Negative Current Pulses Research Articles

Electrophysiological and repetitive firing properties of neurons in the superficial/middle layers of the human neocortex maintained in vitro.

Conventional intracellular recordings were made from neurons located in the superficial/middle layers of human temporal neocortical slices obtained from patients undergoing neurosurgical procedures for the treatment of epilepsy or brain tumour. In most of the neurons, inward membrane rectification was observed when the cell was depolarized or hyperpolarized from rest by intracellular injection of positive or negative current pulses. Bath application of tetrodotoxin abolished the depolarizing inward rectification, but not the "anomalous rectification" in the hyperpolarizing direction. Single action potential firing was followed by a fast afterhyperpolarization, a depolarizing afterpotential and a medium afterhyperpolarization, while a slower afterhyperpolarization was seen following repetitive firing. Blockade of Ca2+ channels with Cd2+ diminished all three types of afterhyperpolarization. Although the repetitive firing pattern in all cells indicated that they discharge in a regular-spiking fashion, 63% of the cells fired tonically in the initial part of discharge, while the remaining 37% of the cells fired phasically. Frequency-current plot for the initial interspike intervals during long depolarizing pulses revealed primary and secondary ranges of firing. Spike frequency adaptation was also observed. In conclusion, our experiments indicate that human neocortical cells in the superficial/middle layers display electrophysiological characteristics that are similar to those described in rodent and feline neocortices.

Experimental confirmation of positive-streamer-like mechanism for negative corona current pulse rise

Current growth waveforms corresponding to initial phases of negative corona current pulses were measured in nitrogenous gas mixtures at pressures of 6.67-40 kPa. The measurements showed that the steep pulse rise is not dependent on cathode secondary emission processes for pressures above 30 kPa. This, together with a discontinuous transition in the current waveforms observed with increasing gap voltage in N2 at pressures of 20-26.6 kPa, implies that the feedback-to-cathode Townsend ionization mechanism was supplanted by a faster streamer-like mechanism. At pressures below 20 kPa a positive-streamer-like wave results in complex Trichel pulse shapes with a step on the pulse leading edge and a hump on the pulse trailing part.

Memory switching effects in a-Si/c-Si heterojunction bipolar structures

The authors report a three-terminal undoped amorphous silicon (a-Si)/p-n crystalline silicon (c-Si) structure, which exhibits OFF and ON states. An OFF state is characterized by a current in the structure in the low nanoampere range due to the large resistance of the undoped a-Si layer, while in the ON state the structure exhibits a large conductance and its current is determined in practice by the load resistance. Reversible switching between the two states with a rise time of about 40 ns and a fall time of about 200 ns was achieved by applying appropriate positive or negative current pulses to the base of the c-Si p-n junction. The structure can be integrated into a standard bipolar process, and, being of a size suitable for VLSI applications, may be useful as a basic three-terminal memory cell. >

Sensorless Position and Speed Control of a Brushless DC Motor from Start-Up to Nominal Speed

SummaryThe brush less DC motors are synchronous motors, self-commutated through a transistor bridge. The aim is to generate a magnetic field always positioned so that the torque is maximum. This principle needs a rotor position detection. Sensors as Hall effect devices, resolvers or optical disks arc generally used. For small motors as those used in computer peripherics, it becomes impossible to implement such devices. Thus, the goal is to realize indirect sensors. The present paper describes a methodology to control a motor from standstill to full speed without direct position and speed sensors.At standstill, the position is determined by short current pulses, enabling to detect indirectly the saturation level of the different phases. In order to avoid the temperature effect on the time constants, the measurement is converted into a current difference, with positive and negative current pulses in each phase. So, a logical state is defined, corresponding to the actual position with an angular accuracy of ...

Negative Corona Current Pulses and Cathode Sheath Instabilities in a Short Point-Plane Gap in CO2

Waveforms ofthe first negative corona current pulses have been measured in C02 and C02+SF6 mixtures as a function of applied voltage and gas pressure in the range 6�67-66�7 kPa. A complex form of the pulses with a step on the pulse leading edge and a current hump on the pulse trailing part has been found at gas pressures below 40 kPa. Changing the cathode surface material from copper to copper iodide, which has an exceptionally high photoelectric yield, resulted in at least a twofold increase in height of the step on the pulse leading edge, but had little effect on the main current rise to the pulse peak. The use of unconditioned cathodes resulted in the appearance of peculiar current spikes on the pulse tail, which are tentatively attributed to positive-streamer-like instabilities of the cathode sheath. The implications of these results for theories of Trichel pulse formation are discussed, and a physical picture of the phenomenon based on the presumed existence of a cathode-directed streamer associated with the steep pulse rise is outlined.

Hyperpolarizing inward rectification in rat neocortical neurons located in the superficial layers

Intracellular recordings were performed in neurons located in layers II III of rat neocortical slices maintained in vitro. High intensity negative current pulses elicited hyperpolarizing responses that were characterized by a sag of membrane potential towards the resting level. Graphically, this inward rectification was reflected as a break in the slope of the I– V plot at membrane level of around −98 mV. The inward rectification was only partially reduced by the bath perfusion of Cs + (3 mM). In contrast, it was abolished by the extracellular addition of Ba 2+ (3 mM) or the intracellular injection of QX-314 (50 mM). Our findings indicate that Ba 2+ and QX-314 are more effective than Cs + in blocking the hyperpolarizing inward rectification generated by rat neocortical cells located in the superficial layers.

Measurement of contractile and electrical properties of single human thenar motor units in response to intraneural motor-axon stimulation

1. A method is described for measuring contractile properties of single human motor units. Conventional human microneurographic techniques were adapted to stimulate individual motor axons in the median nerve, with the use of negative current pulses and a tungsten microelectrode, while recording motor-unit electromyographic activity (EMG) and isometric force responses from the thenar muscles. 2. EMG signals were recorded from both proximal and distal thenar muscle surfaces. Force was recorded in two directions (thumb flexion and abduction). This allowed calculation of the direction and magnitude of resultant force exerted by each unit. 3. Data accepted as originating from a single unit satisfied all the traditional "all-or-none" criteria. Additional criteria also required the following: 1) a wide safety margin between the threshold for unit activation and the current intensity needed to elicit responses from other units; 2) that the characteristic direction in which each unit generated force did not change during the recording period; and 3) whenever F-responses were encountered, the second EMG waveform was identical to the first--a highly improbable event if more than one unit had been excited. 4. Respiration and blood pressure waves introduced baseline fluctuations that distorted the force measurements. These fluctuations were minimized by synchronizing stimuli to the pulse pressure cycle and resetting the baseline electronically just before stimulus onset. 5. Combining motor-axon stimulation at a site remote from the muscle with electronic resetting of the force baseline and delivery of stimuli at fixed intervals after the pulse pressure waves allowed the full time course of human motor-unit twitch and tetanic force and EMG signals to be recorded accurately without signal averaging.

A ‘puff and advance’ technique for visually controlled staining of turtle retinal ganglion cells

We describe a ‘puff and advance’ technique for visually controlled staining of retinal ganglion cells (GCs) in the unfixed, living retina for light and electron microscopy. Glass microelectrodes are filled with rhodamine-isothiocyanate labeled horseradish peroxidase (Rh-HRP), or Lucifer yellow (LY), or a mixture of both, or with 5,6-carboxytetramethylrhodamine (5,6-Rh) and advanced tangentially through the GC layer with microscopic observation using epifluorescence. Brief “puffs” of LY or 5,6-Rh are constantly ejected from the advancing electrode tip by a train of negative current pulses. GC penetration is signaled by virtually instantaneous staining of its soma (and eventually its axon and dendrites if the electrode is not advanced further). An impaled GC can be electron densely stained with the Rh-HRP complex by switching to positive current pulses. The extent of dye filling is monitored through the microscope using a filter combination appropriate for the dye. After fixation, standard histochemical procedures reveal HRP stained GCs in wholemount views for light microscopical examination. Furthermore, the preservation of the labeled cells and the neuropil is of a quality to allow electron microscopic analysis for synaptic input. This technique can be used in combination with LY backfilling of GCs from the optic nerve and with retinas in which GCs have been prelabeled with rhodamine beads retrogradely transported from the optic tectum as well.

First Negative Corona Pulses in 70% N2+30%SF6 Mixture

Waveforms of the first negative corona current pulses in a 70% N2+30% SF6 mixture at pressures of 13.33, 26.66 and 40 kPa have been studied as a function of overvoltage ranging from 0 to 100%. Near the corona threshold, the pulses, taking the form of a simple hump, are governed by the secondary electron emission due to a positive-ion impact. At moderate overvoltage, the formation of a double-peaked pulse was observed. This phenomenon can be explained in terms of the effect of photoemission and secondary electron emission. At higher overvoltages, peculiar current waveforms were observed. This phenomenon was explained by the association of the development of a cathode-directed streamer and its interaction with a cathode surface.

Secondary electron emission in the course of the first Trichel-like pulse development in N/sub 2/+SF/sub 6/ mixtures

Waveforms of the first negative corona current pulses in 70% N/sub 2/+30% SF/sub 6/ mixture at a pressure of 13.33 kPa were investigated as a function of applied gap voltage. Based on the observed changes of the pulse shape, the role of different emission processes in the course of the discharge development was elucidated. At corona threshold, the pulse took the form of a simple hump, consistent with secondary electron emission due to positive-ion impact. A small increase in gap voltage resulted in the formation of a double-peaked pulse form which can be explained in terms of independent photoemission and secondary emission due to positive-ion bombardment. At higher voltage values (1.5 to two times higher than the corona threshold voltage), a peculiar form of the current pulse leading edge was observed and tentatively has been attributed to a field emission.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-power, TIL-type gate-assisted turn-off thyristors

The work reports the development of trial high-voltage (2000–2500 V), highcurrent (up to 500 A) gate-assisted turn-off (GAT) thyristors based on the novel, double-interdigitated or two interdigitation levels (TIL) gate-cathode configuration. The active cathode area of devices is only 1.7 cm2. The test TIL GATTs were both normal, i.e. using no lifetime killers and gold-diffused at several temperatures. The investigations revealed the high efficiency of the negative current pulse, applied to the gate during the time when the forward anode voltage is being reapplied, in the reduction of the turn-off time t q. Thus, the application of a peak gate current I GR of only 2.5 − 4 A leads to a two- to tenfold decrease of t q in TIL GATTs rated at an anode current i T of 200-500 A. Within the aforementioned levels of i T and at constant value of i GR the dependence of t q on i T was found to be relatively weak. The dependence of TIL GATT performance on gold-doping, junction temperature, n-base thickness and the rate of reapplied anode voltage rise is presented in detail. All trial TIL GATTs possess an excellent turn-on sensitivity accompanied by a high immunity to noise. The results presented reveal the ability of TIL patterns with a relatively coarse geometry to perform multiple-end roles in various modes of operation of high-power GATTs.

Chaotic phenomena in an oscillator controlled by DC current pulse

AbstractChaos is a general term for irregular phenomena appearing in deterministic systems which can be represented by differential or difference equations, and it has been studied widely in various fields of natural science. Chaotic phenomena have long been known to occur also in nonlinear electrical circuits. The proof for the generation of chaos, however, is insufficient and circuit experiments are not satisfactory. This paper proposes a chaos generator with which chaos generation can be verified and its confirmation by circuit experiments also is possible. The circuit is constructed from a negative resistance LC oscillator and dc current pulse source. Also, when the amplitude exceeds a certain value, a current pulse of the opposite direction is added, thereby controlling the amplitude. For the analysis a mapping method is used and its Poincaré map is derived strictly as a one‐dimensional mapping having a form similar to the Logistic map. Using this map, such phenomena as period doubling bifurcation, chaos, islands and windows, that are generated in the circuit can be explained. In addition, these phenomena are confirmed experimentally.

Very low resistance ohmic contacts on p-type InP by direct plating

Very low resistance ohmic contacts (ρc ≃4×10−5 Ω cm2) have been formed on p-type InP by light assisted plating of Au and Zn. The plating technique, which uses alternating positive and negative current pulses, is suitable for producing patterned, small care contacts on device structures and is compatible with established n-type ohmic contacting procedures.

Tungsten (W) as electrode material: electrode potential and small-signal impedances.

Tungsten (W) microelectrodes were made with a taper of the tip of about 1∶10 and insulated with lacquer leaving a tip length of about 10–100 μm. The resting potential in saline was about −0·3 V relative to an Ag/AgCl reference electrode for input currents <10−12 A. The small signal impedance was ideally that of a capacitor 0·4 pF/μm2 between 10 Hz and 1000 Hz. Imperfect insulation at the tip caused this impedance to be increasingly resistive. The voltage response to positive and negative current pulses was symmetrical for currents up to 10−8 A. The results obtained in the brain of the conscious monkey and the anaesthetised frog were similar to those obtained in the salines. The electrochemical properties of tungsten were studied using macroelectrodes: the metal behaves as an inert metal within the potentials where it is usually used in biological experiments.

Zeeman modulator for nuclear quadrupole resonance spectroscopy

An instrument has been developed to drive modulation coils to produce Zeeman modulation in nuclear quadrupole resonance spectroscopy. A sequence of alternately negative and positive current pulses of up to 4 A, spaced by no-current intervals, is produced. The frequency may be as high as 500 Hz, since no electromechanical components are employed. The instrument is described in detail and sample resonance curves are presented.

The Mode of Transverse Spread of Contraction Initiated by Local Activation in Single Crayfish Muscle Fibers

Isolated single crayfish muscle fibers were locally activated by applying negative current pulses to a pipette whose tip was in contact with the fiber surface. The contraction initiated by a moderate depolarization spread inwards in a graded manner according to the magnitude and duration of depolarization. Increase of the depolarized area increased the distance of the inward spread for a given amount of depolarization. If a large area of the surface membrane was depolarized with a large pipette for a sufficiently long time, the contraction spread not only inwards, but further transversely passing through the center of the fiber. Successive brief depolarizations given at an appropriate interval could produce contraction more effectively for a given amount of total current than did a prolonged depolarization. On the other hand, the contraction initiated by a strong negative current was observed to spread around the whole perimeter but not through the center of the fiber. Each type of local contraction always spread along the striation pattern and not longitudinally. Possible mechanisms of these responses are discussed in connection with the transverse tubular system of the muscle fibers.

The mode of transverse spread of contraction initiated by local activation in single frog muscle fibers.

Isolated single frog muscle fibers were locally activated by applying negative current pulses to a pipette whose tip was in contact with the fiber surface. In contrast to the graded inward spread of contraction initiated by a moderate depolarization, the contraction in response to a strong negative current was observed to spread transversely around the whole perimeter but not through the center of the fiber. This response was elicited only with pipettes of more than 6 micro diameter. The response was still present if the sodium of the Ringer solution was replaced by choline, or the chloride was replaced by nitrate or propionate. The duration of the response appeared to be independent of the duration of stimulating current in fresh fibers, while the contraction lasted as long as the current went on in deteriorated fibers. The contraction was first initiated at the area of fiber surface covered by the pipette, and spread around the perimeter of the fiber with a velocity of 0.8-6 cm/sec. Possible mechanisms of the response are discussed in connection with the properties of the transverse tubular system, the possibility of some self-propagating process along the walls of the tubules being suggested.