Unexpected Behavior of the Hydrogen Oxidation Reaction on Palladium in Alkaline Solution: A Feasible Kinetic Explanation

The hydrogen oxidation reaction was studied on a thin film palladium electrode supported on a gold substrate in alkaline solution. The hydrogen absorption process was analyzed by open circuit potential transient and the current - overpotential dependences were obtained by chronoamperometry at different rotation rates in a solution saturated with hydrogen. The results obtained show anunexpected profile at low overpotentials, where current decreases as rotation rate increases. A kinetic mechanism was proposed to interpret this behavior and the corresponding equations were derived. They were used to correlate the results, with a good agreement between the experimental and fitted curves.Fil: Montero, María de Los Angeles. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; ArgentinaFil: Gennero, Maria Rosa. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; ArgentinaFil: Chialvo, Abel Cesar. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Programa de Electroquímica Aplicada e Ingeniería Electroquímica; Argentin

Noise in neurons is message-dependent

Neuronal responses are conspicuously variable. We focus on one particular aspect of that variability: the precision of action potential timing. We show that for common models of noisy spike generation, elementary considerations imply that such variability is a function of the input, and can be made arbitrarily large or small by a suitable choice of inputs. Our considerations are expected to extend to virtually any mechanism of spike generation, and we illustrate them with data from the visual pathway. Thus, a simplification usually made in the application of information theory to neural processing is violated: noise {\sl is not independent of the message}. However, we also show the existence of {\sl error-correcting} topologies, which can achieve better timing reliability than their components.Comment: 6 pages,6 figures. Proceedings of the National Academy of Sciences (in press

Intensity Coding in Two-Dimensional Excitable Neural Networks

In the light of recent experimental findings that gap junctions are essential for low level intensity detection in the sensory periphery, the Greenberg-Hastings cellular automaton is employed to model the response of a two-dimensional sensory network to external stimuli. We show that excitable elements (sensory neurons) that have a small dynamical range are shown to give rise to a collective large dynamical range. Therefore the network transfer (gain) function (which is Hill or Stevens law-like) is an emergent property generated from a pool of small dynamical range cells, providing a basis for a "neural psychophysics". The growth of the dynamical range with the system size is approximately logarithmic, suggesting a functional role for electrical coupling. For a fixed number of neurons, the dynamical range displays a maximum as a function of the refractory period, which suggests experimental tests for the model. A biological application to ephaptic interactions in olfactory nerve fascicles is proposed.Comment: 17 pages, 5 figure

Kinetics of passivation and pitting corrosion of polycrystalline copper in borate buffer solutions containing sodium chloride

The pitting corrosion of copper in borate buffer containing sodium chloride is studied by using potentiostatic and potentiodynamic techniques complemented with scanning electron microscopy and EDAX. The breakdown potential shifts towards more negative values as the sodium chloride concentration increases. During pitting both soluble Cu(I) and Cu(II) species are detected. The first stage of pitting is explained through the competition between the passive layer formation and the nucleation and growth of the CuCl layer in equilibrium with Cu(I)-chloride complexes in solution. When salt nuclei reach the metal surface, pit growth under charge-transfer control is found. In the following stage the kinetics of pit growth changes to a diffusion controlled process when the thick CuCl layer is completed. Secondary breakdown of the salt layer results in copper dissolution through Cu(II) soluble species. The corresponding overall process is discussed in terms of a sum of nucleation and growth processes. The reaction model reproduces the potentiostatic current transients of copper in weakly alkaline borate buffer containing sodium chloride.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Kinetics of passivation and pitting corrosion of polycrystalline copper in borate buffer solutions containing sodium chloride

The pitting corrosion of copper in borate buffer containing sodium chloride is studied by using potentiostatic and potentiodynamic techniques complemented with scanning electron microscopy and EDAX. The breakdown potential shifts towards more negative values as the sodium chloride concentration increases. During pitting both soluble Cu(I) and Cu(II) species are detected. The first stage of pitting is explained through the competition between the passive layer formation and the nucleation and growth of the CuCl layer in equilibrium with Cu(I)-chloride complexes in solution. When salt nuclei reach the metal surface, pit growth under charge-transfer control is found. In the following stage the kinetics of pit growth changes to a diffusion controlled process when the thick CuCl layer is completed. Secondary breakdown of the salt layer results in copper dissolution through Cu(II) soluble species. The corresponding overall process is discussed in terms of a sum of nucleation and growth processes. The reaction model reproduces the potentiostatic current transients of copper in weakly alkaline borate buffer containing sodium chloride.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas (INIFTA)Facultad de Ciencias Exacta

Unstable Dynamics, Nonequilibrium Phases and Criticality in Networked Excitable Media

Here we numerically study a model of excitable media, namely, a network with occasionally quiet nodes and connection weights that vary with activity on a short-time scale. Even in the absence of stimuli, this exhibits unstable dynamics, nonequilibrium phases -including one in which the global activity wanders irregularly among attractors- and 1/f noise while the system falls into the most irregular behavior. A net result is resilience which results in an efficient search in the model attractors space that can explain the origin of certain phenomenology in neural, genetic and ill-condensed matter systems. By extensive computer simulation we also address a relation previously conjectured between observed power-law distributions and the occurrence of a "critical state" during functionality of (e.g.) cortical networks, and describe the precise nature of such criticality in the model.Comment: 18 pages, 9 figure

Estudio de la reacción de electrooxidación de acido fórmico sobre Iridio usando una celda de flujo

Estudios recientes de la reacción de electrooxidación de ácido fórmico (FAO) mediante la determinación simultanea de la dependencia corriente-potencial en estado estacionario y del cubrimiento de monóxido de carbono adsorbido COad, han permitido aportar nueva evidencia para la comprensión de los procesos elementales involucrados en las misma [1-3]. Se ha observado que la FAO sobre Pd [3] presenta un comportamiento cinético marcadamente diferente a los observados sobre Pt [1] y Rh [3]. En este contexto, en el presente trabajo se estudia la FAO sobre iridio, metal que se caracteriza por la adsorción de oxhidrilo OHad a bajos potenciales.Fil: Montero, María de Los Angeles. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; ArgentinaFil: Gennero, Maria Rosa. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; ArgentinaFil: Chialvo, Abel Cesar. Universidad Nacional del Litoral. Instituto de Química Aplicada del Litoral. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Química Aplicada del Litoral.; ArgentinaXXII Congreso Argentino de Fisicoquímica y Química InorgánicaLa PlataArgentinaAsociación Argentina de Investigación FisicoquímicaUniversidad Nacional de la Plata. Facultad de Ingenierí

Scale-free correlations in the dynamics of a small (N ~ 10000) cortical network

The advent of novel opto-genetics technology allows the recording of brain activity with a resolution never seen before. The characterisation of these very large data sets offers new challenges as well as unique theory-testing opportunities. Here we discuss whether the spatial and temporal correlation of the collective activity of thousands of neurons are tangled as predicted by the theory of critical phenomena. The analysis shows that both, the correlation length $\xi$ and the correlation time $\tau$ scale as predicted as a function of the system size. With some peculiarities that we discuss, the analysis uncovers new evidence consistent with the view that the large scale brain cortical dynamics corresponds to critical phenomena.Comment: 8 pages, 6 figure

Avalanches in self-organized critical neural networks: A minimal model for the neural SOC universality class

The brain keeps its overall dynamics in a corridor of intermediate activity and it has been a long standing question what possible mechanism could achieve this task. Mechanisms from the field of statistical physics have long been suggesting that this homeostasis of brain activity could occur even without a central regulator, via self-organization on the level of neurons and their interactions, alone. Such physical mechanisms from the class of self-organized criticality exhibit characteristic dynamical signatures, similar to seismic activity related to earthquakes. Measurements of cortex rest activity showed first signs of dynamical signatures potentially pointing to self-organized critical dynamics in the brain. Indeed, recent more accurate measurements allowed for a detailed comparison with scaling theory of non-equilibrium critical phenomena, proving the existence of criticality in cortex dynamics. We here compare this new evaluation of cortex activity data to the predictions of the earliest physics spin model of self-organized critical neural networks. We find that the model matches with the recent experimental data and its interpretation in terms of dynamical signatures for criticality in the brain. The combination of signatures for criticality, power law distributions of avalanche sizes and durations, as well as a specific scaling relationship between anomalous exponents, defines a universality class characteristic of the particular critical phenomenon observed in the neural experiments. The spin model is a candidate for a minimal model of a self-organized critical adaptive network for the universality class of neural criticality. As a prototype model, it provides the background for models that include more biological details, yet share the same universality class characteristic of the homeostasis of activity in the brain.Comment: 17 pages, 5 figure

Emergent complex neural dynamics

A large repertoire of spatiotemporal activity patterns in the brain is the basis for adaptive behaviour. Understanding the mechanism by which the brain's hundred billion neurons and hundred trillion synapses manage to produce such a range of cortical configurations in a flexible manner remains a fundamental problem in neuroscience. One plausible solution is the involvement of universal mechanisms of emergent complex phenomena evident in dynamical systems poised near a critical point of a second-order phase transition. We review recent theoretical and empirical results supporting the notion that the brain is naturally poised near criticality, as well as its implications for better understanding of the brain