Near-threshold Currents Research Articles

Splashing conditions for a liquid metal in vacuum arcs: Cyclic processes in a cathode spot

Molten metal extrusion from craters that form on the cathode during vacuum arc burning is considered. Using the hydrodynamic similarity principle, this process is compared with the well-studied splashing process that can develop within the impact of drops impinging one by one on a solid surface (the arc cycle duration is identified with the inverse frequency of the drop train). Based on this analogy, the conditions under which the regime of spreading of a liquid over the cathode surface will change into the splashing regime (accompanied by the formation of microjets and droplets) are analyzed. As it turns out, the conditions realized in vacuum arc cathode spot at near-threshold currents are close to the splashing threshold of liquid metal. This gives grounds to relate the existence of a threshold arc current to the existence of a threshold for the process of liquid metal jet formation.

Hydrodynamics of the molten metal in a vacuum arc cathode spot at near-threshold currents

The extrusion of the molten metal from a microcrater formed on a metal cathode during the operation of a vacuum arc is considered. The problem is thought to be similar to the classical hydrodynamic problem of a liquid drop impact on a solid surface. Based on this analogy, the conditions are analyzed under which the liquid will change its regular behavior (spreading over the cathode surface) into a singular behavior (formation of microjets and droplets). It is shown that the conditions realized in vacuum arc cathode spots at near-threshold currents are close to the threshold conditions for splashing of the molten metal. This points to a considerable contribution of hydrodynamic processes to the self-sustained operation of a vacuum arc and, in particular, gives grounds to relate the existence of a threshold arc current to the existence of a splashing threshold for liquid metal.

Contribution of near-threshold currents to intrinsic oscillatory activity in rat medial entorhinal cortex layer II stellate cells

The temporal lobe is well known for its oscillatory activity associated with exploration, navigation, and learning. Intrinsic membrane potential oscillations (MPOs) and resonance of stellate cells (SCs) in layer II of the entorhinal cortex are thought to contribute to network oscillations and thereby to the encoding of spatial information. Generation of both MPOs and resonance relies on the expression of specific voltage-dependent ion currents such as the hyperpolarization-activated cation current (I(H)), the persistent sodium current (I(NaP)), and the noninactivating muscarine-modulated potassium current (I(M)). However, the differential contributions of these currents remain a matter of debate. We therefore examined how they modify neuronal excitability near threshold and generation of near-threshold MPOs and resonance in vitro. We found that resonance mainly relied on I(H) and was reduced by I(H) blockers and modulated by cAMP and an I(M) enhancer but that neither of the currents exhibited full control over MPOs in these cells. As previously reported, I(H) controlled a theta-frequency component of MPOs such that blockade of I(H) resulted in fewer regular oscillations that retained low-frequency components and high peak amplitude. However, pharmacological inhibition and augmentation of I(M) also affected MPO frequencies and amplitudes. In contrast to other cell types, inhibition of I(NaP) did not result in suppression of MPOs but only in a moderation of their properties. We reproduced the experimentally observed effects in a single-compartment stochastic model of SCs, providing further insight into the interactions between different ionic conductances.

Dendritic Spikes in Apical Dendrites of Neocortical Layer 2/3 Pyramidal Neurons

Layer 2/3 (L2/3) pyramidal neurons are the most abundant cells of the neocortex. Despite their key position in the cortical microcircuit, synaptic integration in dendrites of L2/3 neurons is far less understood than in L5 pyramidal cell dendrites, mainly because of the difficulties in obtaining electrical recordings from thin dendrites. Here we directly measured passive and active properties of the apical dendrites of L2/3 neurons in rat brain slices using dual dendritic-somatic patch-clamp recordings and calcium imaging. Unlike L5 cells, L2/3 dendrites displayed little sag in response to long current pulses, which suggests a low density of I(h) in the dendrites and soma. This was also consistent with a slight increase in input resistance with distance from the soma. Brief current injections into the apical dendrite evoked relatively short (half-width 2-4 ms) dendritic spikes that were isolated from the soma for near-threshold currents at sites beyond the middle of the apical dendrite. Regenerative dendritic potentials and large concomitant calcium transients were also elicited by trains of somatic action potentials (APs) above a critical frequency (130 Hz), which was slightly higher than in L5 neurons. Initiation of dendritic spikes was facilitated by backpropagating somatic APs and could cause an additional AP at the soma. As in L5 neurons, we found that distal dendritic calcium transients are sensitive to a long-lasting block by GABAergic inhibition. We conclude that L2/3 pyramidal neurons can generate dendritic spikes, sharing with L5 pyramidal neurons fundamental properties of dendritic excitability and control by inhibition.

Long-term enhancement of entorhinal-dentate evoked potentials following `modified' ECS in the rat

Electroconvulsive therapy (ECT) is widely used as a treatment for drug-resistant depression. The animal analogue of ECT is electroconvulsive shock (ECS) seizures. We have recently shown that repeated ECS seizures cause a long-lasting, perhaps permanent, enhancement in entorhinal-dentate evoked potentials in the rat. Our study, however, involved `unmodified' ECS, whereas in clinical practice ECT is now usually given in its `modified' form (with near-threshold currents, a short-acting barbiturate, muscle relaxant and oxygen). We have therefore repeated our experiments using modified ECS. Entorhinal-dentate evoked potentials were measured in Long–Evans rats before and after: (1) eight modified ECS seizures; or (2) eight sham modified ECS trials. Despite the use of the modified procedure, a significant and long-lasting enhancement in population spike amplitude was seen in the ECS group. We conclude that the modified procedure does not protect rats against the long-lasting enhancement of evoked potentials. Similar changes may be occurring in the brains of patients subjected to modified ECT.

Accelographic Train-of-four at Near-threshold Currents

The authors evaluated train-of-four (TOF) fade, as quantified by accelography, in response to neurostimulation at currents ranging from 10 to 60 mA. This was done to determine the range of currents over which measurements of fade remain consistent. In 31 patients (ASA Physical Status 1,2, and 3), anesthesia was induced with fentanyl, midazolam, and thiopental and was maintained with isoflurane and 66% nitrous oxide in oxygen. Surface stimulating electrodes were placed over the ulnar nerve, and an acceleration transducer was placed on the thumb. Succinylcholine was administered to facilitate tracheal intubation; after neuromuscular recovery, a bolus of vecuronium (0.01-0.05 mg.kg-1) and an infusion (0.25-1.5 micrograms.kg-1.min-1) were administered. After documentation of a stable TOF ratio, accelographic TOF responses were quantified in response to 200-microseconds stimulation at 10, 15, 20, 30, 40, 50, and 60 mA, in random order. A total of 95 data sets were collected at different depths of blockade. The TOF ratios maintained intercurrent consistency (P = not significant by nonparametric repeated measures analysis of variance), except at currents near the fourth-twitch (T4) threshold current. This inconsistency was eliminated by testing at greater than or equal to 10 mA above threshold. TOF ratios obtained at 10 mA above T4 threshold correlated highly with those at 60 mA (Spearman r value = 0.94). The authors conclude that the TOF ratio is consistent over a wide range of stimulating currents and that testing with submaximal currents can be performed reliably at greater than or equal to 10 mA above the T4 threshold.(ABSTRACT TRUNCATED AT 250 WORDS)

Medial forebrain bundle units in the rat: dependence of refractory period estimates on pulse duration

The slope of some psychophysically-derived curves describing recovery from refractoriness in medial forebrain bundle (MFB) reward neurons decreases as the pulse duration is lengthened. This effect has been attributed to increased recruitment of small-diameter fibers by the long-duration pulses. Alternatively, this effect might be due to increases in the refractory periods of a fixed population of neurons. By means of extracellular recording, we measured the effect of increasing the pulse duration on recovery from refractoriness in 26 neurons antidromically activated by MFB stimulation. In all cases, recovery from refractoriness was delayed as the pulse duration was increased from 0.1 ms to 2.0 ms. The delay was more pronounced at near-threshold currents than at higher currents and was generally largest in neurons that recovered most rapidly from refractoriness when stimulated with brief pulses. These effects of long duration pulses on recovery of excitability in a fixed population of neurons may have contributed to the delay in recovery implied by the psychophysical data. Thus, delayed recovery does not necessarily imply preferential recruitment of small-diameter fibers by the long-duration pulses.

Development of EEG epileptic activity and seizures during kindling in sensorimotor cortex in cats.

The problem of the relationship between stimulus intensity and kindling effect was studied in three groups of cats with bipolar stimulating electrodes implanted in the right posterior sigmoid gyrus (sensorimotor cortex). Daily stimulation with a 1-sec train of 60-Hz rectangular pulses was carried out in 17 cats over a period of from 27 to 265 days. Group I, 3 cats, was stimulated with a current intensity of 200 microA, peak-to-peak, which was subthreshold for afterdischarges (ADs); 6 animals from group II were stimulated with near-threshold currents (0.8--1.1 mA); and in the 8 animals of the group III, the ADs were evoked by threshold currents of 1.0--1.6 mA. The EEG was recorded from the sensorimotor and visual cortices, hippocampus, caudate nucleus, dentate nucleus, and cerebellar cortex. It was found that the low-current stimulation (200 microV) was not effective in inducing kindling. Near-threshold stimulation (below 1 mA) resulted in the development of bioelectrical epileptic activity in most cats. Threshold stimulation for AD resulted in the development of bioelectrical spontaneous activity and in an increase in the duration of ADs, as well as in generalized tonic-clonic seizures during cortical stimulations, in the majority of cats. Differences in hippocampal and neocortical kindling in cats are discussed in terms of ADs and seizure development. It was found that (1) a longer time was required for neocortical than for hippocampal kindling (3--10 weeks), and (2) there was greater variability in the effects of neocortical kindling. Secondary generalized seizures developed in the group with threshold stimulation for AD and were preceded by an increase in the number of ADs. The interictal epileptic activity developed in some cats in the absence of ADs.