ORCID Profile
0000-0002-5560-4771
Current Organisation
University of New South Wales
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Publisher: Springer Science and Business Media LLC
Date: 17-02-2017
DOI: 10.1038/SREP42682
Abstract: Retinal electrostimulation is promising a successful therapy to restore functional vision. However, a narrow stimulating current range exists between retinal neuron excitation and inhibition which may lead to misperformance of visual prostheses. As the conveyance of representation of complex visual scenes may require neighbouring electrodes to be activated simultaneously, electric field summation may contribute to reach this inhibitory threshold. This study used three approaches to assess the implications of relatively high stimulating conditions in visual prostheses: (1) in vivo , using a suprachoroidal prosthesis implanted in a feline model, (2) in vitro through electrostimulation of murine retinal preparations, and (3) in silico by computing the response of a population of retinal ganglion cells. Inhibitory stimulating conditions led to diminished cortical activity in the cat. Stimulus-response relationships showed non-monotonic profiles to increasing stimulating current. This was observed in vitro and in silico as the combined response of groups of neurons (close to the stimulating electrode) being inhibited at certain stimulating litudes, whilst other groups (far from the stimulating electrode) being recruited. These findings may explain the halo-like phosphene shapes reported in clinical trials and suggest that simultaneous stimulation in retinal prostheses is limited by the inhibitory threshold of the retinal ganglion cells.
Publisher: IEEE
Date: 08-2011
Publisher: IEEE
Date: 09-2009
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 05-2022
Publisher: IEEE
Date: 07-2018
Publisher: IEEE
Date: 08-2012
Publisher: IEEE
Date: 08-2012
Publisher: IOP Publishing
Date: 08-2023
Abstract: Objective. A transverse intrafascicular multichannel electrode (TIME) may offer advantages over more conventional cuff electrodes including higher spatial selectivity and reduced stimulation charge requirements. However, the performance of TIME, especially in the context of non-conventional stimulation waveforms, remains relatively unexplored. As part of our overarching goal of investigating stimulation efficacy of TIME, we developed a computational toolkit that automates the creation and usage of in silico nerve models with TIME setup, which solves nerve responses using cable equations and computes extracellular potentials using finite element method. Approach. We began by implementing a flexible and scalable Python/MATLAB-based toolkit for automatically creating models of nerve stimulation in the hybrid NEURON/COMSOL ecosystems. We then developed a sciatic nerve model containing 14 fascicles with 1,170 myelinated (A-type, 30%) and unmyelinated (C-type, 70%) fibers to study fiber responses over a variety of TIME arrangements (monopolar and hexapolar) and stimulation waveforms (kilohertz stimulation and cathodic r modulation). Main results. Our toolkit obviates the conventional need to re-create the same nerve in two disparate modeling environments and automates bi-directional transfer of results. Our population-based simulations suggested that kilohertz stimuli provide selective activation of targeted C fibers near the stimulating electrodes but also tended to activate non-targeted A fibers further away. However, C fiber selectivity can be enhanced by hexapolar TIME arrangements that confined the spatial extent of electrical stimuli. Improved upon prior findings, we devised a high-frequency waveform that incorporates cathodic DC r to completely remove undesirable onset responses. Conclusion. Our toolkit allows agile, iterative design cycles involving the nerve and TIME, while minimizing the potential operator errors during complex simulation. The nerve model created by our toolkit allowed us to study and optimize the design of next-generation intrafascicular implants for improved spatial and fiber-type selectivity.
Publisher: IEEE
Date: 08-2015
Publisher: IEEE
Date: 07-2017
Publisher: SPIE
Date: 09-12-2016
DOI: 10.1117/12.2242866
Publisher: Hindawi Limited
Date: 2013
DOI: 10.1155/2013/706195
Abstract: A generic cardiomyocyte ionic model, whose complexity lies between a simple phenomenological formulation and a biophysically detailed ionic membrane current description, is presented. The model provides a user-defined number of ionic currents, employing two-gate Hodgkin-Huxley type kinetics. Its generic nature allows accurate reconstruction of action potential waveforms recorded experimentally from a range of cardiac myocytes. Using a multiobjective optimisation approach, the generic ionic model was optimised to accurately reproduce multiple action potential waveforms recorded from central and peripheral sinoatrial nodes and right atrial and left atrial myocytes from rabbit cardiac tissue preparations, under different electrical stimulus protocols and pharmacological conditions. When fitted simultaneously to multiple datasets, the time course of several physiologically realistic ionic currents could be reconstructed. Model behaviours tend to be well identified when extra experimental information is incorporated into the optimisation.
Publisher: Hindawi Limited
Date: 2013
DOI: 10.1155/2013/951234
Abstract: A 3D model of atrial electrical activity has been developed with spatially heterogeneous electrophysiological properties. The atrial geometry, reconstructed from the male Visible Human dataset, included gross anatomical features such as the central and peripheral sinoatrial node (SAN), intra-atrial connections, pulmonary veins, inferior and superior vena cava, and the coronary sinus. Membrane potentials of myocytes from spontaneously active or electrically paced in vitro rabbit cardiac tissue preparations were recorded using intracellular glass microelectrodes. Action potentials of central and peripheral SAN, right and left atrial, and pulmonary vein myocytes were each fitted using a generic ionic model having three phenomenological ionic current components: one time-dependent inward, one time-dependent outward, and one leakage current. To bridge the gap between the single-cell ionic models and the gross electrical behaviour of the 3D whole-atrial model, a simplified 2D tissue disc with heterogeneous regions was optimised to arrive at parameters for each cell type under electrotonic load. Parameters were then incorporated into the 3D atrial model, which as a result exhibited a spontaneously active SAN able to rhythmically excite the atria. The tissue-based optimisation of ionic models and the modelling process outlined are generic and applicable to image-based computer reconstruction and simulation of excitable tissue.
Publisher: IEEE
Date: 07-2017
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 07-2023
Publisher: IEEE
Date: 08-2012
Publisher: IEEE
Date: 08-2016
Publisher: Frontiers Media SA
Date: 11-09-2018
Publisher: Hindawi Limited
Date: 2013
DOI: 10.1155/2013/213563
Publisher: IEEE
Date: 07-2018
Publisher: Wiley
Date: 09-06-2016
DOI: 10.1002/CNM.2794
Abstract: Infarct extension, a process involving progressive extension of the infarct zone (IZ) into the normally perfused border zone (BZ), leads to continuous degradation of the myocardial function and adverse remodelling. Despite carrying a high risk of mortality, detailed understanding of the mechanisms leading to BZ hypoxia and infarct extension remains unexplored. In the present study, we developed a 3D truncated ellipsoidal left ventricular model incorporating realistic electromechanical properties and fibre orientation to examine the mechanical interaction among the remote, infarct and BZs in the presence of varying infarct transmural extent (TME). Localized highly abnormal systolic fibre stress was observed at the BZ, owing to the simultaneous presence of moderately increased stiffness and fibre strain at this region, caused by the mechanical tethering effect imposed by the overstretched IZ. Our simulations also demonstrated the greatest tethering effect and stress in BZ regions with fibre direction tangential to the BZ-remote zone boundary. This can be explained by the lower stiffness in the cross-fibre direction, which gave rise to a greater stretching of the IZ in this direction. The average fibre strain of the IZ, as well as the maximum stress in the sub-endocardial layer, increased steeply from 10% to 50% infarct TME, and slower thereafter. Based on our stress-strain loop analysis, we found impairment in the myocardial energy efficiency and elevated energy expenditure with increasing infarct TME, which we believe to place the BZ at further risk of hypoxia. Copyright © 2016 John Wiley & Sons, Ltd.
Publisher: Walter de Gruyter GmbH
Date: 06-2014
Abstract: The aim of this study was the development of a geometrically simple and highly computationally-efficient two dimensional (2D) biophysical model of whole heart electrical activity, incorporating spontaneous activation of the sinoatrial node (SAN), the specialized conduction system, and realistic surface ECG morphology computed on the torso. The FitzHugh-Nagumo (FHN) equations were incorporated into a bidomain finite element model of cardiac electrical activity, which was comprised of a simplified geometry of the whole heart with the blood cavities, the lungs and the torso as an extracellular volume conductor. To model the ECG, we placed four electrodes on the surface of the torso to simulate three Einthoven leads V I , V II and V III from the standard 12-lead system. The 2D model was able to reconstruct ECG morphology on the torso from action potentials generated at various regions of the heart, including the sinoatrial node, atria, atrioventricular node, His bundle, bundle branches, Purkinje fibers, and ventricles. Our 2D cardiac model offers a good compromise between computational load and model complexity, and can be used as a first step towards three dimensional (3D) ECG models with more complex, precise and accurate geometry of anatomical structures, to investigate the effect of various cardiac electrophysiological parameters on ECG morphology.
Publisher: IEEE
Date: 08-2011
Publisher: Wiley
Date: 23-05-2007
Publisher: Wiley
Date: 25-04-2019
DOI: 10.1002/CNM.3204
Abstract: Flow energetics have been proposed as early indicators of progressive left ventricular (LV) functional impairment in patients with myocardial infarction (MI), but its correlation with in idual MI parameters has not been fully explored. Using electro-fluid-structure interaction LV models, this study investigated the correlation between four MI parameters: infarct size, infarct multiplicity, regional enhancement of contractility at the viable myocardium area (RECVM), and LV mechanical dyssynchrony (LVMD) with intraventricular vortex and flow energetics. In LV with small infarcts, our results showed that infarct appearance lified the energy dissipation index (DI), where substantial viscous energy loss was observed in areas with high flow velocity and near the infarct-vortex interface. The LV with small multiple infarcts and RECVM showed remarkable DI increment during systole and diastole. In correlation analysis, the systolic kinetic energy fluctuation index (E') was positively related to ejection fraction (EF) (R
Publisher: IEEE
Date: 08-2010
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 2022
Publisher: IEEE
Date: 04-2015
Publisher: IEEE
Date: 07-2013
Publisher: Wiley
Date: 29-05-2022
DOI: 10.1002/CNM.3616
Abstract: In this study, we present a varying‐radius cable equation for nerve fibres taking into account the varying diameter along the neuronal segments. Finite element neuronal models utilising the classical (fixed‐radius) and varying‐radius cable formulations were compared using simple and realistic morphologies under intra‐ and extracellular electrical stimulation protocols. We found that the use of the classical cable equation to model intracellular neural electrical stimulation exhibited an error of 17% in a passive resistive cable model with abrupt change in radius from 1 to 2 μm, when compared to the known analytical solution and varying‐radius cable formulation. This error was observed to increase substantially using more realistic neuron morphologies and branching structures. In the case of extracellular stimulation however, the difference between the classical and varying‐radius formulations was less pronounced, but we expect this difference will increase under more complex stimulation paradigms such as high‐frequency stimulation. We conclude that for computational neuroscience applications, it is essential to use the varying‐radius cable equation for accurate prediction of neuronal responses under electrical stimulation.
Publisher: Mary Ann Liebert Inc
Date: 05-2011
Abstract: Despite reduced sympathetic activity below the level of a spinal cord injury (SCI), venoconstriction during autonomic dysreflexia increases venous return to the heart. Here, contractions of isometrically mounted tail veins from rats with spinal transection at T4 performed 8 - 10 weeks earlier are compared with those from sham-operated rats. After SCI, lumen diameter was reduced by ∼30% and the contractions evoked by electrical stimulation of the perivascular axons were larger than control. This augmentation of neurovascular transmission was not associated with enhanced sensitivity to α-adrenoceptor agonists or to adenosine-5'-triphosphate (ATP) although contractions to depolarization with K(+) were larger after SCI. The percentage reduction in nerve-evoked contraction after SCI produced by the α(1)-adrenoceptor antagonist prazosin (10 nM) was unchanged but that by the α(2)-adrenoceptor antagonist rauwolscine (0.1 μM) was reduced. The relative contribution of P2-purinoceptors to nerve-evoked contractions after α-adrenoceptor blockade, revealed by adding suramin (0.1 mM), was unchanged. The greater depolarization-induced contraction and the reduced contribution of α(2)-adrenoceptors to nerve-evoked contraction suggest that changes in the venous smooth muscle underlie the potentiation of neurovascular transmission after SCI. Furthermore, the smaller lumen diameter after SCI will increase the pressure that the veins exert on the luminal contents when they are neurally activated.
Publisher: AIP Publishing
Date: 09-2023
DOI: 10.1063/5.0153753
Publisher: OSA
Date: 2018
Publisher: Wiley
Date: 24-08-2006
Publisher: IEEE
Date: 08-2012
Publisher: IEEE
Date: 08-2010
Publisher: IEEE
Date: 08-2012
Publisher: IOP Publishing
Date: 10-2022
Abstract: Objective. Biomedical instrumentation and clinical systems for electrophysiology rely on electrodes and wires for sensing and transmission of bioelectric signals. However, this electronic approach constrains bandwidth, signal conditioning circuit designs, and the number of channels in invasive or miniature devices. This paper demonstrates an alternative approach using light to sense and transmit the electrophysiological signals. Approach. We develop a sensing, passive, fluorophore-free optrode based on the birefringence property of liquid crystals (LCs) operating at the microscale. Main results. We show that these optrodes can have the appropriate linearity ( µ ± s.d.: 99.4 ± 0.5%, n = 11 devices), relative responsivity ( µ ± s.d.: 57 ± 12%V −1 , n = 5 devices), and bandwidth ( µ ± s.d.: 11.1 ± 0.7 kHz, n = 7 devices) for transducing electrophysiology signals into the optical domain. We report capture of rabbit cardiac sinoatrial electrograms and stimulus-evoked compound action potentials from the rabbit sciatic nerve. We also demonstrate miniaturisation potential by fabricating multi-optrode arrays, by developing a process that automatically matches each transducer element area with that of its corresponding biological interface. Significance. Our method of employing LCs to convert bioelectric signals into the optical domain will pave the way for the deployment of high-bandwidth optical telecommunications techniques in ultra-miniature clinical diagnostic and research laboratory neural and cardiac interfaces.
Publisher: IEEE
Date: 07-2017
Publisher: American Physiological Society
Date: 05-2017
Abstract: Electrical stimulation of neuronal tissue is a promising strategy to treat a variety of neurological disorders. The mechanism of neuronal activation by external electrical stimulation is governed by voltage-gated ion channels. This stimulus, typically brief in nature, leads to membrane potential depolarization, which increases ion flow across the membrane by increasing the open probability of these voltage-gated channels. In spiking neurons, it is activation of voltage-gated sodium channels (Na V channels) that leads to action potential generation. However, several other types of voltage-gated channels are expressed that also respond to electrical stimulation. In this study, we examine the response of voltage-gated potassium channels (K V channels) to brief electrical stimulation by whole cell patch-cl electrophysiology and computational modeling. We show that nonspiking amacrine neurons of the retina exhibit a large variety of responses to stimulation, driven by different K V -channel subtypes. Computational modeling reveals substantial differences in the response of specific K V -channel subtypes that is dependent on channel kinetics. This suggests that the expression levels of different K V -channel subtypes in retinal neurons are a crucial predictor of the response that can be obtained. These data expand our knowledge of the mechanisms of neuronal activation and suggest that K V -channel expression is an important determinant of the sensitivity of neurons to electrical stimulation. NEW & NOTEWORTHY This paper describes the response of various voltage-gated potassium channels (K V channels) to brief electrical stimulation, such as is applied during prosthetic electrical stimulation. We show that the pattern of response greatly varies between K V channel subtypes depending on activation and inactivation kinetics of each channel. Our data suggest that problems encountered when artificially stimulating neurons such as cessation in firing at high frequencies, or “fading,” may be attributed to K V -channel activation.
Publisher: Elsevier BV
Date: 07-2012
DOI: 10.1016/J.BRS.2011.07.004
Abstract: Electroconvulsive therapy (ECT) is a highly effective treatment for severe depressive disorder. Efficacy and cognitive outcomes have been shown to depend on variations in electrode placement and other stimulus parameters, presumably because of differences in the pattern of neuronal activation. This latter effect, however, is poorly understood. In this study, we present an anatomically accurate human head computational model to stimulate neuronal excitation during ECT, to better understand the effects of varying electrode placement and stimulus parameters. Electric field and current density throughout the head, as well as direct neural activation within the brain, were computed using the finite element method. Regions representing passive volume conductors (skin, skull, cerebrospinal fluid) were extracellularly coupled to an excitable neural continuum region representing the brain. The skull was modeled with anistropic electrical conductivity. Simulation results indicated that direct activation of the brain occurred immediately beneath the electrodes on the scalp, consistent with existing imaging studies. In addition, we found that the brainstem was also activated using a right unilateral electrode configuration. Simulation also demonstrated that a reduction in stimulus litude or pulse width led to a reduction in the spatial extent of brain activation. The novel model described in this study was able to simulate direct excitation of the brain during ECT, was useful in characterizing differences in neuronal activation as electrode placement, pulse width, and litude were altered, and is proposed as a tool for further exploring the effects of variations in ECT stimulation approaches. Results from the simulations assist in understanding recently described clinical phenomena, in particular, the reduction in cognitive side effects with ultrabrief pulse width stimulation, and greater effects of the ECT stimulus on cardiovascular function with unilateral electrode placement.
Publisher: Ovid Technologies (Wolters Kluwer Health)
Date: 06-2018
Publisher: Wiley
Date: 28-04-2016
DOI: 10.1111/JCE.12968
Abstract: This study aims to characterize the regional variability in rate-adaptation in the atria. Action potential (AP) responses to pulses with uniform as well as pseudo-random non-uniform pacing intervals were recorded from rabbit sino-atrial node, right and left atrial pectinate as well as pulmonary vein antrum tissue preparations using conventional intracellular glass microelectrodes. Steady-state restitution curves were reconstructed for various AP waveform metrics. We observed significant variability between the four regions under basal pacing representing the rabbit resting heart rate as well as regional variability in rate-adaptation to increased pacing frequencies. Right-left atrial restitution differences were further confirmed using the non-uniform pacing protocol, with significant differences in AP litude, duration (APD) as well as maximum phase 0 depolarization rate restitution curves in response to an identical sequence of non-uniform pacing intervals. In addition, we report regional differences in alternans of AP waveform metrics, over a wide range of pacing frequencies and not simply prior to 1:1 entrainment being lost. We also observed an increase in APD90 along the conduction pathway from the left atrium to pulmonary vein junction. Our results identified significant regional differences in electrical restitution in the rabbit atria and suggest their dependency on both baseline AP morphology and local intrinsic differences in rate-adaptation. We propose that the atrial heterogeneity in rate-adaptation could contribute to arrhythmogenesis and the greater susceptibility of pulmonary vein myocardial sleeves to ectopic foci and reentrant activity.
Publisher: IEEE
Date: 07-2013
Publisher: Public Library of Science (PLoS)
Date: 25-06-2013
Publisher: IEEE
Date: 08-2015
Publisher: Springer Science and Business Media LLC
Date: 16-05-2013
Abstract: Despite the rapid advancement of left ventricular assist devices (LVADs), adverse events leading to deaths have been frequently reported in patients implanted with LVADs, including bleeding, infection, thromboembolism, neurological dysfunction and hemolysis. Cannulation forms an important component with regards to thrombus formation in assisted patients by varying the intraventricular flow distribution in the left ventricle (LV). To investigate the correlation between LVAD cannula placement and potential for thrombus formation, detailed analysis of the intraventricular flow field was carried out in the present study using a two way fluid structure interaction (FSI), axisymmetric model of a passive LV incorporating an inflow cannula. Three different cannula placements were simulated, with device insertion near the LV apex, penetrating one-fourth and mid-way into the LV long axis. The risk of thrombus formation is assessed by analyzing the intraventricular vorticity distribution and its associated vortex intensity, amount of stagnation flow in the ventricle as well as the level of wall shear stress. Our results show that the one-fourth placement of the cannula into the LV achieves the best performance in reducing the risk of thrombus formation. Compared to cannula placement near the apex, higher vortex intensity is achieved at the one-fourth placement, thus increasing wash out of platelets at the ventricular wall. One-fourth LV penetration produced negligible stagnation flow region near the apical wall region, helping to reduce platelet deposition on the surface of the cannula and the ventricular wall.
No related grants have been discovered for Amr Al Abed.