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At least 487 records · Page 27Linked to original sources

PubMed1984

Ephaptic impulse transmission between ventricular myocardial cells in vitro.

Impulse transmission between closely appositioned cylindrical myocardial preparations , from the ferret, was studied in vitro. One preparation was driven, and when the longitudinal extracellular potential difference between electrodes 80 micrometers apart was made larger than 30-60 mV by increasing the resistance of the extracellular space, ephaptic (i.e. non synaptic) impulse transmission from one preparation to another occurred. In 2 out of 8 pairs of preparations examined the impulse transmission was bidirectional. The latency of the transmission varied from 9 to 369 ms. Various rate dependent blocks (sometimes associated with a Wenckebach phenomenon) were observed at stimulation frequencies above 1 Hz. The experimental situation has pathophysiological properties relevant to the analysis of the genesis of arrhythmias. Ephaptic interaction may be involved in the shaping of the wavefront of excitation under both normal and pathological conditions.

Animals↗

PubMed2002

Climate-driven bedrock incision in an active mountain belt.

Measurements of fluvial bedrock incision were made with submillimeter precision in the East Central Range of Taiwan, where long-term exhumation rates and precipitation-driven river discharge are independently known. They indicate that valley lowering is driven by relatively frequent flows of moderate intensity, abrasion by suspended sediment is an important fluvial wear process, and channel bed geometry and the presence of widely spaced planes of weakness in the rock mass influence erosion rate and style.

Journal Article↗

PubMed2000

The solar wind-magnetosphere-ionosphere system

The solar wind, magnetosphere, and ionosphere form a single system driven by the transfer of energy and momentum from the solar wind to the magnetosphere and ionosphere. Variations in the solar wind can lead to disruptions of space- and ground-based systems caused by enhanced currents flowing into the ionosphere and increased radiation in the near-Earth environment. The coupling between the solar wind and the magnetosphere is mediated and controlled by the magnetic field in the solar wind through the process of magnetic reconnection. Understanding of the global behavior of this system has improved markedly in the recent past from coordinated observations with a constellation of satellite and ground instruments.

Journal Article↗

PubMed2004

Ground-based diffusion experiments on liquid Sn-In systems using the shear cell technique of the satellite mission Foton-M1.

This study reported in this paper was aimed at testing the shear cell that was developed for the satellite mission Foton-M1 to measure diffusion coefficients in liquid metals under microgravity (microg)-conditions. Thick Layer diffusion experiments were performed in the system Sn90In10 versus Sn under 1 g-conditions. For this system several microg-diffusion results are available as reference data. This combination provides a low, but sufficiently stable, density layering throughout the entire experiment, which is important to avoid buoyancy-driven convection. The experimental results were corrected for the influences of the shear-induced convection and mixing after the final shearing, both of which are typical for the shear cell technique. As the result, the reproducibility and the reliability of the diffusion coefficients in the ground-based experiments were within the limits of error of microg-data. Based on our results we discuss the necessary conditions to avoid buoyancy-driven convection.

Capillaries↗

PubMed2001

New androgen response elements in the murine pem promoter mediate selective transactivation.

The Pem homeobox transcription factor is expressed under androgen control in the testis and epididymis. It is also transcribed in the ovary, muscle, and placenta. The mouse Pem gene promoter was cloned and sequenced. It was analyzed in transactivation tests using CV-1 and PC-3 cells expressing the AR and found to be strongly stimulated by androgens. EMSAs and mutational analysis of the Pem promoter allowed the identification of two functional androgen response elements named ARE-1 and ARE-2. They both differed from the consensus semipalindromic steroid response element and exhibited characteristics of direct repeats of the TGTTCT half-site. Unlike the steroid response element, both Pem androgen response elements were selectively responsive to androgen stimulation. Specific mutations in the left half-site of Pem ARE-1 and ARE-2, but not of the steroid response element, were still compatible with AR binding in the EMSA. In addition, Pem ARE-1, but not ARE-2 or the steroid response element, showed some flexibility with regard to spacing between half-sites. These results strongly suggest that the AR interacts differently with direct repeats than with inverted repeats, potentially leading to cis element-driven selective properties. Thus, the existence of several classes of DNA response elements might be an essential feature of differential androgen regulation.

Androgens↗

PubMedDate not supplied

Theoretical characterization of dual concentric conductor microwave applicators for hyperthermia at 433 MHz.

Radiation patterns from multi-aperture arrays of Dual Concentric Conductor (DCC) applicators for superficial microwave hyperthermia were calculated numerically using the Finite Difference Time Domain (FDTD) method at an operating frequency of 433 MHz. Different aperture sizes were studied from 3.5-7 cm square in six-element array configurations which may be considered as building block elements of much larger arrays. Once optimum parameters were established for gap width, inter-element spacing, and bolus thickness for each aperture size using repetitive parametric FDTD studies, a direct comparison of power deposition (SAR) patterns was performed for similar DCC arrays driven at 915 MHz and 433 MHz. Results show that clinically desirable uniformity and penetration of SAR can be obtained for square apertures in the range of 4-6 cm per side at 433 MHz, using 9.5-12.5 mm thick water coupling boluses, as compared to the optimum 2-4.5 cm square apertures and 5-10 mm water bolus dimensions determined previously for 915 MHz arrays. With an accompanying reduction in the lateral adjustability of power control, the 433 MHz applicators can provide a significantly larger heating area than arrays with the same number of elements driven at a frequency of 915 MHz. Penetration depth is similar for optimally sized apertures at both frequencies of operation, providing a maximum heating depth of 1-2 cm for the DCC array applicators.

Hyperthermia, Induced↗

PubMed2006

Crystal structure of the trigonal form of isotactic polypropylene as an example of density-driven polymer structure.

Propene-hexene copolymers crystallize in a new polymorphic form of isotactic polypropylene when the concentration of hexene is higher than nearly 10-15 mol %. The hexene units are included in the crystals, inducing an increase of density that allows crystallization of 3-fold helical chains in a trigonal unit cell according to the space group R3c or Rc, where the helical symmetry of the chains is maintained in the crystal lattice. The structure of this new form is similar to those of isotactic polybutene and polystyrene and does not crystallize in polypropylene homopolymer because it would have too low density. The crystal structure of isotactic polypropylene is therefore no longer an exception to the principles of polymer crystallography, but the new structure represents the fulfillment of these principles and indicates that the packing of polymer molecules is mainly driven by density.

Journal Article↗

PubMed2005

Quantum dynamics driven by continuous laser fields under measurements: towards measurement-assisted quantum dynamics control.

We study quantum system dynamics driven by continuous laser fields under the measurement process. In order to take into account the system transition due to the measurement, we define the superoperator which eliminates the coherence relevant to the measured quantum states. We clarify that the dynamics of the measured states is frozen in the frequent measurement limit, while the space spanned by unmeasured states is isolated from the original system. We also derive the effective Liouvillian which governs incoherent population dynamics under the condition, in which measurements are frequently applied. We apply the formulation to two-level and Lambda-type three-level systems and clarify how the quantum measurements hinder the coherent population dynamics driven by the continuous laser fields in practical examples. Analysis on the laser field amplitude dependency of the final distribution in the t-->infinity limit suggests the possibility of the measurement-assisted quantum control.

Journal Article↗

PubMed2005

Functional periplasmic secretion of organophosphorous hydrolase using the twin-arginine translocation pathway in Escherichia coli.

Recombinant Escherichia coli systems expressing organophosphorous hydrolase (OPH) have been proposed for biotransformation of toxic organophosphate compounds. However, whole cell biocatalyst systems are critically disadvantaged due to substrate diffusion limitations. To enhance whole cell biocatalytic efficiency, we engineered E. coli, for the first time, to secrete metal ion cofactor-requiring OPH into the periplasmic space using the twin-arginine translocation (Tat) pathway. In particular, the twin-arginine signal sequence of E. coli trimethylamine N-oxide (TMAO) reductase (TorA) was employed. Even though total OPH activity in the cell lysate fraction was lower in the periplasmic-secreting strain than in the control cytosolic-expressing strain, whole cell OPH activity was approximately 2.8-fold higher due to successful translocation of OPH into the periplasmic space. In addition, whole cell OPH activity in the periplasmic-secreting strain was far more stable than that in the cytosolic-expressing strain. Therefore, Tat-driven periplasmic-secreting E. coli can be successfully employed as efficient whole cell biocatalysts.

Arginine↗

PubMed2004

Polyp oriented modelling of coral growth.

The morphogenesis of colonial stony corals is the result of the collective behaviour of many coral polyps depositing coral skeleton on top of the old skeleton on which they live. Yet, models of coral growth often consider the polyps as a single continuous surface. In the present work, the polyps are modelled individually. Each polyp takes up resources, deposits skeleton, buds off new polyps and dies. In this polyp oriented model, spontaneous branching occurs. We argue that branching is caused by a so called "polyp fanning effect" by which polyps on a convex surface have a competitive advantage relative to polyps on a flat or concave surface. The fanning effect generates a more potent branching mechanism than the Laplacian growth mechanism that we have studied previously. We discuss the application of the polyp oriented model to the study of environmentally driven morphological plasticity in stony corals. In a few examples we show how the properties of the individual polyps influence the whole colony morphology. In our model, the spacing of polyps influences the thickness of coral branches and the overall compactness of the colony. Density variations in the coral skeleton may also be important for the whole colony morphology, which we address by studying two variants of the model. Finally, we discuss the importance of small scale resource translocation in the coral colony and its effects on the morphology of the colony.

Animals↗

PubMed2000

Quantitative analysis of intestinal motor patterns: spatiotemporal organization of nonneural pacemaker sites in the rat ileum.

BACKGROUND & AIMS: Intestinal contractions are triggered by electric activity of pacemaker cells within the smooth muscle. However, the precise spatial organization of the pacemaker system is unknown. We directly assessed the spatiotemporal organization of pacemakers by video image analysis combined with manometry and electromyography. METHODS: Isolated segments of rat ileum were perfused arterially with oxygenated fluorocarbon solution and luminally with saline. Luminal end pressures, extracellular electric activity, and images of the intestine were recorded simultaneously. Tetrodotoxin, N(G)-nitro-L-arginine methyl ester (L-NAME), L-arginine, and eserine were administered arterially. RESULTS: Myogenic contractions originated in discrete areas (dominant pacemakers) and propagated faster in aboral than in oral direction. Dominant pacemakers were distributed along the intestine at regular intervals. The preparations were mostly driven by 1 pacemaker at the time, but 2 or 3 pacemakers with different frequencies could be active simultaneously. Tetrodotoxin decreased aboral propagation velocity and revealed multiple regularly spaced pacemaker areas. Eserine increased, whereas L-arginine decreased, their frequency. After L-NAME, pacemaker activity increased and isolated pacemakers with higher frequency appeared. CONCLUSIONS: Nonneural pacemakers in rat ileum are functionally organized not as a continuous system but show a segmental arrangement, spatially and temporally modulated by neural activity.

Animals↗

PubMed2001

Classical and quantum periodically driven scattering in one dimension.

Irregular scattering at harmonically driven one-dimensional potential wells is studied both on the classical and the quantum level. We show that an ac-driven single square well, and a smooth well with oscillating bottom, are sufficient to generate chaotic scattering. For a square well with oscillating bottom, we introduce the concept of pseudointegrable scattering. The quantum dynamics of these models is treated using Floquet scattering theory, which is exact for arbitrary amplitude and frequency of the driving. In the deep quantum regime, scattering is dominated by multiphoton exchanges with the driving field, leading to complex resonance structures in transmission and reflection. For strong and fast driving, the ac-driven square well develops an effective double-well potential that introduces coherent tunneling in the scattering. We identify signatures of classical chaotic scattering in a phase-space representation of the quantum dynamics.

Journal Article↗

PubMed2002

Partial-trace-free time-convolutionless equation of motion for the reduced density matrix.

Evolution of a system, coupled to its environment and influenced by external driving fields, is an old problem that remains of interest. In this paper, we derive an equation of motion for the reduced system density matrix, which is time convolutionless and free of the partial trace with respect to the environment states. This new approach uses an extension of the projection-operator technique, which incorporates an isomorphism between the system's Liouville space and the unit eigenspace of the projection operator induced by the uniform environment density matrix. Numerical application of the present approach is particularly useful in large externally driven systems, as the partial-trace-free equation is given in terms of submatrices significantly smaller than the matrices in the conventional time-convolutionless approaches, which alleviates the computational burden. We also show that all time-convolutionless approaches, conventional or partial-trace-free, are based upon a hidden underlying assumption of time reversibility of the system's evolution. This feature puts significant constraints on applicability of time-convolutionless approaches when employing approximations that yield time irreversibility. Also, we investigate the application of the approach in the description of far-from-equilibrium systems.

Journal Article↗

PubMed2006

Quasiperiodic and chaotic discrete breathers in a parametrically driven system without linear dispersion.

We study a one-dimensional lattice of anharmonic oscillators with only quartic nearest-neighbor interactions, in which discrete breathers (DB's) can be explicitly constructed by an exact separation of their time and space dependence. Introducing parametric periodic driving, we first show how a variety of such DB's can be obtained by selecting spatial profiles from the homoclinic orbits of an invertible map and combining them with initial conditions chosen from the Poincaré surface of section of a simple Duffing's equation. Placing then our initial conditions at the center of the islands of a major resonance, we demonstrate how the corresponding DB can be stabilized by varying the amplitude of the driving. We thus discover around elliptic points a large region of quasiperiodic breathers, which are stable for very long times. Starting with initial conditions close to the elliptic point at the origin, we find that as we approach the main chaotic layer, a quasiperiodic breather either destabilizes by delocalization or turns into a chaotic breather, with an evidently broadbanded Fourier spectrum before it collapses. For some breather profiles stable quasiperiodic breathers exist all the way to the separatrix of the Duffing equation, indicating the presence of large regions of tori around the DB solution in the multidimensional phase space. We argue that these strong localization phenomena are due to the absence of phonon resonances, as there are no linear dispersion terms in our lattices. We also show, however, that these phenomena persist in more realistic physical models, in which weak linear dispersion is included in the equations of motion, with a sufficiently small coefficient.

Journal Article↗

PubMed2002

Self-organization of a carbide superlattice during deposition of carbon on Mo.

Formation and evolution of a carbide superlattice (SL) during C deposition on Mo have been studied using molecular beam epitaxy techniques. The ordering of the SL is energetically driven, such that the interplay between strain and surface energies determines the length scale of the SL. Surface precipitation of C occurs within a narrow range of SL spacing that appears to control the size and spacing of the precipitates leading to a possible mechanism for nucleation of single-walled carbon nanotubes.

Journal Article↗

PubMedDate not supplied

Influence of natural heterogeneity on the efficiency of chemical floods in source zones.

Several technologies for cleaning up DNAPLs in source zones rely on solubilizing contaminants or destroying them in situ. Typically, these approaches employ an injection/withdrawal system to recirculate this treatment fluids. Our interest is in examining factors that influence delivery efficiency. Although many factors can affect this efficiency, this study looks at the combined influence of density-driven flow and porous media heterogeneities. The analysis is based on a series of numerical simulations of hypothetical chemical floods (e.g., potassium permanganate), which are highly resolved in space and time with a scale that is typical of field installations. Results indicate that the characteristics of convective mixing, i.e., natural, forced, and mixed convection, greatly affect delivery efficiency and patterns of mass transport. The ratio of the Grashof number (Gr) and Reynolds number (Re) proved useful in interpreting the patterns of flooding in a homogeneous porous medium. When higher-permeability layers are included in the domain, they act as conduits, effectively expediting the transport of treatment chemicals from injection to recovery wells. These large fluxes of chemicals in the high-permeability layers produce significant flooding inefficiencies. The problem is less severe in heterogeneous medium where the connectivity through the treatment zone is less well developed. Overall, this paper illustrates that density effects and high-permeability pathways need to be considered in designing chemical floods.

Disasters↗

PubMed2004

Real-time computation at the edge of chaos in recurrent neural networks.

Depending on the connectivity, recurrent networks of simple computational units can show very different types of dynamics, ranging from totally ordered to chaotic. We analyze how the type of dynamics (ordered or chaotic) exhibited by randomly connected networks of threshold gates driven by a time-varying input signal depends on the parameters describing the distribution of the connectivity matrix. In particular, we calculate the critical boundary in parameter space where the transition from ordered to chaotic dynamics takes place. Employing a recently developed framework for analyzing real-time computations, we show that only near the critical boundary can such networks perform complex computations on time series. Hence, this result strongly supports conjectures that dynamical systems that are capable of doing complex computational tasks should operate near the edge of chaos, that is, the transition from ordered to chaotic dynamics.

Animals↗

PubMed2000

Low-cost MR-compatible moving heart phantom.

For the development and evaluation of cardiac magnetic resonance (MR) imaging sequences and methodologies, the availability of a periodically moving phantom to model respiratory and cardiac motion would be of substantial benefit. Given the specific physical boundary conditions in an MR environment, the choice of materials and power source of such phantoms is heavily restricted. Sophisticated commercial solutions are available; however, they are often relatively costly and user-specific modifications may not easily be implemented. We therefore sought to construct a low-cost MR-compatible motion phantom that could be easily reproduced and had design flexibility. A commercially available K'NEX construction set (Hyper Space Training Tower, K'NEX Industries, Inc., Hatfield, PA) was used to construct a periodically moving phantom head. The phantom head performs a translation with a superimposed rotation, driven by a motor over a 2-m rigid rod. To synchronize the MR data acquisition with phantom motion (without introducing radiofrequency-related image artifacts), a fiberoptic control unit generates periodic trigger pulses synchronized to the phantom motion. Total material costs of the phantom are US$ < 200.00, and a total of 80 man-hours were required to design and construct the original phantom. With schematics of the present solution, the phantom reproduction may be achieved in approximately 15 man-hours. The presented MR-compatible periodically moving phantom can easily be reproduced, and user-specific modifications may be implemented. Such an approach allows a detailed investigation of motion-related phenomena in MR images.

Artifacts↗