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PubMed2002

Detailed passive cable models of layer 2/3 pyramidal cells in rat visual cortex at different temperatures.

We present detailed passive cable models of layer 2/3 pyramidal cells based on somatic voltage transients in response to brief current pulses at physiological and room temperatures and demonstrate how cooling alters the shape of postsynaptic responses. Whole cell recordings were made from cells in visual cortical slices from 20- to 22-day-old rats. The cells were filled with biocytin and morphologies were reconstructed from three cells which were representative of the full range of physiological responses. These formed the basis for electrotonic models with four electrical variables, namely membrane capacitance (C(m)), membrane resistivity (R(m)), cytoplasmic resistivity (R(i)) and a somatic shunt conductance (G(sh)). Simpler models, with a single value for R(m) and no G(sh), did not fit the data adequately. Optimal parameter values were derived by simulating the responses to somatic current pulses, varying the parameters to give the best match to the experimental recordings. G(sh) and R(m) were badly constrained. In contrast, the total membrane conductance (G(tot)) was well constrained, and its reciprocal correlated closely with the slowest membrane time constant (tau(0)). The models showed close agreement for C(m) and R(i) (ranges at 36 degrees C: 0.78-0.94 microF cm(-2) and 140-170 Omegacm), but a larger range for G(tot) (7.2-18.4 nS). Cooling produced consistent effects in all three model cells; C(m) remained constant (Q(10) = 0.96), R(i) increased (Q(10) = 0.80), whilst G(tot) dropped (Q(10) = 1.98). In terms of whole cell physiology, the predominant effect of cooling is to dramatically lengthen the decay of transient voltage shifts. Simulations suggest that this markedly increases the temporal summation of postsynaptic potentials and we demonstrate this effect in the responses of layer 2/3 cells to tetanic extracellular stimulation in layer 4.

Algorithms↗
PubMed1995

Optimization of signal-to-noise ratio for multilayer PZT transducers.

In medical ultrasound imaging, two-dimensional (2-D) array transducers are desirable to implement dynamic focusing and phase aberration correction in two dimensions as well as volumetric imaging. Unfortunately, the small size of a 2-D array element results in a small clamped capacitance and a large electrical impedance near the resonance frequency. This results in poor signal-to-noise ratio (SNR) of the array elements. It has previously been demonstrated that transducers made from multilayer PZT ceramics have lower electrical impedance and greater SNR than comparable single layer elements. A simplified circuit model has been developed to optimize the SNR for multilayer ceramic (MLC) transducers. In this model, an electronic transmitter excites the array element and in the receive mode, the element drives a coaxial cable load terminated by a high impedance preamplifier. The transducer impedance is Zt/N2, where N is the number of piezoelectric layers. Maximum transmit signal is obtained when N = Ntx such that the transducer impedance, Zt/Ntx2, is matched to the source impedance. Maximum receive signal is obtained when N = Nrx such that the transducer impedance, Zt/Nrx2, is matched to the coaxial cable reactance. For maximum pulse-echo signal, the transducer should be designed with N = square root of Ntx Nrx, the geometric mean of Ntx and Nrx. Using this optimization technique, a 1.5-D array was designed with 3 layers for maximum pulse-echo SNR. Results of simulations from the simplified circuit analysis were consistent with those of the KLM model. The 3 layer array was fabricated as well as a single layer control array. The measured transmit signal and receive signal agreed with the simulation results.

Ceramics↗
PubMedDate not supplied

Surface transmission probe for noninvasive measurements of dielectric properties of organ tissues at frequencies between 1 MHz and 300 MHz.

A probe is described for determining the dielectric permittivity and dielectric loss of organ tissues at very high frequencies (VHF) in a non-violating manner. Instead of the usual reflection coefficient measurement, with this probe the electromagnetic wave transmission coefficient is measured between two coplanar electrodes on the organ surface which are capacitively coupled by the electric fringing fields extending into the organ. An appropriately constructed transmission probe can sense the dielectric behaviour of tissue into deeper organ regions than a reflection probe of corresponding size. Results are reported of transmission probe measurements on various liquids with known dielectric properties for the purpose of probe testing and on a pig liver during ischemia.

Animals↗
PubMedDate not supplied

[Respiratory mechanics in animals. 2. The forced oscillation technique--review].

The analysis of respiratory mechanics using the forced oscillation technique (FOT) was originally developed for human medicine. Since this technique can be used during spontaneous breathing and does not require patient cooperation, it is generally applicable to conscious animals. In contrast to conventional methods to characterise the mechanics of breathing, FOT has the advantage of being non-invasive. Compared to the classical method which takes mechanical parameters (resistance, inertance, compliance) into account, the forced oscillation techniques utilises electrical parameters (resistance, inductance, capacitance). Briefly, in the forced oscillation technique, externally generated test signals are sent to the respiratory system under investigation. These test signals are superimposed on spontaneous breathing. In order to examine respiratory mechanics, the response of the respiratory system to these test signals is analysed and interpreted. The complex respiratory impedance resulting from this analysis contains information concerning the resistive, capacitive and inductive properties of the respiratory system. In general, FOT is a portable technique which can be applied to several kinds of animals.

Airway Resistance↗
PubMed2007

Electrokinetic measurements of dielectric properties of membrane for apoptotic HL-60 cells on chip-based device.

The specific membrane capacitance and conductance of mammalian cells reflect the surface morphological complexities and barrier functions of cell membrane, respectively, and could potentially respond to cell physiological and pathological changes in a measurable manner. In this study, an electrokinetic system was developed by using negative dielectrophoretic force (nDEP force) assisted positioning and electroroation (ROT) measurement. Numerical simulations regarding the geometric model of the electrode were performed primarily for the electric field analysis. The dielectric responses of membrane for apoptotic HL-60 cells induced by bufalin were detected. The membrane capacitance of the cells was found to fall from an initial value of 15.6 +/- 0.9 mF/cm(2) to 6.4 +/- 0.6 mF/cm(2) after a 48 h treatment with 10 nM bufalin. However, the membrane conductance remained almost constant at (2.25 +/- 1.1) x 10(3) S/m(2) during the first 12 h of bufalin treatment and then increased distinctly to (4.2 +/- 1.3) x 10(3) S/m(2) thereafter. Scan electron microscopy (SEM) studies of the cells revealed a decreased complexity in cell membrane morphology following bufalin treatments, suggesting that the observed changes in the membrane capacitance was dominated by the alterations of cell surface structures. The results demonstrate that the ROT technique gives a quantitative analysis of the toxic damage by chemicals to cells and can be exploited in the testing and development of new pharmaceuticals and active cell agents.

Animals↗
PubMed2005

Systematic evaluation of the determinants of defibrillation efficacy.

OBJECTIVES: We studied the effect of varying shock capacitance, shock impedance, and pulse duration on defibrillation efficacy in a randomized, crossover manner for biphasic shocks. BACKGROUND: The relationship between the electrical determinants of defibrillation efficacy is incompletely understood. METHODS: Biphasic shocks were delivered to 12 dogs through epicardial patches (to vary impedance) after 15 seconds of ventricular fibrillation using one of 100- or 155-muF capacitors at each of four pulse durations (2.5, 5, 10, 20 ms), in a balanced random order. There were two impedance groups: six with higher impedance (mean 97 +/- 15 Omega, range 80-120) and six with lower impedance (mean 39 +/- 3 Omega, range 34-44). Voltage requirements were estimated as the average of three defibrillation threshold (DFT) tests. RESULTS: Shock capacitance, resistance, and pulse duration all had significant effects upon the minimum voltage DFT (P = .0065, P = .0066, and P = .0001, respectively). The tilt associated with the lowest voltage and current requirement for each of the four capacitance/resistance combinations varied widely, between 34 +/- 5% and 63 +/- 3%, depending on capacitance and impedance. The optimal pulse duration associated with minimum DFT lies between 5.11 and 5.34 ms. CONCLUSIONS: Defibrillation voltage requirements for biphasic shocks are affected by pulse duration, capacitance and impedance, but not "tilt."

Animals↗
PubMed1984

Electrophotoluminescence and the electrical properties of the photosynthetic membrane. I. Initial kinetics and the charging capacitance of the membrane.

Preilluminated chloroplast membranes, and particularly hypotonically swollen vesicles (blebs), give rise to a strong characteristic luminescence (electrophotoluminescence, EPL; Ellenson and Sauer, 1976, Photochem. Photobiol., 23:113-123; Arnold and Azzi, 1971, Photochem. Photobiol., 14:233-240) during the application of a strong external electric field. A detailed kinetic study of EPL was carried out and the initial kinetics from the field onset are reported here. The fast rise time (less than 0.2 mus) of the applied external electric field together with a high instrumental time resolution allowed the observation of a characteristic delay (lag time) between the field onset and the appearance of the induced emission. The lag time decreased with increase in the applied field strength and/or the conductivity of the suspension and is interpreted to be a consequence of (a) the necessity to reach a threshold electrical potential difference in the bleb membrane, below which no emission can be triggered, and (b) the finite time required to attain such a transmembranal field during the charging process of the membrane. A quantitative analysis, connecting the lag time, the controllable experimental parameters, and the membrane electrical characteristics is presented. Its verification was carried out in both size-selected and heterogeneous bleb populations. In the latter, experiments were consistent with the assumption that the lag time reflects the charging of the largest blebs. The results indicate (a) the possibility of directly measuring the specific membrane capacitance, yielding an estimate of Cm = 1.2 +/- 0.3 microF/cm2 (the precision being particle size-homogeneity dependent); (b) A minimal transmembranal potential difference (of approximately 240 mV) is necessary to induce electrophotoluminescence; and (c) the lag duration depends on the time elapsed between the preillumination and the external field application. Correlated with the study of ionophore effects on the lag time, this suggests additivity of the light- and field-induced transmembrane potentials in attaining the threshold for emission.

Chloroplasts↗
PubMed1980

I. Novel capacitative electrode with a wide frequency range for measurements of flash-induced changes of interface potential at the oil-water interface. Mechanical construction and electrical characteristics of the electrode.

The mechanical construction and the electrical properties of a new type of capacitative electrode for the oil-water interface are described. The electrode is designed to detect changes of the interface potential induced by photochemical, photophysical, and photobiological reactions occurring at the interface. The construction is based on capacitative coupling of two aqueous compartments separated by a thin Teflon film. Thereby, the oil-water interface is in horizontal position and the electrode is placed with its planar bottom about 10 micrometer above the interface. A main feature of the electrode is the transparency to visible light which is achieved by having a clear electrolyte solution in the inner compartment of the capacitative electrode. The aqueous subphase and the inner electrolyte are connected with Ag/AgCl electrodes to voltage amplifiers. The capacitative electrode is best operated under open circuit conditions. The frequency range experimentally verified is 500 MHz larger than or equal to 0.03 Hz. The sensitivity is mainly determined by the noise of the electronic amplifiers, typical 50-100 muV.

Electric Conductivity↗
PubMedDate not supplied

[Parametric resonance and amplification of periodic disturbances in membranes containing ion channels with inactivation].

Parametric resonance and amplification of periodic perturbations in the membrane transport of ions through channels with inactivation was studied in computational experiments. It has been shown that a periodic change in the membrane capacitance or in the applied electric current with a frequency approximately 2 omega 0 (omega 0--the own angular frequency of the membrane) may excite stable self-oscillations in the membrane with a frequency of approximately omega 0. For this to occur, the degree of the capacitance modulation m or the amplitude of the applied current i0 must exceed some critical values mcr and i0cr. Excitation of self-oscillations by alternating electric current of the frequency approximately 2 omega 0 has the characteristics of parametric resonance. This can be explained by the fact that the equivalent membrane inductance depends on ionic current and displays periodic changes with a frequency approximately 2 omega 0, as also does the current. Small-amplitude periodic changes in the capacitance (m less than mcr) with frequencies approximately 2 omega 0 may result in significant amplification of periodic perturbations with frequencies approximately omega 0.

Biological Transport↗
PubMed1990

On-line bioelectric impedance during haemodialysis: monitoring of body fluids and cell membrane status.

We have measured by a computed integrated system (BIA 109, RJL AKERN) the changes of bio-impedance (BI) deriving from a tetrapolar system working on 800 microA, 50 kHz current, in 23 haemodialysed patients. Resistance (R) and reactance (Xc) have been continuously monitored during haemodialysis in each patient. Resistance was strictly inversely correlated to the decrease of body weight (r = 0.82). Also, Xc increased almost constantly. In most of the patients the increase of Xc was proportionally greater than R, resulting in an increase of phase angle (PA). However, Xc showed a transient decrease in response to seven severe symptomatic hypotensive episodes, whereas R maintained the increasing trend, causing a sharp reduction of phase angle. As Xc is an expression of storage of electrical charge by the cells acting as condensers, and phase angle quantifies the active capacitive component in relation to passive electrical resistance, these parameters may be important to evaluate cell membrane function. In fact, the univocal increase of R, Xc and phase angle observed during normal unevenful haemodialysis probably indicates improvement of cellular activities due to the depurative treatment. On the contrary, the transient reduction of Xc and phase angle observed during hypotensive crises may be an expression of cellular distress because of a too rapid ultrafiltration.

Adult↗
PubMed1993

A GTP-dependent step in the activation mechanism of capacitative calcium influx.

Calcium influx in electrically non-excitable cells is regulated by the filling state of intracellular calcium stores. Depletion of stores activates plasma membrane channels that are voltage-independent and highly selective for Ca2+ ions. We report here that the activation of plasma membrane Ca2+ currents induced by depletion of Ca2+ stores requires a diffusible cytosolic factor that washes out with time when dialyzing cells in the whole-cell configuration of the patch-clamp technique. The activation of calcium release-activated calcium current (ICRAC) by ionomycin- or inositol 1,4,5-trisphosphate-induced store depletion is blocked by guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) and guanyl-5'-yl imidodiphosphate, non-hydrolyzable analogs of GTP, suggesting the involvement of a GTP-binding protein. The inhibition by GTP gamma S occurs at a step prior to the activation of ICRAC and is prevented by the addition of GTP. We conclude that the activation mechanism of depletion-induced Ca2+ influx encompasses a GTP-dependent step, possibly involving an as yet unidentified small GTP-binding protein.

Aluminum Compounds↗
PubMed2002

Classical properties of low-dimensional conductors: giant capacitance and non-ohmic potential drop.

The electrical field arising around an inhomogeneous conductor when an electrical current passes through it is not screened, as distinct from 3D conductors, in low-dimensional conductors. As a result, the electrical field depends on the global distribution of the conductivity sigma(x) rather than on the local value of it, inhomogeneities of sigma(x) produce giant capacitances C(omega) that show frequency dependence at relatively low omega, and electrical fields develop in vast regions around the inhomogeneities of sigma(x). A theory of these phenomena is presented for 2D conductors.

Journal Article↗
PubMed1998

Low-conductance intercellular coupling between mouse chromaffin cells in situ.

1. Patch-clamp experiments were used to compare membrane properties of mouse chromaffin cells in thin tissue slices and of isolated cells in primary culture. The mean membrane input resistance (R(in)) and membrane capacitance were 3.1 +/- 0.6 G omega and 9.1 +/- 0.5 pF in situ and 9.9 +/- 1.8 G omega and 8.2 +/- pH in isolated cells. 2. Spike-like currents were observed on top of the calcium currents during depolarizations in thirty out of forty-nine cells in situ. They were not seen in isolated cells nor after addition of Cd2+ (100 microM) and TTX (10 microM) to the perfusate of the slices. The mean R(in) of cells which displayed current spikes (2.3 +/- 0.18 G omega) was significantly smaller than that of cells lacking spikes (3.9 +/- 0.25 G omega). It is suggested that the current spikes represent intercellular currents which result from action potential firing in neighbouring cells during the depolarization of the patch-clamped cell. 3. Investigation of capacitative currents induced by square voltage pulses showed a slow component in twenty-four out of twenty-seven cells in situ. 4. It is concluded that a large fraction of mouse chromaffin cells in situ are electrically coupled. From the slow capacitative currents and the amplitude of the intercellular current spikes a junctional conductance between chromaffin cells of below 1 nS was deduced. 5. This junctional conductance appears to be too low to support spreading of electrical activity in cases where a single cell is stimulated by an action potential. However, the junctional conductance could allow longer depolarizations of one cell or simultaneous firing of several cells to trigger electrical activity in neighbouring cells.

Animals↗
PubMed1982

Discrete changes of cell membrane capacitance observed under conditions of enhanced secretion in bovine adrenal chromaffin cells.

The capacitance of the surface membrane of small adrenal chromaffin cells was measured with patch-clamp pipettes. Continuous and discrete changes of capacitance were observed. They were interpreted as changes of surface area connected to exocytotic or endocytotic processes. Most of the measurements were performed in the "whole-cell" recording configuration [Hamill, O. P., Marty, A., Neher, E., Sakmann, B. & Sigworth, F. J. (1981) Pflügers Arch. 391, 85-100], which allows the intracellular Ca2+ concentration to be controlled. With an internal solution highly buffered to low values of Ca2+ concentration (10 nM), the surface capacitance usually decreased and could not be markedly changed by electrical stimulation. At low buffering capacity and medium Ca2+ concentrations (0.1-1 microM), the capacitance measurement showed large fluctuations and discrete steps, reflecting both capacitance decrease and increase. A large transient increase of capacitance could be induced by electrical stimulation under these conditions. It was linked to Ca2+ currents through the membrane. Relatively large (2-6 x 10(-14) F) steps of capacitance decrease were common after extensive stimulation. The size distribution of step-like capacitance changes is well compatible with the idea that steps of capacitance increase reflect individual events of exocytosis of chromaffin granules, whereas steps of the opposite polarity reflect the formation of vesicles or vacuoles by endocytosis.

Animals↗
PubMed2006

Capacitance fluctuations causing channel noise reduction in stochastic Hodgkin-Huxley systems.

Voltage-dependent ion channels determine the electric properties of axonal cell membranes. They not only allow the passage of ions through the cell membrane, but also contribute to an additional charging of the cell membrane resulting in the so-called capacitance loading. The switching of the channel gates between an open and a closed configuration is intrinsically related to the movement of gating charge within the cell membrane. At the beginning of an action potential, the transient gating current is opposite to the direction of the current of sodium ions through the membrane. Therefore, the excitability is expected to become reduced due to the influence of a gating current. Our stochastic Hodgkin-Huxley-like modeling takes into account both the channel noise-i.e. the fluctuations of the number of open ion channels-and the capacitance fluctuations that result from the dynamics of the gating charge. We investigate the spiking dynamics of membrane patches of a variable size and analyze the statistics of the spontaneous spiking. As a main result, we find that the gating currents yield a drastic reduction of the spontaneous spiking rate for sufficiently large ion channel clusters. Consequently, this demonstrates a prominent mechanism for channel noise reduction.

Action Potentials↗
PubMed1994

Iontophoresis of a model peptide across human skin in vitro: effects of iontophoresis protocol, pH, and ionic strength on peptide flux and skin impedance.

This study deals with effects of electrical (current density, frequency and duty cycle) and chemical (buffer pH and ionic strength) conditions on the flux of the octapeptide, 9-desglycinamide, 8-arginine-vasopressin (DGAVP), through dermatomed human skin. A pulsed constant current was applied during iontophoresis. The anode faced the anatomical surface of the skin samples inside the diffusion cells. The resistive and capacitative components of the equivalent electrical circuit of human skin could be calculated by fitting the voltage response to a bi-exponential equation. The skin resistance prior to iontophoresis varied between 20 and 60 k omega.cm2. During iontophoresis a decrease of skin resistance and an increase of the series capacitances was observed, which were most pronounced during the first hour of iontophoresis; thereafter both quantities gradually levelled off to an apparent steady state value. The reduction of the resistance during iontophoresis increased non-linearly with increasing current density between 0.013-0.64 mA.cm-2. The steady state resistance and capacitances did not vary significantly with frequency and duty cycle of the current pulse. There was no pH dependence of skin resistance at steady state. Between pH 4 and 10, the steady state peptide flux had a bell-shaped pH-dependence with a maximum of 0.17 nmol.cm-2.h-1 at pH 7.4, which is close to the I.E.P. of the peptide. Lowering the ionic strength from 0.15 to 0.015 M NaCl increased the steady state flux at pH 5 and pH 8 by a factor 5 to 0.28 +/- 0.21 and 0.48 +/- 0.37 nmol.cm-2.h-1, respectively. Together these observations suggested that DGAVP is transported predominantly by volume flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Arginine Vasopressin↗
PubMed2002

Investigation of interfacial capacitance of Pt, Ti and TiN coated electrodes by electrochemical impedance spectroscopy.

Electrochemical processes at the electrode-electrolyte (body fluid) interface are of ultimate importance for stimulating/sensing electrode function. A high electrode surface area is desirable for safe stimulation through double-layer charging and discharging. Pt and Pt-Ir alloys have been the most common electrode materials. The use of TiN coating as the surface layer on the electrode has found increasing interest because of its metal-like conductivity, excellent mechanical and chemical properties, and the fact that it can be deposited with a high surface area. In this work, electrochemical impedance spectroscopy (EIS), which is a sensitive and non-destructive technique and widely used for characterization of electrical properties of electrode-electrolyte interfaces, was applied to investigate pure Pt and Ti, and TiN coated electrodes exposed to a phosphate-buffered-saline (PBS) solution. Platinized Pt and Ti were also studied for comparison. The capacitance value of the electrodes in PBS was obtained through quantitative analysis of the EIS spectra. The results reveal that the capacitance of the TiN coated electrodes with a rough surface is several hundreds times higher than that of a smooth Pt surface. Platinization of Ti can also increase the capacitance to the same extent as platina. EIS has been shown to be a powerful technique for characterization of stimulating/sensing electrodes.

Coated Materials, Biocompatible↗
PubMed2002

Current applications of electrotherapeutics in collagen healing.

Electrical current flow appears to be integral to the healing of collagen containing tissue, i.e., bone, cartilage, ligaments, tendons and skin. Accordingly, it is reasonable to hypothesize that externally applied electrical fields should be able to enhance healing, especially in conditions that have resisted more standard treatments. Nevertheless, applications of electrotherapeutics is challenging because the precise mechanism of action is unknown and, accordingly, there is an almost unlimited combination of stimulation parameters (e.g., type of waveform, voltage, current, phase, frequency, etc.) that can be applied to a treatment site. Presently, of the three major types of electrical stimulation, i.e., direct, and capacitive and inductive coupling, there is a growing trend toward utilization of the latter because of its efficacy and greater margin of safety. Although the mechanisms of action for enhanced healing of all three types remain elusive there is increasing evidence that electrical stimulation exerts its influence via effects at the cellular and/or molecular levels within the tissue. Utilization of electrotherapeutics has been most prevalent in bony injuries resistant to healing, but applications to severe lesions of skin and ligaments, and even to degenerative joint disease seems promising as cartilage has been shown to be more responsive than bone to applied electrical energy. We conclude that there is a clear trend toward greater orthopedic utilization of inductive stimulation and that, despite the lack of definitive guidelines relating specific parameters with specific conditions, electrotherapeutics appears to be a safe and often effective treatment for collagen containing tissues in many cases in which more standard therapies have failed.

Journal Article↗