Spontaneous Voltage Oscillations Research Articles

Oscillatory Reaction Rates in a Direct Formic Acid Fuel Cell (DFAFC)

Formic acid has been studied as a promising fuel for its direct oxidation in fuel cells since, in comparison with other organic molecules, it is more easily oxidized to CO2, has a high thermodynamic voltage and fewer problems associated with crossover [1]. It is accepted that the electro-oxidation of formic acid on platinum proceeds through parallel reaction pathways, including an indirect path through carbon monoxide poisoning, the active intermediate (or formate) pathway and the direct pathway [2]. From another point of view, the electro-oxidation of formic acid on Pt and on Pt-group metals is a typical system showing periodic oscillations. Basically, the mechanism that gives rise to this type of oscillatory dynamics is the interplay between the adsorption isotherm of a catalytic poison, for example θCO, and of oxygenated species, θOH, both of which are potential dependent [3]. A key question underlying this dynamics is how it can affect the efficiency of a DFAFC. The purpose of this study was to evaluate the performance of a DFAFC under oscillatory regime. The experiments were performed using the typical setup of a unit cell, with a special approach (hydrogen on the cathode) that allows reading the anodic overvoltage directly. The conditions were: Pt/C (50 wt%) was used as anodic and cathodic catalysts with a metal loading of 1 mgPt cm-2. The anode was fed with 8 mol L-1 of HCOOH and the cathode was fed with H2. The unit cell (≈ 4.6 cm2) was operated at 30oC under atmosphere pressure. Figure 1 exhibits the current-potential curves obtained under potentiostatic and galvanostatic control modes for the electro-oxidation of formic acid and the theoretical curve for oxygen reduction. It is worth noting that the high overpotential required for the oxygen reduction and for the formic acid oxidation on Pt drastically reduces the DFAFC voltage, as it does for all fuel cells, given the sluggish kinetics. However, during the slow galvanodynamic sweep, spontaneous voltage oscillations emerge and this kinetic instability periodically decreases the anodic overpotential. This fact is interesting since the oscillations increase the average cell voltage and could result in the deceleration of long-term surface deactivation [4]. It also testifies to the importance of evaluating its dynamics to improve the fuel cell efficiency. The occurrence of such oscillatory behavior during the operation of a Direct Methanol Fuel Cell (DMFC) has been report, however, due to the small amplitude and waveform of the oscillations that takes place during the methanol electro-oxidation, no net improvement was observed in this system [5]. Although, a decrease in long-term instabilities can still play an important role. Probably, the self-cleaning process is more efficient for formic acid oxidation than that for methanol due to its larger amplitude and also because during a cycle the anodic overpotential remains longer at lower values [4]. Here we report a self-organization process observed in a DFAFC and its influence on the device efficiency. It was observed that at certain moments the anodic potential is lower than that on a steady state and this phenomena together with a self-cleaning process could improve the overall fuel cell performance. Acknowledgements J.A.N. and H.V. acknowledge São Paulo Research Foundation (FAPESP) for the scholarship (grant #2015/09295-9) and financial support (grants #2012/21204-0, and #2013/16930-7). H.V. (#306151/2010-3) also acknowledges Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for financial support.

Modelling of photo-thermal control of biological cellular oscillators.

We study the transient dynamics of biological oscillators subjected to brief heat pulses. A prospective well-defined experimental system for thermal control of oscillators is the peripheral electroreceptors in paddlefish. Epithelial cells in these receptors show spontaneous voltage oscillations which are known to be temperature sensitive. We use a computational model to predict the effect of brief thermal pulses in this system. In our model thermal stimulation is realized through the light excitation of gold nanoparticles delivered in close proximity to epithelial cells and generating heat due to plasmon resonance. We use an ensemble of modified Morris-Lecar systems to model oscillatory epithelial cells. First, we validate that the model quantitatively reproduces the dynamics of epithelial oscillations in paddlefish electroreceptors, including responses to static and slow temperature changes. Second, we use the model to predict transient responses to short heat pulses generated by the light actuated gold nanoparticles. The model predicts that the epithelial oscillators can be partially synchronized by brief 5 - 15 ms light stimuli resulting in a large-amplitude oscillations of the mean field potential.

Spontaneous voltage oscillations and response dynamics of a Hodgkin-Huxley type model of sensory hair cells

We employ a Hodgkin-Huxley-type model of basolateral ionic currents in bullfrog saccular hair cells for studying the genesis of spontaneous voltage oscillations and their role in shaping the response of the hair cell to external mechanical stimuli. Consistent with recent experimental reports, we find that the spontaneous dynamics of the model can be categorized using conductance parameters of calcium-activated potassium, inward rectifier potassium, and mechano-electrical transduction (MET) ionic currents. The model is demonstrated for exhibiting a broad spectrum of autonomous rhythmic activity, including periodic and quasi-periodic oscillations with two independent frequencies as well as various regular and chaotic bursting patterns. Complex patterns of spontaneous oscillations in the model emerge at small values of the conductance of Ca2+-activated potassium currents. These patterns are significantly affected by thermal fluctuations of the MET current. We show that self-sustained regular voltage oscillations lead to enhanced and sharply tuned sensitivity of the hair cell to weak mechanical periodic stimuli. While regimes of chaotic oscillations are argued to result in poor tuning to sinusoidal driving, chaotically oscillating cells do provide a high sensitivity to low-frequency variations of external stimuli.

Spontaneous low-frequency voltage oscillations in frog saccular hair cells.

Spontaneous membrane voltage oscillations were found in 27 of 130 isolated frog saccular hair cells. Voltage oscillations had a mean peak-to-peak amplitude of 23 mV and a mean oscillatory frequency of 4.6 Hz. When compared with non-oscillatory cells, oscillatory cells had significantly greater hyperpolarization-activated and lower depolarization-activated current densities. Two components, the hyperpolarization-activated cation current, I(h), and the K(+)-selective inward-rectifier current, I(K1), contributed to the hyperpolarization-activated current, as assessed by the use of the I(K1)-selective inhibitor Ba(2+) and the I(h)-selective inhibitor ZD-7288. Five depolarization-activated currents were present in these cells (transient I(BK), sustained I(BK), I(DRK), I(A), and I(Ca)), and all were found to have significantly lower densities in oscillatory cells than in non-oscillatory cells (revealed by using TEA to block I(BK), 4-AP to block I(DRK), and prepulses at different voltages to isolate I(A)). Bath application of either Ba(2+) or ZD-7288 suppressed spontaneous voltage oscillations, indicating that I(h) and I(K1) are required for generating this activity. On the contrary, TEA or Cd(2+) did not inhibit this activity, suggesting that I(BK) and I(Ca) do not contribute. A mathematical model has been developed to test the interpretation derived from the pharmacological and biophysical data. This model indicates that spontaneous voltage oscillations can be generated when the electrophysiological features of oscillatory cells are used. The oscillatory behaviour is principally driven by the activity of I(K1) and I(h), with I(A) playing a modulatory role. In addition, the model indicates that the high densities of depolarization-activated currents expressed by non-oscillatory cells help to stabilize the resting membrane potential, thus preventing the spontaneous oscillations.

Spontaneous voltage oscillations in striatal projection neurons in a rat corticostriatal slice.

In a rat corticostriatal slice, brief, suprathreshold, repetitive cortical stimulation evoked long-lasting plateau potentials in neostriatal neurons. Plateau potentials were often followed by spontaneous voltage transitions between two preferred membrane potentials. While the induction of plateau potentials was disrupted by non-NMDA and NMDA glutamate receptor antagonists, the maintenance of spontaneous voltage transitions was only blocked by NMDA receptor and L-type Ca2+ channel antagonists. The frequency and duration of depolarized events, resembling up-states described in vivo, were increased by NMDA and L-type Ca2+ channel agonists as well as by GABAA receptor and K+ channel antagonists. NMDA created a region of negative slope conductance and a positive slope crossing indicative of membrane bistability in the current-voltage relationship. NMDA-induced bistability was partially blocked by L-type Ca2+ channel antagonists. Although evoked by synaptic stimulation, plateau potentials and voltage oscillations could not be evoked by somatic current injection--suggesting a dendritic origin. These data show that NMDA and L-type Ca2+ conductances of spiny neurons are capable of rendering them bistable. This may help to support prolonged depolarizations and voltage oscillations under certain conditions.

Interactions between multiple rhythm generators produce complex patterns of oscillation in the developing rat spinal cord.

Neural networks capable of generating coordinated rhythmic activity form at early stages of development in the spinal cord. In this study, voltage-imaging techniques were used to examine the spatiotemporal pattern of rhythmic activity in transverse slices of lumbar spinal cord from embryonic and neonatal rats. Real-time images were recorded in slices stained with the voltage-sensitive fluorescent dye RH414 using a 464-element photodiode array. Fluorescence signals showed spontaneous voltage oscillations with a frequency of 3 Hz. Simultaneous recordings of fluorescence and extracellular field potential demonstrated that the two signals oscillated with the same frequency and had a distinct phase relationship, indicating that the fluorescence changes represented changes in transmembrane potentials. The oscillations were reversibly blocked by cobalt (1 mM), indicating a dependence on Ca(2+) influx through voltage-gated Ca(2+) channels. Extracellular field potentials revealed oscillations with the same frequency in both stained and unstained slices. Oscillations were apparent throughout a slice, although their amplitudes varied in different regions. The largest amplitude oscillations were produced in the lateral regions. To examine the spatial organization of rhythm-generating networks, slices were cut into halves and quarters. Each fragment continued to oscillate with the same frequency as intact slices but with smaller amplitudes. This finding suggested that rhythm-generating networks were widely distributed and did not depend on long-range projections. In slices from neonatal rats, the oscillations exhibited a complex spatiotemporal pattern, with depolarizations alternating between mirror locations in the right and left sides of the cord. Furthermore, within each side depolarizations alternated between the lateral and medial regions. This medial-lateral pattern was preserved in hemisected slices, indicating that pathways intrinsic to each side coordinated this activity. A different pattern of oscillation was observed in slices from embryos with synchronous 3-Hz oscillations occurring in limited regions. Our study demonstrated that rhythm generators were distributed throughout transverse sections of the lumbar spinal cord and exhibited a complex spatiotemporal pattern of coordinated rhythmic activity.

Generation of slow waves in membrane potential is an intrinsic property of interstitial cells of Cajal.

To reveal the unique intrinsic properties of interstitial cells of Cajal (ICC), morphological and electrophysiological characteristics of isolated ICC from the adult mouse small intestine were investigated and compared with those of smooth muscle cells. All typical ultrastructural features of in situ ICC were evident in isolated ICC throughout the isolation procedure and short-term culture. With the use of the nystatin perforated patch-clamp technique, ICC demonstrated spontaneous voltage oscillations that were not abolished by hyperpolarization nor by L-type calcium channel blockers. This rhythmic activity occurred at room temperature at a frequency of 13.9 +/- 11.2 cycles/min, with an amplitude of 13.4 +/- 11.2 mV at membrane potentials from -20 to -70 mV. Smooth muscle cells from the same culture only generated voltage-sensitive action potentials above the threshold potential of -35 mV. Hyperpolarization as well as the addition of L-type calcium channel blockers abolished the action potentials. In whole cell voltage-clamp recordings from ICC, a large noninactivating outward current was observed to be activated (5% threshold) at -49.6 mV with a half-activation voltage of -18.7 mV and slope factor of 9.9 mV. In contrast, in smooth muscle cells, smaller outward currents with distinctive transient outward currents were present. In conclusion, the generation of L-type calcium channel blocker-insensitive slow waves in membrane potential is a unique intrinsic property of ICC.

Epitaxial Si Sensors at Low Temperatures: Non-Linear Effects

Cryogenic bolometric sensors made from epitaxially grown Si:As have been tested down to 40 mK. The sensors were grown by chemical vapour deposition with a doped layer 8.4 µm thick. The dopant concentration was measured using SIMS and was constant, ±1%, with an excellent box profile. Arsenic concentrations up to 7.5 × 1018 cm−3 were achieved. Above 100 mK the low power resistance R(T) followed the variable range hopping law, or Efros-Shklovskii law for a Coulomb gap, R(T) = R0 exp(T0/T)1/2 with T0 ≈ 25 K, typically. A double sensor arrangement was used to measure the electron-phonon coupling in the sensors and the phonon coupling to the heat sink. As the dc current bias through a sensor was increased, spontaneous voltage oscillations were observed across the sensor below 100 mK, which limited the sensitivity of the sensors in this region. These are circuit-limited oscillations between high and low resistance states. A phase diagram was established for the spatio-temporal coexistence of the two states, with a critical temperature Tc = 115 mK. We show that this is an intrinsic phase transition within a thermal model of the electron-phonon coupling. For a resistance-temperature characteristic given by the Efros-Shklovskii law we find Tc = 0.00512 T0, independent of R0 and the coupling strength. This predicts Tc = 115 ± 4 mK in this case. The model gives excellent agreement for the critical voltage and current, by assuming that the breakdown occurred via the formation of a filamentary region of high current density and high electron temperature. At higher currents, the response was temperature independent and given by I(E) = I(0) exp{−(E0/E)1/2} where E is the average applied electric field and E0 ≈ 380 V/cm, in agreement with a thermal model which includes the phonon-phonon coupling to the heat sink.

Frequency-locked, quasiperiodic, and chaotic motions of current-density filaments in a semiconductor device.

Silicon multilayered devices biased with a dc voltage exhibit spontaneous voltage oscillations which are connected with spatial, pendulumlike oscillations of a current-density filament. When driven with a superimposed sinusoidal voltage, the oscillations of the device voltage show frequency locking, quasiperiodicity, and chaos. In order to correlate the global voltage oscillations with the filament motion spatially resolved measurements of the filament dynamics have been performed by using two different techniques: measurements of the recombination radiation with a streak camera and measurements of the potential on the device surface between the electrical contacts with an active potential probe. The experimental results clearly reveal that the various kinds of voltage oscillations are caused by corresponding frequency-locked, quasiperiodic, or chaotic filament motions. \textcopyright{} 1996 The American Physical Society.

Modeling the gastric mill central pattern generator of the lobster with a relaxation-oscillator network

1. The gastric mill central pattern generator (CPG) controls the chewing movements of teeth in the gastric mill of the lobster. This CPG has been extensively studied, but the precise mechanism underlying pattern generation is not well understood. The goal of this research was to develop a simplified model that captures the principle, biologically significant features of this CPG. We introduce a simplified neuron model that embodies approximations of well-known membrane currents, and is able to reproduce several global characteristics of gastric mill neurons. A network built with these neurons, using graded synaptic transmission and having the synaptic connections of the biological circuit, is sufficient to explain much of the network's behavior. 2. The cell model is a generalization and extension of the Van der Pol relaxation oscillator equations. It is described by two differential equations, one for current conservation and one for slow current activation. The model has a fast current that may, by adjusting one parameter, have a region of negative resistance in its current-voltage (I-V) curve. It also has a slow current with a single gain parameter that can be regarded as the combination of slow inward and outward currents. 3. For suitable values of the fast current parameter and the slow current parameter, the isolated model neuron exhibits several different behaviors: plateau potentials, postinhibitory rebound, postburst hyperpolarization, and endogenous oscillations. When the slow current is separated into inward and outward fractions with separately adjustable gain parameters, the model neuron can fire tonically, be quiescent, or generate spontaneous voltage oscillations with varying amounts of depolarization or hyperpolarization. 4. The most common form of synaptic interaction in the gastric CPG is reciprocal inhibition. A pair of identical model cells, connected with reciprocal inhibition, oscillates in antiphase if either the isolated cells are endogenous oscillators, or they are quiescent without plateau potentials, or they have plateau potentials but the synaptic strengths are below a critical level. If the isolated cells have widely differing frequencies (or would have if the cells were made to oscillate by adjusting the fast currents), reciprocal inhibition entrains the cells to oscillate with the same frequency but with phases that are advanced or retarded relative to the phases seen when the cells have the same frequency. The frequency of the entrained pair of cells lies between the frequencies of the original cells. The relative phases can also be modified by using very unequal synaptic strengths.(ABSTRACT TRUNCATED AT 400 WORDS)

Nickel induces oscillatory behavior and enhanced synaptic and electrotonic transmission between stomatogastric neurons of Panulirus interruptus

The pyloric pattern generator network of the stomatogastric ganglion uses a mixture of burst-inducing plateau potentials, synaptic transmission, and electrical coupling to produce its patterned output. This study examines the effects of two divalent, calcium channel blockers, nickel and cadmium, on voltage oscillations, synaptic transmission, and electrical coupling between the two pyloric dilator (PD) neurons and lateral pyloric (LP) neuron of Panulirus interruptus. The in vitro stomatogastric ganglion was bathed in saline containing tetrodotoxin (TTX) to eliminate Na-spikes and the spontaneous voltage oscillations of the pyloric rhythm, resulting in a steady resting potential. Addition of 50–100 μ M Ni 2+ to the TTX-saline induced voltage oscillations of similar amplitude and frequency as the endogenous rhythmic activity (before the application of TTX). 25–50 μM nickel enhanced graded synaptic transmission and electrical coupling and altered voltage waveforms, while producing little change in the input resistance measured in the soma. 10–1000 μM Cd 2+ acted as a dose-dependent blocker of graded synaptic transmission, but had no other detectable effects. We propose that nickel, in contrast to cadmium, exerts a modulator-like effect deep in the pyloric neuropil.

SPACE-CHARGE RECOMBINATION OSCILLATIONS IN DOUBLE-INJECTION STRUCTURES

The small-signal impedance is derived for a double-injection structure operating in the low-injection square-law regime. Space-charge effects are found to be essential. Spontaneous voltage oscillations are found to occur above a threshold voltage Vth. The theory gives quantitative agreement with experimental results for Au-doped Si p-i-n diodes. These include the temperature dependence of Vth and the threshold frequency as well as the voltage dependence of the frequency above threshold.