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PubMed2000

Nonlinear interaction of homogeneously oscillating domains in a planar gas discharge system

A planar dc gas discharge system with a high Ohmic semiconductor cathode is investigated with respect to temporal destabilization of the stationary homogeneous state. A subcritical Hopf bifurcation is observed, leading to a spatial homogeneous oscillation. The dependence of the oscillator's properties on control parameters is investigated. By applying spatial nonuniform optical control of the semiconductor cathode, several domains that may oscillate on different frequencies can be created. These spatially homogeneous domains can interact with each other through common boundaries. By adjusting the strength of coupling of the domains, their interaction can be controlled. In this interaction, regularities have been found that are, in some aspects, similar to those observed in externally driven nonlinear oscillators.

Journal Article↗
PubMed2000

Ionization waves in electron-beam-assisted, shielded capillary discharge

A theory of propagation of cathode-directed ionization waves during the early stages of an electrical breakdown in a shielded, low-pressure capillary is developed. The discharge process occurs due to the ionization of the low-density gas in the capillary by an electron beam that is emanating from a hollow cathode. Due to the strong electric field in the capillary the electrons are in the fast acceleration regime. Consequently, the full momentum equation for the electrons is employed, rather than the electron drift velocity approach. The smallness of the ratio of the capillary radius to the characteristic length of the electric potential variation in the axial direction allows the construction of a quasi-one-dimensional model. The latter retains the important two-dimensional nature of the electron flow as well as the electrodynamic boundary conditions at the capillary wall and the conducting shield and results in a set of one-dimensional, time-dependent partial differential equations for the on-axis distributions of the physical quantities. It is shown that those equations admit self-similar solutions that represent ionization waves propagating with constant velocities. The resulting set of ordinary differential equations is solved numerically for various initial conditions representing a nonperturbed steady state ahead of the ionization front and the resulting features of the ionization waves are investigated and discussed. The obtained solutions describe both ionization growth and virtual anode propagation and represent fast ionization waves in plasma waveguides, for which the maximum value of the mean electron velocity is much higher than the wave velocity. The space-charge distribution associated with the ionization waves is found in the form of plasma oscillations with a continuously increasing frequency and a solitary envelope. The calculated wave velocity increases with the gas pressure and this tendency is in agreement with corresponding experimental observations.

Journal Article↗
PubMed2002

Synchronization of coupled assemblies of relaxation oscillatory electrode pairs.

Spatiotemporal patterns emerging through coupling of identical relaxation oscillatory electrode pairs are studied. Each pair, consisting of an iron anode and a copper cathode, oscillates periodically under fixed applied potential difference conditions. It is shown that the system synchronizes rapidly (within few oscillatory cycles) and differences of natural frequencies as well as boundary effects are compensated. The effect of the geometrical configuration on the dynamic modes is investigated for relatively large assemblies of such oscillatory pairs. When oscillators are coupled through neighboring electrodes, the response is synchronized by a simultaneous formation of groups. The formation of groups due to enhancement or inhibition of the oscillations depends on the relative position of interacting anodes and cathodes. The behavior of the system is compared with the response of coupled relaxation cells of neurophysiological interest.

Calcium Channels↗
PubMed2003

Two-dimensional nonlocal model of axially and radially inhomogeneous plasma of cylindrical magnetron discharge.

A cylindrical magnetron discharge (CMD) with two coaxial electrodes, a uniform axially directed magnetic field, and discharge ends closed by the shields biased at the cathode potential is considered. The presence of the shields creates axial inhomogeneity of plasma, which is taken into account in this study. At low pressures and small magnetic fields the pronounced nonlocal regime of the electron distribution function (EDF) formation is realized. The electron component is analyzed on the basis of radially and axially inhomogeneous Boltzmann kinetic equation. Unmagnetized electrons that move in axial direction are trapped in the axial potential well, their energy relaxation length exceeds the discharge vessel length, and the kinetic equation can be averaged over axial flights of electrons. Using a model two-dimensional potential profile, the EDFs at the different axial positions are obtained and two-dimensional distributions of the electron density, ionization rate, and current on cathode are calculated. The results of the modeling and experiments are compared for the dc CMD in Ar at a pressure of 3 Pa, magnetic field strength of 10 mT, and current of 150 mA.

Journal Article↗
PubMed2005

Ionization fronts in planar dc discharge systems with high-ohmic electrode.

Electric breakdown and ionization fronts are considered theoretically in a sandwich-like dc discharge system consisting of two plane-parallel electrodes and a gaseous gap in between. The key system feature is a high-ohmic cathode opposite to an ordinary metal anode. Such systems have received much attention from experimental studies because they naturally support current patterns. Using adiabatic description of electrons and two-scale expansion we demonstrate that in the low-current Townsend mode the discharge is governed by a two-component reaction-diffusion system. The latter provides quantitative system description on the macroscopic time scale (i.e., much larger than the ion travel time). The breakdown appears as an instability of the uniform overvoltage state. A seed current fluctuation triggers a shock-like ionization front that propagates along the discharge plane with constant speed (typically approximately 10(4) cm/s). Depending on the cathode resistivity the front exhibits either monotonic or oscillatory behavior in space. Other breakdown features, such as damping transient oscillations of the global current, can also be found as solutions of the reaction-diffusion equations.

Journal Article↗
PubMed1993

Electrotropism of Maize (Zea mays L.) Roots (Facts and Artifacts).

Intact and decapped primary roots of maize (Zea mays L.) were exposed to DC electric fields of 0.5 to 8.0 V/cm in low-salinity media to resolve conflicting results about the direction of electrotropism. In DC fields of 0.5 V/cm or 1.0 V/cm, intact roots always curved toward the cathode. In a field of 8.0 V/cm, intact roots curved toward the anode and stopped growth. Decapped roots also curved toward the anode both in weak and strong fields. The results indicate that growth toward the cathode is the true response of healthy roots.

Journal Article↗
PubMed1972

Peroxidase isozymes of first internodes of sorghum: tissue and intracellular localization and multiple peaks of activity isolated by gel filtration chromatography.

Electrophoretic analyses using Sepraphore III strips indicate the presence of a minimum of five bands of peroxidase activity detectable with o-dianisidine and H(2)O(2) in extracts from first internodes of Sorghum vulgare var. Wheatland milo. Three of these isozymes were anodic and two were cathodic forms at pH 8.3. The relative amounts of these forms are compared in zero time and incubated excised internodes, stelar and cortical tissues of internodes, and in other parts of the plant. Localization of these isozymes with respect to walls and cytoplasm was characterized by differential centrifugation after grinding of the internodes and by an in situ extraction of walls by centrifugation after vacuum infiltration. Using the latter in situ method, 32% of the total activity of the fast moving cathodic form was exchanged from the wall after infiltration with 50 mm CaCl(2). Only trace amounts of the other isozymes were localized in the walls of the cortex. The isozymes were eluted as two peaks from columns of Sephadex G-100 and three peaks from Agarose A-15m. Although such groupings may be due to asymmetric molecules and ionic interactions as well as to molecular weight differences, they may indicate associations with complexes or membranes of different cytoplasmic constituents.

Journal Article↗
PubMed1973

Changes in Amount of Polyphenols and Activity of Related Enzymes during Growth of Tobacco Flower and Capsule.

Developmental stages of tobacco (Nicotiana tabacum L. cv. Burley 21) flower and capsule were correlated with tissue contents of polyphenols and activities of phenylalanine ammonialyase, polyphenoloxidase, and peroxidase. Chlorogenic acid, scopolin, and scopoletin were present in most tissues, whereas rutin and two dihydroxyphenolic glycosides concentrated primarily in the corolla and placenta, respectively. Ovules contained only chlorogenic acid. As development progressed, polyphenols accounted for nearly 15% of the dry weight in the green capsule of field-grown plants. Fertilization triggered a rapid increase of chlorogenic acid in the ovary. When l-phenylalanine-U-(14)C was fed to the detached green capsules and capsule parts, an incorporation of radioactivity into chlorogenic acid and dihydroxyphenolic glycosides occurred which suggested in situ synthesis of these compounds. This was subtantiated by a positive correlation between phenylalanine ammonia-lyase activity and polyphenol accumulation. High polyphenoloxidase activity was associated mainly with the ovary, whereas peroxidase activity was maximal during senescence of all tissues. Polyacrylamide gel slab electrophoresis revealed five cathodic bands and one diffuse zone with poly-phenoloxidase activity in flower extracts. Two anodic poly-phenoloxidase isozymes appeared only in the fertilized ovary. Among 17 peroxidase isozymes, six cathodic forms were present throughout floral development, and the anodic ones increased in number and activity at the later stages of capsule growth.

Journal Article↗
PubMed1975

Cell Wall and Protoplast Isoperoxidases of Corn Leaves in Relation to Cut Injury and Infection with Helminthosporium maydis.

Young leaves of two corn (Zea mays) inbreds with normal and Texas male-sterile cytoplasm, which differed in their susceptibility to Helminthosporium maydis Nisikado and Miyake race T, showed no significant qualitative or quantitative differences in their isoperoxidase patterns. Of six cathodic and four anodic isoenzymes present in the soluble fraction, four and two, respectively, comprised the fraction ionically bound to the cell wall. Peroxidase fractions ionically and covalently bound to the wall constituted about 20% of the total peroxidase activity. No new isoperoxidases were detected in either inbred line in response to cutting, infection, or detachment only and exposure to darkness for 40 hours. Three isoperoxidases, all cathodic, mainly reacted to cutting injury as well as fungal infection. One of the isoperoxidases appeared responsible for the increase in the peroxidase activity of the soluble fraction while the other two were responsible for the increase in that of the fraction ionically bound to the walls. The relative increase in the latter fraction was greater for infected leaves than for mechanically injured ones. No significant differences were found between the two inbreds in their peroxidase reactions to cutting injury or infection. Thus, the corn leaf isoperoxidases were distinctive in their distribution in the cell and in their reaction to injury. Changes in their activity induced by infection may result from a nonspecific response to injury.

Journal Article↗
PubMed1981

Metal Complexation in Xylem Fluid : III. ELECTROPHORETIC EVIDENCE.

The capacity of ligands in xylem fluid to form metal complexes was tested with a series of in vitro experiments using paper electrophoresis and radiographs. The xylem fluid was collected hourly for 8 hours from soybean (Glycine max L. Merr.) and tomato (Lycopersicon esculentum Mill.) plants grown in normal and Zn-phytotoxic nutrient solutions. Metal complexation was assayed by anodic or reduced cathodic movement of radionuclides ((63)Ni, (65)Zn, (109)Cd, (54)Mn) that were presumed to have formed negatively charged complexes.Electrophoretic migration of Ni, Zn, Cd, and Mn added to xylem exudate and spotted on KCl- or KNO(3)-wetted paper showed that stable Ni, Zn, and Cd metal complexes were formed by exudate ligands. No anodic Mn complexes were observed in this test system. Solution pH, plant species, exudate collection time, and Zn phytotoxicity all affected the amount of metal complex formed in exudate. As the pH increased, there was increased anodic metal movement. Soybean exudate generally bound more of each metal than did tomato exudate. Metal binding usually decreased with increasing exudate collection time, and less metal was bound by the high-Zn exudate.Ni, Zn, Cd, and Mn in exudate added to exudate-wetted paper demonstrated the effect of ligand concentration on stable metal complex formation. Complexes for each metal were demonstratable with this method. Cathodic metal movement increased with time of exudate collection, and it was greater in the high-Zn exudate than in the normal-Zn exudate. A model study illustrated the effect of ligand concentration on metal complex stability in the electrophoretic field. Higher ligand (citric acid) concentrations increased the stability for all metals tested.

Journal Article↗
PubMed1986

Heterogeneity of catalase in maturing and germinated cotton seeds.

To investigate possible charge and size heterogeneity of catalase (EC 1.11.1.6) in cotton (Gossypium hirsutum L. cv Deltapine 62), extracts of cotyledons from different developmental ages were subjected to nondenaturing polyacrylamide gel electrophoresis and isoelectric focusing. Special precautions (e.g. fresh homogenates, reducing media) were necessary to prevent artefacts due to enzyme modification during extraction and storage. When the gels were stained for enzyme activity, two distinct electrophoretic forms of catalase were resolved in extracts of maturing and mature cotton seeds. In germinated seeds, three additional cathodic forms were detected revealing a total of five electrophoretic variants. In green cotyledons, the two anodic forms characteristic of ungerminated seeds were less active; whereas, the most cathodic form was predominant. All forms of catalase were found in isolated glyoxysomes. Corresponding electrophoretic patterns were found on Western blots probed with anticatalase serum; no immunoreactive, catalytically inactive forms were detected. Western blots of sodium dodecyl sulfate-polyacrylamide gels revealed only one immunoreactive (55 kilodaltons) polypeptide in cotton extracts of all developmental ages. Results from isoelectric focusing and Ferguson plots indicate that the electrophoretic variants of catalase are charge isomers with a molecular weight of approximately 230,000.

Journal Article↗
PubMed1999

A computational model of electrical stimulation of the retinal ganglion cell.

Localized retinal electrical stimulation in blind volunteers results in discrete round visual percepts corresponding to the location of the stimulating electrode. The success of such an approach to provide useful vision depends on elucidating the neuronal target of surface electrical stimulation. To determine if electrodes preferentially stimulate ganglion cells directly below them or passing fibers from distant ganglion cells, we developed a compartmental model for electric field stimulation of the retinal ganglion cell (RGC). In this model a RGC is stimulated by extracellular electrical fields with active channels and realistic cell morphology derived directly from a neuronal tracing. Three membrane models were applied: a linear passive model, a Hodgkin-Huxley model with passive dendrites (HH), and a model composed of all active compartments (FCM) with five nonlinear ion channels. Idealized monopolar point and disk stimulating electrodes were positioned above the cell. For the HH and FCM models, the position of lowest cathodal threshold to propagate an action potential was over the soma. Brief (100 microseconds) cathodic stimuli were 20% (HH with disk electrode) to 73% (FCM with point-source) more effective over the soma than over the axon. In the passive model, the axon is preferentially stimulated versus the soma. Although it may be possible to electrically stimulate RGC's near their cell body at lower thresholds than at their axon, these differences are relatively small. Alternative explanations should be sought to explain the focal perceptions observed in previously reported patient trials.

Action Potentials↗
PubMed2002

Extracellular stimulation window explained by a geometry-based model of the neuron-electrode contact.

Extracellular stimulation of single cultured neurons which are completely sealing a microelectrode is usually performed using anodic or biphasic currents of at least 200 nA. However, recently obtained experimental data demonstrate the possibility to stimulate a neuron using cathodic current pulses with less amplitude. Also, a stimulation window is observed. These findings can be explained by a finite-element model which permits geometry-based electrical representation of the neuron-electrode interface and can be used to explore the required conditions for extracellular stimulation in detail. Modulation of the voltage sensitive channels in the sealing part of the membrane appears to be the key to successful cathodic stimulation. Furthermore, the upper limit of the stimulation window can be explained as a normal consequence of the neuronal membrane electrophysiology.

Action Potentials↗
PubMed1996

Imposition of a physiologic DC electric field alters the migratory response of human keratinocytes on extracellular matrix molecules.

Outwardly directed ionic currents have been measured leaving skin wounds in vivo. These currents generate physiologic electric fields of approximately 100 mV/mm, which may function to direct keratinocyte migration toward the healing wound. We investigated whether the substrate on which the keratinocyte migrates modulates the galvanotactic response to an electric migratory signal. Cultured human keratinocytes were plated on different matrices; types I and IV collagen, fibronectin, laminin, and tissue culture plastic. The effect of an applied direct current (DC) electric field on directional migration was monitored by time-lapse video microscopy over a 2-h period. Directionality was quantitated by calculating the cosine of the angle of migration in relation to anodal-cathodal orientation. Migration toward the negative pole was observed on all matrices as compared with controls (no applied field), which displayed random migration. No significant increase in directional response occurred when the field strength was increased by 100 mV/mm (physiologic levels) to 400 mV/mm. The degree of directionality and the average net cell translocation however, varied significantly with the substrate. The greatest cathodal migration in response to a DC electric field was observed with keratinocytes plated on types I and IV collagens and plastic. The directional migratory response was least on a laminin substrate, whereas cells on fibronectin demonstrated a response that was intermediate between those of collagen and laminin. These results suggest that physiologic ionic currents in concert with underlying matrix may influence the rate of reepithelialization of skin wounds.

Cell Movement↗
PubMed2004

Transcranial direct current stimulation disrupts tactile perception.

The excitability of the cerebral cortex can be modulated by various transcranial stimulation techniques. Transcranial direct current stimulation (tDCS) offers the advantage of portable equipment and could, therefore, be used for ambulatory modulation of brain excitability. However, modulation of cortical excitability by tDCS has so far mostly been shown by indirect measures. Therefore, we examined whether tDCS has a direct behavioral/perceptional effect. We compared tactile discrimination of vibratory stimuli to the left ring finger prior to, during and after tDCS applied for 7 min at 1-mA current intensity in 13 subjects. Stimulation was pseudorandomized into cathodal, anodal and sham conditions in a within-subject design. The active electrode was placed over the corresponding somatosensory cortex at C4 according to the 10-20 EEG system and the reference electrode at the forehead above the contralateral orbita. Cathodal stimulation compared with sham induced a prolonged decrease of tactile discrimination, while anodal and sham stimulation did not. Thus, cortical processing can be modulated in a behaviorally/perceptually meaningful way by weak transcranial current stimulation applied through portable technology. This finding offers a new perspective for the treatment of conditions characterized by alterations of cortical excitability.

Adult↗
PubMed1977

The spatial correlation of fluctuations of the blood flow in forearm skin.

The oxygen tension at the skin surface, with the atmospheric oxygen supply excluded, may be measured polarographically on the human forearm. Under suitable environmental conditions, spontaneous fluctuations in skin surface oxygen tension can be recorded and these are the result of fluctuating blood flow in the superficial skin vessels. An array of separate cathodes in a single polarographic electrode assembly has been used to measure the fluctuations in blood flow which are occurring simultaneously at seven different points in the skin. Spectral analysis of a long recording of such fluctuations shows that most of the power is concentrated at the lower frequencies but does not reveal any clearly defined dominant frequency. It is clear, however, from inspection of the fluctuation patterns, that recurring bursts of activity occur with quite well-defined periodicities in the range of 30s to 5 min. There is a stronger correlation between the fluctuations occurring at points on the skin surface less than 1 mm apart than between points further apart, although there are clearly some components which are common to points separated by up to 5 mm which was the largest separation of cathodes used in these experiments. These observations are supported by the value of cross correlation coefficients between the channels. These results suggest that the observed fluctuations are the result of at least two factors. The first one is common to all the channels but the effect of this may be overridden by a second and more local one controlling the blood flow.

Arm↗
PubMed1978

Continuous comparison of transcutaneous and arterial oxygen tension in newborn infants with respiratory illnesses.

Transcutaneous oxygen tension (tcPO2) measured by two skin electrodes of different design, and arterial oxygen tension (PaO2) measured by an intravascular oxygen electrode, were continuously compared for periods of 6--27 h in 45 newborn infants with respiratory illnesses. One skin electrode (Dräger) had three microcathodes surrounded by a heated ring-shaped anode and the other (Roche) a large heated cathode. The electrodes were calibrated in vitro; 44 degrees C was found to be a suitable electrode temperature for the estimation of Pao2. tcPo2 recorded by the electrode with the microcathodes estimated Pao2 reasonably accurately for 6 h without the necessity for re-siting. tcPao2 recorded by the electrode with the large cathode gave a similar estimate of Pao2 for 3 h, but then tcPao2 often fell relative to Pao2. This fall was probably caused by skin changes at the electrode site. Poor skin perfusion causing underestimation of Pao2 by tcPo2 could not be identified by a number of methods tried.

Asphyxia Neonatorum↗
PubMed1977

The purification and properties of human alpha1-antitrypsin (alpha1-antiprotease), variant Z.

After three stages of preliminary purification, variant Z was chromatographed on a DEAE-cellulose column. Upon elution with a linearly increasing concentration of NaCl, variant Z was recovered in two separate peaks, the first of which contained 81% and the second 19% of the total. The preparation corresponding to the first peak was homogeneous by various criteria. The trypsin and chymotrypsin inhibiting capacities and the specific antigenic activity of the preparation were nearly the same as those of an authentic sample of variant M. Variant Z contained 8 or 9 more gycine residues than variant M, but no appreciable difference was found between their carbohydrate contents. By analytical isoelectrofocusing the isoinhibitors of purified variant Z overlapped with those in the plasma of the donor and were cathodal to, but partially overlapped with purified variant M. After desialysation, the overlap between the different variants became complete, but variant Z contained a larger proportion of cathodal and smaller proportion of anodal components than variant M. Both variants formed five distinct isoinhibitor-protease complexes after incubation with trypsin and chymotrypsin and the corresponding complexes in the different variants completely coincided.

Amino Acids↗