On characterizing skeletal muscle contraction properties. Experimental and simulation methodology

The main objective of this work is to characterize the fatigue contractile properties of threedifferent rat muscles (Tibialis Anterior, Extensor Digitorium Longus and Soleus) in order toobtain experimental parameters for numerical simulations.Experiments were conducted “in vivo" on three groups (n = 6) of male Wistar rats (210 +/- 11g)using a protocol developed by authors in previous works. Muscles were subjected to anelectrical stimulus to achieve tetanic contraction during ten seconds. Digital Image Correlationwas used during tests for 3D strain and displacements measurement that allow the correlationwith the finite element simulations.By means of Computed Tomography, a precise reconstruction of both bone and muscle of therat hindlimb geometry was obtained. The methodology proposed allows to obtain and validatecomputational simulations of skeletal muscle fatigue under different characteristics related tofiber types.6

A numerical investigation of changes in lens shape during accommodation

The purpose of this study was to investigate how the mechanical properties and geometry of the lens influence the changes in lens shape during accommodation. To do so, ex vivo stretching tests of the isolated lens were simulated via finite element analysis. In these tests, the lens is stretched from the accommodated state to the non-accommodated state. Several key characteristics of the lens were studied: the stiffness gradient of the lens material, the distribution of the capsule thickness, the mechanical properties of the capsule and the material comprising the lens, nucleus and cortex, and the influence of two different age-related lens geometries (17 and 29 y/o subjects). To determine the effects on the changes in lens shape during accommodation, changes in the anterior and posterior radius, the lens and nucleus thicknesses and the equatorial lens diameter were analysed. The results suggest that multiple factors exert statistically significant influences on how the lens changes its shape, but two factors predominate over the rest: the stiffness ratio between the nucleus and cortex and the stiffness of the capsule, specifically the posterior surface

Reconstruction and numerical modelling of the abdominal wall. Application to hernia surgery

Routine hernia repair surgery involves the implant of synthetic mesh. However, this proceduremay give rise to several problems causing considerable patient disability. With the aim ofimproving surgical procedures, the healthy and the herniated human abdomen are simulatedusing finite element (FE) simulations. For that purpose, a reconstruction of the physiologicalgeometry of a human abdomen was created using magnetic resonance images. Besides,following the anatomy of the abdomen, the different muscles and aponeurosis were defined.Furthermore, collagen fibres were included in each muscle layer and their passive anisotropicmechanical contribution was modelled within the framework of hyperelasticity. In the FEsimulation of the abdomen, the constraint imposed by the shoulder is applied and an internalpressure of 23 kPa was applied to the interior abdominal wall to reproduce the abdominal loadwhen standing jumping. After generating a hernia in the front of the abdomen, differentprostheses (Surgipro®, Optilene® and Infinit®), modelled using a membrane model, are placedin the defect to simulate the behaviour of the abdomen after the surgical procedure. In thehealthy abdomen, maximal principal stresses (MPS) and displacements (MD) appear in thefront of the belly. On the other hand, once the hernia is created and the mesh is placed, theMD and MPS are higher than those attained in the healthy abdomen. Thus, just after surgery,surgical repair procedure does not fully restore normal physiological conditions and the risk ofhernia recurrence by the suture is high due to the stress concentration

Chemo-mechanical model for skeletal muscle contraction

Generation of force in skeletal muscle tissue depends on both chemical and mechanical phenomena. In this work, considering the kinetics of cross bridges, a one dimensional model has been proposed for predicting the isometric force according to the intracellular calcium ion distribution

Assessing the role of Ca2+ in skeletal muscle fatigue using a multi-scale continuum model

The Calcium ion Ca2+ plays a critical role as an initiator and preserving agent of the cross-bridge cycle in the force generation of skeletal muscle. A new multi-scale chemo-mechanical model is presented in order to analyze the role of Ca2+ in muscle fatigue and to predict fatigue behavior. To this end, a cross-bridge kinematic model was incorporated in a continuum based mechanical model, considering a thermodynamic compatible framework. The contractile velocity and the generated active force were directly related to the force-bearing states that were considered for the cross-bridge cycle. In order to determine the values of the model parameters, the output results of an isometric simulation were initially fitted with experimental data obtained for rabbit Extensor Digitorum Longus muscle. Furthermore, a simulated force-velocity curve under concentric contractions was compared with reported experimental results. Finally, by varying the Ca2+ concentration level and its kinetics in the tissue, the model was able to predict the evolution of the active force of an experimental fatigue protocol. The good agreement observed between the simulated results and the experimental outcomes proves the ability of the model to reproduce the fatigue behavior and its applicability for more detailed multidisciplinary investigations related to chemical conditions in muscle performance

Toll-like receptor 2 modulates the inhibitory motor response induced by hydrogen sulphide in mouse colon

Introduction: The recognition of intestinal microbiota is in part carried out by toll-like receptors (TLR), which are responsible for initiating the innate immune response. Alterations in the intestinal microbiota and its recognition may contribute to the development of intestinal inflammatory pathologies. Otherwise, hydrogen sulphide (H2S) is an endogenous gaseous signalling molecule and it potentially plays a relevant role in the intestinal motility. In mammals, two pyridoxalphosphate-dependent enzymes are responsible for H2S synthesis: cystathionine b-synthase (CBS) and cystathionine -lyase (CSE)..

Effect of cryopreserved amniotic membrane on the mechanical properties of skeletal muscle after strabismus surgery in rabbits

Purpose: To study the functional recovery of the superior rectus muscle (SRM) after its partial resection in a rabbit model with and without cryopreserved amniotic membrane (AM). Material and methods: Resection of the right and left SRMs of 30 rabbits was performed. On the left eyes, a single sheet of equine cryopreserved AM was placed covering the muscle edge sutured. Active and passive mechanical properties of muscles operated with and without AM were monitored over time at 30 (n = 10), 60 (n = 10), and 90 (n = 10) days after surgery. Muscle samples were extracted and electrically stimulated to register the force exerted by the samples, characterizing its active behavior. They were, then, subjected to stretching test to obtain its resistance to deformation, known as passive behavior. Moreover, right and left eyes of a control group (n = 5) were equally subjected to active and passive tests to characterize the physiological behavior of SRM muscles. Results: On active function examination, statistically significant differences were documented between the following: control vs AM and no AM at 30 days (p = 0.002 and p = 0.04, respectively). All other comparisons were insignificant (p > 0.05). On passive function analysis, significant differences were only found between control vs. no AM at 30 days (p = 0.004) and between AM vs. no AM at 30 days (p = 0.002). Indeed, muscle operated without AM did not recover a normal passive function until 60 days after surgery. Conclusion: Cryopreserved AM is effective in accelerating recovery of SRM passive function in rabbits. Nevertheless, AM produced no significant effect on recovery of SRM active function.

Active behavior of abdominal wall muscles: Experimental results and numerical model formulation

In the present study a computational finite element technique is proposed to simulate the mechanical response of muscles in the abdominal wall. This technique considers the active behavior of the tissue taking into account both collagen and muscle fiber directions. In an attempt to obtain the computational response as close as possible to real muscles, the parameters needed to adjust the mathematical formulation were determined from in vitro experimental tests. Experiments were conducted on male New Zealand White rabbits (2047. ±. 34. g) and the active properties of three different muscles: Rectus Abdominis, External Oblique and multi-layered samples formed by three muscles (External Oblique, Internal Oblique, and Transversus Abdominis) were characterized. The parameters obtained for each muscle were incorporated into a finite strain formulation to simulate active behavior of muscles incorporating the anisotropy of the tissue. The results show the potential of the model to predict the anisotropic behavior of the tissue associated to fibers and how this influences on the strain, stress and generated force during an isometric contraction

Toll-like receptors 2 and 4 modulate intestinal IL-10 differently in ileum and colon

Background: Inflammatory bowel diseases are consequence of an intestinal homeostasis breakdown in which innate immune dysregulation is implicated. Toll-like receptor (TLR)2 and TLR4 are immune recognition receptors expressed in the intestinal epithelium, the first physical-physiological barrier for microorganisms, to inform the host of the presence of Gram-positive and Gram-negative organisms. Interleukin (IL)-10 is an essential anti-inflammatory cytokine that contributes to maintenance of intestinal homeostasis. Aim: Our main aim was to investigate intestinal IL-10 synthesis and release, and whether TLR2 and TLR4 are determinants of IL-10 expression in the intestinal tract. Methods: We used Caco-2 cell line as an enterocyte-like cell model, and also ileum and colon from mice deficient in TLR2, TLR4 or TLR2/4 to test the involvement of TLR signaling. Results: Intestinal epithelial cells are able to synthesize and release IL-10 and their expression is increased after TLR2 or TLR4 activation. IL-10 regulation seems to be tissue specific, with IL-10 expression in the ileum regulated by a compensation between TLR2 and TLR4 expression, whereas in the colon, TLR2 and TLR4 affect IL-10 expression independently. Conclusions: Intestinal epithelial cells could release IL-10 in response to TLR activation, playing an intestinal tissue-dependent and critical intestinal immune role