Spark channel

For simplicity of the model, it is assumed that the natural convection, radiation, and ionic wind effect are ignored. The ignorance of the radiation loss from the spark channel during the discharge may be reasonable, because the radiation heat loss is found to be negligibly small in the previous studies [5,6]. The amount of heat transfer from the flame kernel to the spark electrodes, whose temperature is 300 K, is estimated by Fourier s law between the electrode surface and an adjacent cell. 

Sparks Spark discharges are most common between solid conductors, although one electrode may be a conduc tive liquid. They appear as a narrow, luminous channel, and cany a large peak current for a few microseconds or less. Sparks are the only form of discharge for which a maximum spark energy can be calculated, using the expression ... 

This is a transient discrete electric discharge which takes place between two conductors which are at different potentials, bridging the gap in the form of a single ionization channel (Plate 4). Based on light emission measurements of sparks with symmetrical electrode geometry, the energy is dissipated approximately uniformly along the channel. This is in contrast with asym-... 

In addition to intracellular heme-containing proteins, big-conductance calcium-dependent K+ (BKCa) channels and calcium-spark activated transient Kca channels in plasma membrane are also tar geted by CO [3]. As well known, nitric oxide (NO) also activates BKca channels in vascular smooth muscle cells. While both NO and CO open BKCa channels, CO mainly acts on alpha subunit of BKCa channels and NO mainly acts on beta subunit of BKca channels in vascular smooth muscle cells. Rather than a redundant machinery, CO and NO provide a coordinated regulation of BKca channel function by acting on different subunits of the same protein complex. Furthermore, pretreatment of vascular smooth muscle... 

The knowledge of turbulent premixed flames has improved from this very simple level by following the progress made in experimental and numerical techniques as well as theoretical methods. Much employed in early research, the laboratory Bunsen burners are characterized by relatively low turbulence levels with flow properties that are not constant everywhere in the flame. To alleviate these restrictions, Karpov et al. [5] pioneered as early as in 1959 the studies of turbulent premixed flames initiated by a spark in a more intense turbulence, produced in a fan-stirred quasi-spherical vessel. Other experiments carried out among others by Talantov and his coworkers allowed to determine the so-called turbulent flame speed in a channel of square cross-section with significant levels of turbulence [6]. 

Compared to flame excitation, random fluctuations in the intensity of emitted radiation from samples excited by arc and spark discharges are considerable. For this reason instantaneous measurements are not sufficiently reliable for analytical purposes and it is necessary to measure integrated intensities over periods of up to several minutes. Modern instruments will be computer controlled and fitted with VDUs. Computer-based data handling will enable qualitative analysis by sequential examination of the spectrum for elemental lines. Peak integration may be used for quantitative analysis and peak overlay routines for comparisons with standard spectra, detection of interferences and their correction (Figure 8.4). Alternatively an instrument fitted with a poly-chromator and which has a number of fixed channels (ca. 30) enables simultaneous measurements to be made. Such instruments are called direct reading spectrometers. 

Recent work using confocal microscopy has found localized increases of [Ca2+]j named Ca2+ sparks which are due to the release of Ca2+ from one or a small number of RyRs (Jaggar et al 2000). These localized releases of Ca2+ activate Ca2+-dependent channels in the surface membrane (Perez et al 2001). Activation of the Ca2+-activated K+ current will hyperpolarize the membrane potential (Herrera et al 2001) and thereby decrease Ca2+ entry into the cell on voltage-dependent Ca2+ channels. This provides a mechanism whereby Ca2+ release from the SR can decrease contraction. It is therefore important, in different smooth muscles, to consider to what extent SR Ca2+ release activates rather than decreases contraction. It is, of course, possible that, in the same smooth muscle, SR release may sometimes directly activate contraction and, at other times, decrease it by activating K+ channels. 

Herrera GM, Heppner TJ, Nelson MT 2001 Voltage dependence of the coupling of Ca2+ sparks to BKCa channels in urinary bladder smooth muscle. Am J Physiol Cell Physiol 280 ... 

Perez GJ, Bonev AD, Nelson MT 2001 Micromolar Ca2+ from sparks activates Ca2+-sensitive K+ channels in rat cerebral artery smooth muscle. Am J Physiol Cell Physiol 281 C1769-C1775 Rembold CM 1992 Regulation of contraction and relaxation in arterial smooth muscle. Hypertension 20 129-137... 

Nelson Relating to the proximity, we have probed this area by looking at sparks and the communication in the large conductance Ca2+-activated K+ (BK) channels. The measurements are consistent with close apposition of the RyRs and spark sites to the BK channels. This would be consistent with a proximity of 10—20 nm seen by electron microscopy. Also, mobile buffers are unable to compete with Ca2+ at the BK channel, which is also consistent with this idea. As a probe of what happens to local Ca2+ we have looked at the decay of a spark. Nothing we could do, such as zero Na+ or lanthanum, had any effect on the decay, suggesting that diffusion was responsible. 

STOCs arise from the concerted activation of up to 100 Ca2+-activated K+ channels (KCa) in the sarcolemma as a consequence of Ca2+ release from the SR. This release may take the form either of non-propagating focal events such as sparks or puffs , or of more regenerative Ca2+ waves. Since a temporal correlation between them exists, STOCs have been attributed to focal nonpropagating Ca2+ release events. However, this view requires that each STOC is a spatially restricted membrane current that occurs at selected areas of membrane closely apposed to the SR. While essential to the prevalent hypothesis for STOCs,... 

Nelson We have several papers on simultaneous measurements of sparks and large conductance KCa channel (BK) currents (Perez et al 1999, 2001). These studies point to Ca2+ being in the order of 10—100/tM when it activates the channels. We have no evidence that argues in favour of a cluster of BK channels. Given the density of channels, a uniform distribution would be sufficient to... 

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