Time Constant Of Current Decay Research Articles

ArF laser-induced discharge interruption in a mixture of C/sub 2/H/sub 3/Cl/CF/sub 4//CH/sub 4/, CF/sub 4//CH/sub 4/, and C/sub 2/H/sub 3/Cl/He

The ArF-laser (193-nm)-induced discharge interruption is comparatively described in a mixture of C/sub 2/H/sub 3/Cl/CF/sub 4//CH/sub 4/, CF/sub 4//CH/sub 4/, and C/sub 2/H/sub 3/Cl/He using a hollow-cathode-type discharge tube. The unfocused ArF laser (typically 27 mJ/cm/sup 2/ and 10 ns duration) illuminated both the negative glow inside the hollow cathode and the column between the two electrodes. With a 1.8-torr mixture of C/sub 2/H/sub 3/Cl/CF/sub 4/, the discharge current of 1 mA was decreased rapidly with a current-decay time constant of 1 mu s and a current-decay rate of 1 kA/s. With a 1.5-torr mixture of CF/sub 4//CH/sub 4/, 2.2 mu s and 450 A/s were obtained. With a 1.5-torr mixture of CH/sub 2/H/sub 3/Cl/He, a time constant of over 3 mu s and a rate of less than 300 A/s were obtained. An analysis of the discharge interruption induced by ArF-laser irradiation is given in terms of photon-plasma kinetics. The current peak induced by ArF irradiation is also explained for a mixture of C/sub 2/H/sub 3/Cl/He in terms of photoionization and photodetachment processes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Discharge interruption induced by an ArF laser in a C2H3Cl/CF4/CH4 mixture

We have successfully demonstrated discharge interruption induced by ArF laser irradiation in a discharge medium of C2H3Cl/CF4/CH4, using a hollow cathode type discharge tube. With a mixture of C2H3Cl/CF4/CH4 (1.8 Torr of CH4 with 0.7% of C2H3Cl and 10% of CF4), a discharge current of 1 mA (0.14 mA/cm2), and after laser irradiation of 27 mJ/cm2 (10 ns full width at half-maximum), first a discharge current peak appeared and then the discharge current was decreased rapidly with a current decay time constant of 1.0 μs. A corresponding discharge current decay rate (dI/dt) of 1 kA/s was obtained. The discharge interruption condition was limited by the gas mixture and the total pressure. Moreover, dependences of dI/dt and the time constant (τ) on both laser fluence and discharge current were observed.

Interaction of narcotic antagonist naltrexone with nicotinic acetylcholine receptor

The interactions of naltrexone with the nicotinic acetylcholine receptor were studied electrophysiologically using the frog sciatic nerve-sartorius muscle and biochemically using membranes from the electric organ of Torpedo ocellata. At nanomolar concentrations naltrexone increased the peak amplitude of endplate currents with little change in the decay time constant. At micromolar concentrations there was a concentration-dependent depression of endplate current and miniature endplate current amplitudes and decay time constants. Decay time constant depression was enhanced with hyperpolarization. Only marginal curvature was induced in peak endplate current amplitude versus membrane voltage plots by naltrexone. Naltrexone had no effect on single channel conductance but decreased open channel lifetime, according to fluctuation analysis. Naltrexone alone (⩽ 3 μM) did not impair binding of [ 125I]α-bungarotoxin to the receptor in a fast pre-equilibration assay, but increased the ability of acetylcholine to displace [ 125I]α-bungarotoxin. The drug displaced the agonist-stimulated binding of [ 3H]perhydrohistrionicotoxin to the channel site. Biphasic functional changes in neuromuscular transmission can be attributed to an allosteric mechanism with increased agonist binding to the nicotinic receptor at nanomolar concentrations and caused a non-competitive blockade of the ionic channel at micromolar concentrations.

The effects of sample capacitance and light intensity on the time-of-flight signal in a-Si:H

The capacitance of sandwich-type a-Si:H samples results in a distortion of drift-current signal if the time constant of dark current decay (τ d ) becomes comparable with the transit time (t T ). Distorted signals have apparently dispersive character and 1/t T vs applied field has sublinear dependence. Furthermore, charge collection is field dependent. Similar relations characterizing the transport in a-Si:H were sometimes published, however, the problem of possible distortion of current signal was not discussed.

A kinetic analysis of the endplate ion channel blocking action of disopyramide and its optical isomers

The effects of the antiarrhythmic agent disopyramide was studied on responses from voltage-clamped endplates at the neuromuscular junction of the garter snake. Disopyramide reduced endplate current amplitude and decay time constant in a concentration- and voltage-dependent manner. Endplate current decays remained monophasic in the presence of the drug. These results were interpreted in terms of the drug blocking the open form of the acetylcholine receptor-ion channel complex. Disopyramide produced a greater reduction of the amplitude of endplate currents than of miniature endplate currents. The reduction in miniature endplate current amplitude was not voltage-dependent. Analysis of endplate current driving functions showed that this was due to the rapid occurrence of channel block during the rising phase of the endplate current. The residual reduction, apart from that produced by channel block, is most probably due to receptor block. Disopyramide had a voltage-dependent blocking rate constant of about 10(7) M-1 S-1 at -90 mV. The unblocking rate constant was estimated from the results of experiments using paired ionophoretically applied pulses of acetylcholine. This value was again voltage-dependent and approximately 1 s-1. The actions of the (+)- and (-)-stereoisomers of disopyramide on endplate current decay were identical, indicating that the channel binding site at the neuromuscular junction is not stereoselective.

Adaptation in Helix pomatia neurons

1. 1. Spike frequency adaptation has been studied on neurons of Helix pomatia subesophageal ganglia and interpreted by means of a behavioural model describing the phenomenon in neurons either silent or autorhythmic at rest. 2. 2. At low stimulating currents the initial discharge frequency F(0) is linearly related to the current strength G. 3. 3. In the linearity range F(0)/ G each neuron was characterized by means of four model parameters: the proportionality constant between F(0) and G, the decay constant of the frequency, the inhibitory current from a single nerve impulse and the decay time constant of the inhibitory current. 4. 4. The four parameters varied in different cells with a range of 0.18–4.98 Hz/nA, 1.02–3.85 sec, 0.05–0.95 nA and 1.74–22.33 see, respectively. 5. 5. Experimental results have been analyzed considering inhibitory current, electrogenie sodium pump and other proposed adaptation parameters.

Design and related experiments of JT-60 divertor coils.

The JT-60 divertor coils produce a separatrix configuration in divertor operations of JT-60. A suitable separatrix configuration was obtained for a plasma current of 2.1 MA with coil ampere turns of ± 0.755 MAT. A high primary membrane stress of 52 MPa was permissible at the welded joints of the copper conductor made on the site. The mechanical strength of the joints welded in a factory was also improved by means of a press treatment. Electric insulation materials were selected considering a degradation of with stand voltage characteristics due to high cyclic mechanical strain. Vacuum-tight coil cases were composed of rigid rings and U-shaped bellows made of Inconel-625 alloy, and designed to withstand plasma disruption with a current decay time constant of 3 ms. The maximum temperature of the conductor in the periodic operation of divertor discharges was below 155°C which was the allowable temperature of the coil insulation. Molybdenum armor plates coated with titanium carbide and Inconel-625 bellows cov...

Toxins That Modulate the Sodium Channel Gating Mechanisma

A variety of toxins and chemicals has been shown to modulate the gating kinetics of the sodium channel. Studies of batrachotoxin, grayanotoxins and pyrethroids are summarized here as examples. Batrachotoxin and grayanotoxins eliminate the sodium channel inactivation thereby causing a prolonged, steady-state sodium current to flow during a depolarizing step. The sodium channel activation kinetics are not affected markedly. Batrachotoxin appears to bind to a site in the sodium channel to which the inactivation gate normally binds, thus causing an inhibition of sodium inactivation. Single channel recording experiments have shown that the mean open time of individual sodium channels is greatly prolonged by batrachotoxin. It appears that individual sodium channels are modified by batrachotoxin in an all-or-none manner. Pyrethroids which are synthetic derivatives of pyrethrins also modify the kinetics of sodium channels in a very drastic manner. In the presence of type I pyrethroids which lack a cyano group at the alpha position (e.g., allethrin and tetramethrin), a large steady-state sodium current appears during a step depolarization and a large slowly decaying sodium tail current appears upon repolarization. Thus both the activation and inactivation kinetics are slowed. Type II pyrethroids which contain an alpha-cyano group (e.g., deltamethrin, cyphenothrin, and fenvalerate) exert effects on sodium channels qualitatively similar to those of type I pyrethroids. However, the amplitudes of the steady-state sodium current and sodium tail current are smaller and the time constant of tail current decay is much longer. The mean open time of single sodium channels is greatly prolonged by the pyrethroids, and the effect is much more pronounced in type II than in type I pyrethroids. A high degree of stereospecificity has been found among four isomers of tetramethrin, (+)-trans and (+)-cis isomers being highly active and (-)-trans and (-)-cis isomers almost totally inactive. The inactive isomers bind to the sodium channel sites, thus preventing the action of the active isomers. Because of the unique action of pyrethroids in modulating the sodium channels, they are becoming useful tools for channel physiology and pharmacology.

The energizing of a NMR superconducting coil by a superconducting rectifier

NMR magnets require a good homogeneity within a certain volume and an excellent field stability. The homogeneity can be met using a superconducting shim coil system. The field stability requires a constant current, although in many cases the current decay time constant is too low, due to imperfections in the superconducting wire and joints. This can be overcome using a rectifier. The rectifier can also be used to load the coil. The combination and interaction of the superconducting NMR coil (2.0 Tesla and 0.35 m cold bore) and the rectifier (20 W / 1 kA) is tested. The safety of the system is discussed. The shim coil system can compensate the strayfield of the rectifier. The field decay compensation will be discussed.

The ionic channel of the nicotinic acetylcholine receptor is unable to differentiate between the optical antipodes of perhydrohistrionicotoxin

The enantiomers of perhydrohistrionicotoxin were studied in their effects on endplate currents recorded at the junctional region of sartorius muscles of Rana pipiens. The two optical antipodes progressively decreased the peak amplitude of the endplate currents and were indistinguishable from each other at all times. The enantiomers shortened equally the time constants for endplate current decay, but did not alter their voltage sensitivities. Although perhydrohistrionicotoxin contains 4 chiral centers, complete steric inversion does not alter its effects on the acetylcholine receptor—ion channel complex. By contrast the recognition site of the AcChR is extremely sensitive to any change in the chirality of agonists.

Voltage clamp study of stimulant-evoked currents in mouse pancreatic acinar cells.

Isolated segments of mouse pancreas were placed in a perspex bath and superfused with physiological saline solution. Acinar units were voltage-clamped with the help of two intracellular microelectrodes. Voltage homogeneity was in some experiments checked with a third microelectrode inserted into the same unit. The currents associated with hyper- or depolarizing voltage jumps were recorded in the absence or presence of sustained stimulation with acetylcholine (ACh), caerulein or bombesin nonapeptide. ACh (2 x 10(-7)-10(-8) M) evoked a dose-dependent inward current and an increase in the membrane conductance. The steady state ACh-evoked current (control current subtracted from total current in presence of ACh) depended linearly on voltage within the range -100 to +20 mV and its polarity reversed at about -25 mV. The effects of caerulein and bombesin nonapeptide were indistinguishable from those of ACh. Voltage homogeneity in the acinar unit was attained earlier than 1 ms after a hyper- or depolarizing voltage jump. The current transients associated with voltage jumps decayed according to single exponential functions both in the absence and presence of ACh. The time constant of the single exponential current decay after a voltage jump was the same (1-3 ms) in the absence or presence of ACh. The amplitude of the current transient was, however, reduced by ACh. The time constant of the current decay following voltage jumps was independent of the voltage in the range +60 to -60 mV, both in the absence and presence of ACh. The ACh-evoked reduction in the amplitude of the transient current following voltage jumps dependent linearly on the voltage.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 20,25-diazacholesterol treatment on the decay of end-plate currents

The open time of the acetylcholine-activated end-plate channel (τ) was estimated as the time constant of end-plate current decay in magnesium-depressed nervemuscle preparations. Pretreatment of rats with 20,25-diazacholesterol, which is known to lower membrane concentrations of cholesterol, significantly decreased τ. Thus the membrane milieu of the channel influences the functional properties of the cholinergic receptor.

Block and inactivation of sodium channels in nerve by amino acid derivatives. II. Dependence on temperature and drug concentration

The interaction of n-propylguanidinium (nPG) with sodium channels has been further characterized. From experiments at varying temperatures, the Q10 for the sodium current decay time constant in the two [Na+] gradients is 2.6-2.9 independent of drug. Testing several nPG concentrations we find that peak sodium current declines sharply with [nPG] at all levels, but the decay time constant approaches an asymptote above 4 mM. No "hooks" in sodium tail currents are seen. If the sodium current is allowed to decay completely before repolarization no tail current is observed. We have developed a kinetic model in which nPG acts at a single site within the sodium channel. Reaction of nPG with its receptor requires two steps. Fitting the temperature data shows that the first step involves diffusion of the drug to the site and close association with it. The second step may include molecular reorganization of the complex. The rate constants for the reaction are all simple exponential functions of voltage. Using them, the model successfully predicts decay time constants and peak currents, and their dependence on potential, [Na+] gradient, temperature, and nPG concentration. The results are consistent with the idea that an arginine residue may be closely associated with inactivation.

Voltage and time characteristics of the potassium mechanoreceptor current in the ciliateStylonychia

1. The membrane current underlying the hyperpolarizing receptor response following a mechanical stimulus to the cell posterior of the hypotriche ciliateStylonychia mytilus was investigated with a two microelectrode voltage-clamp technique. 2. The relationship between the amplitude and the time course of the receptor current and the wave form of the voltage pulse driving the mechanical stimulation system was studied (Figs. 2, 3, 4). The amplitude of the receptor current increased approximately linearly with the driving pulse, but both amplitude and time course of the receptor current were unaffected by varying the duration of the driving pulse. 3. The mean maximum amplitude of the receptor current elicited by mechanical stimuli was 20.4 ± 5.6 nA (± S.D., n=19) at the normal resting potential of −51.4±1.6 mV (n=20). This corresponds to an averagemaximum conductance increase of 0.61 μS. The receptor current flow reversed its direction at a membrane potential of −87.7±3.3 mV (n=18; Figs. 5, 6). At the extracellular K+ concentration of 1 mM, the intracellular K+ concentration was calculated to be 33.3 mM. 4. The amplitude of the receptor current changed linearly with the membrane potential in the hyperpolarizing direction. In the depolarizing direction the receptor current amplitude increased less than expected from the increased driving force. The conductance increase following a mechanical stimulus was in average less than 50% at zero mV as compared with that at the normal resting potential of around −50 mV. 5. The receptor current decayed with a single exponential time course, its mean time constant was 7.3±1.2 ms (n=15). The time course of the receptor current changed with the membrane voltage. The rise time and the time constant of decay of the receptor current increased with hyperpolarization towards the reversal potential, and both decreased with depolarization (Figs. 7, 8). These changes with membrane potential were approximately exponential; the voltage displacement to achieve an e-fold increase of the rise time was −410 mV, and of the decay time constant −110 mV. At membrane potentials just beyond the reversal potential both rise time and decay time constant of the receptor current, now flowinginward, were reduced by 20 to 50%, before increasing again with further hyperpolarization (Fig. 9). 6. The results indicate that the life time and possibly the conductance of mechanically activated ionic channels are dependent on the membrane voltage. The implications of these findings for a mechanoreceptor are compared with data obtained at chemoreceptors, e.g. at neuromuscular junctions and other chemical synapses.

Properties of end-plate channels in rats immunized against acetylcholine receptors.

1. Rats injected with purified acetylcholine receptors (AChR) extracted from electric organs of Torpedo marmorata showed clinical symptoms consistent with the development of experimental myasthenia gravis.2. Sera of rats with this disease contain high levels of anti-AChR antibodies. However, no simple correlation was found between antibody titre and miniature end-plate current (m.e.p.c.) amplitude.3. M.e.p.c.s. at the end-plates of rats injected with AChR (Anti-R), emulsified in complete Freund Adjuvant (CFA), were reduced to about one third the size of controls taken from rats injected only with CFA (Anti-CFA). Mean m.e.p.c. (Anti-R) = 0.73 +/- 0.06 nA; mean m.e.p.c. (Anti-CFA) = 2.43 +/- 0.12 nA (V(m) = -80 mV, T = 20 degrees C).4. The m.e.p.c. decay time constant, tau(m.e.p.c.), is similar at immunized and control rat end-plates. tau(m.e.p.c.) (Anti-R) = 1.32 +/- 0.06 msec; tau(m.e.p.c.) (Anti-CFA) = 1.31 +/- 0.06 msec (V(m) = -80 mV, T = 20 degrees C).5. The end-plate current decay time constant, tau(e.p.c.), is similar at immunized and control end-plates and in both cases depends exponentially on membrane potential. The change in membrane potential required to produce an e-fold change in tau(e.p.c.) is 102.0 +/- 5.72 mV at immunized (Anti-R) end-plates and 92.3 +/- 6.14 mV at control (Anti-CFA) end-plates at T = 10 degrees C.6. Acetylcholine noise was examined at immunized and control rat end-plates at 10 degrees C. Analysis of noise indicates that the single channel conductance, gamma, and mean channel life-time, tau(noise), are essentially unchanged by immunization against AChR. gamma (Anti-R) = 13.15 +/- 0.53 pS; gamma (Anti-CFA) = 12.50 +/- 0.50 pS; tau(noise) (Anti-R) = 2.9 +/- 0.18 msec; tau(noise) (Anti-CFA) = 2.68 +/- 0.14 msec (V(m) = -80 mV, T = 10 degrees C).7. Mean quantal content and Ca(2+) dependence of the end-plate potential are unchanged at immunized end-plates.8. It is concluded that at immunized end-plates the number of activated receptor-channel complexes is reduced without modification of single channel properties. In this respect the immunized rat end-plate is a good model for myasthenia gravis affected human end-plates.

Accumulation of acetylcholinesterase at newly formed nerve-muscle synapses

The early appearance and functional significance of synaptic acetylcholinesterase (AChE) were examined in embryonic chick spinal cord-muscle cocultures. Synapses were identified by focal extracellular recording and divided into three categories based on their mean time constant of synaptic current decay ( \\ ̄ gt syn ): fast ( 1.0 msec < \\ ̄ gt syn < 1.8 msec ), intermediate ( 1.8 msec < \\ ̄ gt syn < 2.6 msec ), and slow ( \\ ̄ gt syn > 2.6 msec ). Patches of AChE histochemical reaction product were evident at identified synapses within 24 hr of coculture. After 4 days, 74% of the fast synapses stained for AChE, whereas none of the slow synapses was stained. Thus, it is likely that AChE serves to limit the duration of ACh action at newly formed nerve-muscle synapses, just as it does at the adult neuromuscular junction. This conclusion is supported by the fact that methanesulfonyl fluoride, an AChE inhibitor, prolonged synaptic currents at fast, but not at slow, synapses. Moreover, the mean channel open time of activated ACh receptors estimated by fluctuation analysis was the same at fast and slow synapses.

A simple method for measuring magnetization of superconducting wires

A technique is described which allows an easy and precise loss measurement of superconducting high-Tc tapes in an alternating magnetic field in the temperature range between 4.2 K and the critical temperature (∼115 K for Bi tapes). It combines a calorimetric measuring technique with a magnetisation measurement. While the magnetisation measurement is faster and more sensitive at 77 K, the calorimetric measurement which is performed in the same measuring run allows an exact calibration. A field orientation parallel or perpendicular to the broad face of the tapes can be chosen without warming up the sample. Measurements of coupling current decay time constants are performed in the same device. Coupling currents are induced in the tapes by exponential field decay or by half-cycle sinusoidal magnetic field pulses. Time constants can be measured over a wide range between ∼20 μs and hundreds of milliseconds. Measured voltage traces are well described by a theoretical model, if an average effective permeability μef is used as a fit parameter. This procedure therefore allows determination of the sample μef values.

Pulse and steady photoconductivities of colored alkali halides

Measurements of pulse and steady photoconductivity were made on colored KBr and KCl in an attempt to investigate various kinds of electron traps in colored alkali halides. To investigate the space charge polarization effect which makes it difficult to measure the true photoconductivity of specimens, photocurrents produced by repeated light pulses under various conditions were observed. The frequency dependence of a.c. photocurrents under steady illumination was also investigated. From the above results, the condition of space charge free was found. The temperature dependence of the pulse photocurrent, the time constants of current-decay, and the a.c. photoconductivity under steady F-light illumination vs. its illumination level were measured under this condition. By applying trapping kinetics to the above results, three kinds of electron traps were found in colored KBr, one corresponding to F-centers and the other two to unknown centers. In KCl traps produced by photo-chemical reaction were found in addition to F-centers.