Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Electric Capacitance”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,459 records · Page 81Linked to original sources
PubMed1995

Toward the ultimate metal microelectrode.

The performance of metal microelectrodes for stimulating and recording neuronal action potentials depends on precise control of their geometrical, electrical and mechanical properties. We describe a combination of materials whose properties approach fundamental physical limitations on achievable performance and reproducible fabrication techniques that provide probes with very small dimensions. Pure iridium wire is electrolytically sharpened, vapor-coated with Parylene-C insulation and the tip exposed using an automatically steerable UV laser. Electrochemical activation of the iridium increases the capacitance of the metal-electrolyte interface so that the overall impedance in the relevant frequency band (100-10,000 Hz) is dominated by the access resistance of the surrounding tissues.

Electric Conductivity↗
PubMed1974

Studies of impedance in cardiac tissue using sucrose gap and computer techniques. II. Circuit simulation of passive electrical properties and cell-to-cell transmission.

The impedance measured in a strip of heart tissue from the moth Hyalophora cecropia is fitted by circuit models of several configurations. The circuits include: (a) a single R-C circuit (b) a double R-C circuit (c) terminated transmission lines, and (d) a pattern of cells with cell-to-cell transmission paths. The last of these is found to give the best fit. Calculation of the model impedances and optimization of element values are performed by a computer. The possibility that the mechanism of cell-to-cell transmission may be capacitative rather than conductive is explored using values of capacitance derived from the circuit models to calculate the effect of capacitative coupling alone on signal transmission. The calculations show that sufficient voltage can be transmitted from the excited cell to an adjacent cell to effect excitation.

Animals↗
PubMed1975

Circulatory responses to stimulation of the carotid body chemoreceptors in the cat.

Cardiovascular responses to carotid body chemoreceptor stimulation were followed in the 'curairzed', vagotomized, artificially ventilated cat. Stimulation of the chemoreceptors by perfusion of the carotid sinus regions with venous blood induced a reflex vasconstriction in skeletal muscle, kidney, intestine and skin, and, in most cases, an increased heart rate. A comparison of the chemoreceptor reflex responses with those obtained by direct electrical stimulation of the regional vasomotor fibres indicated that in chemorecptor reflexes the vasomotor fibre activity increased to the same extent in skeletal muscle and intestinal resistance vessels and, probably, in the nutritional skin vessels but to a smaller extent in the renal vessels and the skeletal muscle capacitance vessels. The renal vessels and the muscle capacitance vessels seemed, however, to respond more during chemoreceptor stimulation than when the barorecptors were unloaded, indicating that anexcitation of the bulbar vasomotor centre is more effective than the withdroawal of an inhibitory restraint in activating the vasomotor fibres to these vascular sections.

Animals↗
PubMed1984

Action potential and membrane currents of single pacemaker cells of the rabbit heart.

Single, viable pacemaker cells were isolated from sinoatrial (S-A) and atrioventricular (A-V) nodes by treating with collagenase. In normal Tyrode solution containing 1.8 mM Ca2+, these pacemaker cells had a round configuration and contracted rhythmically at a frequency of about 150-260/min. The amplitude, duration, and maximum rate of rise of the spontaneous action potentials recorded using patch clamp electrodes were similar to those obtained from multicellular preparations. Amplitudes of the recorded membrane current were normalized with reference to the surface area of the cell by assuming the cell shape as a plane oblate spheroid. The membrane resistance of the isolated nodal cells was 14.9 +/- 4.0 k omega . cm2 (n = 12) at about -35 mV and the membrane capacitance was 1.30 +/- 0.24 microF/cm2 (n = 18). The inactivation time course of the slow inward current, isi, was fitted with a sum of two exponentials with time constants of 6.7 +/- 0.6 ms and 46.6 +/- 15.3 ms (n = 4) at +10 mV. The amplitude of isi peaked at 0 approximately +10 mV in the current-voltage relationship and was 18.2 +/- 8.4 microA/cm2. The potassium current, iK, was activated in the voltage range positive to -50 mV and was saturated at about +20 mV. The amplitude of the fully-activated iK at -40 mV was 3.3 +/- 1.4 microA/cm2 (n = 10) and showed an inward-going rectification. The activation of the hyperpolarization-activated current was observed at potentials negative to -70 mV in seven of 14 experiments. The current density and membrane capacitance calculated could be overestimated and the membrane resistance underestimated, because of the presence of caveolae on the cell surface. However, these data give the nearest possible estimates of the electrical constants in the nodal cells, which cannot be measured accurately in the conventional multicellular preparations.

Animals↗
PubMed2001

Robust, high-resolution, whole cell patch-clamp capacitance measurements using square wave stimulation.

High-resolution, whole cell capacitance measurements are usually performed using sine wave stimulation using a single frequency or a sum of two frequencies. We present here a high-resolution technique for whole-cell capacitance measurements based on square-wave stimulation. The square wave represents a sum of sinusoidal frequencies at odd harmonics of the base frequency, the amplitude of which is highest for the base frequency and decreases as the frequency increases. The resulting currents can be analyzed by fitting the current relaxations with exponentials, or by a phase-sensitive detector technique. This method provides a resolution undistinguishable from that of single-frequency sine wave stimulation, and allows for clear separation of changes in capacitance, membrane conductance, and access resistance. In addition, it allows for the analysis of more complex equivalent circuits as associated with the presence of narrow fusion pores during degranulation, tracking many equivalent circuit parameters simultaneously. The method is insensitive to changes in the reversal potential, pipette capacitance, or widely varying cell circuit parameters. It thus provides important advantages in terms of robustness for measuring cell capacitances, and allows analysis of complicated changes of the equivalent circuits.

Animals↗
PubMed1993

Charge transport by ion translocating membrane proteins on solid supported membranes.

A new method for the investigation of ion translocating membrane proteins is presented. Protein containing membrane fragments or vesicles are adsorbed to a solid supported membrane. The solid supported membrane consists of a lipid monolayer on a gold evaporated or gold sputtered glass substrate which is coated with a long chained mercaptan (CH3(CH2)mSH, m = 15, 17). Specific conductance and specific capacitance of the solid supported membrane are comparable to those of a black lipid membrane. However, the solid supported membrane has the advantage of a much higher mechanical stability. The electrical activity of bacteriorhodopsin, Na,K-ATPase, H,K-ATPase, and Ca-ATPase on the solid supported membrane is measured and compared to signals obtained on a conventionally prepared black lipid membrane. It is shown that both methods yield similar results. The solid supported membrane therefore represents an alternative method for the investigation of electrical properties of ion translocating transmembrane proteins.

Adenosine Triphosphatases↗
PubMed2000

A nanometre-scale electronic switch consisting of a metal cluster and redox-addressable groups.

So-called bottom-up fabrication methods aim to assemble and integrate molecular components exhibiting specific functions into electronic devices that are orders of magnitude smaller than can be fabricated by lithographic techniques. Fundamental to the success of the bottom-up approach is the ability to control electron transport across molecular components. Organic molecules containing redox centres-chemical species whose oxidation number, and hence electronic structure, can be changed reversibly-support resonant tunnelling and display promising functional behaviour when sandwiched as molecular layers between electrical contacts, but their integration into more complex assemblies remains challenging. For this reason, functionalized metal nanoparticles have attracted much interest: they exhibit single-electron characteristics (such as quantized capacitance charging) and can be organized through simple self-assembly methods into well ordered structures, with the nanoparticles at controlled locations. Here we report scanning tunnelling microscopy measurements showing that organic molecules containing redox centres can be used to attach metal nanoparticles to electrode surfaces and so control the electron transport between them. Our system consists of gold nanoclusters a few nanometres across and functionalized with polymethylene chains that carry a central, reversibly reducible bipyridinium moiety. We expect that the ability to electronically contact metal nanoparticles via redox-active molecules, and to alter profoundly their tunnelling properties by charge injection into these molecules, can form the basis for a range of nanoscale electronic switches.

Journal Article↗
PubMed1997

Effects of chronic growth hormone treatment in aged rats on the biophysical and pharmacological properties of skeletal muscle chloride channels.

1. The effects of a 4-month daily treatment with recombinant human growth hormone (GH) (150 micrograms kg-1) to aged rats were evaluated on the passive and active membrane electrical properties of extensor digitorum longus (EDL) muscle fibres in vitro by means of a two intracellular microelectrode technique. 2. Chronic GH treatment completely restored the diameter and the membrane capacitance of aged EDL muscle fibres and significantly lowered the membrane resistance towards the adult value. There was also an increase of the threshold current, a shortening of the latency and an increase of the amplitude of the action potential and a significant amelioration of the membrane firing capability. 3. The effects were almost fully attributable to a significant 50% increase of resting conductance to chloride ions (GCl), although an observed restoration of potassium conductance and a possible effect on voltage-activated sodium channels could contribute to the effects. 4. EDL muscle fibres of untreated aged rats showed a different pharmacological response to 2-(p-chlorophenoxy) propionic acid (CPP) enantiomers from that seen in adult rats; the S-(-) isomer was less potent in blocking GCl and the R-(+) isomer always increased GCl instead of producing the typical biphasic effect observed in adult fibres (an increase of GCl at 1-10 microM and a decrease at higher concentrations). The 4-month-GH-treated aged rats showed a pharmacological sensitivity to CPP enantiomers similar to that of adults. 5. The in vitro application of insulin-like growth factor I (IGF-I), the peripheral mediator of GH, produced a significant and irreversible increase of GCl of EDL muscle of EDL muscle of untreated aged rats, an effect not observed in adults. This effect was completely inhibited by preincubation with 0.5 microM okadaic acid, suggesting that the IGF-I receptor transduction pathway can act on the phosphorylation state of the chloride channel through a serine-threonine protein phosphatase. 6. The results show that the skeletal muscle chloride channel is a target of the impairment of GH/IGF-I axis occurring in aged subjects. The acute and chronic effects observed on GCl of aged muscle fibres suggest that the GH/IGF-I stimuli act through a modulation of channel phosphorylation state and through the synthesis of 'adult'-like type chloride channels.

Aging↗
PubMed1983

Continuous registration of membrane input resistances of small plant cells using a double-pulse current clamp technique for single-electrode impalements : comparison with the conventional two-electrode method.

To measure the cell input resistance in Elodea leaf cells, a new single-microelectrode method was explored by comparing the results with conventional two-microelectrode experiments. The new method takes advantage of the difference in the frequency response curves between electrode and cell impedances. By application of electrical stimuli, which contain specific frequency bands, the different impedances can be analyzed separately. To get a distinct separation in the frequency response of cell and electrode, respectively, the electrode capacitance has to be compensated during the impalement. Different time constants of the cell membrane can be accounted for by adjustment of the stimulus length. It is shown that both the single- and the double-electrode method yield the same results, even if the cell input resistances change considerably during the course of the experiment. This demonstrates the usefulness of the new single-electrode method for continuous measurements of cell membrane resistances, especially in cells so small that the double-electrode method is no longer applicable.

Journal Article↗
PubMed2000

ATP-sensitive potassium channels in capillaries isolated from guinea-pig heart.

The full-length cDNAs of two different alpha-subunits (Kir6.1 and Kir6.2) and partial cDNAs of three different beta-subunits (SUR1, SUR2A and SUR2B) of ATP-sensitive potassium (KATP) channels of the guinea-pig (gp) were obtained by screening a cDNA library from the ventricle of guinea-pig heart. Cell-specific reverse-transcriptase PCR with gene-specific intron-spanning primers showed that gpKir6.1, gpKir6.2 and gpSUR2B were expressed in a purified fraction of capillary endothelial cells. In cardiomyocytes, gpKir6.1, gpKir6.2, gpSUR1 and gpSUR2A were detected. Patch-clamp measurements were carried out in isolated capillary fragments consisting of 3-15 endothelial cells. The membrane capacitance measured in the whole-cell mode was 19.9 +/- 1.0 pF and was independent of the length of the capillary fragment, which suggests that the endothelial cells were not electrically coupled under our experimental conditions. The perforated-patch technique was used to measure the steady-state current-voltage relation of capillary endothelial cells. Application of K+ channel openers (rilmakalim or diazoxide) or metabolic inhibition (250 microM 2,4-dinitrophenol plus 10 mM deoxyglucose) induced a current that reversed near the calculated K+ equilibrium potential. Rilmakalim (1 microM), diazoxide (300 microM) and metabolic inhibition increased the slope conductance measured at -55 mV by a factor of 9.0 (+/-1.8), 2.5 (+/-0.2) and 3.9 (+/-1.7), respectively. The effects were reversed by glibenclamide (1 microM). Our results suggest that capillary endothelial cells from guinea-pig heart express KATP channels composed of SUR2B and Kir6.1 and/or Kir6.2 subunits. The hyperpolarization elicited by the opening of KATP channels may lead to an increase in free cytosolic Ca2+, and thus modulate the synthesis of NO and the permeability of the capillary wall.

Adenosine Triphosphate↗
PubMed1990

The fundamental law of electrostimulation and its application to defibrillation.

Around the turn of the last century, there was an intensive discussion among physiologists as to whether there is a law describing the phenomena of electrostimulation and which formula may best approximate it mathematically. J.L. Hoorweg found in 1892 that the voltage at which a capacitor must be charged to elicit an excitation, was a function of the capacitance in an inverse correlation. G. Weiss reported in 1901 that according to his investigations a linear relationship existed between the duration of a pulse and the corresponding quantity of electricity applied and called it "formule fondamentale." We are now able to give the "fundamental formula" a physical interpretation that yields, as result, the electric field produced by the electrode acting on the excitable membrane. The electric field in the extracellular space is transformed by the cell geometry ratio: cell length to membrane thickness yielding a high transmembrane field capable of reducing the inherent electric field to its threshold level. The consequences drawn from this hypothesis are remarkable and (should) have an influence on all applications of electrostimulation including the discussions on defibrillation. The application of the stimulation theory to defibrillation yields as results: (1) The basic engineering principle of defibrillation is to produce an electric field within the ventricles of 400 V/m or more. An orthogonal pulse application may reduce the energy requirements, as more fibers are longitudinally reached by the electric field; (2) The shape of the defibrillation pulse and its polarity plays no role. Consequently it follows that biphasic pulses must be less efficient than monophasic pulses, if they are close to the chronaxie; and (3) The most serious disadvantage in today's defibrillation practice is its dose characterization in "energy"; but this physical quantity cannot be justified in the light of the fundamental law of electrostimulation.

Animals↗
PubMed1986

Electrophysiological characterization of single pregnant rat myometrial cells in short-term primary culture.

Smooth muscle cells were isolated from the longitudinal layer of pregnant rat myometrium (18-19 days) and studied either freshly dissociated or during short-term primary culture (until 30 h) using intracellular microelectrode techniques and direct microscopic observation. The isolated myometrial cells excluded trypan blue vital stain and could repetitively contract in response to various stimuli. Electrophysiological studies at 37 degrees C showed normal resting potential (-54.5 +/- 7.5 mV, n = 71). Action potentials with overshoot (+7.8 +/- 4.6 mV, n = 71) could be elicited by intracellular stimulation. Moreover, the membrane potential was largely dependent on the external K+ concentration. The action potential was suppressed in a Ca2+-free solution [with 0.1 mM ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid], and the overshoot amplitude was clearly Ca2+ dependent. The action potential was inhibited by Mn2+ ions (1 mM), Co2+ ions (1 mM), and D 600 (1 microM) but was unaffected by tetrodotoxin (2 microM) and external Na+ removal. Tetraethylammonium chloride (TEA, 10 mM) and 4-aminopyridine (4-AP, 10 mM) increased both overshoot amplitude and duration of the electrical responses. When the cell surface area was measured with light microscopy, the mean specific membrane resistance was 14.8 +/- 4.6 k omega . cm2 (n = 14), and the mean specific membrane capacitance was 2.3 +/- 0.7 microF/cm2 (n = 14). Outward-going rectification was consistently observed in all cells examined. This was either inhibited by TEA and 4-AP (10 mM each) or reduced in the presence of 1 mM Mn2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗
PubMed2006

Factors affecting the stimulus artifact tail in surface-recorded somatosensory-evoked potentials.

Surface-recorded somatosensory-evoked potentials (SEPs) are neural signals elicited by an external stimulus. In the case of electrically induced SEPs, the artifact generated by the stimulation process can severely distort the signal. In some cases, the artifact tail often lasts well into the initiation of the SEP making the determination of absolute latency very difficult. In this work, a new approach was taken to identify factors that affect the tail of the artifact. The methodology adopted was the development of a lumped electrical circuit model of the artifact generation process. While the modeling of the instrumentation hardware is relatively simple, this is not the case with tissue and electrode/skin interface effects. Consequently, this paper describes a novel tissue modeling approach that uses an autoregressive moving average (ARMA) parametric technique and an artificial neural network (ANN) to estimate tissue parameters from experimental data. This coupled with an estimation of the stimulation electrode-skin impedance completes the lumped circuit model. Simulink (The Mathworks Inc.) was used to evaluate the model under several different conditions. These results show that both the stimulation electrode-skin interface impedance and nature of the body tissue directly under the recording electrodes have a profound effect on the appearance of the stimulus artifact tail. This was verified by experimentally recorded data obtained from the median nerve using surface electrodes. Conclusions drawn from this work include that stimulation electrodes with low series capacitance should be used whenever possible to minimize the duration of the artifact tail.

Arm↗
PubMed2001

Development of electrophysiological and morphological diversity in autonomic neurons.

The generation of neuronal diversity requires the coordinated development of differential patterns of ion channel expression along with characteristic differences in dendritic geometry, but the relations between these phenotypic features are not well known. We have used a combination of intracellular recordings, morphological analysis of dye-filled neurons, and stereological analysis of immunohistochemically labeled sections to investigate the development of characteristic electrical and morphological properties of functionally distinct populations of sympathetic neurons that project from the celiac ganglion to the splanchnic vasculature or the gastrointestinal tract of guinea pigs. At early fetal stages, neurons were significantly more depolarized at rest compared with neurons at later stages, and they generally fired only a single action potential. By mid fetal stages, rapidly and slowly adapting neurons could be distinguished with a topographic distribution matching that found in adult ganglia. Most rapidly adapting neurons (phasic neurons) at this age had a long afterhyperpolarization (LAH) characteristic of mature vasomotor neurons and were preferentially located in the lateral poles of the ganglion, where most neurons contained neuropeptide Y. Most early and mid fetal neurons showed a weak M current, which was later expressed only by rapidly-adapting and LAH neurons. Two different A currents were present in a subset of early fetal neurons and may indicate neurons destined to develop a slowly adapting phenotype (tonic neurons). The size of neuronal cell bodies increased at a similar rate throughout development regardless of their electrical or neurochemical phenotype or their topographical location. In contrast, the rate of dendritic growth of neurons in medial regions of the ganglion was significantly higher than that of neurons in lateral regions. The apparent cell capacitance was highly correlated with the surface area of the soma but not the dendritic tree of the developing neurons. These results demonstrate that the well-defined functional populations of neurons in the celiac ganglion develop their characteristic electrophysiological and morphological properties during early fetal stages of development. This is after the neuronal populations can be recognized by their neurochemical and topographical characteristics but long before the neurons have finished growing. Our data provide strong circumstantial evidence that the development of the full phenotype of different functional classes of autonomic final motor neurons is a multi-step process likely to involve a regulated sequence of trophic interactions.

Action Potentials↗
PubMed1999

Modeling electroporation in a single cell. I. Effects Of field strength and rest potential.

This study develops a model for a single cell electroporated by an external electric field and uses it to investigate the effects of shock strength and rest potential on the transmembrane potential V(m) and pore density N around the cell. As compared to the induced potential predicted by resistive-capacitive theory, the model of electroporation predicts a smaller magnitude of V(m) throughout the cell. Both V(m) and N are symmetric about the equator with the same value at both poles of the cell. Larger shocks do not increase the maximum magnitude of V(m) because more pores form to shunt the excess stimulus current across the membrane. In addition, the value of the rest potential does not affect V(m) around the cell because the electroporation current is several orders of magnitude larger than the ionic current that supports the rest potential. Once the field is removed, the shock-induced V(m) discharges within 2 micros, but the pores persist in the membrane for several seconds. Complete resealing to preshock conditions requires approximately 20 s. These results agree qualitatively and quantitatively with the experimental data reported by Kinosita and coworkers for unfertilized sea urchin eggs exposed to large electric fields.

Cell Membrane↗
PubMed1996

Experimental model for an ectopic focus coupled to ventricular cells.

BACKGROUND: We used a mathematical model of a sinoatrial nodal cell (SAN model) electrically coupled to real ventricular cells (VCs) to investigate action potential conduction from an automatic focus. METHODS AND RESULTS: Since input resistance of a VC is less than that of an SAN cell, coupling of the SAN model, with a size factor of 1, to a VC produced either (1) spontaneous pacing at the slower rate of the SAN model but without driving (activation) of the VC for lower values of coupling conductance (Gj) or (2) inhibition of pacing of the SAN model by electrical coupling to the VC for higher values of Gj. When the SAN model was adjusted in size to be 3 to 5 times larger than a sinoatrial nodal cell, thus making effective SAN model capacitance 3 to 5 times larger and input resistance 3 to 5 times smaller, the SAN model propagated activity to the coupled VC for Gj above a critical value. When the VC was paced at 1 Hz, the coupled cell pair demonstrated a stable rhythm of alternating cycle lengths and alternating conduction directions. By increasing pacing frequency to 2 Hz, we converted this rhythm to a regular 2-Hz frequency in which each action potential originated in the VC. More complex periodic interactions were observed at intermediate cycle lengths and lower or higher values of Gj. CONCLUSIONS: The phenomena we observed demonstrate the critical role of the size of an automatic focus as well as the coupling in the propagation of activity from the focus into surrounding myocardium.

Action Potentials↗