Biomedical subjects

Kyoung J Lee

Publications and source records attributed to Kyoung J Lee.

11 recordsLinked to original sources

PubMed2006

Line-defects-mediated complex-oscillatory spiral waves in a chemical system.

In this paper, we summarize our experimental observations on complex-oscillatory spiral waves that arise in a Belousov-Zhabotinsky (BZ) reaction-diffusion system. The observed wave structures generically bear line defects across which the phase of local oscillation changes by a multiple of 2 pi. The local oscillation at every spatial point along a line defect of period-2 (P-2) oscillatory media is period-1 (P-1) oscillatory. For the homogeneous BZ reaction can be excitable, simply periodic, complex periodic, or chaotic as the control parameters are tuned, a number of different complex wave states are revealed. A two-dimensional phase diagram, which includes domains of P-2 oscillatory spirals, intermittently breathing spirals, period-3 (P-3) oscillatory spirals, two different types of mixed-mode periodic spirals, and line-defect-mediated turbulence, is constructed. Several different transitions among different dynamic states are described systematically. In all cases, line defects are found to play an important role.

Journal Article↗

PubMed2005

Complex-periodic spiral waves in confluent cardiac cell cultures induced by localized inhomogeneities.

Spatiotemporal wave activities in excitable heart tissues have long been the subject of numerous studies because they underlie different forms of cardiac arrhythmias. In particular, understanding the dynamics and the instabilities of spiral waves have become very important because they can cause reentrant tachycardia and their subsequent transitions to fibrillation. Although many aspects of cardiac spiral waves have been investigated through experiments and model simulations, their complex properties are far from well understood. Here, we show that intriguing complex-periodic (such as period-2, period-3, period-4, or aperiodic) spiral wave states can arise in monolayer tissues of cardiac cell culture in vitro, and demonstrate that these different dynamic states can coexist with abrupt and spontaneous transitions among them without any change in system parameters; in other words, the medium supports multistability. Based on extensive image data analysis, we have confirmed that these spiral waves are driven by their tips tracing complex orbits whose unusual, meandering shapes are formed by delicate interplay between localized conduction blocks and nonlinear properties of the culture medium.

Action Potentials↗

PubMed2005

Dynamical heterogeneity of suprachiasmatic nucleus neurons based on regularity and determinism.

The suprachiasmatic nucleus (SCN) is known to be the master biological clock in mammals. Despite the periodic mean firing rate, interspike interval (ISI) patterns of SCN neurons are quite complex and irregular. The aim of the present study was to investigate the existence of nonlinear determinism in the complex ISI patterns of SCN neurons. ISI sequences were recorded from 173 neurons in rat hypothalamic slice preparations using a cell-attached patch recording technique. Their correlation dimensions (D2) were estimated, and were then compared with those of the randomly-shuffled surrogate data. We found that only 16 neurons (16/173) exhibited deterministic ISI patterns of spikes. In addition, clustering analysis revealed that SCN neurons could be divided into two subgroups of neurons each having distinct values of coefficient of variation (CV) and skewness (SK). Interestingly, most deterministic SCN neurons (14/16) belonged to the group of irregularly spiking neurons having large CV and SK values. To see if the neuronal coupling mediated by the gamma-aminobutyric acid (GABA), the major neurotransmitter in the SCN, contributed to the deterministic nature, we examined the effect of the GABAA receptor antagonist bicuculline on D2 values of 56 SCN neurons. 8 SCN neurons which were originally stochastic became to exhibit deterministic characteristics after the bicuculline application. This result suggests that the deterministic nature of the SCN neurons arises not from GABAergic synaptic interactions, but likely from properties inherent to neurons themselves.

Action Potentials↗

PubMed2005

Fractal stochastic modeling of spiking activity in suprachiasmatic nucleus neurons.

Individual neurons in the suprachiasmatic nucleus (SCN), the master biological clock in mammals, autonomously produce highly complex patterns of spikes. We have shown that most (approximately 90%) SCN neurons exhibit truly stochastic interspike interval (ISI) patterns. The aim of this study was to understand the stochastic nature of the firing patterns in SCN neurons by analyzing the ISI sequences of 150 SCN neurons in hypothalamic slices. Fractal analysis, using the periodogram, Fano factor, and Allan factor, revealed the presence of a 1/f-type power-law (fractal) behavior in the ISI sequences. This fractal nature was persistent after the application of the GABAA receptor antagonist bicuculline, suggesting that the fractal stochastic activity is an intrinsic property of individual SCN neurons. Based on these physiological findings, we developed a computational model for the stochastic SCN neurons to find that their stochastic spiking activity was best described by a gamma point process whose mean firing rate was modulated by a fractal binomial noise. Taken together, we suggest that SCN neurons generate temporal spiking patterns using the fractal stochastic point process.

Action Potentials↗

PubMed2004

Transverse instability of line defects of period-2 spiral waves.

Spiral waves that arise in period-2 oscillatory media extended in space generically bear "line defects" along which the local kinetics exhibits a period-1 oscillation. Locally, these defect structures can be viewed as a front separating two period-2 oscillatory domains oscillating 2pi out of phase. Here we show that their shape can become sinusoidal with a transverse instability as in bistable fronts. This instability eventually leads to a line-defect filled spatiotemporal chaotic state having erratic proliferations, annihilations, and regenerations of line defects. The same sequence of phenomena is observed in a model reaction-diffusion system as well as in an experimental system.

Biophysics↗

PubMed2004

Regular and alternant spiral waves of contractile motion on rat ventricle cell cultures.

We demonstrate that meandering as well as regular spiral waves can form in a well-controlled culture layer of rat ventricle cells and that the meandering spiral wave, in particular, can generate an alternant rhythm. These observations are made possible by a newly developed, noninvasive phase contrast macro-optics that is simple but highly effective in visualizing the contractile motion of the populations of cardiac cells.

Animals↗

PubMed2003

Spiral waves in a coupled network of sine-circle maps.

A coupled two-dimensional lattice of sine-circle maps is investigated numerically as a simple model for coupled network of nonlinear oscillators under a spatially uniform, temporally periodic, external forcing. Various patterns, including quasiperiodic spiral waves, periodic, banded spiral waves in several different polygonal shapes, and domain patterns, are observed. The banded spiral waves and domain patterns match well with the results of earlier experimental studies. Several transitions are analyzed. Among others, the source-sink transition of a quasiperiodic spiral wave and the cascade of "side-doubling" bifurcations of polygonal spiral waves are of great interest.

Journal Article↗

PubMed2003

Parametrically forced surface wave with a nonmonotonic dispersion relation.

Surface wave patterns that arise in a mechanically driven ferrofluid system under constant magnetic field are investigated (1) to find out what kind of spatial patterns emerge when the system acquires a nonmonotonic dispersion relation and (2) to compare its surface wave patterns with those produced in the magnetically driven system studied earlier. As the strength of the applied magnetic field increases, the initial subharmonic square lattice formed by the Faraday instability first transforms to rolls, then becomes a rhomboid lattice. The rolls and the rhomboid lattice are found to coexist for a finite range of parameter space forming patterns with mixed domains. Possible underlying mechanisms for the observed rhomboid lattice is discussed. None of the diverse superlattices observed in the magnetically driven ferrofluid system appears in the mechanically driven system studied here.

Journal Article↗

PubMed2002

Subharmonic bifurcations of standing wave lattices in a driven ferrofluid system.

Superlattice standing waves arising on the surface of ferrofluids that are driven by an ac magnetic field are investigated experimentally. Several different types are obtained through successive spatial period doublings, which are mediated by resonant mode interactions. The observed superlattices are quite diverse, depending on the relevant base Fourier modes, the orientation and the number of emerged subharmonic modes, and the phase difference among the involved modes all together. On the other hand, their temporal evolutions are all either period-1 (harmonic) or period-2 (subharmonic).

Journal Article↗

PubMed2002

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

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

Journal Article↗

PubMed2002

Unusual spiral wave tip trajectories in a parametrically forced nonequilibrium system.

The effect of parametric modulations on spiral waves that form in a thin layer of liquid crystal under rotating magnetic field is studied in laboratory experiments and numerical simulations. Parametric forcings that are sinusoidal in time make a simply rotating spiral tip to meander, rendering a compound tip trajectory that is composed of a main circular orbit and a satellite orbit with an unusual crescent shape. No evidence of frequency locking is found. The underlying mechanism for the unusual shape of the satellite orbit is elucidated, and the dependence of the rotation frequency of the main circular orbit f(o) on the perturbation parameters are discussed.

Journal Article↗