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Propagating Wave (propagating + wave)
Selected AbstractsEntrainment by an Extracellular AC Stimulus in a Computational Model of Cardiac TissueJOURNAL OF CARDIOVASCULAR ELECTROPHYSIOLOGY, Issue 10 2001JASON M. MEUNIER B.S. Sinusoidal Stimulation of Cardiac Sheet.Introduction: Cardiac tissue can be entrained when subjected to sinusoidal stimuli, often responding with action potentials sustained for the duration of the stimulus. To investigate mechanisms responsible for both entrainment and extended action potential duration, computer simulations of a two-dimensional grid of cardiac cells subjected to sinusoidal extracellular stimulation were performed. Methods and Results: The tissue is represented as a bidomain with unequal anisotropy ratios. Cardiac membrane dynamics are governed by a modified Beeler-Reuter model. The stimulus, delivered by a bipolar electrode, has a duration of 750 to 1,000 msec, an amplitude range of 800 to 3,200 ,A/cm, and a frequency range of 10 to 60 Hz. The applied stimuli create virtual electrode polarization (VEP) throughout the sheet. The simulations demonstrate that periodic extracellular stimulation results in entrainment of the tissue. This phase-locking of the membrane potential to the stimulus is dependent on the location in the sheet and the magnitude of the stimulus. Near the electrodes, the oscillations are 1:1 or 1:2 phase-locked; at the middle of the sheet, the oscillations are 1:2 or 1:4 phase-locked and occur on the extended plateau of an action potential. The 1:2 behavior near the electrodes is due to periodic change in the voltage gradient between VEP of opposite polarity; at the middle of the sheet, it is due to spread of electrotonic current following the collision of a propagating wave with refractory tissue. Conclusion: The simulations suggest that formation of VEP in cardiac tissue subjected to periodic extracellular stimulation is of paramount importance to tissue entrainment and formation of an extended oscillatory action potential plateau. [source] Androgenic suppression of spreading depression in familial hemiplegic migraine type 1 mutant mice,ANNALS OF NEUROLOGY, Issue 4 2009Katharina Eikermann-Haerter MD Familial hemiplegic migraine type 1 (FHM1), a severe migraine with aura variant, is caused by mutations in the CACNA1A gene. Mutant mice carrying the FHM1 R192Q mutation exhibit increased propensity for cortical spreading depression (CSD), a propagating wave of neuroglial depolarization implicated in migraine aura. The CSD phenotype is stronger in female R192Q mutants and diminishes after ovariectomy. Here, we show that orchiectomy reciprocally increases CSD susceptibility in R192Q mutant mice. Chronic testosterone replacement restores CSD susceptibility by an androgen receptor-dependent mechanism. Hence, androgens modulate genetically-enhanced CSD susceptibility and may provide a novel prophylactic target for migraine. Ann Neurol 2009;66:564,568 [source] Generation and propagation of gastric slow wavesCLINICAL AND EXPERIMENTAL PHARMACOLOGY AND PHYSIOLOGY, Issue 4 2010Dirk F Van Helden Summary 1. Mechanisms underlying the generation and propagation of gastrointestinal slow wave depolarizations have long been controversial. The present review aims to collate present knowledge on this subject with specific reference to slow waves in gastric smooth muscle. 2. At present, there is strong agreement that interstitial cells of Cajal (ICC) are the pacemaker cells that generate slow waves. What has been less clear is the relative role of primary types of ICC, including the network in the myenteric plexus (ICC-MY) and the intramuscular network (ICC-IM). It is concluded that both ICC-MY and ICC-IM are likely to serve a major role in slow wave generation and propagation. 3. There has been long-standing controversy as to how slow waves ,propagate' circumferentially and down the gastrointestinal tract. Two mechanisms have been proposed, one being action potential (AP)-like conduction and the other phase wave-based ,propagation' resulting from an interaction of coupled oscillators. Studies made on single bundle gastric strips indicate that both mechanisms apply with relative dominance depending on conditions; the phase wave mechanism is dominant under circumstances of rhythmically generating slow waves and the AP-like propagation is dominant when the system is perturbed. 4. The phase wave mechanism (termed Ca2+ phase wave) uses cyclical Ca2+ release as the oscillator, with coupling between oscillators mediated by several factors, including: (i) store-induced depolarization; (ii) resultant electrical current flow/depolarization through the pacemaker cell network; and (iii) depolarization-induced increase in excitability of downstream Ca2+ stores. An analogy is provided by pendulums in an array coupled together by a network of springs. These, when randomly activated, entrain to swing at the same frequency but with a relative delay along the row giving the impression of a propagating wave. 5. The AP-like mechanism (termed voltage-accelerated Ca2+ wave) propagates sequentially like a conducting AP. However, it is different in that it depends on regenerative store Ca2+ release and resultant depolarization rather than regenerative activation of voltage-dependent channels in the cell membrane. 6. The applicability of these mechanisms to describing propagation in large intact gastrointestinal tissues, where voltage-dependent Ca2+ entry is also likely to be functional, is discussed. [source] The patterns of spontaneous Ca2+ signals generated by ventral spinal neurons in vitro show time-dependent refinementEUROPEAN JOURNAL OF NEUROSCIENCE, Issue 8 2009Sara Sibilla Abstract Embryonic spinal neurons maintained in organotypic slice culture are known to mimic certain maturation-dependent signalling changes. With such a model we investigated, in embryonic mouse spinal segments, the age-dependent spatio-temporal control of intracellular Ca2+ signalling generated by neuronal populations in ventral circuits and its relation with electrical activity. We used Ca2+ imaging to monitor areas located within the ventral spinal horn at 1 and 2 weeks of in vitro growth. Primitive patterns of spontaneous neuronal Ca2+ transients (detected at 1 week) were typically synchronous. Remarkably, such transients originated from widespread propagating waves that became organized into large-scale rhythmic bursts. These activities were associated with the generation of synaptically mediated inward currents under whole-cell patch-clamp. Such patterns disappeared during longer culture of spinal segments: at 2 weeks in culture, only a subset of ventral neurons displayed spontaneous, asynchronous and repetitive Ca2+ oscillations dissociated from background synaptic activity. We observed that the emergence of oscillations was a restricted phenomenon arising together with the transformation of ventral network electrophysiological bursting into asynchronous synaptic discharges. This change was accompanied by the appearance of discrete calbindin immunoreactivity against an unchanged background of calretinin-positive cells. It is attractive to assume that periodic oscillations of Ca2+ confer a summative ability to these cells to shape the plasticity of local circuits through different changes (phasic or tonic) in intracellular Ca2+. [source] Wavelet analysis and the governing dynamics of a large-amplitude mesoscale gravity-wave event along the East Coast of the United StatesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 577 2001Fuqing Zhang Abstract Detailed diagnostic analyses are performed upon a mesoscale numerical simulation of a well-observed gravity-wave event that occurred on 4 January 1994 along the East Coast of the United States. The value of using wavelet analysis to investigate the evolving gravity-wave structure and of using potential vorticity (PV) inversion to study the nature of the flow imbalance in the wave generation region is demonstrated. The cross-stream Lagrangian Rossby number, the residual in the nonlinear balance equation, and the unbalanced geopotential-height field obtained from PV inversion are each evaluated for their usefulness in diagnosing the flow imbalance. All of these fields showed clear evidence of strong imbalance associated with a middle-to-upper tropospheric jet streak, and tropopause fold upstream of the large-amplitude gravity wave several hours before the wave became apparent at the surface. Analysis indicates that a train of gravity waves was continuously generated by geostrophic adjustment in the exit region of the unbalanced upper-level jet streak as it approached the inflection axis in the height field immediately downstream of the maximum imbalance associated with the tropopause fold. A split front in the middle troposphere, characterized by the advance of the dry conveyor belt above the warm front, was overtaken by one of these propagating waves. During this merger process, a resonant interaction resulted, which promoted the rapid amplification and scale contraction of both the incipient wave (nonlinear wave development) and the split front (frontogenesis). The gravity wave and front aloft became inseparable following this merger. The situation became even more complex within a few hours as the vertical motion enhanced by this front-wave interaction acted upon a saturated, potentially unstable layer to produce elevated moist convection. An analysis of the temporal changes in the vertical profile of wave energy flux suggests that moist convective downdraughts efficiently transported the wave energy from the midlevels downward beneath the warm-front surface, where the wave became ducted. However, pure ducting was not sufficient for maintaining and amplifying the waves; rather, wave-CISK (Conditional Instability of the Second Kind) was crucial. This complex sequence of nonlinear interactions produced a long-lived, large-amplitude gravity wave that created hazardous winter weather and disrupted society over a broad and highly populated area. Although gravity waves with similar appearance to this large-amplitude wave of depression occasionally have been seen in other strong cyclogenesis cases involving a jet streak ahead of the upper-level trough axis, it is unknown whether other such events share this same sequence of interactions. [source] | ||||||||||