Half Of Duration Research Articles

Propagating features of upward positive leaders in the initial stage of rocket-triggered lightning

The propagation characteristics of upward positive leaders in the initial stage of rocket-triggered lightning flashes have been analyzed using high-speed video images, channel base currents, and electric field changes. The luminous intensity at the tip of leader steps was stronger than that in the channel behind it. The average 2-D speed of the leader was 1.0×105m/s with partial speeds ranging from 2.0×104m/s to 1.8×105m/s between 130m and 730m above ground. The leader speed showed an obvious irregularity and an accelerating tendency with height and time. Unipolar current pulses, with peak currents ranging from tens of amperes to about 150A, were observed at the ground during the initial stage of leader development. The electric field change showed stepped waveforms at this stage and it was evident that the leader involved a stepwise propagating mode. For 34 current pulses occurring in the initial stage of the leader development, the geometric mean values of the peak current, rise time, half peak width, duration, charge transfer and pulse interval were 45.0A, 0.49μs, 0.99μs, 3.2μs, 4.8μC and 19.9μs, respectively. After the leaders extended to several hundred meters above ground, fluctuations were found to be superimposed on the continuous current and the associated electric field.

Source parameters determination for earthquakes in Kushiro, Japan considering source time function

This paper applies a new formulation to do moment tensor inversion for earthquakes in the Kushiro area of Japan. Comparing with conventional moment tensor inversion method, the new one takes the effect of source time function into consideration. For the inversion, best solution is obtained by minimizing the difference between the observed seismograms and the synthetic ones. And the best-fitting focal depth is determined from the variance reduction. The results indicate that half duration of source time function is proportional to the magnitude of earthquakes. Large earthquakes have long half duration, whereas that of moderate-small earthquakes is comparatively shorter. The focal mechanisms of all three earthquakes are of thrust fault type, which is mainly ascribed to the collision of the North American plate with the Eurasia plate in the late Cretaceous or Paleogene.

W phase source inversion for moderate to large earthquakes (1990-2010)

Rapid characterization of the earthquake source and of its effects is a growing field of interest. Until recently, it still took several hours to determine the first-order attributes of a great earthquake (e.g. M_w ≥ 7.5), even in a well-instrumented region. The main limiting factors were data saturation, the interference of different phases and the time duration and spatial extent of the source rupture. To accelerate centroid moment tensor (CMT) determinations, we have developed a source inversion algorithm based on modelling of the W phase, a very long period phase (100–1000 s) arriving at the same time as the P wave. The purpose of this work is to finely tune and validate the algorithm for large-to-moderate-sized earthquakes using three components of W phase ground motion at teleseismic distances. To that end, the point source parameters of all M_w ≥ 6.5 earthquakes that occurred between 1990 and 2010 (815 events) are determined using Federation of Digital Seismograph Networks, Global Seismographic Network broad-band stations and STS1 global virtual networks of the Incorporated Research Institutions for Seismology Data Management Center. For each event, a preliminary magnitude obtained from W phase amplitudes is used to estimate the initial moment rate function half duration and to define the corner frequencies of the passband filter that will be applied to the waveforms. Starting from these initial parameters, the seismic moment tensor is calculated using a preliminary location as a first approximation of the centroid. A full CMT inversion is then conducted for centroid timing and location determination. Comparisons with Harvard and Global CMT solutions highlight the robustness of W phase CMT solutions at teleseismic distances. The differences in M_w rarely exceed 0.2 and the source mechanisms are very similar to one another. Difficulties arise when a target earthquake is shortly (e.g. within 10 hr) preceded by another large earthquake, which disturbs the waveforms of the target event. To deal with such difficult situations, we remove the perturbation caused by earlier disturbing events by subtracting the corresponding synthetics from the data. The CMT parameters for the disturbed event can then be retrieved using the residual seismograms. We also explore the feasibility of obtaining source parameters of smaller earthquakes in the range 6.0 ≤_Mw < 6.5. Results suggest that the W phase inversion can be implemented reliably for the majority of earthquakes of Mw= 6 or larger.

A model of the physiological basis of a multivariate phenotype that is mediated by Ca 2+ signaling and controlled by ryanodine receptor composition

Calcium-signals occur in a wide variety of tissue types – from skeletal, smooth and cardiac muscle to pancreatic and brain tissues. Ca 2+ signals regulate diverse processes including muscle contraction, hormone secretion, neural communication and gene expression. Together these different tissues and processes form the basis of a multivariate trait. Calcium signals are characterized by Ca 2+ transients, which are sharp increases in Ca 2+ concentration over a short period of time. In this paper we derive and analyze a model of Ca 2+ transients for skeletal muscle, neurons and cardiac tissue based on underlying biophysical principles. Tissue differentiation in our model and in nature comes about by varying the ryanodine receptor (RyR) channel composition of tissues. In vertebrates, there are typically three types of RyR channels (labeled RyR1, RyR2 and RyR3 in mammals and α ‐ RyR , cardiac-RyR and β ‐ RyR in birds, amphibians and fish). Different compositions of these three RyR channels generate different Ca 2+ transient properties. There are four Ca 2+ transient properties that we measure: maximum amplitude, duration, half duration ( D 50) and integrated concentration. In agreement with experimental work, our results find that the addition of RyR3 amplifies Ca 2+ transients in skeletal muscle. An important consequence of shared molecular components between tissue types in a multivariate setting is that the shared components cause individual traits of a multivariate trait to be correlated in function. Here we show how correlations in Ca 2+ transient properties between tissues can be predicted using an underlying biophysical model.

Role of K+ Channels in Regulating Spontaneous Activity in Detrusor Smooth Muscle In Situ in the Mouse Bladder

We investigated the functional role of K(+) channels for regulating spontaneous activity in mouse bladder detrusor smooth muscle. The effects of different K(+) channels blockers on spontaneous changes in membrane potential and intracellular Ca(2+) dynamics were examined using intracellular recording techniques and Ca(2+) imaging with fluo-4 fluorescence, respectively. Detrusor smooth muscle generated spontaneous action potentials and Ca(2+) transients. Iberiotoxin (0.1 microM), charybdotoxin (0.1 microM) or tetraethylammonium (1 mM) increased the amplitude of action potentials and prolonged their repolarizing phase without inhibiting their after-hyperpolarization. Tetraethylammonium (10 mM) but not stromatoxin (0.1 microM) suppressed after-hyperpolarization and further increased the amplitude and half duration of action potentials. Apamin (0.1 microM) increased the frequency of action potentials but had no effect on their configuration. Spontaneous Ca(2+) transients were generated in individual detrusor smooth muscle cells and occasionally propagated to neighboring cells to form intercellular Ca(2+) waves. Transmural nerve stimulations invariably initiated synchronous Ca(2+) transients within and across muscle bundles. Charybdotoxin (0.1 microM) increased the amplitude of spontaneous Ca(2+) transients, while the subsequent application of tetraethylammonium (10 mM) increased their half duration. In addition, tetraethylammonium increased the synchronicity of Ca(2+) transients in muscle bundles. These results suggest that large and intermediate conductance Ca(2+) activated K(+) channels contribute to action potential repolarization and restrict the excitability of detrusor smooth muscle in the mouse bladder. In addition, the activation of voltage dependent K(+) channels is involved in repolarization and after-hyperpolarization, and it has a fundamental role in stabilizing detrusor smooth muscle excitability.

How do synapses measure milliseconds?

How do synapses measure milliseconds?

Oral arginine attenuates the growth hormone response to resistance exercise

This study investigated the combined effect of resistance exercise and arginine ingestion on spontaneous growth hormone (GH) release. Eight healthy male subjects were studied randomly on four separate occasions [placebo, arginine (Arg), placebo + exercise (Ex), arginine + exercise (Arg+Ex)]. Subjects had blood sampled every 10 min for 3.5 h. After baseline sampling (30 min), subjects ingested a 7-g dose of arginine or placebo (blinded, randomly assigned). On the exercise days, the subject performed 3 sets of 9 exercises, 10 repetitions at 80% one repetition maximum. Resting GH concentrations were similar on each study day. Integrated GH area under the curve was significantly higher on the Ex day (508.7 +/- 169.6 min.ng/ml; P < 0.05) than on any of the other study days. Arg+Ex (260.5 +/- 76.8 min.ng/ml) resulted in a greater response than the placebo day but not significantly greater than the Arg day. The GH half-life and half duration were not influenced by the stimulus administered. The GH secretory burst mass was larger, but not significantly, on the Arg, Ex, and Arg+Ex day than the placebo day. Endogenous GH production rate (Ex > Arg+Ex > Arg > placebo) was greater on the Ex and Arg+Ex day than on the placebo day (P < 0.05) but there were no differences between the Ex and Arg+Ex day. Oral arginine alone (7 g) stimulated GH release, but a greater GH response was seen with exercise alone. The combined effect of arginine before exercise attenuates the GH response. Autonegative feedback possibly causes a refractory period such that when the two stimuli are presented there will be suppression of the somatotrope.

Abstracts of the 8th Meeting of the Italian Peripheral Nerve Study Group: 61

We describe a patient affected by MMN with CB exhibiting unusual progression and clinical features. MMN is a chronic immune mediated demyelinating neuropathy characterized by slowly progressive painless asymmetrical weakness usually in distal limb muscles. GM1 antibodies are found in high titer in 22%‐ 84% of cases. Electrophysiological findings are CB not at usual sites of compression, abnormal temporal dispersion and F‐waves, normal or near‐normal sensory conduction studies. This 63 year old woman was referred for evaluation of one year and half duration weakness which began in right upper limb and progressed to contralateral and to lower extremities. On first admission (July 2001) neurological examination showed normal cranial nerves and sensory testing, severe asymmetric weakness mainly proximal in the upper extremities and distal in the lower. There were atrophy of small hand muscles and weak deep reflexes. She denied sensory symptoms. There were no upper motor neuron signs. Laboratory tests including CK, tumor marker, CSF, anti‐GM1 antibodies were unremarkable. Electrophysiological study was consistent with MMN with CB. Patient underwent IVIG (0.4 g/kg/daily for five days) and PE without significant improvement. In October 2002, her conditions rapidly worsened: she became quadriplegic, unable to swallow, to chew and to protrude her tongue. Admitted to ICU because of respiratory failure, tracheotomy was performed. Repeated searches for GM1 antibodies gave negative results. CSF was still normal. IVIG was administered twice with partial benefit although severe disability persisted. The case presented has notable features: the rapid monophasic progression to quadriplegia within two years and half, the acute worsening in absence of precipitating events and of systemic illness, the severe involvement of cranial nerves and respiratory muscles. The late response to IVIG is in favor of an immune mediated disorder despite the absence of GM1 antibodies.

Apparatus and method for playback of waveform sample data and sequence playback of waveform sample data

An audio record/playback system is configured using a RAM containing PC buffers and a sound input/output board which is equipped with another RAM containing P buffers and R buffers as well as a digital audio circuit. At a playback mode, waveform sample data consisting of waveform samples are subjected to burst transfer using a PCI bus from the PC buffer to the P buffer in a first half duration of each sampling period with respect to one channel. In a second half duration, one of the waveform samples is transferred from the P buffer to the digital audio circuit, wherein it is subjected to digital audio processing. Thus, the waveform sample data are played back in response to prescribed timings synchronized with sampling periods. At a record mode, waveform sample data corresponding to sounds to be picked up are supplied to the digital audio circuit, from which they are transferred to the R buffer. Then, the waveform sample data are transferred to the PC buffer. At a sequence playback mode, a set of WAVE data are loaded from an external storage unit to the RAM with respect to multiple channels in response to a sequence playback instruction. Then, the WAVE data are transferred to the sound input/output board, by which they are played back. Incidentally, when playback is completed with respect to one channel, next WAVE data regarding another channel are automatically loaded to the RAM.

Daily low dose recombinant hCG and recombinant FSH: a novel stimulation protocol for IVF

Objective: Stimulation protocols often utilize oral contraceptive pre-treatment prior to initiation of GnRH agonists or antagonists to facilitate scheduling of the IVF cycle. In some cases, this leads to profound suppression of endogenous gonadotropins. Prior investigation has demonstrated that initiation of FSH prior to resolution of this suppression is associated with delayed follicular growth and increased consumption of gonadotropins. It has been proposed that LH deficiency is responsible for this delayed response. Many IVF programs currently employ ’mixed’ stimulation protocols using HMG together with FSH preparations in attempt to overcome this problem. Despite this approach, some patients may continue to have a suboptimal response. This is likely due to the short half-life of the LH component of hMG preparations. Recombinant HCG has recently become available. This drug may serve as a source of LH activity with a significantly longer half life and duration of action than the LH component of HMG. This study was undertaken to evaluate the potential benefit of using a daily low dose of recombinant hCG as an adjunct to recombinant FSH treatment for ovulation induction in IVF patients.

Daily low dose recombinant hCG and low dose recombinant FSH: a novel treatment for hypogonadotropic hypogonadism

Objective: LH in the follicular phase is known to be important to the production of C-19 steroids that serve as precursors for estrogen formation. Estrogen produced by the follicle has been speculated to have a major role in oocyte maturation. Treatment of hypogonadotropic hypogonadism with recombinantly derived FSH has previously met with poor response secondary to LH deficiency. The administration of recombinant FSH in the absence of significant endogenous LH can lead to slow follicle development associated with extremely low estradiol production. Because of this, patients with hypogonadotropic amenorrhea have classically been treated with hMG. Despite this approach, these patients may require high doses of gonadotropin and prolonged treatment. This is likely due to the short half-life of the LH component of hMG preparations. Recombinant HCG has recently become available. This drug may serve as a source of LH activity with a significantly longer half life and duration of action than the LH component of HMG. This study was undertaken to evaluate the potential benefit of using a daily low dose of recombinant hCG as an adjunct to recombinant FSH treatment for ovulation induction in a patient with severe LH deficiency.

Audio system for playback of waveform sample data

An audio record/playback system is configured using a RAM containing PC buffers and a sound input/output board which is equipped with another RAM containing P buffers and R buffers as well as a digital audio circuit. At a playback mode, waveform sample data consisting of waveform samples are subjected to burst transfer using a PCI bus from the PC buffer to the P buffer in a first half duration of each sampling period with respect to one channel. In a second half duration, one of the waveform samples is transferred from the P buffer to the digital audio circuit, wherein it is subjected to digital audio processing. Thus, the waveform sample data are played back in response to prescribed timings synchronized with sampling periods. At a record mode, waveform sample data corresponding to sounds to be picked up are supplied to the digital audio circuit, from which they are transferred to the R buffer. Then, the waveform sample data are transferred to the PC buffer. At a sequence playback mode, a set of WAVE data are loaded from an external storage unit to the RAM with respect to multiple channels in response to a sequence playback instruction. Then, the WAVE data are transferred to the sound input/output board, by which they are played back. Incidentally, when playback is completed with respect to one channel, next WAVE data regarding another channel are automatically loaded to the RAM.

Cessation of low-dose gonadotropin releasing hormone agonist therapy followed by high-dose gonadotropin stimulation yields a favorable ovarian response in poor responders.

This study is a prospective nonrandomized study to determine the effect of a new protocol of controlled ovarian hyperstimulation (COH) using low doses and a half-period of gonadotropin releasing hormone agonist (GnRHa) followed by high doses of gonadotropin in patients who were supposed to be poor responders to standard long protocols of GnRHa administration. From Dec 1996 to Nov 1998, 50 patients who were classified as "poor responders" were scheduled for 52 cycles of a modified controlled ovarian hyperstimulation protocol. They were categorized into 3 groups: a group of poor responders to COH in the previous IVF or IUI cycles, a group with elevated Day 3 FSH levels, and a group over the age of 40 years. All patients received GnRH agonist from the midluteal phase of the previous cycle to the onset of menstruation in the next cycle. Then high doses of gonadotropins (HMG/FSH) were given. The patients then had standard courses of in vitro fertilization and embryo transfer (IVF-ET) or transfallopian embryo transfer (TET). Six of the 52 cycles of the modified protocols were cancelled because of poor ovarian response. One premature ovulation was noted before ovum retrieval was performed. In the other 45 cycles, an average of 6.3 mature oocytes were retrieved. The total pregnancy rate and implantation rate were 20.5 and 11.5%, respectively. The low dose and half duration of GnRHa therapy lessened the suppression of the response of the ovaries to COH compared with the regular long protocol of GnRHa down regulation therapy. This resulted in a low cancellation rate (11.8%), a favorable embryo implantation rate (11.5%), and an acceptable clinical pregnancy rate (20.5%).

Interdomain interactions within ryanodine receptors regulate Ca2+ spark frequency in skeletal muscle.

DP4 is a 36-residue synthetic peptide that corresponds to the Leu(2442)-Pro(2477) region of RyR1 that contains the reported malignant hyperthermia (MH) mutation site. It has been proposed that DP4 disrupts the normal interdomain interactions that stabilize the closed state of the Ca(2)+ release channel (Yamamoto, T., R. El-Hayek, and N. Ikemoto. 2000. J. Biol. Chem. 275:11618-11625). We have investigated the effects of DP4 on local SR Ca(2)+ release events (Ca(2)+ sparks) in saponin-permeabilized frog skeletal muscle fibers using laser scanning confocal microscopy (line-scan mode, 2 ms/line), as well as the effects of DP4 on frog SR vesicles and frog single RyR Ca(2)+ release channels reconstituted in planar lipid bilayers. DP4 caused a significant increase in Ca(2)+ spark frequency in muscle fibers. However, the mean values of the amplitude, rise time, spatial half width, and temporal half duration of the Ca(2)+ sparks, as well as the distribution of these parameters, remained essentially unchanged in the presence of DP4. Thus, DP4 increased the opening rate, but not the open time of the RyR Ca(2)+ release channel(s) generating the sparks. DP4 also increased [(3)H]ryanodine binding to SR vesicles isolated from frog and mammalian skeletal muscle, and increased the open probability of frog RyR Ca(2)+ release channels reconstituted in bilayers, without changing the amplitude of the current through those channels. However, unlike in Ca(2)+ spark experiments, DP4 produced a pronounced increase in the open time of channels in bilayers. The same peptide with an Arg(17) to Cys(17) replacement (DP4mut), which corresponds to the Arg(2458)-to-Cys(2458) mutation in MH, did not produce a significant effect on RyR activation in muscle fibers, bilayers, or SR vesicles. Mg(2)+ dependence experiments conducted with permeabilized muscle fibers indicate that DP4 preferentially binds to partially Mg(2)+-free RyR(s), thus promoting channel opening and production of Ca(2)+ sparks.

BepiColombo, ESA's Mercury Cornerstone mission

The paper presents the results of the definition studies performed for the European Space Agency (ESA) on system architectures and enabling technologies for “BepiColombo”, a Cornerstone class mission to be launched in the 2007–2009 time frame. The scientific mission comprises 1-year observations by a Mercury Planetary Orbiter (MPO), dedicated to remote sensing, and a Mercury Magnetospheric Orbiter (MMO), dedicated to particles and fields, plus short-duration in situ analysis by a Mercury surface element (MSE). A flexible approach to the programme has been developed, comprising two alternative launch scenarios. In the first option (2009), the 2500-kg class satellite composite, including two propulsion modules and three scientific modules, is launched by an Ariane-5. The trajectory design is based on Venus and Mercury gravity assists plus the thrust provided by a Solar Electric Propulsion Module (SEPM), that is jettisoned before being captured into Mercury orbit. Capture and orbit insertion, executed by successive manoeuvres of a Chemical Propulsion Module (CPM), occur less than 2.5 yr after launch. In the second scenario, the mission is split into two launches of a small launch vehicle. Two 1200-kg class composites are launched either in the same one-month window or at an interval of 1.6 yr . One composite comprises the SEPM, CPM, MMO and MSE and the other comprises duplicate SEPM+CPM and the MPO. The trajectory design follows the same principles as the Ariane-5 mission, with the SEPM thrust reduced by half and cruise duration ranging between 2.3 and 3.5 yr . Whatever be the implementation, the mission is expected to return about 1700 Gbit of scientific data during the one-year observation phase. The crucial aspects of the spacecraft design are associated with, and constrained by, the high-temperature and high-radiation environment. Basic feasibility has been demonstrated by an extensive design and analysis exercise, and the focus of the programme has now moved to a 3-year preparatory programme dedicated for developing the enabling technologies.

Source parameters and three‐dimensional attenuation structure from the inversion of microearthquake pulse width data: Method and synthetic tests

We propose a new method to determine source parameters and attenuation structure of a three‐dimensional medium based on first P rise time and total pulse width measurements from microearthquake data. The effects of fault finiteness on seismic radiation are taken into account by assuming the rupture model for a circular crack of Sato and Hirasawa (1973). Ray theory synthetic seismograms in a constant Q anelastic medium are computed to derive a set of nonlinear equations which relate the source and attenuation parameters (fault radius, orientation of the fault plane, and quality factor) to the pulse width data (half and total duration of the P waveforms).The numerically built relationships are used to compute the direct problem in the framework of a nonlinear inversion scheme, based on the modified downhill Simplex method. The validity and robustness of the inversion method are tested by synthetic simulations by assuming the sources and receivers configuration of the seismic passive experiment conducted in the Campi Flegrei caldera (southern Italy) during the last microearthquake crisis (1982–1984). Different heterogeneous Q models have been considered in order to assess the uncertainty and resolution of source and attenuation parameters for the given acquisition layout. The results of this simulation study indicate that first pulse width data from a local network permit retrieval with sufficient accuracy of the heterogeneous Q structure and fault radii. A rather dense azimuthal coverage of the sources is instead needed to recover the angles (in particular, the fault strike) which define the fault orientation.

Estimating source parameters of the earthquake swarm in the Hida Mountains Region, central Japan, using near‐field waveforms

We estimated source durations for larger events in a strong swarm that occurred from August 7, 1998 in the Hida mountains region, central Japan. We used broadband seismograms recorded at the Kamitakara observatory at distances of about 30 km. The events had seismic moments from 1014 to 1017 Nm. To determine the source durations, we systematically searched for the half duration of source time functions which minimized the difference between synthetic waveforms and observed data. In the data we could clearly see the near‐field contributions to the waveforms, suggesting that the durations were longer than expected for typical events, considering their size and the distance to the station. We estimated the source durations (half duration of source time functions), source dimensions, and stress drops to study the source parameters of these event. Source dimensions are determined from these source durations under the assumption of circular faults and seismic moments are estimated from maximum amplitudes. These results show that source durations are longer and source dimensions are larger compared to most earthquakes with the same seismic moments.

A hippocampal interneuron associated with the mossy fiber system

Network properties of the hippocampus emerge from the interaction of principal cells and a heterogeneous population of interneurons expressing gamma-aminobutyric acid (GABA). To understand these interactions, the synaptic connections of different types of interneurons need to be elucidated. Here we describe a type of inhibitory interneuron of the hippocampal CA3 region that has an axon coaligned with the mossy fibers. Whole-cell patch-clamp recordings, in combination with intracellular biocytin filling, were made from nonpyramidal cells of the stratum lucidum under visual control. Mossy fiber-associated (MFA) interneurons generated brief action potentials followed by a prominent after-hyperpolarization. Subsequent visualization revealed an extensive axonal arbor which was preferentially located in the stratum lucidum of CA3 and often invaded the hilus. The dendrites of MFA interneurons were mainly located in the strata radiatum and oriens, suggesting that these cells are primarily activated by associational and commissural fibers. Electron microscopic analysis showed that axon terminals of MFA interneurons established symmetric synaptic contacts predominantly on proximal apical dendritic shafts, and to a lesser degree, on somata of pyramidal cells. Synaptic contacts were also found on GABAergic interneurons of the CA3 region and putative mossy cells of the hilus. Inhibitory postsynaptic currents (IPSCs) elicited by MFA interneurons in simultaneously recorded pyramidal cells had fast kinetics (half duration, 3.6 ms) and were blocked by the GABA(A) receptor antagonist bicuculline. Thus, MFA interneurons are GABAergic cells in a position to selectively suppress the mossy fiber input, an important requirement for the recall of memory traces from the CA3 network.

Modulation of the frequency of spontaneous sarcoplasmic reticulum Ca2+ release events (Ca2+ sparks) by myoplasmic [Mg2+] in frog skeletal muscle.

The modulation by internal free [Mg2+] of spontaneous calcium release events (Ca2+ "sparks") from the sarcoplasmic reticulum (SR) was studied in depolarized notched frog skeletal muscle fibers using a laser scanning confocal microscope in line-scan mode (x vs. t). Over the range of [Mg2+] from 0.13 to 1.86 mM, decreasing the [Mg2+] induced an increase in the frequency of calcium release events in proportion to [Mg2+]-1.6. The change of event frequency was not due to changes in [Mg-ATP] or [ATP]. Analysis of individual SR calcium release event properties showed that the variation in event frequency induced by the change of [Mg2+] was not accompanied by any changes in the spatiotemporal spread (i.e., spatial half width or temporal half duration) of Ca2+ sparks. The increase in event frequency also had no effect on the distribution of event amplitudes. Finally, the rise time of calcium sparks was independent of the [Mg2+], indicating that the open time of the SR channel or channels underlying spontaneous calcium release events was not altered by [Mg2+] over the range tested. These results suggest that in resting skeletal fibers, [Mg2+] modulates the SR calcium release channel opening frequency by modifying the average closed time of the channel without altering the open time. A kinetic reaction scheme consistent with our results and those of bilayer and SR vesicle experiments indicates that physiological levels of resting Mg2+ may inhibit channel opening by occupying the site for calcium activation of the SR calcium release channel.

Submillisecond AMPA Receptor-Mediated Signaling at a Principal Neuron–Interneuron Synapse

Glutamatergic transmission at a principal neuron–interneuron synapse was investigated by dual whole-cell patch-clamp recording in rat hippocampal slices combined with morphological analysis. Evoked EPSPs with rapid time course (half duration ≈ 4 ms; 34°C) were generated at multiple synaptic contacts established on the interneuron dendrites close to the soma. The underlying postsynaptic conductance change showed a submillisecond rise and decay, due to the precise timing of glutamate release and the rapid deactivation of the postsynaptic AMPA receptors. Simulations based on a compartmental model of the interneuron indicated that the rapid postsynaptic con-ductance change determines the shape and the somatodendritic integration of EPSPs, thus enabling interneurons to detect synchronous principal neuron activity.