Apparent Amplitude Research Articles

Rapid inversion of 2-D geoelectrical data by multichannel deconvolution

Modern geoelectrical data acquisition systems can record more than 100 000 data values per field day. Despite the growth in computer power and the development of more efficient numerical algorithms, interpreting such data volumes remains a nontrivial computational task. We present a 2-D one‐pass inversion procedure formulated as a multichannel deconvolution. It is based on the equation for the electrical potential linearized under the Born approximation, and it makes use of the 2-D form of the Fréchet derivatives evaluated for the homogeneous half‐space. The inversion is formulated in the wavenumber domain so that the 2-D spatial problem decouples into many small 1-D problems. The resulting multichannel deconvolution algorithm is very fast and memory efficient. The inversion scheme is stabilized through covariance matrices representing the stochastic properties of the earth resistivity and data errors. The earth resistivity distribution is assumed to have the statistical characteristics of a two‐parameter, self‐affine fractal. The local apparent amplitude and fractal dimension of the earth resistivity are estimated directly from geoelectrical observations. A nonlinearity error covariance matrix is added to the conventional measurement error covariance matrix. The stochastic model for the dependence of nonlinearity error on electrode configuration as well as resistivity amplitude and fractal dimension is determined pragmatically through nonlinear simulation experiments. Tests on synthetic examples and field cases including well control support the conclusion that for long data profiles this method automatically produces linearized resistivity estimates which faithfully resolve the main model features.

Modulation of Primary Radical Pair Kinetics and Energetics in Photosystem II by the Redox State of the Quinone Electron Acceptor Q A

Time-resolved photovoltage measurements on destacked photosystem II membranes from spinach with the primary quinone electron acceptor Q A either singly or doubly reduced have been performed to monitor the time evolution of the primary radical pair P680 +Pheo −. The maximum transient concentration of the primary radical pair is about five times larger and its decay is about seven times slower with doubly reduced compared with singly reduced Q A. The possible biological significance of these differences is discussed. On the basis of a simple reversible reaction scheme, the measured apparent rate constants and relative amplitudes allow determination of sets of molecular rate constants and energetic parameters for primary reactions in the reaction centers with doubly reduced Q A as well as with oxidized or singly reduced Q A. The standard free energy difference Δ G° between the charge-separated state P680 +Pheo − and the equilibrated excited state (Chl NP680)* was found to be similar when Q A was oxidized or doubly reduced before the flash (∼−50 meV). In contrast, single reduction of Q A led to a large change in Δ G° (∼+40 meV), demonstrating the importance of electrostatic interaction between the charge on Q A and the primary radical pair, and providing direct evidence that the doubly reduced Q A is an electrically neutral species, i.e., is doubly protonated. A comparison of the molecular rate constants shows that the rate of charge recombination is much more sensitive to the change in Δ G° than the rate of primary charge separation.

Dropout-tolerant read channels

Dropouts are intermittent losses of signal commonly seen in magnetic tape recording readout. The main reason for such losses is the increased spacing between the head and the medium due to media defects or debris particles. The resulting signal is not only degraded by the apparent amplitude loss, but the characteristics of the pulses due to transitions are also changed. Moreover, in many cases, the locations of the pulses are altered, causing excessive amounts of peakshift. In this paper, a model, linking the liftoff to these effects, is presented. Experimental verification of the model using actual signals from a test tape drive is also given. Artificial dropout waveforms generated using this model are used to test two different read channel strategies. The first approach is an equalization-based correction scheme that attempts to undo the dropout. Results indicate that the dropout effects can be almost completely eliminated if an appropriate equalization procedure is applied. As an alternative approach, it is shown that the use of turbo coding in the presence of dropouts appears to be promising.

Optimum excitation signal for long-range inspection of steel wires by longitudinal waves

Optimization of an excitation signal is proposed for long-range inspection of thin steel wires by ultrasonic waves. Due to its sensitivity to flaws normal to the axial direction, a longitudinal wave is a useful tool for flaw detection of wires. We can launch and detect longitudinal waves at arbitrary positions on the wire by using electromagnetic acoustic transducers. The broadband pulse is suitable for achieving high spatial resolution in locating flaws. However, the dispersion of the longitudinal wave distorts the waveform, and consequently decreases the apparent signal amplitude. If the effect of the dispersion is removed, the signal to noise ratio can be expected to be improved and the high flaw detectability can be obtained. In this study, we suggest using the optimum excitation signal, which has been modified to compensate for the dispersion during the propagation of the assumed path length. It is possible that the flaw, which occupies 10% of the cross-sectional area of the wire, can easily be detected from a distance of 10 m.

The Anomalous Mole Fraction Effect in Chara: Gating at the Edge of Temporal Resolution

The anomalous mole fraction effect (AMFE) of the K + channel in excised patches of the tonoplast of Chara showed a minimum of apparent open-channel current at 20 mM Tl + and 230 mM K +. Time series obtained at a sampling rate of 100 kHz (filter 25 kHz) were analyzed by three methods to find out whether the AMFE results from an effect on gating or on the conductivity of the open state. Fitting the amplitude histograms by a superposition of gaussians showed a broadening in the presence of Tl +. Dwell-time analysis based on an O–O–C–C–C model failed to evaluate rate constants above the filter frequency. Thus, the absence of any reduction of apparent open-channel current in time series simulated with the evaluated rate constants could not be taken as evidence against the hypothesis of gating. Finally, a direct fit of the measured time series using five different 5-state Hidden Markov models revealed that the presence of Tl + changed the rate constants in such a way that the number of transitions into the short-lived open state (30 μs) increased strongly compared to those in the absence of Tl +. These models explain 25% reduction of apparent single-channel current amplitude through a rapid gating mechanism.

Spectroscopy and Component Masses of the Eclipsing Dwarf Nova HS 0907+1902

ABSTRACTHS 0907+1902 was recently discovered to be one of a handful of deeply eclipsing dwarf novae with periods longward of the 2–3 hr “gap.” This paper presents orbit‐resolved spectra and time‐series photometry of an eclipse. The apparent velocity amplitude of the M‐dwarf secondary is K2 = 297 ± 15 km s−1. The phase of the radial velocities of the Hα emission‐line wings agrees accurately (for once) with the phase of the white dwarf motion deduced from the eclipse, and an estimate of the emission‐line velocity amplitude yields K1 = 115 ± 7 km s−1. The eclipse width is Δϕ = 0.060 ± 0.005. At face value, these measurements yield mass estimates of M1 = 0.99 ± 0.12 M⊙ for the white dwarf and M2 = 0.38 ± 0.06 M⊙ for the secondary. The eclipse width and nominal mass ratio constrain the binary inclination i to be 77.°3 ± 0 .°9. The influence of systematic uncertainties on these values is discussed; the conclusion that the white dwarf is somewhat more massive than typical field white dwarfs appears to be robust.The Hα emission‐line profile out of eclipse is only slightly double‐peaked, but the line shows a strong rotational disturbance in eclipse. Models of the line profile through eclipse using a flat, Keplerian disk do not give a good quantitative match to the observations.

Low-Cycle Fatigue Characteristics of Precipitation-Hardened Superalloys at Cryogenic Temperatures

Abstract In order to examine the axial-strain-controlled low-cycle fatigue characteristics of precipitation-hardened superalloys at cryogenic temperatures, Ni-base alloy Inconel 718 and iron-base alloy A286 were selected as superalloys for cryogenic use. Low-cycle fatigue characteristics of these alloys including the weld were investigated at 300, 77, and 4 K, in order to clarify the relation between mechanical properties and fatigue strength, especially in the high-cycle fatigue life regime. As for Inconel 718, the modified alloy improving the weldability and manufacturing of large-sized ingots was used. The tests clarified that fatigue strength of both alloys increases with decreasing temperature on the basis of a relation between total strain range and fatigue life (Nƒ). However, even though apparent stress amplitudes are normalized by 0.2% yield strength at the same temperatures, the fatigue strength of these alloys was found to decrease with the increase of temperature in the high Nƒ regime. Taking 105 cycles as a typical fatigue life in the high Nƒ regime and adjusting the data for the relation between normalized apparent stress amplitude at 105 cycles and 0.2% yield strength, including the reference data of 300-series stainless steels, it was made clear that the higher the static strength of the alloy with stable austenite, the relatively lower the fatigue strength in the high Nƒ regime tends to be, when it is normalized by 0.2% yield strength at the same temperatures. Further, focusing on the alloys with stable austenite at 4 K, the relation between normalized apparent stress amplitude at 105 cycles and 0.2% yield strength was generally characterized, and an upper bound of 0.2% yield strength for providing apparent stress amplitude value at a fatigue life of 105 cycles, which is larger than 0.2% yield strength, was established.

Properties of saccade-related neurons in the cat superior colliculus: patterns of movement fields and discharge timing.

The discharge characteristics of saccade-related neurons in the superior colliculi (SC) of trained cats were investigated in order to understand the involvement of the SC in dynamic control of saccades. In particular, two aspects of saccade-related collicular activity in relation to visually guided saccades were quantitatively analyzed: the bounded pattern of amplitude tuning and the timing of neural activity in relation to saccade offset. In order to determine whether the amplitude tuning of SC cells is bounded or unbounded, we characterized it with a quantitative index, the unbounded-field (UF) index. Analysis of the characteristics of the distribution of this index, including the absence of unbounded fields for cells within the central 10 degrees, suggested that all cells form a single class and that apparent unbounded amplitude tuning is an artifact occurring when the range of gaze shift is truncated by restraining the head. This interpretation is supported by the results of a computer simulation based on truncation of gaze-shift range that closely replicated the experimental data. Analyses of relative location of peak motor activity in the spatial and temporal dimensions revealed that the peak discharge was more closely linked to the midpoint of the saccade than to the offset of saccade, regardless of the pattern of amplitude tuning. These results do not support the proposal advanced by the moving-hill hypothesis: the bounded pattern of amplitude tuning is inconsistent with rostral migration of the activity locus during execution of a saccade, and the relative location of the peak motor activity is inconsistent with the idea that the SC activity encodes dynamic motor error (the difference between desired and current gaze directions).

The Oblique Pulsator Model for the Blazhko Effect in RR Lyrae Stars. Theory of Amplitude Modulation I

AbstractBy assuming that RR Lyrae stars have fairly strong dipole magnetic fields with symmetry axis oblique to the rotation axis of the star, I show that the oscillation mode which would be a pure radial oscillation in absence of the magnetic field has a quadrupole component, whose symmetry axis coincides with the magnetic axis. The aspect angle of the quadrupole component changes due to the stellar rotation, and this apparent amplitude variation is interpreted as the Blazhko effect in RR Lyrae stars. It is shown that, in the case of off-axis magnetic field, phase modulation is expected. I list up some theoretical predictions based on this model, which would observationally examine this hypothesis.

Rescue of dysfunctional deltaF508-CFTR chloride channel activity by IBMX.

Nucleotide-dependent gating of DeltaF508-CFTR was evaluated in membrane patches excised from HEK 293 and mouse L-cells and compared to observations on wt-CFTR channels recorded in the same expression systems. DeltaF508-CFTR exhibited PKA activated, ATP-dependent channel gating. When compared to wt-CFTR, the Km for ATP was increased by ninefold (260 micron vs. 28 micron) and maximal open probability (Po) was reduced by 49% (0.21 +/- 0.06 vs. 0.41 +/- 0. 02). Additionally, in the absence of PKA, DeltaF508-CFTR inactivated over a 1 to 5 min period whereas wt-CFTR remained active. Time-dependent inactivation could be mimicked in wt-CFTR by the intermittent absence of ATP in the cytosolic solution. The effects of 3-isobutyl-1-methyl xanthine (IBMX), a compound reported to stimulate DeltaF508-CFTR, were evaluated on wt- and DeltaF508-CFTR channels. At concentrations up to 5 mm, IBMX caused a concentration dependent reduction in the observed single channel amplitude (i) of wt-CFTR (maximal observed reduction 35 +/- 3%). However, IBMX failed to significantly alter total patch current because of a concomitant 30% increase in Po. The effects of IBMX on DeltaF508-CFTR were similar to effects on wt-CFTR in that i was reduced and Po was increased by similar magnitudes. Additionally, DeltaF508-CFTR channel inactivation was dramatically slowed by IBMX. These results suggest that IBMX interacts with the ATP-bound open state of CFTR to introduce a short-lived nonconducting state which prolongs burst duration and reduces apparent single channel amplitude. A secondary effect observed in DeltaF508-CFTR, which may result from this interaction, is a prolongation of the activated state. In light of previously proposed linear kinetic models of CFTR gating, these results suggest that IBMX traps CFTR in an ATP-bound state which may preclude inactivation of DeltaF508-CFTR.

Substrates of multidrug resistance-associated proteins block the cystic fibrosis transmembrane conductance regulator chloride channel.

1. The effects of physiological substrates of multidrug resistance-associated proteins (MRPs) on cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel currents were examined using patch clamp recording from CFTR-transfected mammalian cell lines. 2. Two MRP substrates, taurolithocholate-3-sulphate (TLCS) and beta-estradiol 17-(beta-D-glucuronide) (E217betaG) caused a voltage-dependent block of macroscopic CFTR Cl- currents when applied to the intracellular face of excised membrane patches, with mean apparent dissociation constants (KDs) of 96+/-10 and 563+/-103 microM (at 0 mV) respectively. The unconjugated bile salts taurocholate and cholate were also effective CFTR channel blockers under these conditions, with KDs of 453+/-44 and 3760+/-710 microM (at 0 mV) respectively. 3. Reducing the extracellular Cl- concentration from 154 to 20 mM decreased the KD for block intracellular TLCS to 54+/-1 microM, and also significantly reduced the voltage dependence of block, by suggesting that TLCS blocks Cl- permeation through CFTR by binding within the channel pore. 4. Intracellular TLCS reduced the apparent amplitude of CFTR single channel currents, suggesting that the duration of block is very fast compared to the gating of the channel. 5. The apparent affinity of block by TLCs is comparable to that of other well-known CFTR channel blockers, suggesting that MRP substrates may comprise a novel class of probes of the CFTR channel pore. 6. These results also suggest that the related proteins CFTR and MRP may share a structurally similar anion binding site at the cytoplasmic face of the membrane.

Time-Series Analysis for Determining Vertical Air Permeability in Unsaturated Zones

The air pressure in the unsaturated subsurface changes dynamically as the barometric pressure varies with time. Depending on the material properties and boundary conditions, the intensity of the correlation between the atmospheric and subsurface pressures may be evidenced in two persistent patterns: (1) the amplitude attenuation; and (2) the phase lag for the principal modes, such as the diurnal, semidiurnal, and 8-h tides. The amplitude attenuation and the phase lag generally depend on properties that can be classified into two categories: (1) The barometric pressure parameters, such as the apparent pressure amplitudes and frequencies controlled by the atmospheric tides and others; and (2) the material properties of porous media, such as the air viscosity, air-filled porosity, and permeability. Based on the principle of superposition and a Fourier time-series analysis, an analytical solution for predicting the subsurface air pressure variation caused by the atmospheric pressure fluctuation is presented. The air permeability (or pneumatic diffusivity) can be quantitatively determined by using the calculated amplitude attenuations (or phase lags) and the appropriate analytical relations among the parameters of the atmosphere and the porous medium. An analysis using the field data shows that the Fourier time-series analysis may provide a potentially reliable and simplemore » method for predicting the subsurface barometric pressure variation and for determining the air permeability of unsaturated zones.« less

New observational results on the colliding wind WR+O binary WR 30a

WR30a (MS 4) was first suspected as a possible Wolf-Rayet star by McConnell & Sanduleak (1970) during a deep objective-prism survey performed around the Carina Nebula region. From subsequent studies, the spectrum turned out to be composite, with a WR component classified W04 (or alternatively WC4, W05 and WC3) and an 0 component classified 04. Niemela (1995) reported the first, and up-to-now unique, radial velocity study for that star. She noticed large variations of CIV λ4658 from night to night, suggesting that WR30a might be a short period binary. A plot of the C IV emission-line velocities versus the He II absorption ones clearly indicates that they are exhibiting opposite binary motions. The apparent velocity amplitudes imply a rather low mass ratio Mwo4/Mo4 = 0.15, although the radial velocities of the C IV line could be distorted due to line-profile variations.

Clearance of large Ca2+loads in a single smooth muscle cell: examination of the role of mitochondrial Ca2+uptake and intracellular pH

Redistribution of cytosolic free Ca2+following Ca2+influx into the cytoplasm was studied in single smooth muscle cells isolated from guinea-pig urinary bladder. Voltage-clamped cells were loaded with a low-affinity fluorophore Indo-1FF. A decay of free intracellular Ca2+([Ca2+]i) after the termination of the depolarizing pulse (1 s from –50 mV to +20 mV) was fitted with a single exponential and the effect of various substances on the time constant was compared. At a holding potential of +80 mV the [Ca2+]idecay was 1.56 times slower compared to that at –50 mV suggesting the presence of a voltage-dependent process redistributing Ca2+. In the presence of cyclopiazonic acid (CPA, 10 μM), an inhibitor of sarco(endo)plasmatic Ca2+pump (SERCa), the [Ca2+]idecay was 3.93 times slower than that in the absence of the inhibitor. Introduction of a polycation Ruthenium Red (RR) (20 μM), an inhibitor of the mitochondrial Ca2+uniporter, into a cell or collapsing a transmitochondrial H+gradient with the protonophore CCCP (2 μM) slowed down the [Ca2+]idecay 6.05-fold and 9.78-fold, respectively. The apparent amplitude of [Ca2+]iincrements was also increased by CCCP. Increasing H+buffering power in the intracellular solution from 10 mM to 40 mM of HEPES greatly reduced the effect of CCCP on [Ca2+]idecay. A further increase in HEPES concentration to 100 mM eliminated the effects of CCCP both on the time course of [Ca2+]idecay and on the amplitude of [Ca2+]iincrement. Perfusion of RR together with 100 mM HEPES into the cytoplasm was without effect on the decay time course of [Ca2+]i. The effect of CPA on [Ca2+]idecay was also reduced in cells loaded with 100 mM HEPES; the time constant in the presence of CPA was slowed down by a factor of 2.18. Application of 10 mM Na+– butyrate to the cells loaded with 10 mM HEPES resulted in a slowing down of [Ca2+]idecay: the time constant was increased by a factor of 5.84. Measurement of intracellular pH with SNARF-1 confirmed cytoplasmic acidification during application of Na+-butyrate and CCCP. It is concluded that the contribution of mitochondrial Ca2+uptake to the rapid [Ca2+]idecay is much less than could be extrapolated from action of protonophores in these smooth muscle cells. The results also demonstrate the importance of intracellular pH for Ca2+handling in the cytoplasm of smooth muscle cells.

Kinetics of the light-induced proton translocation associated with the pH-dependent formation of the metarhodopsin I/II equilibrium of bovine rhodopsin.

The kinetics of the formation of the metaII (MII) state of bovine rhodopsin was investigated by time-resolved electrical and absorption measurements with rod outer segment (ROS) fragments. Photoexcitation leads to proton transfer in the direction from the cytosolic to the intradiscal side of the membrane, probably from the Schiff base to the acceptor glutamate 113. Two components of comparable amplitude are required to describe the charge movement with exponential times of 1.1 (45%) and 3.0 ms (55%) (pH 7.8, 22 degreesC, 150 mM KCl). The corresponding activation energies are 86 and 123 kJ/mol, respectively (150 mM KCl). The time constants and amplitudes depend strongly on pH. Between pH 7.1 and 3.8 the kinetics becomes much faster, with the faster and slower components accelerating by factors of about 8 and 2, respectively. Complementary single-flash absorption experiments at 380 nm and 10 degreesC show that the formation of MII also occurs with two components with similar time constants and pH dependence. This suggests that both signals monitor the same molecular events. The pH dependence of the two apparent time constants and amplitudes of the optical data can be described well over the pH range 4-7.5 by two coupled equilibria between MI and two isochromic MII species MIIa and MIIb: MI MIIa(380) MIIb(380), with k0 proportional to the proton concentration. This model implies that deprotonation of the Schiff base and proton uptake are tightly coupled in ROS membranes. Models with k2 proportional to the proton concentration cannot describe the data. Photoreversal of MII by blue flashes (420 nm) leads to proton transfer in a direction opposite to that of the signal associated with MII formation. In this transition the Schiff base is reprotonated, most likely from glutamate 113. At pH 7.3, 150 mM KCl, 22 degreesC, this electrical charge reversal has an exponential time constant of about 30 ms and is about 10 times slower than the forward charge motion.

Properties and functions of calcium-activated K+ channels in small neurones of rat dorsal root ganglion studied in a thin slice preparation.

1. Properties, kinetics and functions of large conductance calcium-activated K+ channels (BKCa) were investigated by the patch-clamp technique in small neurones (Adelta- and C-type) of a dorsal root ganglion (DRG) thin slice preparation without enzymatic treatment. 2. Unitary conductance of BKCa channels measured in symmetrical high K+ solutions (155 mM) was 200 pS for inward currents, and chord conductance in control solution was 72 pS. Potentials of half-maximum activation (V ) of the channels were linearly shifted by 43 mV per log10 [Ca2+]i unit (pCa) in the range of -28 mV (pCa 4) to +100 mV (pCa 7). Open probabilities increased e-times per 15-32 mV depolarization of potential. 3. In mean open probability, fast changes with time were mainly observed at pCa > 6 and at potentials > +20 mV, without obvious changes in the experimental conditions. 4. BKCa channels were half-maximally blocked by 0.4 mM TEA, measured by apparent amplitude reductions. They were completely blocked by 100 nM charybdotoxin and 50 nM iberiotoxin by reduction of open probability. 5. Two subtypes of small DRG neurones could be distinguished by the presence (type I) or absence (type II) of BKCa channels. In addition, less than 10 % of small neurones showed fast (approximately 135 V s-1) and short ( approximately 0.8 ms) action potentials (AP). 6. The main functions of BKCa channels were found to be shortening of AP duration, increasing of the speed of repolarization and contribution to the fast after-hyperpolarization. As a consequence, BKCa channels may reduce the amount of calcium entering a neurone during an AP. 7. BKCa channel currents suppressed a subsequent AP and prolonged the refractory period, which might lead to a reduced repetitive activity. We suggest that the BKCa current is a possible mechanism of the reported conduction failure during repetitive stimulation in DRG neurones.

Dynamic structure of subtilisin-eglin c complex studied by normal mode analysis.

Normal mode analysis of subtilisin-eglin c complex was performed to investigate the dynamics at the interface between the enzyme and the inhibitor. The internal motions of the complex calculated from the normal modes were divided into three parts: the internal motions changing the shape of each molecule, the external rigid-body motions changing their mutual dispositions, and the coupling between the internal and external motions. From the results of the analysis, the following characteristic features were found in the dynamics at the interface regions: 1) negative correlation between the internal and external motions within each molecule, and 2) positive correlation between the external motions of the two molecules. The former decreases the apparent amplitudes of motions at the interface. The latter minimizes the interference between individual motions of the two molecules. These dynamic characteristics allow the enzyme and the inhibitor to move as freely as possible. This finding suggests that the experimental evidence of the large entropy gain on binding should be attributed not only to strong hydrophobic interactions, but also to the dynamic structure of the complex, which is found to minimize an unavoidable loss of the conformational entropy on binding.

Astrocytic gap junction blockage and neuronal Ca2+ oscillation in neuron-astrocyte cocultures in vitro

We have investigated the effects of gap junction inhibitors, octanol, halothane, sodium propionate and lindane, on neuronal periodic Ca2+ 1 transients in neuron–astrocyte coculture systems. Octanol reduced the amplitude and frequency of Ca2+ oscillations in dose–dependent manner. One mM octanol caused a complete disappearance of Ca2+ oscillations. Similar suppressions were obtained by halothane (1 mM) and sodium propionate (25 mM). In contrast, lindane (300 nM) uniquely raised the basal level of [Ca2+]i in oscillating neurons as well as the height of apparent amplitude without changes in the frequency. The current results imply that octanol, halothane and sodium propionate might lower the frequency of spontaneous Ca2+ oscillations by blocking the gap junctional communication of neighboring astrocytes and that lindane, though also blocking the gap junctions, might not affect the frequency but reversely increase both the basal [Ca2+]i and the amplitude, probably due to an increase of neuronal [Ins (1.4.5)P3]i. These findings strongly suggest that astrocytes contribute to the generation of periodic neuronal Ca2+ oscillations through astrocytic gap junctional communications and\\or other signaling components between astrocytes and neurons. © 1988 Elsevier Science Ltd. All rights reserved.

Effects of viscoelasticity on rubber vibration isolator design

The influence of viscoelasticity of rubber materials on the performance of rubber isolators is presented. Dynamic properties of both neat rubber and filled rubber with filler content φ<φc are linear viscoelastic while those of filled rubbers with filler content φ⩾φc are strongly strain dependent and nonlinear viscoelastic. The Kramers & Kronig relationship and its approximation is used to simulate the performance of a linear viscoelastic rubber isolator. Two specific conditions for viscoelastic rubber systems, (a) systems with constant loss tangent and (b) systems in the region of ω0.5 frequency dependence, are studied. The limitation on the transmissibility of a linear viscoelastic rubber isolator is also illustrated through the Kramers & Kronig relationship. The performance of a vibration isolator made from filled rubbers with filler content φ⩾φc, however, can be simulated by the L-N-B model. When the strain amplitude at natural frequency is greater than the apparent strain amplitude εapp, the stability problems such as Jump effect are likely to happen.

Large conductance Ca(2+)-activated K+ channels in the soma of rat motoneurones.

Properties of large conductance Ca(2+)-activated K+ channels were studied in the soma of motoneurones visually identified in thin slices of neonatal rat spinal cord. The channels had a conductance of 82 +/- 5 pS in external Ringer solution (5.6 mM Ko+/(/)155 mM Ki+) and 231 +/- 4 pS in external high-Ko solution (155 mM Ko+/(/)155 mM Ki+). The channels were activated by depolarization and by an increase in internal Ca2+ concentration. Potentials of half-maximum channel activation (E50) were -13, -34, -64 and -85 mV in the presence of 10(-6), 10(-5), 10(-4) and 10(-3) M internal Ca2+, respectively. Using an internal solution containing 10(-4) M Ca2+, averaged KCa currents showed fast activation within 2-3 msec after a voltage step to +50 mV. Averaged KCa currents did not inactivate during 400 msec voltage pulses. External TEA reduced the apparent single-channel amplitude with a 50% blocking concentration (IC50) of 0.17 +/- 0.02 mM. KCa channels were completely suppressed by externally applied 100 mM charybdotoxin. It is concluded that KCa channels activated by Ca2+ entry during the action potential play an important role in the excitability of motoneurones.