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At least 397 records · Page 22Linked to original sources

PubMed1991

Graphite: a mimic for DNA and other biomolecules in scanning tunneling microscope studies.

Highly ordered pyrolytic graphite (HOPG) is the substrate often used in scanning tunneling microscope (STM) studies of biomolecules such as DNA. All of the images presented in this article are of freshly cleaved HOPG surfaces upon which no deposition has occurred. These images illustrate features previously thought to be due to biological molecules, such as periodicity and meandering of "molecules" over steps. These features can no longer be used to distinguish real molecules from features of the native substrate. The feasibility of the continued use of HOPG as a substrate for biological STM studies is discussed.

Adsorption↗

PubMed1999

Complexity in natural landform patterns

Patterns in nature, such as meandering rivers and sand dunes, display complex behavior seemingly at odds with their simplicity of form. Existing approaches to modeling natural landform patterns, reductionism and universality, are incompatible with the nonlinear, open nature of natural systems. An alternative modeling methodology based on the tendency of natural systems to self-organize in temporal hierarchies is described.

Journal Article↗

PubMed2000

Altered river morphology in south africa related to the permian-triassic extinction

The Permian-Triassic transition in the Karoo Basin of South Africa was characterized by a rapid and apparently basin-wide change from meandering to braided river systems, as evidenced by preserved sedimentary facies. This radical changeover in river morphology is consistent with geomorphic consequences stemming from a rapid and major die-off of rooted plant life in the basin. Evidence from correlative nonmarine strata elsewhere in the world containing fluvial Permian-Triassic boundary sections suggests that a catastrophic terrestrial die-off of vegetation was a global event, producing a marked increase in sediment yield as well as contributing to the global delta(13)C excursion across the Permian-Triassic boundary.

Journal Article↗

PubMed2002

Fouling dynamics in suspension flows.

A particle suspension flowing in a channel in which fouling layers are allowed to form on the channel walls is investigated by numerical simulation. A two-dimensional phase diagram with at least four different behaviors is constructed. The fouling is modeled by attachment during collision with the deposits and by detachment caused by large enough hydrodynamic drag. For fixed total number of particles and small Reynolds numbers, the relevant parameters governing the fouling dynamics are the solid volume fraction of the suspension and the detachment drag force threshold. Below a critical curve in this 2D phase space only transient fouling takes place when the suspension is accelerated from rest by a pressure gradient. Above the fouling transition line, persistent fouling layers are formed via ballistic deposition for low and via homogeneous deposition for large solid volume fractions. Close to the fouling transition line, the flow path between the deposited layers meanders, while necking appears for increasing distance from the transition. Finally, another transition to a fully blocked flow path takes place. As determined by the estimated amount of deposited particles at saturation, both transitions seem to be discontinuous. Large fluctuations and long saturation times are typical of the dynamics of the system.

Journal Article↗

PubMed2005

Substrate size as a determinant of fibrillatory activity maintenance in a mathematical model of canine atrium.

Tissue size has been considered an important determinant of atrial fibrillation (AF), but recent work has questioned the critical size hypothesis. Here, we use a previously developed mathematical model of the two-dimensional canine atrium with realistic action potential, ionic, and conduction properties to address substrate size effects on the maintenance of fibrillatory activity. Cholinergic AF was simulated at different acetylcholine (ACh) concentrations ([ACh]) and distributions, with substrate area varied 11.1-fold. Automated phase singularity detection was used to facilitate the analysis of arrhythmic activity. The duration of activity induced by a single extrastimulus increased with increasing substrate dimensions. Two general mechanisms underlying activity were observed and were differentially affected by substrate size. For large mean [ACh], single primary rotors anchored in low-[ACh] zones maintained activity and substrate dimensions were not critical. At lower mean [ACh], extensive spiral wave meander prevented the emergence of single stable rotors. Prolonged activity was favored when substrate size permitted a sufficiently large number of simultaneous longer-lasting rotors that extinction of all was unlikely. Thus either single dominant rotor or multiple reentrant spiral generator mechanisms could maintain fibrillatory activity in this model and were differentially dependent on substrate size. These results speak to recent debates about the role in AF of single driver rotors versus multiple reentrant circuit mechanisms by suggesting that either may maintain fibrillatory atrial activity depending on atrial size and electrophysiological properties.

Action Potentials↗

PubMed2003

A model for human ventricular tissue.

The experimental and clinical possibilities for studying cardiac arrhythmias in human ventricular myocardium are very limited. Therefore, the use of alternative methods such as computer simulations is of great importance. In this article we introduce a mathematical model of the action potential of human ventricular cells that, while including a high level of electrophysiological detail, is computationally cost-effective enough to be applied in large-scale spatial simulations for the study of reentrant arrhythmias. The model is based on recent experimental data on most of the major ionic currents: the fast sodium, L-type calcium, transient outward, rapid and slow delayed rectifier, and inward rectifier currents. The model includes a basic calcium dynamics, allowing for the realistic modeling of calcium transients, calcium current inactivation, and the contraction staircase. We are able to reproduce human epicardial, endocardial, and M cell action potentials and show that differences can be explained by differences in the transient outward and slow delayed rectifier currents. Our model reproduces the experimentally observed data on action potential duration restitution, which is an important characteristic for reentrant arrhythmias. The conduction velocity restitution of our model is broader than in other models and agrees better with available data. Finally, we model the dynamics of spiral wave rotation in a two-dimensional sheet of human ventricular tissue and show that the spiral wave follows a complex meandering pattern and has a period of 265 ms. We conclude that the proposed model reproduces a variety of electrophysiological behaviors and provides a basis for studies of reentrant arrhythmias in human ventricular tissue.

Action Potentials↗

PubMed2002

Electrical refractory period restitution and spiral wave reentry in simulated cardiac tissue.

Theoretical and experimental studies have shown that restitution of the cardiac action potential (AP) duration (APD) plays a major role in predisposing ventricular tachycardia to degenerate to ventricular fibrillation, whereas its role in atrial fibrillation is unclear. We used the Courtemanche human atrial cell model and the Luo-Rudy guinea pig ventricular model to compare the roles of electrical restitution in destabilizing spiral wave reentry in simulated two-dimensional homogeneous atrial and ventricular tissue. Because atrial AP morphology is complex, we also validated the usefulness of effective refractory period (ERP) restitution. ERP restitution correlated best with APD restitution at transmembrane potentials greater than or equal to -62 mV, and its steepness was a reliable predictor of spiral wave phenotype (stable, meandering, hypermeandering, and breakup) in both atrial and ventricular tissue. Spiral breakup or single hypermeandering spirals occurred when the slope of ERP restitution exceeded 1 at short diastolic intervals. Thus ERP restitution, which is easier to measure clinically than APD restitution, is a reliable determinant of spiral wave stability in simulated atrial and ventricular tissue.

Animals↗

PubMed2004

Dynamics of virtual electrode-induced scroll-wave reentry in a 3D bidomain model.

Functional reentry in the heart can be caused by a wave front of excitation rotating around its edge. Previous simulations on the basis of monodomain cable equations predicted the existence of self-sustained, vortex-like wave fronts (scroll waves) rotating around a filament in three dimensions. In our simulations, we used the more accurate bidomain model with modified Beeler-Reuter ionic kinetics to study the dynamics of scroll-wave filaments in a 16 x 8 x 1.5-mm slab of ventricular tissue with straight fibers. Wave fronts were identified as the areas with inward current. Their edges represented the filaments. Both transmural and intramural reentries with I- and U-shaped filaments, respectively, were obtained by the S1-S2 point stimulation protocol through the virtual electrode-induced phase singularity mechanism. The filaments meandered along elongated trajectories and tended to attach to the tissue boundaries exposed to air (no current flow) rather than to the bath (zero extracellular potential). They completely detached from electroporated (zero transmembrane potential) boundaries. In our simulations, the presence of the bath led to generation of only U-shaped filaments, which survived for the 1.5-mm-thick slab but not for the slabs of 0.5- or 3-mm thicknesses. Thus boundary conditions may be another determinant of the type and dynamics of reentry.

Action Potentials↗

PubMed2001

Electrophysiological heterogeneity and stability of reentry in simulated cardiac tissue.

Generation of wave break is a characteristic feature of cardiac fibrillation. In this study, we investigated how dynamic factors and fixed electrophysiological heterogeneity interact to promote wave break in simulated two-dimensional cardiac tissue, by using the Luo-Rudy (LR1) ventricular action potential model. The degree of dynamic instability of the action potential model was controlled by varying the maximal amplitude of the slow inward Ca(2+) current to produce spiral waves in homogeneous tissue that were either nearly stable, meandering, hypermeandering, or in breakup regimes. Fixed electrophysiological heterogeneity was modeled by randomly varying action potential duration over different spatial scales to create dispersion of refractoriness. We found that the degree of dispersion of refractoriness required to induce wave break decreased markedly as dynamic instability of the cardiac model increased. These findings suggest that reducing the dynamic instability of cardiac cells by interventions, such as decreasing the steepness of action potential duration restitution, may still have merit as an antifibrillatory strategy.

Action Potentials↗

PubMed1987

Implications of rotational kinematics for the oculomotor system in three dimensions.

1. This paper develops three-dimensional models for the vestibuloocular reflex (VOR) and the internal feedback loop of the saccadic system. The models differ qualitatively from previous, one-dimensional versions, because the commutative algebra used in previous models does not apply to the three-dimensional rotations of the eye. 2. The hypothesis that eye position signals are generated by an eye velocity integrator in the indirect path of the VOR must be rejected because in three dimensions the integral of angular velocity does not specify angular position. Computer simulations using eye velocity integrators show large, cumulative gaze errors and post-VOR drift. We describe a simple velocity to position transformation that works in three dimensions. 3. In the feedback control of saccades, eye position error is not the vector difference between actual and desired eye positions. Subtractive feedback models must continuously adjust the axis of rotation throughout a saccade, and they generate meandering, dysmetric gaze saccades. We describe a multiplicative feedback system that solves these problems and generates fixed-axis saccades that accord with Listing's law. 4. We show that Listing's law requires that most saccades have their axes out of Listing's plane. A corollary is that if three pools of short-lead burst neurons code the eye velocity command during saccades, the three pools are not yoked, but function independently during visually triggered saccades. 5. In our three-dimensional models, we represent eye position using four-component rotational operators called quaternions. This is not the only algebraic system for describing rotations, but it is the one that best fits the needs of the oculomotor system, and it yields much simpler models than do rotation matrix or other representations. 6. Quaternion models predict that eye position is represented on four channels in the oculomotor system: three for the vector components of eye position and one inversely related to gaze eccentricity and torsion. 7. Many testable predictions made by quaternion models also turn up in models based on other mathematics. These predictions are therefore more fundamental than the specific models that generate them. Among these predictions are 1) to compute eye position in the indirect path of the VOR, eye or head velocity signals are multiplied by eye position feedback and then integrated; consequently 2) eye position signals and eye or head velocity signals converge on vestibular neurons, and their interaction is multiplicative.(ABSTRACT TRUNCATED AT 400 WORDS)

Computer Simulation↗

PubMedDate not supplied

Prenatal diagnosis of a congenital bladder diverticulum. Case report and benefits of prenatal diagnosis.

A case of congenital bladder diverticulum diagnosed at 37 weeks of gestation (measured from the first day after the last day of the last menstrual period) is reported. Delivery took place 24 h later. A postnatal urologic work-up confirmed the diagnosis of asymptomatic congenital bladder diverticulum. The infant underwent laparotomic surgery at the age of 6 months, with an extravesical diverticulectomy and ureteral reimplantation. There were no complications. This is the first case reported in the literature of a prenatal diagnosis of a congenital bladder diverticulum. This new aspect allows early management and avoidance of the diagnostic meanders to which the discovery of a pelvic mass might lead, as well as the complications that can follow bladder diverticula.

Adult↗

PubMed1986

Systematisation of the arteries in the splenic hilus.

156 abdominal preparations were explored by arteriography, corrosion and dissection. Within the meanders of the splenic artery a basic pattern can be traced. The division of the artery can easily be summarized by introducing the term of a truncus lienogastroepiploicus. The short gastric arteries are always present, and usually they are longer and more important than may appear from their name. In nearly half of the preparations (52%) an artery for the extremitas lienalis posterior is found; an artery for the extremitas lienalis anterior is very exceptional. A posterior gastric artery can be identified (36%) but it must be distinguished from several other vessels. The arteria colli pancreatis (for the neck of the pancreas) is a constant vessel that may have a superior (usually arteria lienalis) or an inferior origin (arteria mesenterica superior). It commonly supplies the transverse pancreatic artery and often the prepancreatic arcade. The arteria corporis pancreatis, too, is a constant artery. The pancreatic tail is thoroughly irrigated by usually more than one arteria caudae pancreatis. An important part of the greater curvature is not accompanied by the left gastroepiploic artery, but is supplied by the arteriae gastricae breves.

Arteries↗

PubMed1996

Riverine barriers and gene flow in Amazonian saddle-back tamarins.

We describe patterns of genotypic and phenotypic variation in saddle-back tamarin (Saguinus fuscicollis) populations along the central and upper Rio Juruá, western Brazilian Amazonia. The genetic data are sequence haplotypes of the mitochondrial cytochrome b gene; phenotypic data are pelage colour variants that define sharply demarcated subspecies of this extremely variable tamarin species. We show that gene flow occurs between adjacent subspecies, but that this phenomenon is restricted to the headwater section of the river, which is consistent with expectations from the riverine barrier hypothesis. In this model, the major first-order tributaries of the Amazon form effective barriers to dispersal, with between-bank gene flow limited to the narrowed sections of headwater streams and parallel divergence increasing along both banks from the headwaters to the mouth of a given river. In meandering rivers such as the Rio Juruá, we suggest passive transfer through river channel dynamics as the main mechanism permitting genetic contact between populations on opposite banks of the river. Finally, we argue that in the case of plant and animal species that are largely restricted to unflooded (terra firme) forests, such as tamarins, seasonally flooded (várzea) forest can operate as a critical additional barrier to between-bank gene flow.

Animals↗

PubMedDate not supplied

Capillary microscopy of the nailfold in healthy subjects.

In a total of 100 healthy subjects (36 males, 64 females) aged between 5 and 58 years, capillary microscopy of the nailfold was performed in all the fingers of both hands. The aim of this study was to determine whether capillary abnormalities, hitherto regarded as pathological, were to be found, and if so, to establish their frequency and nature. Capillary branchings - on average 4-5/person - were seen in 78% of the subjects investigated, and proved to be most common in digits IV and V of both hands. Of the subjects investigated, 94% showed meandering, 64% tortuous loops, 25% hemorrhagic extravasations, and 19% apical dilatations of the capillaries. Only a single case of a giant loop was seen. Neither avascular fields, stasis nor reversion of blood flow were seen. Maricq's venule index was increased in 3 subjects. These findings clearly indicate that the significance of deviating capillary patterns need to be reconsidered, for branchings or hemorrhagic extravasations can certainly be found in healthy subjects.

Adolescent↗

PubMed1988

New canine model of chronic pancreatitis due to chronic ischemia with incomplete pancreatic duct obstruction.

A new experimental model of chronic pancreatitis was produced by a combination of chronic ischemia and incomplete obstruction of the pancreatic duct. Ischemia was induced by ligation and separation of branches flowing into the left pancreatic lobe from the splenic artery. Incomplete ductal obstruction was achieved by ligation and separation of the minor pancreatic duct and placement of a polyethylene tube in the major pancreatic duct. Macroscopic examination at 6 months after model preparation showed that the pancreas was hard, with severe inflammatory change. In the secretin test, the flow rate of pancreatic juice, amylase output and bicarbonate concentration were significantly reduced as compared with the controls. Pancreatography revealed dilatation and meandering of the major pancreatic duct and poor visualization of its secondary and tertiary bifurcations. The histopathological findings consisted of a decrease in the pancreatic parenchyma, replacement of fat, severe inflammatory cell infiltration, extensive fibrosis and tubular complexes. This model most closely resembles human chronic pancreatitis, and is a very useful instrument.

Animals↗

PubMed2003

Spatial distribution of phase singularities in ventricular fibrillation.

BACKGROUND: Multiple excitation wavelets are present during ventricular fibrillation (VF). The underlying wavelet organization of VF is unclear. Phase singularities (PSs)-locations of ambiguous activation state-underlie reentry and wavelet splitting and represent the sources of VF. Understanding the mechanisms of PS formation might be important in the development of effective therapies for sudden death. METHODS AND RESULTS: We performed voltage, phase, and PS mapping in fibrillating ventricles, applying an automated PS detection algorithm to optically recorded fibrillation signals. PS clustering was noted along epicardial vessels, ridges of endocardial trabeculae, and papillary muscle insertions. Microscopically, these locations correlated with areas of apposition of fibers with different angulations and intramural vessels. A total of 83.2% of PSs were formed at and meandered about these anatomic structures, which acted as stabilizers: PSs colocalizing at anatomic substrates had longer life spans than nonanatomic PS (82.46+/-60.8 versus 40.5+/-31.9 ms, P<0.01). The RV endocardium had a higher PS incidence than the epicardium (42.3+/-9.2 versus 23.5+/-11.6 PS/s, P<0.01). Autocorrelation showed that irregular behavior was spatially restricted to anatomic heterogeneities compared with other areas, which had nearly periodic behaviors. Simple spatial PS distributions underlay complex and variable activation patterns attributable to variable PS behaviors, life spans, and inter-PS interactions. CONCLUSIONS: PSs occur in a nonrandom spatial distribution and colocalize with normal anatomic heterogeneities. Varying PS behaviors and life spans but stable PS spatial distributions cause ever-changing activation patterns that characterize VF.

Animals↗

PubMed1995

Nonstationary vortexlike reentrant activity as a mechanism of polymorphic ventricular tachycardia in the isolated rabbit heart.

BACKGROUND: Ventricular tachycardia may result from vortexlike reentrant excitation of the myocardium. Our general hypothesis is that in the structurally normal heart, these arrhythmias are the result of one or two nonstationary three-dimensional electrical scroll waves activating the heart muscle at very high frequencies. METHODS AND RESULTS: We used a combination of high-resolution video imaging, electrocardiography, and image processing in the isolated rabbit heart, together with mathematical modeling. We characterized the dynamics of changes in transmembrane potential patterns on the epicardial surface of the ventricles using optical mapping. Image processing techniques were used to identify the surface manifestation of the reentrant organizing centers, and the location of these centers was used to determine the movement of the reentrant pathway. We also used numerical simulations incorporating Fitzhugh-Nagumo kinetics and realistic heart geometry to study how stationary and nonstationary scroll waves are manifest on the epicardial surface and in the simulated ECG. We present epicardial surface manifestations (reentrant spiral waves) and ECG patterns of nonstationary reentrant activity that are consistent with those generated by scroll waves established at the right and left ventricles. We identified the organizing centers of the reentrant circuits on the epicardial surface during polymorphic tachycardia, and these centers moved during the episodes. In addition, the arrhythmias that showed the greatest movement of the reentrant centers displayed the largest changes in QRS morphology. The numerical simulations showed that stationary scroll waves give rise to monomorphic ECG signals, but nonstationary meandering scroll waves give rise to undulating ECGs characteristic of torsade de pointes. CONCLUSIONS: Polymorphic ventricular tachycardia in the healthy, isolated rabbit heart is the result of either a single or paired ("figure-of-eight") nonstationary scroll waves. The extent of the scroll wave movement corresponds to the degree of polymorphism in the ECG. These results are consistent with our numerical simulations that showed monomorphic ECG patterns of activity for stationary scroll waves but polymorphic patterns for scroll waves that were nonstationary.

Animals↗

PubMed1998

Transmembrane potential properties at the core of functional reentrant wave fronts in isolated canine right atria.

BACKGROUND: The characteristics of transmembrane potential (TMP) at the core of functional reentry in the atrium are not well understood. METHODS AND RESULTS: In protocol 1 (11 dogs), isolated perfused canine right atria were mapped from the endocardial surface while simultaneous TMPs were recorded from the epicardial surface. Episodes of reentry (n=64) were induced in the presence of 1 to 5 micromol/L acetylcholine. Successful simultaneous TMP recordings and activation maps were made in 8 episodes. The TMP was "near the core" if it was within 3.2 mm of the core; otherwise, it was considered to be "in the periphery." The mean cycle length of reentry was 110+/-35 ms. The TMP amplitude, duration (90% repolarization), and (dV/dt)max near the core (n=106) were 58+/-22 mV, 46+/-14 ms, and 33+/-20 V/s, respectively, significantly less than those in the periphery (n=241): 70+/-8 mV, 94+/-32 ms, and 55+/-10 V/s (P<0.001 for all). In 2 episodes of reentry, the cell at the core remained unexcited at its resting membrane potential. In protocol 2 (2 dogs), we performed simultaneous high-density mapping in 4 episodes of reentry and showed synchronous activation patterns on both surfaces with similar locations of the core. CONCLUSIONS: During meandering functional reentry in isolated canine right atria, (1) TMPs of cells near the core have a reduced amplitude, duration, and (dV/dt)max, and (2) cells at the core may remain unexcited at their resting membrane potential. These findings are compatible with the spiral wave concept of functional reentry in the atrium.

Acetylcholine↗