripheral currents is, in general, different from mixing between the circulations and the jet. The transport properties are characterized by probability distribution functions (PDFs) of durations and lengths of flights. Both the PDFs exhibit at their tails power-law decay with different values of exponents.

Models, Statistical↗

PubMed1990

Meander tail reveals a discrete developmental unit in the mouse cerebellum.

Analyses of phenotypic mutations with altered patterns of cellular organization in invertebrate systems have lead to the identification of genes important to histogenesis. Efforts to identify genes defining segments or compartments in mammalian systems and demonstrate a role for such genes in the establishment of the cellular architectonics of the brain have been hampered by the absence of phenotypic mutations that reveal compartments. Evidence is presented that in the cerebellum of the mutant mouse, meander tail, there is an abrupt transition from the normal cytoarchitecture seen in the posterior lobes to a severely disorganized cellular pattern. Thus, in the anterior lobes the Purkinje cells are positioned randomly throughout the cortex, and other cellular layers, including the internal granule cell layer, do not form. In addition, radial Bergmann glial processes are virtually absent. One explanation for the discrete boundary of the affected area in the anterior portion of this mutant cerebellar cortex is that the meander tail gene influences compartmental cellular organization in mammalian brain.

Animals↗

PubMed1991

Modeling gamma absorbed dose due to meandering plumes.

A theory was developed to predict the average gamma absorbed dose rates due to a continuous Gaussian plume that meanders due to variations in the wind's direction caused by large-scale eddies. Such meandering often occurs under very stable, low-wind conditions. The theory is based on Gifford's fluctuating Gaussian plume model. Approximate calculations based on this theory were compared to traditional approaches based on an average concentration distribution.

Accidents↗

PubMed2003

Interrupted coarsening of anisotropic step meander.

We report on the effect of anisotropy on the step meandering instability on vicinal surfaces during molecular beam epitaxy growth. A scenario of interrupted coarsening is found: the lateral length scale of the structure first significantly increases with time and then freezes at a larger length scale. The wavelength selection mechanism results from a nontrivial nonlinear effect of anisotropy. Anisotropy also leads to solutions which drift sideways, resulting from the loss of the back-front symmetry of the meander and the nonvariational character of dynamics.

Journal Article↗

PubMed2005

Scroll waves meandering in a model of an excitable medium.

We study numerically the dynamics of a scroll wave in a three-dimensional (3D) excitable medium in the presence of substantial meandering of the corresponding 2D spiral wave in the Aliev-Panfilov model. We identify three types of dynamics of the scroll wave filament--quasi-2D, periodic, and aperiodic meandering--and we study their dependence on parameter settings and thickness of the medium.

Animals↗

PubMed2006

Convective nature of the planimetric instability in meandering river dynamics.

The convective nature of the linear instability of meandering river dynamics is analytically demonstrated and the corresponding Green's function is derived. The wave packet due to impulsive disturbance migrates along a river either downstream or upstream, depending on the subresonant or superresonant conditions. The influence of the parameters that govern the meandering process is shown and the role of the fluid dynamic detail used to describe the morphodynamic problem is discussed. A numerical simulation of the river planimetry is also developed.

Journal Article↗

PubMed2001

Superspiral structures of meandering and drifting spiral waves.

The spatiotemporal superstructure of meandering and drifting spiral waves is explained analytically. It is also demonstrated that the Hopf eigenmode that causes the transition to meandering waves is weakly exponentially localized at onset but grows exponentially slightly before onset.

Animals↗

PubMed2001

Unstable step meandering with elastic interactions.

We report on theoretical investigations of the influence of step interactions due to elasticity on unstable step meandering during molecular beam homoepitaxy. It is shown that elasticity causes coarsening of the cellular structure of the meander found in a previous work. The time dependence of step roughness is found to be robust, behaving as t(1/2). The lateral length scale coarsening is shown to be sensitive to the underlying physical mechanisms. In particular, the typical length follows the law t(alpha), with alpha = 1/6 or 1/4 depending on whether line diffusion is negligibly small or not.

Journal Article↗

PubMed2002

Formation of a spiraling line defect and its meandering transition in a period-2 medium.

The instability of a period-1 spiral wave resulting in a period-2 spiral wave with a line defect is investigated for the first time in a laboratory system. At the very onset the transition proceeds by an emergence of a spiraling line defect, "breathing" intermittently while retaining its symmetry of a period-1 spiral wave. With a further change in a control parameter, the line defect undergoes a meandering transition producing a compound tip trajectory, following a dynamic shape transition. The observed transitions have a strong analogy to the phase synchronization transition of two coupled nonlinear oscillators and the meandering transition of a period-1 spiral wave.

Journal Article↗

PubMed2003

From meandering to faceting, is step flow growth ever stable?

Based on helium atom beam diffraction and scanning tunneling microscopy data, the coexistence of a meandering and a bunching instability during homoepitaxial step flow growth is established in a class of nonreconstructed, metallic vicinal surfaces, Cu (1,1,n), n=5,9,17. Specifically, the meandering instability is shown to act as a precursor to the bunching instability, indicating that a one-dimensional treatment of bunching in step flow growth is not sufficient. Our findings might be generic to step flow growth in kinetically restricted systems.

Journal Article↗

PubMed2005

Wave grouping of a meandering spiral induced by Doppler effects and oscillatory dispersion.

A new type of meandering spiral pattern, in which the dense waves form groups while the sparse waves keep evenly spaced, is observed in a spatial open reactor using a ferroin-catalyzed Belousov-Zhabotinsky reaction. Such a phenomenon is related to both the Doppler effect of a meandering spiral and the oscillatory dispersion relation of the system. Simulation in the two-dimensional Oregonator reaction-diffusion model with an oscillatory dispersion relation gives very similar results.

Colloids↗