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Topic: Excitable medium

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	Definition of Excitable medium
		An excitable medium is a nonlinear dynamical system which has the capacity to propagate a wave of some description, and which cannot support the passing of another wave until a certain amount of time has passed (known as the refractory time).
		A forest is an example of an excitable medium: if a wildfire burns through the forest, no fire can return to a burnt spot until the vegetation has gone through its refractory period and regrown.
		Each cell of the automaton is made to represent some section of the medium (for example, a patch of trees in a forest, or stress in heart tissue).
	www.wordiq.com /definition/Excitable_medium (687 words)

	Medium - Wikipedia, the free encyclopedia
		In astronomy, the interplanetary medium or the interstellar medium.
		The medium of instruction is the language used to teach in.
		In parapsychology, a spiritual medium is a person who claims to serve as an intermediary between the living and the dead.
	en.wikipedia.org /wiki/Medium (435 words)

	Excitable medium - Wikipedia, the free encyclopedia
		An excitable medium is a nonlinear dynamical system which has the capacity to propagate a wave of some description, and which cannot support the passing of another wave until a certain amount of time has passed (known as the refractory time).
		A forest is an example of an excitable medium: if a wildfire burns through the forest, no fire can return to a burnt spot until the vegetation has gone through its refractory period and regrown.
		A group of spectators at a sporting event are an excitable medium, as can be observed in a Mexican wave (so-called from its initial appearance in the 1986 World Cup in Mexico).
	en.wikipedia.org /wiki/Excitable_medium (657 words)

	1. General Introduction
		An excitable medium is a medium which has two main properties, namely the ability to conduct pulses of excitation, and the ability to recover its properties after some period of time, called the refractory period.
		Excitable media can be regarded as a generic concept for pattern formation: it consists of a whole class of systems, which have to possess these few basic properties in order to produce the same common generic behaviour.
		(1993) showed that in a tube-shaped excitable medium with decreasing excitability (which is a caricature for a slug), a twisted scroll wave in the highly excitable prestalk zone breaks up into plane waves in the less-excitable prespore zone.
	www-binf.bio.uu.nl /stan/Thesis/Thesis/node2.htm (8151 words)

	Excitable medium (Site not responding. Last check: 2007-09-05)
		An excitable medium is a nonlinear dynamical system which has the capacity to propagate a wave of some description, andwhich cannot support the passing of another wave until a certain amount of time has passed (known as the refractory time).
		A forest is an example of an excitable medium: if a wildfire burns through theforest, no fire can return to a burnt spot until the vegetation has gone through its refractory period and regrown.
		Each cell of the automaton is made torepresent some section of the medium (for example, a patch of trees in a forest, or stress in heart tissue).
	www.therfcc.org /excitable-medium-124410.html (601 words)

	Excitable Media
		Excitable media are spatially distributed systems which have the ability to propagate signals without damping.
		As each element undergoes an excursion from steady state, it causes its neighbors to move over threshold at a rate determined by the diffusion coefficient (a `passive' property of the media), and the rate of rise of the diffused species of the excited element (a `active' property of the media).
		The propagation of electrical activity in cardiac muscle involves the interaction of different ion species across a combination of active and passive ion channels and diffusion of charge through a heterogeneous substrate with dynamically changing conductances.
	www.cnd.mcgill.ca /bios/bub/excitablemain.html (288 words)

	The Cardiac Vulnerable Period, Spiral Waves, Monomorphic and Polymorphic ECGs, Ventricular Fibrillation and the Guarded ... (Site not responding. Last check: 2007-09-05)
		a region of an excitable medium where excitation results in a wave the propagates in some directions and fails to propagate in other directions, is poorly understood.
		All reveal the essential properties of an excitable cell: a threshold of excitability, a refractory period, a vulnerable period and propagation.
		The vulnerable period is easily understood by recognizing that there is a "critical" point in the recovery of cellular excitability that separates the "excitable" state from the "refractory" state.
	monitor.admin.musc.edu /~cfs/sample.html (4094 words)

	Gaseous laser medium and means for excitation - Patent 4075579 (Site not responding. Last check: 2007-09-05)
		The laser medium is added in quantity sufficient to absorb the vacuum ultraviolet radiation efficiently from the noble gas before the radiation strikes the walls of the confining chamber and is wasted.
		The partial pressure of the laser medium should be from about 0.01 to 1% of the partial pressure of the noble gas and may be from 0.5 to 10 torr, preferably 1 to 5 torr.
		In accordance with this aspect of the present invention, a laser medium is selected which, upon photodissociation, produces a particle which requires, for excitation to the desired upper laser state, photon energy of vacuum ultraviolet radiation above the binding energy of the molecules of the laser medium and their photofragment excitation energy.
	www.freepatentsonline.com /4075579.html (4674 words)

	Excitable Media (Site not responding. Last check: 2007-09-05)
		In the basic excitable medium, a cell has three states, namely, ready, firing, and recovering; the firing of a cell is stimulated by the firing of adjacent cells.
		The precise conditions for wave front formation and subsequent wave propagation in an excitable medium are critical for understanding the genesis and possible control of reentrant (spiral wave) cardiac arrhythmias.
		For every such wave there is a small region of vulnerability, where stimulation often results in the formation of a new front that can evolve into a spiral wave, a precursor of sudden cardiac death.
	www.gg.caltech.edu /~zhukov/research/excitable_medium/excitabl.htm (160 words)

	[No title] (Site not responding. Last check: 2007-09-05)
		The FN model shows excitable behavior when $\epsilon \ll 1$ so that there is a separation between the fast evolution of the active component, $C_1$, and the slow evolution of $C_2$, the passive one or {\sl inhibitor}.
		What is distinct of the excitable dynamics is that excitation under the closed flow is a transient, so that the system finally recovers the rest state, at variance with the bistable and autocatalytic behaviour.
		Also a consequence of the recovery behaviour that characterizes the excitable dynamics, and that distinguishes it from the otherwise rather similar bistable dynamics, is the loss of coherence occurring at large $\Da$.
	www.imedea.uib.es /physdept/publications/downfile.php?fid=2810 (8416 words)

	Java Greenberg-Hastings
		The Greenberg-Hastings (GH) Model is perhaps the simplest CA prototype for an excitable medium.
		Real world excitable media include the Belousov - Zhabotinski reaction (see also ff.) and Slime Mold.
		The brave hearted can download our research paper, Threshold-Range Scaling of Excitable Cellular Automata, for an in-depth empirical and theoretical analysis of GH and related dynamics.
	psoup.math.wisc.edu /java/jgh.html (773 words)

	RamosJI ARTICULOS Compendex (Site not responding. Last check: 2007-09-05)
		AB: The propagation of spiral waves in excitable media with locally inhomogeneous time-periodic modulations is studied numerically, and it is shown that the size of the local inhomogeneity and the amplitude and frequency of the periodic forcing play a major role in determining the spiral wave dynamics, suppression and breakup.
		In the presence of several holes and external forcing across the excitable medium, the spiral wave is suppressed and the activator exhibits a breathing behaviour characterized by fronts that propagate towards the boundaries of the excitable medium, and complex patterns when the front is located near the holes.
		AB: The propagation of spiral waves in excitable media with the Belousov-Zhabotinsky reactions in a non-solenoidal, time-independent velocity field is studied numerically as a function of the amplitude and frequency of the velocity.
	www.lcc.uma.es /~villa/GTCI/RamosJIcompendex.html (6454 words)

	A biased history of the physics of Neuro- and Cardiac Electrophysiology
		With their ideas, some derived from the theory of explosives and forest fires, Josef developed a quasi-analytic equation for the critical region in 1 d excitable medium that was remarkably useful for understanding liminal lengths, vulnerability and spiral formation.
		An interesting aspect of studies of excitable media, particularly spiral wave behavior, is that initial attention was focused on the behavior of the waves and their interaction with inexcitable boundaries.
		The path of the spiral is determined by the energy in the front and the state of the excitable medium.
	www.cs.duke.edu /~cfs/biblio.html (12603 words)

	Virtual Electrodes
		The goal of this paper is to use a simple cellular excitable medium to clarify the role of virtual electrodes and deexcitation during cardiac defibrillation and the induction of reentry.
		Elementary models of cellular excitable media (sometimes called cellular automata) provide valuable insight into the electrical behavior of the heart (Kaplan et al., 1988; Saxberg and Cohen, 1991; or see the web-based book by Weimar).
		During a cathodal stimulus, the state of the cell directly under the electrode and its four nearest neighbors in the direction perpendicular to the fibers change to the excited state, and the two remaining nearest neighbors in the direction parallel to the fibers change to the quiescent state, regardless of their previous state.
	sprojects.mmi.mcgill.ca /heart/pages/rot/rothom.html (2838 words)

	Excitable medium - Encyclopedia, History, Geography and Biography
		Excitable medium - Encyclopedia, History, Geography and Biography
		Java applets that show excitable media in 0, 1, and 2D (http://arrhythmia.hofstra.edu/java/applets.html)
		Excitable medium, Modelling excitable media, With cellular automata, Geometries of waves, One-dimensional waves, Two-dimensional waves, References and External links.
	www.arikah.net /encyclopedia/Excitable_medium (697 words)

	Calcium Waves in Agarose Gel with Cell Organelles: Implications of the Velocity Curvature Relationship -- Wussling et ...
		waves is the existence of an appropriate excitable medium.
		Due to the big size of the excitable medium, the picture was composed of several frames.
		Zykov, V. Analytical evaluation of the dependence of the speed of an excitation wave in two-dimensional excitable medium on the curvature of its front.
	www.biophysj.org /cgi/content/full/80/6/2658 (4662 words)

	Pricing Mortgage-Backed Securities and Collateralized Mortgage Obligations (Site not responding. Last check: 2007-09-05)
		An excitable medium generally refers to a medium that is capable of generating traveling waves.
		Excitable neural media are often modeled by integro-differential equations (IDEs).
		Such patterns occur in an excitable medium due to the existence of an inhomogeneous but stationary forcing.
	www.pims.math.ca /science/2003/ubc-scaims/li-10-01.html (282 words)

	Wave Propagation in an Excitable Medium: Conditions for front formation (Site not responding. Last check: 2007-09-05)
		Knowledge of the role of medium properties in modulating the vulnerable period and front formation resulting from stimulation during the VP would facilitate the design and testing of new drugs and electrical stimulators.
		It was assumed that reducing the excitability of cardiac tissue by lowering the sodium channel conductance (equivalent to reducing the amplitude of the reaction term, f(U)) would reduce the incidence of fatal cardiac arrhythmias.
		However, it was shown recently, that reducing the excitability reduces the propagation velocity, which has the undesirable effect of extending the period of vulnerability [4-10].
	www.cs.duke.edu /~cfs/wave/wave_formation.html (4193 words)

	Nonlinearity in the heart (November 1998) - Physics World - PhysicsWeb
		This knock-on effect can be represented as a lattice of coupled excitation equations, and in the continuum limit the "excitable medium" of the heart can be represented by a partial differential equation analogous to the nonlinear cable equation proposed for nerve impulse propagation by the physiologists Alan Hodgkin and Andrew Huxley in 1952.
		In a homogeneous medium the spiral can either rotate around a circular core, or the tip of the spiral can "meander" around the central core in a motion that can be biperiodic, quasi-periodic and perhaps even chaotic (figure 3).
		To terminate this continuous process, all of the excitable tissue of the heart must be driven into the same state.
	physicsweb.org /article/world/11/11/10 (3442 words)

	Reflected Waves in an Inhomogeneous Excitable Medium
		Propagation can be encumbered in an excitable cable in which intrinsic properties change abruptly.
		A sudden increase in diameter or a decrease in conductivity or excitability can lead to propagation block or delay in propagation with or without reflection.
		Finally, we argue that reflection phenomena occur more robustly when excitability is due to saddle-type threshold behavior (type I excitability in the sense of Rinzel and Ermentrout [in {Methods in Neuronal Modeling: From Synapses to Networks, C. Koch and I. Segev, eds., MIT Press, Cambridge, MA, 1989]).
	epubs.siam.org /sam-bin/dbq/article/27679 (230 words)

	2. Spiral Breakup in Excitable Tissue due to Lateral Instability
		In a simple two-component model of excitable tissue a spiral wave is found to break up into a large number of small spirals.
		We studied the dependence of the lateral instability and spiral breakup on the parameters of the excitable medium.
		We expect the phenomenon of spiral breakup by lateral instability to be not uncommon in excitable media and that it is probably easy to find comparable behaviour in other models of excitable media with non-fixed time-scales for the recovery variable.
	www-binf.bio.uu.nl /stan/Thesis/Thesis/node4.htm (2308 words)

	Control of reentry (Site not responding. Last check: 2007-09-05)
		Cardiac tissue can be idealised as an excitable medium, modelled by reaction-diffusion equations in R^2 or R^3, with anisotropy specified by a tensor of diffusion coefficents [1].
		Re-entry is then a spiral wave in R^2, or a scroll wave in R^3, and the position of meandering spiral waves can be controlled by spatially uniform perturbations at a frequency close to the rotation frequncy of the spiral.
		Such resonant drifts allows us to eliminate re-entrant sources from an excitable medium, and so may provide a means of defibrillating cardiac tissue.
	www.cbiol.leeds.ac.uk /control.html (421 words)

	Winfree seminar on optical tomography of 3D excitable media (Site not responding. Last check: 2007-09-05)
		The idea of excitability is that a small but not too small stimulus provokes big local reaction, which later fades back to original quiescent steady state attractor.
		It looks like this cut open in a computer,and like this in the chemically excitable medium cut open at the glass wall of a test-tube.
		Here it is again in the chemical medium as a completely closed ring not touching the glass wall.
	eebweb.arizona.edu /faculty/Winfree/tomog.html (3167 words)

	James D. Lechleiter, Ph.D.
		The patterns are initiated from discrete foci in Xenopus oocytes, and propagate in circular and spiral waves, suggesting that this intracellular Ca2+ signalling system functions as an excitable medium.
		Spiral pattern formations are the trademark of excitable media and have been described in other systems such as the classic Belousov-Zhabotinsky chemical reaction, aggregating slime mold, and electrical activity in neuronal tissue (see Figure).
		The fundamental property responsible for excitability is the Ca2+ dependency of the IP3-bound IP3R.
	www.uthscsa.edu /faculty/lechleit.html (515 words)

	Social behaviourMexican waves in an excitable medium : Nature (Site not responding. Last check: 2007-09-05)
		The Mexican wave, or La Ola, which rose to fame during the 1986 World Cup in Mexico, surges through the rows of spectators in a stadium as those in one section leap to their feet with their arms up, and then sit down again as the next section rises to repeat the motion.
		To interpret and quantify this collective human behaviour, we have used a variant of models that were originally developed to describe excitable media such as cardiac tissue.
		Modelling the reaction of the crowd to attempts to trigger the wave reveals how this phenomenon is stimulated, and may prove useful in controlling events that involve groups of excited people.
	www.nature.com /doifinder/10.1038/419131a (187 words)

	News in Science - Riding the Mexican Wave - 12/09/2002
		Such models are often used to describe phenomena such as a forest fire (which travels as a wave from its initiation point across the forest, regenerating with every tree it ignites), or cardiac tissue, where a current carried by sodium and potassium ions moves to neighbouring cells to generate heart contractions.
		In this case, each person in the crowd was described as an excitable unit, and the signal being passed to one another was to stand and wave.
		The Mexican Wave was the wave activity, and the trigger to begin the wave was a group of active people jumping up and down with their hands in the air.
	www.abc.net.au /science/news/stories/s673650.htm (435 words)

	List of Publications of V.N.Biktashev
		Elkin and V.N. Biktashev ``On the drift of large-core spiral waves in inhomogeneous excitable media'', in Nonlinear Phenomena in Biology, June 23-28, 1998, Institute of Cell Biophycis of R.A.S, Pushchino, Russia, p.
		V.N. Biktashev, ``Resonant drift of an autowave vortex in inhomogeneous or bounded medium'' preprint, Pushchino, 1989, 14 pp.
		V.N. Biktashev, ``Drift of a reverberator in an active medium due to interaction with boundaries'' preprint, Pushchino, 1986, 23 pp.
	www.maths.liv.ac.uk /~vadim/publ.html (2499 words)

	Encyclopedia: Excitable medium (Site not responding. Last check: 2007-09-05)
		Updated 272 days 10 hours 41 minutes ago.
		Java applets that show excitable media in 0, 1, and 2D (http://arrhythmia.hofstra.edu/java/applets.html)
		Click for other authoritative sources for this topic (summarised at Factbites.com).
	www.nationmaster.com /encyclopedia/Excitable-medium (668 words)

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