bluuebunny/crossref_metadata_2025 huggingface dataset:
Bifurcation Control of Nonlinear Systems With Time-Periodic Coefficients
Authors: Da´vid Alexandra , Sinha S. C.
Published: January 2004

In this study, a method for the nonlinear bifurcation control of systems with periodic coefficients is presented. The aim of bifurcation control is to stabilize post bifurcation limit sets or modify other nonlinear characteristics such as stability, amplitude or rate of growth by employing purely nonlinear feedback controllers. The method is based on an application of the Lyapunov-Floquet transformation that converts periodic systems into equivalent forms with time-invariant linear parts. Then, through applications of time-periodic center manifold reduction and time-dependent normal form theory completely time-invariant nonlinear equations are obtained for codimension one bifurcations. The appropriate control gains are chosen in the time-invariant domain and transformed back to the original variables. The control strategy is illustrated through the examples of a parametrically excited simple pendulum undergoing symmetry-breaking bifurcation and a double inverted pendulum subjected to a periodic load in the case of a secondary Hopf bifurcation.
Observability, Reconstructibility and Observer Design for Linear Mechanical Systems Unobservable in Absence of Impacts†
Authors: Martinelli Francesco , Menini Laura , Tornambe` Antonio
Published: January 2004

This paper deals with a class of mechanical systems, constituted by two composite bodies which interact with each other only during collisions: since only one position variable of only one body is measured, the whole system would not be observable without impacts. Each body is possibly constituted by several masses connected by linear springs, so that the internal deformations can be taken into account (the so-called multiple-degrees-of-freedom impacts). For two relevant and different cases, the structural properties of observability and reconstructibility are studied, and observers are proposed in order to estimate all the state variables, including those that would be unobservable in absence of impacts.
Actuator Placement and Micro-Actuation Efficiency of Adaptive Paraboloidal Shells
Authors: Tzou H. S. , Ding J. H.
Published: January 2004

Paraboloidal shells of revolution are commonly used in communication systems, precision opto-mechanical systems and aerospace structures. This study is to investigate the precision distributed control effectiveness of adaptive paraboloidal shells laminated with segmented actuator patches. Mathematical models of the paraboloidal shells laminated with distributed actuator layers subjected to mechanical, temperature, and control forces are presented first. Then, formulations of distributed actuating forces with their contributing micro-meridional/circumferential membrane and bending components are derived using an assumed mode shape function. Studies of actuator placements, actuator induced control forces, micro-contributing components, and normalized actuation authorities of paraboloidal shells are carried out. These forces and membrane/bending components basically exhibit distinct modal characteristics influenced by shell geometries and other design parameters. Analyses suggest that the membrane-contributed components dominate the overall control effect. Locations with larger normalized forces indicate the areas with high control efficiencies, i.e., larger induced actuation force per unit actuator area. With limited actuators, placing actuators at those locations would lead to the maximal control effects of paraboloidal shells.
Estimation of the Maximum Tire-Road Friction Coefficient
Authors: Mu¨ller Steffen , Uchanski Michael , Hedrick Karl
Published: January 2004

We develop and test a “slip-based” method to estimate the maximum available tire-road friction during braking. The method is based on the hypothesis that the low-slip, low-μ parts of the slip curve used during normal driving can indicate the maximum tire-road friction coefficient, μmax. We find support for this hypothesis in the literature and through experiments. The friction estimation algorithm uses data from short braking maneuvers with peak accelerations of 3.9 m/s2 to classify the road surface as either dry μmax≈1 or lubricated μmax≈0.6. Significant measurement noise makes it difficult to detect the subtle effect being measured, leading to a misclassification rate of 20%.
Model Synthesis for Design of Switched Systems Using a Variable Structure System Formulation
Authors: Kypuros Javier A. , Longoria Raul G.
Published: January 2004

A Variable Structure System (VSS) formulation and bond graph approach are used to develop a novel scheme for synthesizing reduced order models of switched systems. Switches are physical mechanisms that discontinuously redirect power, and switched systems employ switches for more efficient power conversion and/or to incorporate a control structure. A variety of power converters including electronic converters, hydraulic pumps, and clutched transmissions/differentials employ switches. It is shown how reduced order models can be derived using bond graphs and a VSS formulation to provide insight for optimizing design of switched systems. Experimental results are presented to validate this utility.
The Development of Anti-Windup Scheme for Time Delay Control With Switching Action Using Integral Sliding Surface
Authors: Lee Sung-Uk , Chang Pyung Hun
Published: January 2004

The Time Delay Control with Switching Action (TDCSA) method, which consists of Time Delay Control (TDC) and a switching action of sliding mode control (SMC), has been proposed as a promising technique in the robust control area where the plant has an unknown dynamics with parameter variations and substantial disturbances are preset. When TDCSA is applied to the plant with saturation nonlinearity, however, the so-called windup phenomenon is observed to arise, causing excessive overshoot and instability. The integral element of TDCSA and the saturation element of a plant cause the windup phenomenon. There are two integral effects in TDCSA. One is the integral effect caused by time delay estimation of TDC. The other is the integral term of an integral sliding surface. To solve this problem, we have proposed an anti-windup scheme method for TDCSA. The stability of the overall system has been proved for a class of nonlinear system. Experimental results show that the proposed method overcomes the windup problem of the TDCSA.
l 1 Controller Design for a High-Order 5-Pool Irrigation Canal System
Authors: Malaterre Pierre-Olivier , Khammash Mustafa
Published: January 2004

The aim of this work is to present an application of recent methods for solving the l1 design problem, based on the Scaled-Q approach, on a high-order, nonminimum phase system. We start by describing the system which is an open-channel hydraulic system (e.g., an irrigation canal). From the discretization and linearization of the set of two partial-derivative equations, a state-space model of the system is generated. This model is a high-order MIMO system (five external perturbations w, five control inputs u, 10 controlled outputs z, five measured outputs y, 65 states x) and is nonminimum phase. A controller is then designed by minimizing the l1 norm of the impulse response of the transfer matrix between the perturbations w and the outputs z. Time-domain constraints are added into the minimization problem in order to force integrators into the controller. The numerical resolution of the problem proved to be efficient, despite of the characteristics of the system. The obtained results are compared in the time-domain to classical PID and LQG controllers on the nonlinear system. The results are good in terms of performance and robustness, in particular for the rejection of the worst-case perturbation.
A Combined Scheme for Identification and Robust Torque Control of Hydraulic Actuators
Authors: Namvar Mehrzad , Aghili Farhad
Published: January 2004

This paper presents a combined scheme of identification and robust torque control for rotary hydraulic actuators. The composite controller consists of a dynamic feedback linearizing inner loop cascaded with an optimal l1-H∞ feedback outer loop. The proposed controller allows the actuator to generate desired torque irrespective of the actuator motion. In fact, the controller reduces significantly the impedance of the actuator as seen by its external load, making the system an ideal source of torque suitable for many robotics and automation applications. The stability analysis of internal unobservable dynamics is presented. An identification method to extract the parameters of nonlinear model of actuator dynamics and to estimate a bound for modeling uncertainty, used for synthesis of the outer optimal controller, is also presented. The theoretical results of the paper are illustrated experimentally on pitch actuator of the Schilling industrial robot.
Dynamic Responses of a Self-Moving Precision Positioning Stage Impacted by a Spring-Mounted Piezoelectric Actuator
Authors: Fung, Rong-Fong , Liu, Yung-Tien , Huang, Tai-Kun , Higuchi, Toshiro
Published: January 2004

The piezoelectric actuator (PA) has been used for precision positioning from micrometer down to nanometer scale. In this paper, a spring-mounted PA is designed to achieve a high accuracy and self-moving ability in precision positioning motion. The contact force between the hammer and the self-moving stage, and the friction force of Leuven’s model caused between the grinded groove and the self-moving stage are considered. The governing equations of the system are formulated by using the finite-element method (FEM). The numerical solutions are provided to compare with the experimental results, and demonstrate the well agreement of the present theoretical formulations.
Adaptive Coupling Control of Two Working Operations in CNC Integrated Machines*
Authors: Sun Dong
Published: January 2004

To reduce work in progress in the plastic industry, integrated CNC punching and cutting machines have been used recently to combine a number of individual operations into one operation. Application of such integrated machines is confronted with a challenging problem of synchronizing two-axis motions. In this paper, an adaptive coupling controller is utilized to synchronize punching and cutting operations of the integrated CNC machine. The controller incorporates cross-coupling technology into adaptive control design, through feedback of position errors of two motion axes and the differential position error between two axes. It has been shown that the proposed controller guarantees asymptotic convergence to zero of both position errors and the synchronization error. Implementation of this new controller in real-time operations of commercial integrated CNC machines, highlights advantages of the proposed approach.
Discrete-Event IC Engine Models: Why the Constant Speed Assumption is Valid
Authors: Guzzella Lino
Published: January 2004

Internal combustion (IC) engine models are often formulated as discrete-event models. This formulation captures well the reciprocating behavior of the engine and often leads to simpler system equations. The independent variable of these models is the engine’s crank angle which, because in general the engine’s speed is not constant, is not increasing linearly with time. Nevertheless, it is standard practice in industry and academia to apply discrete-time control system analysis and synthesis tools to these systems. Of course, this approach is justifiable only if the engine speed is varying very slowly compared to the other dynamic effects. In this paper it will be shown that this assumption is indeed valid in most cases, i.e., that for typical engine systems the speed dynamics are at least one order of magnitude slower than the manifold dynamics.
Crosstalk Between the EGFR and LIN-12/Notch Pathways in
C. elegans
Vulval Development
Authors: Yoo Andrew S. , Bais Carlos , Greenwald Iva
Published: January 2004

The
Caenorhabditis elegans
vulva is an important paradigm for cell-cell interactions in animal development. The fates of six vulval precursor cells are patterned through the action of the epidermal growth factor receptor–mitogen-activated protein kinase (EGFR-MAPK) inductive signaling pathway, which specifies the 1° fate, and the LIN-12/Notch lateral signaling pathway, which specifies the 2° fate. Here, we provide evidence that the inductive signal is spatially graded and initially activates the EGFR-MAPK pathway in the prospective 2° cells. Subsequently, this effect is counteracted by the expression of multiple new negative regulators of the EGFR-MAPK pathway, under direct transcriptional control of the LIN-12–mediated lateral signal.
PEST Domain-Enriched Tyrosine Phosphatase (PEP) Regulation of Effector/Memory T Cells
Authors: Hasegawa Kiminori , Martin Flavius , Huang Guangming , Tumas Dan , Diehl Lauri , Chan Andrew C.
Published: January 2004

Protein tyrosine kinases and phosphatases cooperate to regulate normal immune cell function. We examined the role of PEST domain–enriched tyrosine phosphatase (PEP) in regulating T cell antigen–receptor function during thymocyte development and peripheral T cell differentiation. Although normal naïve T cell functions were retained in
pep
-deficient mice, effector/memory T cells demonstrated enhanced activation of Lck. In turn, this resulted in increased expansion and function of the effector/memory T cell pool, which was also associated with spontaneous development of germinal centers and elevated serum antibody levels. These results revealed a central role for PEP in negatively regulating specific aspects of T cell development and function.
Heterochromatic Silencing and HP1 Localization in
Drosophila
Are Dependent on the RNAi Machinery
Authors: Pal-Bhadra Manika , Leibovitch Boris A. , Gandhi Sumit G. , Rao Madhusudana , Bhadra Utpal , Birchler James A. , Elgin Sarah C. R.
Published: January 2004

Genes normally resident in euchromatic domains are silenced when packaged into heterochromatin, as exemplified in
Drosophila melanogaster
by position effect variegation (PEV). Loss-of-function mutations resulting in suppression of PEV have identified critical components of heterochromatin, including proteins HP1, HP2, and histone H3 lysine 9 methyltransferase. Here, we demonstrate that this silencing is dependent on the RNA interference machinery, using tandem
mini-white
arrays and
white
transgenes in heterochromatin to show loss of silencing as a result of mutations in
piwi, aubergine
, or
spindle-E
(
homeless
), which encode RNAi components. These mutations result in reduction of H3 Lys
9
methylation and delocalization of HP1 and HP2, most dramatically in
spindle-E
mutants.
An ab Initio Molecular Dynamics Study of the Aqueous Liquid-Vapor Interface
Authors: Kuo I-Feng W. , Mundy Christopher J.
Published: January 2004

We present an ab initio molecular dynamics simulation of the aqueous liquid-vapor interface. Having successfully stabilized a region of bulk water in the center of a water slab, we were able to reproduce and further quantify the experimentally observed abundance of surface “acceptor-only”(19%) and “single-donor”(66%) moieties as well as substantial surface relaxation approaching the liquid-vapor interface. Examination of the orientational dynamics points to a faster relaxation in the interfacial region. Furthermore, the average value of the dipole decreases and the average value of the highest occupied molecular orbital for each water molecule increases approaching the liquid-vapor interface. Our results support the idea that the surface contains, on average, far more reactive states than the bulk.
Local Gate Control of a Carbon Nanotube Double Quantum Dot
Authors: Mason N. , Biercuk M. J. , Marcus C. M.
Published: January 2004

We have measured carbon nanotube quantum dots with multiple electrostatic gates and used the resulting enhanced control to investigate a nanotube double quantum dot. Transport measurements reveal honeycomb charge stability diagrams as a function of two nearly independent gate voltages. The device can be tuned from weak to strong interdot tunnel-coupling regimes, and the transparency of the leads can be controlled independently. We extract values of energy-level spacings, capacitances, and interaction energies for this system. This ability to control electron interactions in the quantum regime in a molecular conductor is important for applications such as quantum computation.
Multiferroic BaTiO
3
-CoFe
2
O
4
Nanostructures
Authors: Zheng H. , Wang J. , Lofland S. E. , Ma Z. , Mohaddes-Ardabili L. , Zhao T. , Salamanca-Riba L. , Shinde S. R. , Ogale S. B. , Bai F. , Viehland D. , Jia Y. , Schlom D. G. , Wuttig M. , Roytburd A. , Ramesh R.
Published: January 2004

We report on the coupling between ferroelectric and magnetic order parameters in a nanostructured BaTiO
3
-CoFe
2
O
4
ferroelectromagnet. This facilitates the interconversion of energies stored in electric and magnetic fields and plays an important role in many devices, including transducers, field sensors, etc. Such nanostructures were deposited on single-crystal SrTiO
3
(001) substrates by pulsed laser deposition from a single Ba-Ti-Co-Fe-oxide target. The films are epitaxial in-plane as well as out-of-plane with self-assembled hexagonal arrays of CoFe
2
O
4
nanopillars embedded in a BaTiO
3
matrix. The CoFe
2
O
4
nanopillars have uniform size and average spacing of 20 to 30 nanometers. Temperature-dependent magnetic measurements illustrate the coupling between the two order parameters, which is manifested as a change in magnetization at the ferroelectric Curie temperature. Thermodynamic analyses show that the magnetoelectric coupling in such a nanostructure can be understood on the basis of the strong elastic interactions between the two phases.
Fatal head injury from boxing: a case report from Greece: Figure 1
Authors: Constantoyannis C , Partheni M
Published: January 2004

The case is described of a fatal injury to a young amateur boxer. The number of deaths from boxing have decreased in recent years. However, new rules devised to promote safety, such as limitation of the number of rounds, mandatory use of safety head guards, and better medical control during the match, seem to be of limited value in preventing severe head injuries from boxing.
The Victorian Active Script Programme: promising signs for general practitioners, population health, and the promotion of physical activity
Authors: Sims J , Huang N , Pietsch J , Naccarella L
Published: January 2004

Background:The Active Script Programme (ASP) aimed to increase the number of general practitioners (GPs) in Victoria, Australia who deliver appropriate, consistent, and effective advice on physical activity to patients. To maximise GP participation, a capacity building strategy within Divisions of General Practice (DGPs) was used. The objectives of the programme were to (a) train and support GPs in advising sedentary patients, and (b) develop tools and resources to assist GPs.

Objective:To evaluate the effectiveness of the ASP.

Methods:A systems approach was used to promote capacity in Victorian general practice. Economic analyses were incorporated into the programme’s evaluation. Participants were selected DGPs and their GP members. The programme worked with DGPs to train GPs and provide relevant resources. The main outcome measures were (a) changes in GP knowledge and behaviour and (b) cost effectiveness, based on modelled estimates of numbers of patients advised and adopting physical activity and gaining the associated health benefits.

Results:GP awareness and provision of physical activity advice increased. Although the programme’s reach was modest, based on actual GP involvement, the cost effectiveness figures ($138 per patient to become sufficiently active to gain health benefits, and $3647 per disability adjusted life year saved) are persuasive.

Conclusions:The ASP increased DGPs’ capacity to support GPs to promote physical activity. There is a strong economic argument for governments to invest in such programmes. However, caution is warranted about the maintenance of patients’ activity levels. Programme refinement to encourage GPs to use community supports more effectively will guide future development. Further research on long term patient adherence through a multisectorial approach is warranted.
Effect of prolonged exercise in a hypoxic environment on cardiac function and cardiac troponin T: Table 1
Authors: Shave R E , Dawson E , Whyte G , George K , Gaze D , Collinson P
Published: January 2004

Background:Exercise induced cardiac fatigue has recently been observed after prolonged exercise. A moderate to high altitude has been suggested as a possible stimulus in the genesis of such cardiac fatigue.

Objective:To investigate if exercise induced cardiac fatigue and or cardiac damage occurs after prolonged exercise in a hypoxic environment.

Methods:Eight trained male triathletes volunteered for the study. Each completed two 50 mile cycle trials, randomly assigned from normobaric normoxia and normobaric hypoxia (15% Fio2). Echocardiographic assessment and whole blood collection was completed before, immediately after, and 24 hours after exercise. Left ventricular systolic and diastolic functional variables were calculated, and serum was analysed for cardiac troponin T. Results were analysed using a two way repeated measures analysis of variance, with α set at 0.05.

Results:No significant differences were observed in either systolic or diastolic function across time or between trials. Cardiac troponin T was detected in one subject immediately after exercise in the normobaric hypoxic trial.

Conclusions:A 50 mile cycle trial in either normobaric normoxia or normobaric hypoxia does not induce exercise induced cardiac fatigue. Some people, however, may exhibit minimal cardiac damage after exercise in normobaric hypoxia. The clinical significance of this is yet to be elucidated.
Effect of ultramarathon cycling on the heart rate in elite cyclists
Authors: Neumayr G , Pfister R , Mitterbauer G , Maurer A , Hoertnagl H
Published: January 2004

Objectives:To analyse the heart rate (HR) response and estimate the ultraendurance threshold—the optimum maintainable exercise intensity of ultraendurance cycling—in ultraendurance elite cyclists competing in the Race across the Alps.

Methods:HR monitoring was performed in 10 male elite cyclists during the first Race across the Alps in 2001 (distance: 525 km; cumulative altitude difference: 12 600 m) to investigate the exercise intensity of a cycle ultramarathon and the cardiopulmonary strains involved. Four different exercise intensities were defined as percentages of maximal HR (HRmax) as follows: recovery HR (HRre), <70% of HRmax; moderate aerobic HR (HRma), 70–80%; intense aerobic HR (HRia), 80–90%; and high intensity HR (HRhi), >90%.

Results:All athletes investigated finished the competition. The mean racing time was 27 hours and 25 minutes, and the average speed was 18.6 km/h. The mean HRmaxwas 186 beats/min, and the average value of measured HRs (HRaverage) was 126 beats/min resulting in a mean HRaverage/HRmaxratio of 0.68, which probably corresponds to the ultraendurance threshold. The athletes spent 53% (14 hours 32 minutes) of total race time within HRre, 25% (6 hours 51 minutes) within HRma, 19% (5 hours 13 minutes) within HRia, and only 3% (49 minutes) within HRhi, which shows the exercise intensity to be predominantly moderate (HRre+ HRma =  78% or 21 hours 23 minutes). The HR response was influenced by the course profile as well as the duration. In all subjects, exercise intensity declined significantly during the race, as indicated by a decrease in HRaverage/HRmaxof 23% from 0.86 at the start to 0.66 at the end.

Conclusions:A substantial decrease (10% every 10 hours) in the HR response is a general cardiovascular feature of ultramarathon cycling, suggesting that the ultraendurance threshold lies at about 70% of HRmaxin elite ultramarathon cyclists.
A comparison of the sports safety policies and practices of community sports clubs during training and competition in northern Sydney, Australia
Authors: Donaldson A , Forero R , Finch C F , Hill T
Published: January 2004

Objectives:To compare the safety policies and practices reported to be adopted during training and competition by community sports clubs in northern Sydney, Australia.

Methods:This cross sectional study involved face to face interviews, using an 81 item extensively validated questionnaire, with representatives of 163 community netball, rugby league, rugby union, and soccer clubs (response rate 85%). The study was undertaken during the winter sports season of 2000. Two separate 14 item scales were developed to analyse the level of safety policy adoption and safety practice implementation during training and competition. The statistical analysis comprised descriptive and inferential analysis stratified by sport.

Results:The reliability of the scales was good: Cronbach’s α  =  0.70 (competition scale) to 0.81 (training scale). Significant differences were found between the safety scores for training and competition for all clubs (mean difference 11.2; 95% confidence interval (CI) 10.0 to 12.5) and for each of the four sports: netball (mean difference 14.9; 95% CI 12.6 to 17.2); rugby league (mean difference 10.3; 95% CI 7.1 to 13.6); rugby union (mean difference 9.4; 95% CI 7.1 to 11.7); and soccer (mean difference 8.4; 95% CI 6.5 to 10.3).

Conclusions:The differences in the mean competition and training safety scores were significant for all sports. This indicates that safety policies were less often adopted and practices less often implemented during training than during competition. As injuries do occur at training, and sports participants often spend considerably more time training than competing, sporting bodies should consider whether the safety policies and practices adopted and implemented at training are adequate.
Evaluation of isokinetic force production and associated muscle activity in the scapular rotators during a protraction-retraction movement in overhead athletes with impingement symptoms
Authors: Cools A M , Witvrouw E E , Declercq G A , Vanderstraeten G G , Cambier D C
Published: January 2004

Objectives:To determine if the muscle force and electromyographic activity in the scapular rotators of overhead athletes with impingement symptoms showed differences between the injured and non-injured sides.

Methods:Isokinetic peak force was evaluated during protraction and retraction of the shoulder girdle, with simultaneous recording of electromyographic activity of the three trapezius muscles and the serratus anterior muscle, in 19 overhead athletes with impingement symptoms.

Results:Pairedttests showed significantly lower peak force during isokinetic protraction at high velocity (p<0.05), a significantly lower protraction/retraction ratio (p<0.01), and significantly lower electromyographic activity in the lower trapezius muscle during isokinetic retraction on the injured side than on the non-injured side (p<0.05).

Conclusion:These results confirm that patients with impingement symptoms show abnormal muscle performance at the scapulothoracic joint.
Blood boosting
Authors: Leigh-Smith S
Published: January 2004

This article reviews the history, technique, effects, side effects, and detection of blood boosting. It also considers whether or not this particular performance enhancement technique is a thing of the past or a continuing form of abuse among athletes.
THE ASYMPTOTIC STRUCTURE OF THE HODGKIN–HUXLEY EQUATIONS
Authors: SUCKLEY REBECCA , BIKTASHEV VADIM N.
Published: January 2004

We analyze the asymptotic structure of the Hodgkin–Huxley system of equations, in terms of the concepts of slow manifold and fast foliation, based on Tikhonov's theorem on asymptotics of solutions of slow–fast systems of differential equations. We test Zeeman's conjecture that the jump onset–slow return structure of the action potential in realistic equations of biological excitability may be due to a cusp singularity of the slow manifold with respect to the fast foliation. We find that although the cusp singularity can appear in such equations, the characteristic features in question cannot be reproduced within the Tikhonov scheme and require development of different asymptotic approaches.
A SIMPLE MODEL FOR INTERACTION OF VOLTAGE AND CALCIUM DYNAMICS IN VIRTUAL VENTRICULAR TISSUE
Authors: ASLANIDI O. V. , HOLDEN A. V.
Published: January 2004

A simple two-variable model is used to replace the formulation of calcium dynamics in the Luo–Rudy ventricular cell model. Virtual ventricular cell and tissue are developed and validated to reproduce restitution properties and calcium-dependent voltage patterns present in the original model. Basic interactions between the membrane potential and Ca 2+ dynamics in the virtual cell and a strand of the virtual tissue are studied. Intracellular calcium waves can be linked to both action potentials (APs) and delayed afterdepolarizations (DADs). An intracellular calcium wave propagating from the cell interior can induce an AP upon reaching the cell membrane. The voltage and the intracellular Ca 2+ patterns within the same cell can be highly desynchronized. In a one-dimensional strand of the virtual tissue calcium motion is driven by the AP propagation. However, calcium release can be induced upon certain conditions (e.g. Na + overload of the cells), which results in DADs propagating in the wake of AP. Such propagating DADs can reach the excitation threshold, generating a pair of extrasystolic APs. Collision of a propagating AP with a site of elevated intracellular Ca 2+ concentration does not affect the propagation under the normal conditions. Under Na + overload local elevation of the intracellular Ca 2+ leads to generation of an extrasystolic AP, which destroys the original propagating AP.
PROGRESS AND GAPS IN UNDERSTANDING THE ELECTROPHYSIOLOGICAL PROPERTIES OF MORPHOLOGICALLY NORMAL CELLS FROM THE CARDIAC ATRIOVENTRICULAR NODE
Authors: HANCOX JULES C. , YUILL KATHRYN H. , MITCHESON JOHN S. , CONVERY MARY. K.
Published: January 2004

The atrioventricular node (AVN) is a small but critically important component of the cardiac electrical conduction system and is located at the junction between right atrium and right ventricle of the heart. It plays important roles in normal and abnormal impulse propagation. Mathematical models of the conduction properties of the AVN have been made, but detailed in silico reconstruction of AVN electrophysiology lags behind that of other cardiac regions. One important facet of detailed reconstruction of AVN electrical activity is the development of comprehensive, ionic conductance-based models of single cell electrophysiology. With a view to facilitating the construction of such models, this article reviews progress made regarding single AVN cell electrophysiological data during the last decade, predominantly focusing on that derived from morphologically normal AVN cells. Properties and potential roles of a range of currents are discussed: including L-type and T-type calcium currents (ICa,Land ICa,Trespectively), background current, hyperpolarization-activated current (If), delayed and transient outward potassium currents (IKrand Ito, respectively), sodium–calcium exchanger current (INaCa), the sustained inward current (Ist) and acetylcholine-activated potassium current (IKACh).
ELECTRICAL ACTIVITY AND REENTRY DURING ACUTE REGIONAL MYOCARDIAL ISCHEMIA: INSIGHTS FROM SIMULATIONS
Authors: FERRERO JOSÉ M. , TRÉNOR BEATRIZ , RODRÍGUEZ BLANCA , SÁIZ JAVIER
Published: January 2004

In this work, the authors use computer modeling to theoretically investigate the mechanisms involved in figure-of-eight reentry during acute regional myocardial ischemia, a pattern of excitation which may lead to ventricular fibrillation and sudden cardiac death. For this purpose, a modified version of the Luo–Rudy dynamic model for the action potential and ionic currents has been used, together with a two-dimensional model of the regionally ischemic ventricle. The virtual tissue comprises several realistically dimensioned and located transitional border zones for hyperkalemia, hypoxia and acidosis, simulating the substrate heterogeneity created by acute ischemia. Different types of patterns of excitation following the delivery of a premature stimulus were obtained, including figure-of-eight reentry. Action potentials and selected ionic currents which explain the reentry process are analyzed. The effect of the degree of ATP-sensitive current activation in the vulnerability to reentry is also studied. The results are in accordance with experimental observations, and demonstrate the ability of second-generation mathematical models to analyze and explain the mechanisms involved in ischemic reentry.
NEW DEVELOPMENTS IN AN ANATOMICAL FRAMEWORK FOR MODELING CARDIAC ISCHEMIA
Authors: SMITH NICOLAS , STEVENS CAREY , PULLAN ANDREW , HUNTER PETER , MULQUINEY PETER
Published: January 2004

A new, anatomically accurate, mathematical model of the right and left porcine ventricular myocardium is described based on measurements of the geometry and fibrous-sheet structure. Passive and active properties of the myocardium are calculated using an orthotropic constitutive law based on the fibrous-sheet structure and a biophysical cellular based model of cardiac contraction. Using Galerkin finite element techniques, the equations of finite deformation are solved to determine deformation and regional wall stress through the heart cycle. The mechanics model is coupled via myocardial wall stress, to a one-dimensional coronary blood flow model embedded in the myocardium. Bidomain electrical activation of the myocardium is also modeled, with ionic current based electrophysiological equations and reaction–diffusion equations based on orthotropic conductivity tensors referred to the fibrous-sheet material axes. Metabolic models are used to couple energy supply to contraction and excitation in the heart, and at the body surface, a framework for quantifying the effect of ischemic heart disease is developed.
CELLULAR OPEN RESOURCE (COR): A PUBLIC CELLML BASED ENVIRONMENT FOR MODELING BIOLOGICAL FUNCTION
Authors: GARNY A. , KOHL P. , NOBLE D.
Published: January 2004

Computer models have proved to be invaluable to experimental and theoretical research in cardiac electrophysiology, and several environments have been designed to aim at easing the development of such models. Recent advances in computing have made the development of graphical user interfaces much simpler, which has lead to the arrival of new software packages that are used primarily for teaching, not research.

Here, we introduce Cellular Open Resource (COR), a modeling environment that runs under Microsoft Windows® and that can be used for both research and teaching. It is built around CellMLTM, which provides COR with "out of the box" access to an increasing database of single cell models. Though COR was initially developed to handle cardiac modeling, it may also be applied to other types of reaction (and diffusion) problems. The interface is designed with user friendliness in mind (e.g. an equation viewer can be used to graphically visualize an equation as it would appear in a publication). All the cell models used in a single cell or a multicellular problem are checked for correctness, before being dynamically compiled and converted into machine code, in order to maximize computing efficiency. The computation of a problem is both interactive and event-driven. One can, thus, modify the properties of a problem at any point in time and/or subject to a particular condition.

COR is freely available for academic use from .
DYNAMICS OF CARDIAC INTRACELLULARCa2+HANDLING — FROM EXPERIMENTS TO VIRTUAL CELLS
Authors: ZHANG H. , NOBLE D. , CANNELL M. , ORCHARD C. H. , LANCASTER M. , JONES S. A. , BOYETT M. R. , HOLDEN A. V. , SALEET JAFRI M. , SOBIE E. A. , LEDERER W. J. , DEMIR S. S. , MICHAILOVA A. , DELPRINCIPE F. , EGGER M. , NIGGLI E. , SMITH G. L. , LOUGHREY C. M. , MACQUAIDE N. , DEMPSTER J. , TRAFFORD A. W.
Published: January 2004

Intracellular cardiac Ca2+handling involves interactions of numerous distinct cellular and macromolecular structures. Such interactions coordinate the complicated behaviors of individual processes into spatial and temporal coherent patterns of Ca2+sparks, oscillations and traveling waves. Understanding the dynamical behaviors and functional roles of intracellular Ca2+handling requires not only detailed experimental information about subcellular structures and dynamics, but also useful tools to integrate and analyze this information. This requires interactions between experimental and simulation results within a virtual cell.
MODELING OF PROTEIN INTERACTIONS INVOLVED IN CARDIAC TENSION DEVELOPMENT
Authors: SACHSE F. B. , GLÄNZEL K. G. , SEEMANN G.
Published: January 2004

Modeling of protein interactions responsible for cardiac tension development can enhance the understanding of physiological and pathophysiological phenomena of the heart. Principal components of muscular tension development are the proteins actin, myosin, troponin and tropomyosin. The tension is produced by cross-bridge cycling of actin and myosin using adenosine triphosphate as energy source. The cross-bridge cycling is initiated by binding of intracellular calcium to troponin, resulting in configuration changes of tropomyosin.

In this work a hybrid model of protein interactions in cardiac tension development is derived on basis of recent measurements and descriptions on protein level. Dependencies on intracellular calcium concentration, sarcomere stretch and stretch velocity as well as cooperativity mechanisms are incorporated. The model quantifies the tension development by states associated to configurations of the involved proteins. The model enables in conjunction with electrophysiological models of cardiac myocytes the reconstruction of electro-mechanical phenomena. Numerical simulations with the hybrid model were performed, which illustrated the reconstruction of steady state and length switches experiments. The steady state experiments describe the force-cytosolic [ Ca2+] relationship in intact rat cardiac trabeculae. The length switch experiments provide data on the redevelopment of force after sudden stretch in rabbit right ventricular papillary muscles. Results of the numerical simulations show quantitative agreement with experimental studies.

The hybrid model of cardiac tension development offers interfaces to further models of cardiac electro-mechanics. The hybrid model can be coupled with models of cellular electrophysiology and passive mechanics of myocardium allowing the inclusion of mechano-electrical feedback mechanisms. The hybrid model can be applied to elucidate cooperativity mechanisms, pathophysiological changes and metabolism of tension development.
ONSET AND TERMINATION OF REENTRANT EXCITATION IN HOMOGENEOUS HUMAN VIRTUAL ATRIAL TISSUE
Authors: ZHANG H. , GARRAT C. J. , HOLDEN A. V.
Published: January 2004

Multicellular models of homogeneous and isotropic human atria have been developed by incorporating cellular models of membrane electrical activity of single human atrial myocyte into a parabolic partial differential equation. These models are used to study the rate dependent conduction velocity of excitation wave, vulnerability of tissue to reentry and dynamical behaviors of reentry. Bidomain models were also developed to study the actions of a large and brief external electrical stimulus on wave propagation in human atria. These studies provide basic insights to understand the onset and termination of atrial arrhythmias in the human heart.
ANATOMY OF THE ATRIOVENTRICULAR NODE AND ATRIOVENTRICULAR CONDUCTION SYSTEM
Authors: HO SIEW YEN , McCARTHY KAREN P. , ANSARI ANEEL , THOMAS PENNY S. , SÁNCHEZ-QUINTANA DAMIAN
Published: January 2004

The anatomy of the atrioventricular conduction system was first described nearly a hundred years ago. Since then, it has been an occasional subject of controversy mainly through a lack of adherence to the original definitions based on histology. The gross landmarks for locating the atrial component of the conduction system are found in the right atrium. The components and structure of the system in human are comparable to that found in commonly used laboratory animals. The conduction system is composed of specialized myocytes. Its atrial component, the atrioventricular node, is in contact with atrial myocardium. Having penetrated the atrioventricular insulating plane, the major ventricular bundles are encased in fibrous sheaths that separate the specialized myocytes from the ordinary myocardium. Only at the distal ramifications of the bundle branches do the fibrous sheaths disappear, allowing continuity with ventricular myocardium. Being the only muscular pathway connecting atrial with ventricular myocardium, knowledge of its structure can help in developing potential therapies for some forms of cardiac arrhythmias.
DYNAMICS AND INTERACTION OF FILAMENTS DURING REENTRY AND FIBRILLATION IN MAMMALIAN VIRTUAL VENTRICULAR TISSUE
Authors: CLAYTON R. H. , HOLDEN A. V.
Published: January 2004

Despite its importance, the mechanisms underlying ventricular fibrillation (VF) remain poorly understood. Experimental studies are mainly limited to the study of activity on the heart surface, yet the reentrant mechanisms believed to sustain VF are three-dimensional. In 3D, a reentrant wave rotates around a linear filament, and the properties of these filaments can contribute to the stability of reentry. In this paper we describe how filaments can be detected during reentry in cardiac virtual tissues, and how their length and twist can be extracted from numerical results. As an example, we present some results showing the behavior of reentry in 3D virtual tissues with both idealized slab geometry, and anatomically detailed right ventricular (RV) free wall geometry derived from the Auckland canine ventricles. In the slab geometry reentry was stable even with 120° rotational anisotropy, but with a transmural gradient of action potential duration (APD) to mimic the normal distribution of APD across the ventricular wall, reentry became unstable. In contrast, we found that reentry in the anisotropic RV free wall with uniform APD was unstable, possibly because the rotation of fibers is not uniform as in the slab geometry. With transmural differences in APD the instability was augmented, indicating that transmural differences can act synergistically with other mechanisms to increase instability of reentry. This study shows that numerical models can generate information about the behavior of filaments during reentry in 3D, and highlights the need for a theoretical foundation to explain this observed behavior.
DISSIPATION OF THE EXCITATION FRONT AS A MECHANISM OF SELF-TERMINATING ARRHYTHMIAS
Authors: BIKTASHEVA I. V. , BIKTASHEV V. N. , DAWES W. N. , HOLDEN A. V. , SAUMAREZ R. C. , SAVILL A. M.
Published: January 2004

The dissipation of the excitation wavefronts is a specific mechanism of propagation failure if the sharp gradient of the transmembrane voltage at the wavefront smears out and spread of voltage becomes diffusive, as the main excitation current becomes inactivated. This is produced by the normal kinetics of the ionic currents underlying the action potential. Here we demonstrate that the dissipation of the excitation wavefront can cause arrhythmia as well as lead to its self-termination. We use Courtemanche et al. model of human atrial action potential to demonstrate how reentry creates dynamic electrophysiologic inhomogeneity of the tissue. Local dissipation of the excitation front causes wave breaks and instantaneous displacement of the tip of the reentry, and the same mechanism can lead to elimination of all wavelets, as the inhomogeneity creates conditions for simultaneous dissipation of their excitation fronts.
CONSTRUCTIVE VOLUME GEOMETRY APPLIED TO VISUALIZATION OF CARDIAC ANATOMY AND ELECTROPHYSIOLOGY
Authors: CHEN MIN , TUCKER JOHN V. , CLAYTON RICHARD H. , HOLDEN ARUN V.
Published: January 2004

The complete virtual engineering of the entire beating heart requires the support of sophisticated visualization tools that can effectively extract meaningful visual information from a combination of data sets and depict complex spatial, temporal and physical properties encoded in the data sets. Traditional surface graphics techniques have not been able to serve such requirements adequately, largely because of the necessity of transforming field-based data sets (simulated computationally and recorded clinically or experimentally) to surface representations, and because of the information loss during the process. Constructive volume geometry is a new computer graphics technique, and like the virtual tissue engineering of the beating heart, it employs field-based data types as its intrinsic primitives. In this paper, we describe the application of constructive volume geometry to the visualization of cardiac anatomy and electrophysiology. We demonstrate the capability of this technique in generating visualizations that can depict multiple structures in a combination of data sets and heterogenous interior structure through effective use of opacity and combinational operators. This is applied to tracking and mapping intramural bundles of muscle fibres, and to map voltage isosurfaces of reentrant activity, and the filaments around which reentrant waves propagate.
A SIMPLIFIED MODEL OF PROPAGATION AND DISSIPATION OF EXCITATION FRONTS
Authors: BIKTASHEV V. N.
Published: January 2004

An excitation wave in nerve or cardiac tissue may fail to propagate if the temporal gradient of the transmembrane voltage at the front becomes too small to excite the tissue ahead of it. A simplified mathematical model is suggested, that reproduces this phenomenon and has exact traveling front solutions. The spectrum of possible propagation speeds is bounded from below. This causes a front to dissipate if it is not allowed to propagate quickly enough. A crucial role is played by the Na inactivation gates, even if their dynamics are by an order of magnitude slower than the dynamics of the voltage.