Depolarization Currents Research Articles

Doublet of action potentials evoked by intracellular injection of rectangular depolarization current into rat motoneurones

A doublet of action potentials is frequently observed at the beginning of motoneuronal discharge patterns and its appearance leads to a considerable increase in the motor unit force. The aims of this study were (1) to determine the relationship between the intensity of rectangular depolarization currents injected into motoneurones and their ability to generate doublets and (2) to evaluate the influence of the initial doublets on changes in motoneuronal firing frequency. Experiments were performed on anesthetized rats, and recordings were taken from motoneurones located in the L4-L5 segments of the spinal cord. The depolarization current necessary to evoke the initial doublet of action potentials was measured and expressed in multiples of the rheobase. A gradual increase in the intensity of current injected into motoneurones resulted in initial doublets in 80% of the cases studied, at doublet threshold ranges between 1.25 and 4.0 times the rheobase. This suggests that doublets are an effect of strong synaptic excitation of motoneurones that may precede a sudden change in force during a movement. Moreover, in the great majority of the studied motoneurones, this initial doublet caused changes in the subsequent firing rate by the prolongation of the first interspike interval.

Thermally Stimulated Currents of SiO2/Low‐density Polyethylene Micro‐ and Nanocomposites

AbstractComposite samples of low‐density polyethylene (LDPE)/nano‐SiO2 and LDPE/micro‐SiO2 were prepared with the method of double‐solution mixture. Depolarization currents of all samples were investigated with thermally stimulated depolarization current (TSDC). It was found that the currents of both composites increased with the loading level of nano‐SiO2 and/or micro‐SiO2, and that the peak width of each composite is greater than that of pure LDPE. In addition, the peak position of the nanocomposite shifts as the loading level increases, while that of the microcomposite does not shift significantly. In order to understand activation energy of both composites and pure LDPE, the initial‐rise method was used to analyze the depolarization current. It was found that LDPE has the greatest activation energy among all samples and the activation energy of both composites decreases with increasing loading levels. Moreover, the activation energy of the nanocomposite is less than that of the microcomposite at each of the same loading level. As the nano‐SiO2 loading level reaches 5.0%wt, the composite has the lowest activation energy of 0.25 eV. In addition, dielectric spectra of all samples were investigated in the range of 10−4 to 107 Hz, and it was found that the peak position of loss tangent varied consistently with the TSDC curves as the loading levels of nano‐SiO2 and/or micro‐SiO2 were increased. Copyright © 2010 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Molecular mobility studies on the amorphous state of disaccharides. I—thermally stimulated currents and differential scanning calorimetry

The relaxational processes in amorphous solid gentiobiose and cellobiose are studied by thermally stimulated depolarization currents (TSDC) in the temperature region from 108 K up to 423 K. The slow molecular mobility was characterized in the crystal and in the glassy state. The features of different motional components of the secondary relaxation have been monitored as a function of time as the glass structurally relaxes on aging. It is concluded that some modes of motion of this mobility are aging independent, while others are affected by aging. The value of the steepness index or fragility ( T g–normalized temperature dependence of the relaxation time) was obtained by differential scanning calorimetry (DSC) from the analysis of the scanning rate dependency of T g.

Performance of oil and paper in transformers based on IEC 61620 and dielectric response techniques*

Conduction and polarization mechanisms in the oil and paper of a power transformer are discussed in this article. Some of the electrical characteristics of an insulating oil sample can be determined accurately at room temperature using the IEC 61620 test procedure. High voltage stress and high temperature may modify these characteristics. Deterioration products, both acid and non-acid types, influence the measured conductivity, but moisture or contaminants do not do so. Reasonable upper limits have been suggested for new and in-service oil. Pressboard moisture content and conductivity of the oil in the main ducts of a transformer may be measured using dielectric response techniques, which also give information on the state of the insulation at interfaces. Measurement of polarization and depolarization currents in a transformer without oil allows conduction and polarization phenomena occurring in the insulating paper to be identified. The depolarization current characterizes the type of aging in the paper, provided the applied voltage stress is very low.

Particulate matter exposure decreased spiking response to depolarization and increased potassium current of cardiac vagal neurons in mice

Epidemiologic studies show that exposure to fine particulate matter (PM2.5) increases the total daily cardiovascular mortality. W previously showed that 3 d of PM2.5 exposure (6 hr/d) in the form of iron‐soot reduced heart rate variability in mice. We further showed that PM2.5 exposure reduced neuronal responsiveness to excitation in cardiac vagal neurons in the nucleus ambiguus. Here, we tested the hypothesis that PM2.5 exposure‐induced increases in potassium currents contribute to the decreased neuronal excitability. Using patch‐clamp techniques, we determined the spiking response to depolarization and potassium currents in anatomically identified cardiac vagal neurons, retrogradely labeled with a fluorescent dye applied to the SA node. PM2.5 exposure decreased spiking responses of identified cardiac vagal neurons to depolarization in the presence of glutamatergic and GABAergic receptor antagonists (p < 0.05). PM exposure also increased the peak (by 24%) and stead state (by 17%) potassium currents. The data suggest that PM2.5 exposure‐induced increase in potassium channel conductance maybe one mechanism contributing to the decreased heart rate variability. (Support R01 ES012957)

Coronoelectrets based on polypropylene composites dispersed by a TlIn x Ce1 − x Se2 semiconductor filler

The electret properties in the heterogeneous polymer-semiconductor system have been investigated by the method of the electret-thermal analysis (ETA) of the PP composites with TlInSe2 and TlInxCe1 − xSe2 semiconductor fillers. It is shown that a small change in the filler structure causes a significant alteration of the spectrum of the thermostimulated depolarization currents. As the investigation shows, the TlInSe2 and TlInxCe1 − xSe2 fillers having the p-type conductivity play the role of the structure-forming agent in the polypropylene matrix changing the structure of the polymer and the near-boundary layer of the polymer and the filler.

Dopamine D1-like receptor activation depolarizes medium spiny neurons of the mouse nucleus accumbens by inhibiting inwardly rectifying K + currents through a cAMP-dependent protein kinase A-independent mechanism

Dopamine/cAMP signaling has been reported to mediate behavioral responses related to drug addiction. It also modulates the plasticity and firing properties of medium spiny neurons (MSNs) in the nucleus accumbens (NAc), although the effects of cAMP signaling on the resting membrane potential (RMP) of MSNs has not been specifically defined. In this study, activation of dopamine D1-like receptors (D1Rs) by SKF-38393 elicited membrane depolarization and inward currents in MSNs from the NAc core of 14–17 day-old mice. Similar results were obtained following stimulation of adenylyl cyclase (AC) activity with forskolin or application of exogenous cAMP. Forskolin occluded SKF-38393's effects, thus indicating that D1R action is mediated by AC/cAMP signaling. Accordingly, AC blockade by SQ22536 significantly inhibited the responses to SKF-38393. Effects elicited by D1R stimulation or increased cAMP levels were unaffected by protein kinase A (PKA) or protein kinase C (PKC) blockade and were not mimicked by the Epac agonist, 8CPT-2Me-cAMP. Responses to forskolin were also not significantly modified by cyclic nucleotide-gated (CNG) channel blockade. Forskolin-induced membrane depolarization was associated with increased membrane input resistance. Voltage-clamp experiments revealed that forskolin and SKF-38393 effects were due to inhibition of resting K + currents exhibiting inward rectification at hyperpolarized potentials and a reversal potential (around −90 mV) that shifted with the extracellular K + concentration. Forskolin and D1R agonist effects were abolished by the inward rectifier K + (Kir)-channel blocker, BaCl 2. Collectively, these data suggest that stimulation of postsynaptic D1Rs in MSNs of the NAc core causes membrane depolarization by inhibiting Kir currents. This effect is mediated by AC/cAMP signaling but it is independent on PKA, PKC, Epac and CNG channel activation, suggesting that it may stem from cAMP's direct interaction with Kir channels. D1R/cAMP-mediated excitatory effects may influence the generation of output signals from MSNs by facilitating their transition from the quiescent down-state to the functionally active up-state.

The assessment of the influence of temperature of selected parameters of the approximation method of depolarization current analysis of paper–oil insulation

The paper presents the results of experimental investigations of paper–oil insulation samples from the point of view of the assessment of the influence of temperature on the parameters of the approximation method of the time characteristics analysis of depolarization current. Three types of equations: Debye’s function with a single relaxation time, Curie–von Schweidler’s function with triple Debye’s function (taking into account short, average and long relaxation times of paper–oil insulation), and Jonscher’s function with Debye’s function with a single relaxation time were selected as approximation functions, as recommended by literature. The investigated insulation samples were diversified with respect to the degree of dampness and aging. Summing up, the paper focuses on verification of the determined parameters of approximation equations in relation to the influence of insulation temperature and presents recommendations for their application in diagnostics carried out by the PDC (Polarization and Depolarization Current) Method.

Structure and dynamics in poly(L-lactide) copolymer networks

Poly(L-lactide) networks (PmLA) hydrophilized with different amounts of 2-hydroxyethyl acrylate (HEA) were investigated by dielectric relaxation spectroscopy, thermally stimulated depolarization currents, and differential scanning calorimetry. The incorporation of HEA units in the PmLA network, with the aim of modulating the water sorption capacity of the system, results in a material with a complex behavior. The system consists of phase-separated microdomains richer in one or the other comonomers that constitute the network. Initially, the addition of smalls amount of HEA units in the network gives rise to a one-phase, two-component system; however, when the amount of HEA in the system increases, a new phase (HEA-rich one) is formed containing some mLA chains that modify the main relaxation mode of these domains and the local dynamics of the system. The structure of the system has been analyzed by comparing the relaxational modes in the PmLA and PHEA homonetworks with those in the copolymer networks.

INFLUENCE OF POLARIZING ELECTRIC FIELDS ON THE ELECTRICAL AND OPTICAL PROPERTIES OF POLYMER–CLAY COMPOSITE SYSTEM

We examined the electrical and optical properties of a new material containing polymer–clay nano-composite dispersed in a nematic liquid. The clay (Cloisite-type) was modified by copolymerization of maleic anhydride and divinyl benzene, using azobisisobutytironitrile as the initiator. The final polymer–clay nano-composite has an intercalated structure according to XRD patterns. We measured the thermally stimulated depolarization currents and determined the activation energies. Simultaneously we measured the optical transmission and we studied the influence of the previously applied polarizing electric fields.

Thermally Stimulated Depolarization Currents (TSDC): A sensitive technique for analyzing protein structure

The water molecules surrounding proteins as a thin layer and those packaged in pockets and cavities shape and control their structure. Thermally Stimulated Depolarization Currents (TSDC) technique has been applied to investigate the hydration structure of six proteins with different structural motifs: pepsin, β-lactoglobulin, α-chymotrypsin, bovine serum albumin, human serum albumin and myoglobin, at very low hydration level (water vapor activity aw≈0.80) both in the native state and after treatment in trifluoroethanol/water mixture 80% (v/v). A combined approach based on the use of the TSDC technique, able to distinguish H2O dipoles belonging to the solvation shell in terms of their order degree and mobility, and of FTIR and CD spectroscopies has allowed us to reexamine the problem of conformational stability of macromolecules as a function of their hydration.

Thermally Stimulated Depolarization Currents (TSDC): A sensitive technique for analyzing protein structure

The water molecules surrounding proteins as a thin layer and those packaged in pockets and cavities shape and control their structure. Thermally Stimulated Depolarization Currents (TSDC) technique has been applied to investigate the hydration structure of six proteins with different structural motifs: pepsin,β-lactoglobulin,α-chymotrypsin, bovine serum albumin, human serum albumin and myoglobin, at very low hydration level (water vapor activity aw≈0.80) both in the native state and after treatment in trifluoroethanol/water mixture 80% (v/v). A combined approach based on the use of the TSDC technique, able to distinguish H2O dipoles belonging to the solvation shell in terms of their order degree and mobility, and of FTIR and CD spectroscopies has allowed us to reexamine the problem of conformational stability of macromolecules as a function of their hydration.

Polyurethane–POSS hybrids: Molecular dynamics studies

A series of hybrid polyurethane–POSS materials have been synthesized on the basis of poly(tetramethylene glycol) (Terathane 1400 ®) as soft component, 4,4′-methylenebis(phenylisocyanate) (MDI) as hard component, and 1,4-butanediol as chain extender. POSS particles properly modified have been tethered on the main chain by substitution of the chain extender to weight fractions up to 10%. AFM measurements indicate the formation of POSS crystallites in the PU matrix, extended structures at low POSS content and more regular, smaller structures at higher POSS content. A detailed investigation of molecular mobility by means of Differential Scanning Calorimetry (DSC), Thermally Stimulated Depolarization Currents (TSDC) and, mainly, Broadband Dielectric Relaxation Spectroscopy (DRS) has been conducted in all samples of the series and in addition in neat Terathane, as reference. Four relaxations have been studied in detail: two secondary relaxations γ and β, the segmental α relaxation (dynamic glass transition) and an α′ relaxation slower than α associated with crystallinity in neat Terathane and with the presence of hard microdomains in the polyurethane and the hybrids. Secondary relaxations remain unaffected by POSS. The glass transition temperature rises by a few degrees and, in consistency with that, segmental dynamics slightly slows down with increasing POSS content. In addition, the dielectric strength of the segmental relaxation decreases with increasing POSS content, suggesting that a fraction of polymer is immobilized, making no contribution to the relaxation. These results are discussed in relation to morphology.

Slow Molecular Mobility in the Amorphous Solid State of Fructose: Fragility and Aging

The molecular mobility of beta-D-fructose was studied by thermally stimulated depolarization currents (TSDC) in the amorphous solid state. The amorphous solid samples were prepared in such a way that the tautomeric mixture was near the equilibrium composition. A broad secondary relaxation was observed, that merges, at high temperatures, with the alpha relaxation. The alpha relaxation temperature provided by the TSDC technique is T(g) = 13 degrees C (at 4 K min(-1)). The fragility index calculated from TSDC data is m = 34, significantly lower when compared with the values reported in the literature obtained from Dielectric Relaxation Spectroscopy (DRS). The physical significance of the fragility obtained by the 2 dielectric techniques is discussed. The influence of physical aging on the secondary relaxation in amorphous fructose was analyzed as the glass structurally relaxes. A complex behavior was observed such that the faster components (lower temperature) of the secondary relaxation are negligibly dependent on aging and may be ascribed to intramolecular modes of motion, while the slower motional modes (higher temperature) show a significant dependence on aging and correspond to the genuine Johari-Goldstein beta-relaxation.

Study of charge relaxations after thermal aging in poly (methyl methacrylate)

Effects of thermal aging on physico–chemical and electrical properties of polymethyl methacrylate (PMMA) are reported in this paper. PMMA samples are submitted to successive heat–cooling cycles (Tmin=20 ∘C and Tmax=90 ∘C) in the ambient air, each cycle lasts 12h. Different techniques are thus employed to investigate structural modifications, dielectric relaxations and conduction processes. These are the Fourier Transform Infrared (FTIR) spectroscopy, thermal step method (TSM), thermally stimulated depolarization currents (TSDC), dielectric spectroscopy (DS) and current–voltage technique. Results are discussed and a relationship between structural modifications and charge relaxations was emphasized. We demonstrated that thermal aging favors oxidation phenomenon. Three distinguishable charge relaxations (β, α and ρ) have been highlighted by TSDC.

Water sorption and polymer dynamics in hybrid poly(2-hydroxyethyl-co-ethyl acrylate)/silica hydrogels

This work probes the hydration properties and molecular dynamics of hybrid poly(hydroxyethyl-co-ethyl acrylate)/silica hydrogels. Two series of hybrid copolymers were prepared by simultaneous polymerization and silica preparation by sol–gel method, the first with hydroxyethyl acrylate/ethyl acrylate (HEA/EA) composition at 100/0, 90/10, 70/30, 50/50, 30/70, 10/90 and fixed silica content at 20 wt.%, and the second with fixed HEA/EA organic composition at 70/30 and 0, 5, 10 and 20 wt.% of silica. The hydration properties of these systems were studied at 25 °C by exposure to several controlled water vapor atmospheres (water activities 0–0.98) in sealed jars and by immersion in distilled water. Finally, the molecular dynamics of the hydrated hybrids at several levels of hydration was probed with Thermally Stimulated Depolarization Currents (TSDC) in the temperature interval between −150 and 20 °C. The results indicate that a critical region of silica content between 10 and 20 wt.% exists, above which silica is able to form an inorganic network. This silica network prevents the expansion of water clusters inside the hydrogels and subsequently the total stretching of the polymer network without obstructing the water sorption at the first stages of hydration from the dry state. As concerns the copolymer composition, the presence of EA reduces water sorption and formation of water clusters affecting directly to the hydrophilic regions. The TSDC thermograms reveal the presence of a single primary main broad peak denoted as α cop relaxation process, which is closely related to the copolymer glass transition, and of a secondary relaxation process denoted as β sw relaxation, which originates from the rotational motions of the lateral hydroxyl groups with attached water molecules. The single α cop implies structural homogeneity at the nanoscale in HEA-rich samples ( x HEA > 0.5), while for high EA content ( x EA ⩾ 0.5) phase separation is detected. Both relaxation processes show strong dependence on water content and organic phase composition.

Low temperature and moisture effects on polarization and depolarization currents of oil-paper insulation

In the last decades, dielectric testing techniques are being used and investigated as potential tools for condition assessment of oil-paper insulation. From fields and laboratory investigations these techniques were found to be highly operating conditions (moisture, ageing, temperature, etc.) dependant. Because field measurements (generally performed after de-energizing the transformer), last hours after de-energizing the transformer, the ambient temperature may affect the results. Especially in cold regions of the world, extreme care is required to interpret the results when performing tests at surrounding low temperatures. A better understanding and analysis of the dielectric test results are therefore only possible with a clear understanding of the physical behaviour of the insulation system in response to the ambient conditions. In the current research project, a series of experiments have been performed under controlled laboratory conditions with preset moisture content inside the insulation. This paper reports the effects of low temperature on the time domain dielectric response of oil impregnated paper insulation.

Study of Ester-Type Liquid Crystals by TSDC and Optical Investigations

We study ester-type liquid crystal, 4′-cyanophenyl 4-n-hexyl benzoate and its homologue 4′-cyanophenyl 4-n-pentyl benzoate. We used the Thermally Stimulated Depolarization Currents method and found information about phase transition temperature, and activation energies. The temperatures of phase transitions were compared to those obtained by classical DSC/DTA measurements. Simultaneously with the TSDC, we measured the optical transmission; we found a decrease of the optical transmission when it was measured after a greater pre-applied polarizing electric field.

The mechanisms of thermally stimulated depolarization in poly(diphenylene phthalide)

The nature of the charge formed in a thin poly(diphenylene phthalide) film was studied by thermally stimulated depolarization over the temperature range 170–373 K. The influence of the strength of a polarizing electric field and the rate of temperature changes on the depolarization current spectrum was demonstrated. Polarizing electric field intensity changed from 3 × 105 to 7 × 105 V/cm. The rates of temperature changes were 3, 5, and 10 K/min. The depolarization current spectra contained three regions. The first two low-temperature regions were interpreted using the dipole and volume charge approximations, respectively. It was assumed that, in the third region, where charge inversion was observed, near the electrode polarization occurred. The experimental data were used to estimate activation energies and the relative charge released during relaxation in the first and second regions. The suggestion was made that the ratio between various contributions to depolarization current could be changed by varying the polarization field value.

Revisitation of the molecular mobility of the amorphous solid 4,4′-methylenebis(N,N-diglycidylaniline) (MBDA): New contributions from the TSDC technique

In the present work we revisit the previously published study by Thermally Stimulated Depolarisation Currents (TSDC) of the slow molecular mobility in the amorphous solid state of 4,4′-methylenebis(N,N-diglycidylaniline), MBDA (H. P. Diogo, J. J. Moura Ramos, J. Mol. Liq. 129(2006)138–146) in order to add two important points dealing with recent uses of the TSDC technique. One of them concerns a new method to account for nonexponentiality in Thermally Stimulated Depolarisation Currents (TSDC) data treatment which affects the analysis of the alpha relaxation peak and the determination of the fragility index of the glass-forming system. The other point concerns the analysis of the secondary relaxations in MBDA. It is shown that two kinds of secondary relaxations are differently influenced by aging. The faster components of the secondary relaxation are negligibly dependent on aging and may be ascribed to intramolecular modes of motion, while the slower motional modes show a significant dependence on aging and correspond to the genuine Johari-Goldstein β-relaxation.