Mechanics of motility initiation and motility arrest in crawling cells

Motility initiation in crawling cells requires transformation of a symmetric state into a polarized state. In contrast, motility arrest is associated with re-symmetrization of the internal configuration of a cell. Experiments on keratocytes suggest that polarization is triggered by the increased contractility of motor proteins but the conditions of re-symmetrization remain unknown. In this paper we show that if adhesion with the extra-cellular substrate is sufficiently low, the progressive intensification of motor-induced contraction may be responsible for both transitions: from static (symmetric) to motile (polarized) at a lower contractility threshold and from motile (polarized) back to static (symmetric) at a higher contractility threshold. Our model of lamellipodial cell motility is based on a 1D projection of the complex intra-cellular dynamics on the direction of locomotion. In the interest of analytical transparency we also neglect active protrusion and view adhesion as passive. Despite the unavoidable oversimplifications associated with these assumptions, the model reproduces quantitatively the motility initiation pattern in fish keratocytes and reveals a crucial role played in cell motility by the nonlocal feedback between the mechanics and the transport of active agents. A prediction of the model that a crawling cell can stop and re-symmetrize when contractility increases sufficiently far beyond the motility initiation threshold still awaits experimental verification

Are there reliable constitutive laws for dynamic friction?

Structural vibration controlled by interfacial friction is widespread, ranging from friction dampers in gas turbines to the motion of violin strings. To predict, control or prevent such vibration, a constitutive description of frictional interactions is inevitably required. A variety of friction models are discussed to assess their scope and validity, in the light of constraints provided by different experimental observations. Three contrasting case studies are used to illustrate how predicted behaviour can be extremely sensitive to the choice of frictional constitutive model, and to explore possible experimental paths to discriminate between and calibrate dynamic friction models over the full parameter range needed for real applications.A.M. and T.P., respectively, acknowledge support from the CUED EPSRC Doctoral Training award and the EPSRC programme grant ‘Engineering Nonlinearity’ (ref. EP/K003836/1).This is the final version of the article. It first appeared from the Royal Society via http://dx.doi.org/10.1098/rsta.2014.040

Local impacts of climate change on winter wheat in Great Britain

Under future CMIP5 climate change scenarios for 2050, an increase in wheat yield of about 10% is predicted in Great Britain (GB) as a result of the combined effect of CO2 fertilization and a shift in phenology. Compared to the present day, crops escape increases in the climate impacts of drought and heat stresses on grain yield by developing before these stresses can occur. In the future, yield losses from water stress over a growing season will remain about the same across Great Britain with losses reaching around 20% of potential yield, while losses from drought around flowering will decrease and account for about 9% of water limited yield. Yield losses from heat stress around flowering will remain negligible in the future. These conclusions are drawn from a modelling study based on the response of the Sirius wheat simulation model to local-scale 2050-climate scenarios derived from 19 Global Climate Models from the CMIP5 ensemble at 25 locations representing current or potential wheat-growing areas in GB. However, depending on susceptibility to water stress, substantial interannual yield variation between locations is predicted, in some cases suggesting low wheat yield stability. For this reason, local-scale studies should be performed to evaluate uncertainties in yield prediction related to future weather patterns

Réverbérations d'ondes et frottement

Nous proposons qu'une origine possible de glissement saccadé et erratique de couches élastiques cisaillées l'une contre l'autre est associée à la réverbération des ondes qui rayonnent et se réfléchissent aux niveaux des interfaces en frottement solide. Dans le cadre des lois de frottement à variable interne, nous montrons que la dynamique du glissement interfacial est déterminée par un système différentiel avec délai correspondant au temps caractéristique des réverbérations fixé par l'épaisseur des couches. La stabilité du glissement stationnaire et des solutions périodiques est analysée

Conveyance of texture signals along a rat whisker

Neuronal activities underlying a percept are constrained by the physics of sensory signals. In the tactile sense such constraints are frictional stick–slip events, occurring, amongst other vibrotactile features, when tactile sensors are in contact with objects. We reveal new biomechanical phenomena about the transmission of these microNewton forces at the tip of a rat’s whisker, where they occur, to the base where they engage primary afferents. Using high resolution videography and accurate measurement of axial and normal forces at the follicle, we show that the conical and curved rat whisker acts as a sign-converting amplification filter for moment to robustly engage primary afferents. Furthermore, we present a model based on geometrically nonlinear Cosserat rod theory and a friction model that recreates the observed whole-beam whisker dynamics. The model quantifies the relation between kinematics (positions and velocities) and dynamic variables (forces and moments). Thus, only videographic assessment of acceleration is required to estimate forces and moments measured by the primary afferents. Our study highlights how sensory systems deal with complex physical constraints of perceptual targets and sensors

Quelques applications d'un modèle mécanique de la lithosphère

Ce travail propose un modèle mécanique de la lithosphère qui interprète le concept d'enveloppe de résistance de la manière la plus simple possible. Le comportement fragile qui dépend de la pression et le comportement ductile fortement variable avec la température sont les seuls pris en compte. Les effets de ce modèle sur la stratification thermomécanique de la lithosphère et sur la dynamique de l'expansion océanique sont présentés. Nous étudions un problème modèle unidimensionnel qui combine un mouvement de cisaillement pur et un mouvement de glissement simple variable avec la profondeur. En plus de la transition fragile/ductile, nous mettons en évidence une couche de transition à travers laquelle s'opère un changement de comportement de la réponse des roches en écoulement ductile. Cette couche sépare une zone où le cisaillement pur domine, d'une zone plus profonde où le glissement simple l'emporte. L'application de ce problème à la lithosphère suggère que cette couche jouerait un rôle essentiel comme mécanisme de décollement. Ce problème modèle a été de plus appliqué aux lithosphères océanique et continentale pour lesquelles les géothermes classiques ont été utilisés. Concernant l'expansion océanique, nous développons une théorie analytique de couche limite avec raccordement. Nous montrons comment le couplage entre la lithosphère et la convection mantellique est non linéaire et non local et que l'écoulement gravitaire de la lithosphère océanique suffit à entraîner la remontée passive de l'asthénosphère. À la différence des études antérieures, notre analyse montre clairement les rôles de la force déviatorique et de la partie fragile. En considérant la dilatation thermique dans le loi de comportement, la théorie prédit un taux de déformation et un champ de contrainte en accord avec les observations de Stein & Paleyo (1991) et de Kreemer et al. (2002). La corrélation entre le déviateur des contraintes et les mécanismes au foyer est mise en évidence.LYON1-BU.Sciences (692662101) / SudocSudocFranceF