scholarly journals Confined variational calculation of positronium-hydrogen scattering below the positronium excitation threshold

2021 ◽  
Vol 103 (2) ◽  
Author(s):  
M.-S. Wu ◽  
J.-Y. Zhang ◽  
Y. Qian ◽  
K. Varga ◽  
U. Schwingenschlögl ◽  
...  
Keyword(s):  
Variational Calculation ◽  
Excitation Threshold

scholarly journals Peculiarities of Neurostimulation by Intense Nanosecond Pulsed Electric Fields: How to Avoid Firing in Peripheral Nerve Fibers

2021 ◽  
Vol 22 (13) ◽  
pp. 7051
Author(s):  
Vitalii Kim ◽  
Emily Gudvangen ◽  
Oleg Kondratiev ◽  
Luis Redondo ◽  
Shu Xiao ◽  
...  
Keyword(s):  
Side Effects ◽  
Electric Fields ◽  
Pulsed Electric Fields ◽  
Opposite Polarity ◽  
Nerve Fibers ◽  
High Rate ◽  
Excitation Threshold ◽  
Compound Action Potentials ◽  
Supramaximal Stimulation ◽  
The Impact

Intense pulsed electric fields (PEF) are a novel modality for the efficient and targeted ablation of tumors by electroporation. The major adverse side effects of PEF therapies are strong involuntary muscle contractions and pain. Nanosecond-range PEF (nsPEF) are less efficient at neurostimulation and can be employed to minimize such side effects. We quantified the impact of the electrode configuration, PEF strength (up to 20 kV/cm), repetition rate (up to 3 MHz), bi- and triphasic pulse shapes, and pulse duration (down to 10 ns) on eliciting compound action potentials (CAPs) in nerve fibers. The excitation thresholds for single unipolar but not bipolar stimuli followed the classic strength–duration dependence. The addition of the opposite polarity phase for nsPEF increased the excitation threshold, with symmetrical bipolar nsPEF being the least efficient. Stimulation by nsPEF bursts decreased the excitation threshold as a power function above a critical duty cycle of 0.1%. The threshold reduction was much weaker for symmetrical bipolar nsPEF. Supramaximal stimulation by high-rate nsPEF bursts elicited only a single CAP as long as the burst duration did not exceed the nerve refractory period. Such brief bursts of bipolar nsPEF could be the best choice to minimize neuromuscular stimulation in ablation therapies.

A SIMPLE MODEL FOR INTERACTION OF VOLTAGE AND CALCIUM DYNAMICS IN VIRTUAL VENTRICULAR TISSUE

2003 ◽  
Vol 13 (12) ◽  
pp. 3873-3886
Author(s):  
O. V. ASLANIDI ◽  
A. V. HOLDEN
Keyword(s):  
Intracellular Calcium ◽  
Calcium Release ◽  
Cell Model ◽  
Excitation Threshold ◽  
Calcium Dynamics ◽  
Variable Model ◽  
Ventricular Cell ◽  
Calcium Dependent ◽  
Intracellular Ca ◽  
A Site

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.

scholarly journals Stress-free state of the red blood cell membrane and the deformation of its skeleton

2012 ◽  
Vol 17 (2) ◽  
Author(s):  
Tjaša Švelc ◽  
Saša Svetina
Keyword(s):  
Blood Cell ◽  
Red Blood Cell ◽  
Variational Calculation ◽  
Lateral Distribution ◽  
Free State ◽  
Initial State ◽  
Proposed Model ◽  
Deformed State ◽  
Concentric Circles ◽  
Stress Free State

AbstractThe response of a red blood cell (RBC) to deformation depends on its membrane, a composite of a lipid bilayer and a skeleton, which is a closed, twodimensional network of spectrin tetramers as its bonds. The deformation of the skeleton and its lateral redistribution are studied in terms of the RBC resting state for a fixed geometry of the RBC, partially aspirated into a micropipette. The geometry of the RBC skeleton in its initial state is taken to be either two concentric circles, a references biconcave shape or a sphere. It is assumed that in its initial state the skeleton is distributed laterally in a homogeneous manner with its bonds either unstressed, presenting its stress-free state, or prestressed. The lateral distribution was calculated using a variational calculation. It was assumed that the spectrin tetramer bonds exhibit a linear elasticity. The results showed a significant effect of the initial skeleton geometry on its lateral distribution in the deformed state. The proposed model is used to analyze the measurements of skeleton extension ratios by the method of applying two modes of RBC micropipette aspiration.

Variational calculation of the dynamic third-order susceptibility of water

1995 ◽  
Vol 357 (3) ◽  
pp. 199-205 ◽  
Author(s):  
J.-R. Gabryl ◽  
C. Barbier ◽  
Ph. Lemaire ◽  
E.Nørby Svendsen
Keyword(s):  
Variational Calculation ◽  
Third Order ◽  
Order Susceptibility

Improved defibrillator waveform safety factor with biphasic waveforms

1983 ◽  
Vol 245 (1) ◽  
pp. H60-H65 ◽  
Author(s):  
J. L. Jones ◽  
R. E. Jones
Keyword(s):  
Safety Factor ◽  
Voltage Gradient ◽  
Excitation Threshold ◽  
Efficacy And Safety ◽  
Myocardial Cells ◽  
Safety Factors ◽  
Adult Type ◽  
Intracellular Microelectrodes ◽  
Cultured Myocardial Cells

Excitation thresholds and arrhythmias were studied in "adult-type" cultured chick embryo myocardial cells after electric field stimulation with biphasic, truncated, and rectified underdamped RLC (resistance-inductance-capacitance) type waveforms, to test the hypothesis that the negative phase of biphasic waveforms ameliorates membrane dysfunction induced by the initial positive portion. Photocell mechanograms and intracellular microelectrodes monitored extrasystoles and depolarization-induced arrhythmias. Rectifying or truncating biphasic waveforms did not alter the excitation threshold. However, shock intensities producing specific postshock arrhythmias or a specific severity of postshock prolonged depolarization differed significantly when biphasic waveforms were truncated or rectified. The voltage gradient producing a specific dysfunction was 12-14% lower for the truncated version than for the biphasic; that for the rectified version was 17-27% lower than for the biphasic version (although both contained the same energy). Safety factor, the ratio between shock intensity producing specific dysfunction and that producing excitation, was determined for each waveform. Biphasic waveforms had larger safety factors than truncated or rectified waveforms. Since safety factor, as measured in cultured myocardial cells, closely corresponds with in situ defibrillating effectiveness (14), the significantly higher safety factors of biphasic waveforms suggest that carefully shaped biphasic waveforms might improve the efficacy and safety of cardiac defibrillation procedures.

scholarly journals Tissue engineering of functional cardiac muscle: molecular, structural, and electrophysiological studies

2001 ◽  
Vol 280 (1) ◽  
pp. H168-H178 ◽  
Author(s):  
M. Papadaki ◽  
N. Bursac ◽  
R. Langer ◽  
J. Merok ◽  
G. Vunjak-Novakovic ◽  
...  
Keyword(s):  
Cardiac Muscle ◽  
Cardiac Myocytes ◽  
Conduction Velocity ◽  
Capture Rate ◽  
Signal Amplitude ◽  
Model System ◽  
Neonatal Rat ◽  
Excitation Threshold ◽  
Electrophysiological Properties

The primary aim of this study was to relate molecular and structural properties of in vitro reconstructed cardiac muscle with its electrophysiological function using an in vitro model system based on neonatal rat cardiac myocytes, three-dimensional polymeric scaffolds, and bioreactors. After 1 wk of cultivation, we found that engineered cardiac muscle contained a 120- to 160-μm-thick peripheral region with cardiac myocytes that were electrically connected through gap junctions and sustained macroscopically continuous impulse propagation over a distance of 5 mm. Molecular, structural, and electrophysiological properties were found to be interrelated and depended on specific model system parameters such as the tissue culture substrate, bioreactor, and culture medium. Native tissue and the best experimental group (engineered cardiac muscle cultivated using laminin-coated scaffolds, rotating bioreactors, and low-serum medium) were comparable with respect to the conduction velocity of propagated electrical impulses and spatial distribution of connexin43. Furthermore, the structural and electrophysiological properties of the engineered cardiac muscle, such as cellularity, conduction velocity, maximum signal amplitude, capture rate, and excitation threshold, were significantly improved compared with our previous studies.

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