On the coupling between mechanical properties and electrostatics in biological membranes

Cell membrane structure is proposed as a lipid matrix with embedded proteins, and thus, their emerging mechanical and electrostatic properties are commanded by lipid behavior and their interconnection with the included and absorbed proteins, cytoskeleton, extracellular matrix and ionic media. Structures formed by lipids are soft, dynamic and viscoelastic, and their properties depend on the lipid composition and on the general conditions, such as temperature, pH, ionic strength and electrostatic potentials. The dielectric constant of the apolar region of the lipid bilayer contrasts with that of the polar region, which also differs from the aqueous milieu, and these changes happen in the nanometer scale. Besides, an important percentage of the lipids are anionic, and the rest are dipoles or higher multipoles, and the polar regions are highly hydrated, with these water molecules forming an active part of the membrane. Therefore, electric fields (both, internal and external) affects membrane thickness, density, tension and curvature, and conversely, mechanical deformations modify membrane electrostatics. As a consequence, interfacial electrostatics appears as a highly important parameter, affecting the membrane properties in general and mechanical features in particular. In this review we focus on the electromechanical behavior of lipid and cell membranes, the physicochemical origin and the biological implications, with emphasis in signal propagation in nerve cells.Fil: Galassi, Vanesa Viviana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Interdisciplinario de Ciencias Básicas. - Universidad Nacional de Cuyo. Instituto Interdisciplinario de Ciencias Básicas; ArgentinaFil: Wilke, Natalia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Centro de Investigaciones en Química Biológica de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Centro de Investigaciones en Química Biológica de Córdoba; Argentin

Traveling Wave Solutions to a Coupled System of Spatially Discrete Nagumo Equations

This is the published version, also available here: http://dx.doi.org/10.1137/050624352.We consider a coupled system of discrete Nagumo equations and derive traveling wave solutions to this system using McKean's caricature of the cubic. A certain form of this system is used to model ephaptic coupling between pairs of nerve axons. We study the difference $g(c)=a_1-a_2$ between the detuning parameters $a_i$ that is required to make both waves move at the same speed c. Of particular interest is the effect of a coupling parameter $\alpha$ and an "alignment" parameter A on the function g. Numerical investigation indicates that for fixed A, there exists a time delay value $\beta$ that results in $g=0$, and for large enough wave speeds, multiple such $\beta$ values exist. Also, numerical results indicate that the perturbation of $\alpha$ away from zero will yield additional solutions with positive wave speed when $A={1\over 2}$. We employ both analytical and numerical results to demonstrate our claims

Delay dynamics of neuromorphic optoelectronic nanoscale resonators: Perspectives and applications

With the recent exponential growth of applications using artificial intelligence (AI), the development of efficient and ultrafast brain-like (neuromorphic) systems is crucial for future information and communication technologies. While the implementation of AI systems using computer algorithms of neural networks is emerging rapidly, scientists are just taking the very first steps in the development of the hardware elements of an artificial brain, specifically neuromorphic microchips. In this review article, we present the current state of the art of neuromorphic photonic circuits based on solid-state optoelectronic oscillators formed by nanoscale double barrier quantum well resonant tunneling diodes. We address, both experimentally and theoretically, the key dynamic properties of recently developed artificial solid-state neuron microchips with delayed perturbations and describe their role in the study of neural activity and regenerative memory. This review covers our recent research work on excitable and delay dynamic characteristics of both single and autaptic (delayed) artificial neurons including all-or-none response, spike-based data encoding, storage, signal regeneration and signal healing. Furthermore, the neural responses of these neuromorphic microchips display all the signatures of extended spatio-temporal localized structures (LSs) of light, which are reviewed here in detail. By taking advantage of the dissipative nature of LSs, we demonstrate potential applications in optical data reconfiguration and clock and timing at high-speeds and with short transients. The results reviewed in this article are a key enabler for the development of high-performance optoelectronic devices in future high-speed brain-inspired optical memories and neuromorphic computing. (C) 2017 Author(s).Fundacao para a Ciencia e a Tecnologia (FCT) [UID/Multi/00631/2013]European Structural and Investment Funds (FEEI) through the Competitiveness and Internationalization Operational Program - COMPETE 2020National Funds through FCT [ALG-01-0145-FEDER-016432/POCI-01-0145-FEDER-016432]European Commission under the project iBROW [645369]project COMBINA [TEC2015-65212-C3-3-PAEI/FEDER UE]Ramon y Cajal fellowshipinfo:eu-repo/semantics/publishedVersio

Finite element methods for computational nonlinear optics

Unlike linear systems, where knowledge of the eigenvalues and eigenvectors allows one to write a closed-form solution, few nonlinear systems posses closed-form analytical solutions, and therefore numerical simulations play a crucial role in the process of finding and analysing nonlinear phenomena. For the theoretical study of the complex spatial, temporal and spatiotemporal behaviour of nonlinear optical systems, mathematical modelling of the problem under consideration by efficient stepping algorithms is necessary. For the past decade the Finite Element Method has proved to be a very efficient and versatile method in linear and nonlinear modal analysis with the use of variable meshes and infinite elements as some of its greatest strengths, but little work has been done on its application to evolutional analysis in nonlinear optics. This thesis describes a finite-element-based computer modelling of a wide range of nonlinear optical systems, with a view to developing an understanding of some of the complex but exciting spatial, temporal and spatiotemporal propagation dynamics in such systems. The computer simulation of a wide range of nonlinear optical waveguides and systems in those major areas of nonlinear optics which include nonlinear integrated-optics, nonlinear fiber-optics and nonlinear dynamic systems has been performed. This is carried out through numerical solutions of appropriate wave equations such as the paraxial wave equation, the Maxwell-Debye equations, the infinite-dimensional map of a ring resonator derived from the Maxwell-Bloch equations and coupled nonlinear Schroedinger equations that may include gain terms. Two well defined problems are addressed in detail. First, the determinations of the modes or characteristic solutions by solving the stationary wave equations through modal analysis of different types of nonlinear optical waveguides. Second, the determination of the paraxial propagation solutions along a nonlinear medium by solving the wave equation as step-by-step initial-boundary value problems through beam propagation analysis. For this task, current and novel 2D- and 3D- schemes based on the finite element method are presented and described. Particularly, a novel robust time-dependent code which is a combination of the finite-element propagation algorithm coupled to unconditionally stable difference schemes for marching the solutions along the characteristics of the (z,t)-domain is developed as well as accurate propagation schemes for solving generalized coupled nonlinear Schroedinger equations. Additionally, several novel specific applications involving nonlinear media are thoroughly described. These include the study of nonlinear supermodes of integrated-optics directional couplers, the nonlinear dispersion characteristics of multiple-quantum well waveguides and graded-index fibers with saturable nonlinear cores, controlled spatiotemporal soliton emission, switching and demultiplexing in nonlinear tapered waveguides, temporal optical soliton dynamics in active three-core nonlinear fiber directional couplers and two-dimensional solitary-wave optical memory in fibers and bistable ring cavities. The generation of ultrafast soliton-like pulsetrains from a c.w. dual-frequency input signal with sinusoidal modulation using a proposed novel dual-channel erbium-doped fiber coupler laser is also demonstrated

Two methods of modelling electric current systems by analysis of magnetic field data, with particular reference to the quasi-dc magnetic field of the human leg

This thesis deals with two approaches to the problem of obtaining information about electric current distributions by analysing the associated magnetic field. Both methods have been developed within the context of a particular biomagnetic study, the analysis of the quasi dc magnetic field of the human leg. The techniques have been designed to deal with data sensed by a gradiometer in a series of horizontal scans above the current—carrying region and take full account of the gradiometer configuration. Method 1, the line-dipole technique, analyses each scan individually and calculates the dipole term of a multipole expansion which best characterises the current distribution cross-section immediately below the line of scan. Method 2, the line current loop iterative-perturbative algorithm, uses data from all the scans to compute the coordinates of the best fit line current loop for the whole data map. Both methods have been extensively tested with computer simulated data and with real data from current-carrying wire loops and the results show that both methods are capable of producing an accurate replication of the target system provided it satisfies the initial model assumptions. The dc magnetic field of the human leg has been investigated for a number of normal subjects. The line- dipole technique provides a useful method of characterising the data and indicates regions of high current density which allow inferences to be drawn about the physiological nature of the current generators. Analysis of the field from a leg with a fibula fracture shows significant differences from the normal pattern, although a direct, causal connection with the fracture is not necessarily implied. The line current loop technique has been less successful in achieving a high quality fit to the leg data but this lack of success is consistent with a physiologically reasonable model of the source currents. Although both methods have been designed for this rather specialised biomagnetic inverse problem, they are of more general applicability and may be useful in other fields such as geophysics or non-destructive testing

Biological samples studied by optical nanospectroscopy

The different parts of the electromagnetic spectrum result in diverse effects upon interaction with matter: according to the wavelength, the radiation has energy appropriate for the excitation of a specific physical process. X-rays can be used as a tool to analyze the structure of matter since their wavelength is comparable with the interatomic distances. Infrared light is in the spectral region that excites molecular vibrations and is employed to investigate the chemical composition of a material. Visible radiation can study the optical properties of a sample, such as the fluorescence and the absorbance, and provide a chemical fingerprint when the inelastically scattered light is detected. In this thesis work these light sources are used in diverse experimental approaches to study structured biological specimens, resulting in a detailed chemical and physical characterization at the atomic and molecular scale. Conventional spectroscopy is often not enough sensitive and spatially resolved to detect specific elements or domains in a sample. The need of imaging objects on increasingly finer scales and spatially localize specific molecules, brought to combine infrared, visible and Raman spectroscopy with scanning near-field microscopy giving rise to a powerful nanospectroscopic tool used to perform simultaneous topographical measurements and optical/chemical characterizations with subwavelength resolution, overcoming the diffraction limit of light. Our study combines X-ray diffraction and reflectivity with optical nanospectroscopy to investigate the order and clustering of lipid bilayers, the interaction between solid-supported membranes and embedded alamethicin peptides, the optical and chemical properties of hippocampal neuron cells and the trafficking mechanism of specific neuron receptors

Test of multiple sensor set-up for head motion characterization during MRI acquisition

L'Imaging a Risonanza Magnetica (MRI) è una tecnica di imaging ampiamente utilizzata in ambito medico. La ricerca in questo campo si sta focalizzando sullo sviluppo di scanner a campi molto intensi, come lo scanner a 7 T utilizzato in questa tesi. La risoluzione delle immagini e l'entità degli artefatti creati dai movimenti involontari del paziente sono proporzionali all'intensità di campo magnetico e diventano rilevanti ad intensità molto elevate. Le tecniche di Motion Correction, nota la cinetica dei movimenti, permettono di correggere queste distorsioni. La tesi è inserita in un progetto che ha come scopo la misura indiretta dei movimenti della testa durante la scansione MRI. In particolare, mi sono concentrata sui miglioramenti da apportare al set-up e sulla caratterizzazione dei tre strumenti usati per la misura: la telecamera di campo magnetico (Clip on Camera Head, CCH) formata da 16 sonde fissate in una struttura cilindrica posizionata attorno alla testa del paziente; la telecamera ottica (Moiré Phase Tracking System, MPT) che misura i movimenti tramite l'immagine di un marker olografico supportato da un bite tenuto nella bocca del volontario; il dispositivo (Physlog) dello scanner che fornisce i parametri fisiologici (respirazione e battito cardiaco). La comunicazione hardware degli strumenti avviene grazie a un segnale di trigger, di cui ho ottimizzato la sincronizzazione. Inoltre, abbiamo acquisito dataset completi di tre volontari, a diverse condizioni. I dati sono stati sincronizzati e analizzati, tramite analisi multivariate, per caratterizzare la risposta e la stabilità del sistema e la variabilità individuale dei pazienti. L'analisi ha permesso di capire meglio le proprietà dello strumento e ha consentito di associare le misure del campo magnetico al di fuori del cranio ai valori fisiologici dei volontari