Le système urinaire inférieur : modélisation et validation expérimentale. Étude de son activation sélective.

The Lower Urinary Tract (LUT) stores and evacuates urine. Its neural control is complex and its mechanisms are scarcely described. Among patients with Spinal Cord Injury (SCI) two deficiencies with respect to bladder control are common : detrusor over-activity (DOA) and detrusor-sphincter dyssynergia. Functional Electrical Stimulation (FES) can be used to counteract DOA and artificially manage the bladder. However it requires a better understanding of the system and to be able to activate selectively the detrusor and the striated sphincter. The contribution of this thesis can be divided in three parts : the development of a LUT model particularly focused on the detrusor, the experimental validation of this model, and finally the study of the recruitment in selective multi-polar electrical stimulation. The LUT model proposed here takes into account the specificity of the smooth muscle (the detrusor). It is a multi-scale model. It describes the system from the biochemical reactions of the muscle activation to the macroscopical physics at the global scale of the bladder+sphincter system. A study of model sensitivity with respect to its main parameters was conducted using a numerical implementation. Identification and validation of the referred model was performed based on animal experiments in isometric condition. Finally, we set up a toolchain to simulate selective multi-polar electrical stimulation and inverse recruitment techniques. Simulations results are presented and an experimental protocol to evaluate functionally these methods on animal model is proposed.Le système urinaire inférieur (SUI) réalise le stockage et l'évacuation de l'urine. C'est un système dont le contrôle est complexe et le fonctionnement global insuffisamment décrit. Chez les blessés médulaires deux déficiences du contrôle de la vessie sont communes : l'hyper-réflexie (HR) et la dyssyenergie detrusorsphincter. La stimulation électrofonctionnelle peut être utilisée pour contrer l'HR et rétablir artificiellement un contrôle synergique à condition de pouvoir activer indépendamment le sphincter strié et le detrusor. Globalement, cela nécessite aussi une meilleur connaissance du système à stimuler. La contribution apportée par cette thèse se scinde en trois parties : la modélisation du SUI et notamment du détrusor, la validation expérimentale de ce modèle et enfin l'étude du recrutement sélectif en stimulation électrique multi-polaire. Le modèle du SUI proposé ici s'attache à rendre compte des spécificités des muscles lisses en faisant partie. C'est un modèle multi-échelle, partant des réactions biochimiques dictant l'activation des muscles jusqu'à la physique macroscopique à l'échelle globale du système, en passant par l'intégration à l'échelle de la cellule. Nous avons réalisé une étude numérique sur l'implémentation de ce modèle et en avons tiré les paramètres les plus sensibles. Nous avons enfin validé expérimentalement notre modèle sur l'animal et nous avons identifié les paramètres principaux utilisés pour simuler la contraction isovolumique . Enfin, nous avons mis en place une chaine logicielle pour simuler la stimulation électrique sélective multipolaire, ainsi que des techniques de recrutement inverse. Nous proposons un protocole expérimental pour évaluer fonctionnellement ces méthodes sur l'animal

Smooth Muscle Model for Functional Electrical Stimulation applications : Simulation of realistic bladder behavior under FES

International audienceWe present here a major update of our smooth muscle model. It's aim is to enable the simulation of smooth muscle contraction under functional electrical stimulation (FES). The main addition is a model of calcium dynamics in the smooth muscle cell. It links the calcium concentration to the electrical potential of the membrane cell, therefore to the neural stimulation. This concentration is used to command the microscopic mechanics of the muscle, leading to contraction. We also refined the mechanical model. As smooth muscle have way slower dynamics than striated ones, it is possible to neglected the second order terms. We choose to simulate a well known example : the behavior of the bladder under the stimulation of a Finetech / Brindley implant. This way we can compare our qualitative results with the experimental data available in the literature. They show good consistency both in shape and time course. To obtain quantitative simulations, we need to perform the identification of the model through in-vivo testing on animals. Once this validation step done, it will be possible to apply our methodology to human bladder

Smooth Muscle Model For Functional Electrical Stimulation Applications

International audienceWe present a new model of smooth muscle that can be used to simulate the effect of Functional Electrical Stimulation (FES). It is composed of a set of differential equations based on the physiological reality so that parameter's values are meaningful and can be used for quantitative and objective evaluation of the muscle state. Moreover, the model has an input controlled by an FES signal so that it can simulate the behavior of the muscle under artificial stimulation. We apply this model to the simulation of the bladder contraction through the detrusor stimulation. It shows that the model is able to predict the time response, the intravesical pressure and the necessary time to empty the bladder. Simulations show consistent data with the literature. These preliminary results, after in vivo validations, will be used to caracterise bladders that need to be stimulated, for instance for paraplegics, and then to optimize the needed stimulation when neuroprosthesis are used to restore the emptying function

Speedup computation of HD-sEMG signals using a motor unit-specific electrical source model

International audienceNowadays, bio-reliable modeling of muscle contraction is becoming more accurate and complex. This increasing complexity induces a significant increase in computation time which prevents the possibility of using this model in certain applications and studies. Accordingly, the aim of this work is to significantly reduce the computation time of high-density surface electromyogram (HD-sEMG) generation. This will be done through a new model of motor unit (MU)-specific electrical source based on the fibers composing the MU. In order to assess the efficiency of this approach, we computed the normalized root mean square error (NRMSE) between several simulations on single generated MU action potential (MUAP) using the usual fiber electrical sources and the MU-specific electrical source. This NRMSE was computed for five different simulation sets wherein hundreds of MUAPs are generated and summed into HD-sEMG signals. The obtained results display less than 2% error on the generated signals compared to the same signals generated with fiber electrical sources. Moreover, the computation time of the HD-sEMG signal generation model is reduced to about 90% compared to the fiber electrical source model. Using this model with MU electrical sources, we can simulate HD-sEMG signals of a physiological muscle (hundreds of MU) in less than an hour on a classical workstation

A toolchain to simulate and investigate selective stimulation strategies for FES

International audienceWhen contracting a muscle using NFES (Neural Functional Electrical Stimulation), the stimulus always activates the axons of greater diameter first. Also selective activation of given fascicle inside a nerve is not possible with classical cuff electrode as the recruitment is performed uniformly around the nerve. These limits lead to poorly selective muscle recruitment, inducing fatigue and possible pain. To overcome this, selective stimulation strategies can be used. We propose a toolchain to investigate, simulate and tune selective stimulation strategies. It consists of a conduction volume model to compute the electric field generated in the nerve by a cuff electrode surrounding it; an axon model to predict the effect of the field on the nerve fibre --~the generation, propagation and possible block of action potentials; and an interface script that links the two models and generates the code of the input function for the nerve fibre model. We present some simulation results to illustrate the possibilities of the toolchain to simulate such strategies. Ongoing experimental validations are also discussed. They will enable us to tune the model and may lead to further improvements

Co-Simulation of Electrical and Mechanical Models of the Uterine Muscle

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Multi-scale and multi-physics model of the uterine smooth muscle with mechanotransduction

International audiencePreterm labor is an important public health problem. However, the efficiency of the uterine muscle during labor is complex and still poorly understood. This work is a first step towards a model of the uterine muscle, including its electrical and mechanical components, to reach a better understanding of the uterus synchronization. This model is proposed to investigate, by simulation, the possible role of mechanotransduction for the global synchronization of the uterus. The electrical diffusion indeed explains the local propagation of contractile activity, while the tissue stretching may play a role in the synchronization of distant parts of the uterine muscle. This work proposes a multi-physics (electrical, mechanical) and multi-scales (cell, tissue, whole uterus) model, which is applied to a realistic uterus 3D mesh. This model includes electrical components at different scales: generation of action potentials at the cell level, electrical diffusion at the tissue level. It then links these electrical events to the mechanical behavior, at the cellular level (via the intracellular calcium concentration), by simulating the force generated by each active cell. It thus computes an estimation of the intra uterine pressure (IUP) by integrating the forces generated by each active cell at the whole uterine level, as well as the stretching of the tissue (by using a viscoelastic law for the behavior of the tissue). It finally includes at the cellular level stretch activated channels (SACs) that permit to create a loop between the mechanical and the electrical behavior (mechanotransduction). The simulation of different activated regions of the uterus, which in this first ”proof of concept” case are electrically isolated, permits the activation of inactive regions through the stretching (induced by the electrically active regions) computed at the whole organ scale. This permits us to evidence the role of the mechanotransduction in the global synchronization of the uterus. The results also permit us to evidence the effect on IUP of this enhanced synchronization induced by the presence of SACs. This proposed simplified model will be further improved in order to permit a better understanding of the global uterine synchronization occurring during efficient labor contractions

An electro-mechanical multiscale model of uterine pregnancy contraction

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Fast generation model of high density surface EMG signals in a cylindrical conductor volume

International audienceIn the course of the last decade, fast and qualitative computing power developments have undoubtedly permitted for a better and more realistic modeling of complex physiological processes. Due to this favorable environment, a fast, generic and reliable model for high density surface electromyographic (HD-sEMG) signal generation with a multilayered cylindrical description of the volume conductor is presented in this study. Its main peculiarity lies in the generation of a high resolution potential map over the skin related to active Motor Units (MUs). Indeed, the analytical calculus is fully performed in the frequency domain. HD-sEMG signals are obtained by surfacic numerical integration of the generated high resolution potential map following a variety of electrode shapes. The suggested model is implemented using parallel computing techniques as well as by using an object-oriented approach which is comprehensive enough to be fairly quickly understood, used and potentially upgraded. To illustrate the model abilities, several simulation analyses are put forward in the results section. These simulations have been performed on the same muscle anatomy while varying the number of processes in order to show significant speed improvement. Accuracy of the numerical integration method, illustrating electrode shape diversity , is also investigated in comparison to analytical transfer functions definition. An additional section provides an insight on the volume detection of a circular electrode according to its radius. Furthermore, a large scale simulation is introduced with 300 MUs in the muscle and a HD-sEMG electrode grid composed of 16 Â 16 electrodes for three constant isometric contractions in 12 s. Finally, advantages and limitations of the proposed model are discussed with a focus on perspective works

Gom2n: A Software to Simulate Multipolar Neural Stimulation for Cochlear Implants

International audienceAchieving more focused and accurately controlled stimulation has been a trend in electrical stimulation over the last few years. By simultaneously stimulating adjacent electrodes, a more controlled electrical field can be created. This has led to a growing complexity in stimulator hardware and generation of stimulation patterns. Designing and tuning the devices is a complex problem that can be addressed with simulations. We propose a graphical, open-source software to simulate complex neural stimulations, and discuss the design and use of the software. Finally, we present some simulations performed with Gom2n to illustrate its capabilities and performances