Browsing by Author "Cavero, Miguel."

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Extracellular potentials from action potentials of anatomically realistic neurons and neuronal populations.
(2005) Cavero, Miguel.; Chetty, Nithaya.
Extracellular potentials due to firing of action potentials are computed around cortical neurons and populations of cortical neurons. These extracellular potentials are calculated as a sum of contributions from ionic currents passing through the cell membrane at various locations using Maxwell's equations in the quasi-static limit. These transmembrane currents are found from simulations of anatomically reconstructed cortical neurons implemented as multi-compartmental models in the simulation tool NEURON. Extracellular signatures of action potentials of single neurons are calculated both in the immediate vicinity of the neuron somas and along vertical axes. For the neuronal populations only vertical axis distributions are considered. The vertical-axis calculations were performed to investigate the contributions of action potential firing to laminar-electrode recordings. Results for high-pass (750 - 3000 Hz) filtered potentials are also given to mimic multi-unit activity (MUA) recordings. Extracellular traces from single neurons and populations (both synchronous and asynchronous) of neurons are shown for three different neuron types: layer 3 pyramid, layer 4 stellate and layer 5 pyramid cell. The layer 3 cell shows a 'closed-field' configuration, while the layer 5 pyramid demonstrates an 'open-field' appearance for singe neuron simulations which is less apparent in population simulations. The layer 4 stellate cell seems to fall somewhere in between the open- and closed-field scenarios. Comparing single neuron and synchronous populations, the amplitudes of the extracellular traces increase as population radii increase, though the shapes are generally similar. Asynchronous populations produce small amplitudes due to a time convolution of various neuron contributions.
The structural and mechanical properties of the Pt-Ti and Ir-Ti systems.
(2011) Cavero, Miguel.; Pierrus, John.
Ab initio plane wave based density functional calculations within the generalised gradient approximation (GGA) have been carried out on a wide range of phases and stoichiometries for the platinum-titanium (Pt-Ti) and iridium-titanium (Ir-Ti) alloy systems, using the Vienna Ab Initio Simulation Package (VASP) with projector augmented wave (PAW) potentials. For all of the phases in this work, the equilibrium structures were found by performing a full relaxation of the atoms. There were 20 di erent phases considered for varying atomic percentage compositions for each alloy system. Energy-volume calculations and heats of formations were used to determined the equilibrium structures at each atomice percentage composition and to determine if there were high temperature phases at that composition. The elastic constants and elastic moduli are calculated and the electronic structure and density of states (DOS) were considered to understand the hardness and stability properties of the alloys. For the Pt-Ti system, the low and high temperature phases at di erent compositions agreed with previously published results in the literature. Intermediate phases at 50% were also determined, in agreement with previous results. Alloying Pt with Ti resulted in a decrease in the bulk modulus, i.e. not adding strength to the metal. However, the shear modulus increased for most of the alloys compared to bulk Pt and it was found that in general, alloying may increase the resistance to shear. PtTi alloys were found to be ductile in nature, as with both constituent metals in their bulk form. In the Ir-Ti system, bulk Ir was found to have the highest bulk, shear and Young's modulus with each of these values decreasing with increasing percentage Ti in the alloy. IrTi alloys with 66.7% Ir composition or higher were found to be brittle in nature, similar in behaviour to bulk Ir; alloys with a higher percentage concentration of Ti were found to be ductile.