234 research outputs found
Bi-stability resistant to fluctuations
We study a simple micro-mechanical device that does not lose its snap-through
behavior in an environment dominated by fluctuations. The main idea is to have
several degrees of freedom that can cooperatively resist the de-synchronizing
effect of random perturbations. As an inspiration we use the power stroke
machinery of skeletal muscles, which ensures at sub-micron scales and finite
temperatures a swift recovery of an abruptly applied slack. In addition to
hypersensitive response at finite temperatures, our prototypical Brownian snap
spring also exhibits criticality at special values of parameters which is
another potentially interesting property for micro-scale engineering
applications
Muscle as a meta-material operating near a critical point
Passive mechanical response of skeletal muscles at fast time scales is
dominated by long range interactions inducing cooperative behavior without
breaking the detailed balance. This leads to such unusual "material properties"
as negative equilibrium stiffness and different behavior in force and
displacement controlled loading conditions. Our fitting of experimental data
suggests that "muscle material" is finely tuned to perform close to a critical
point which explains large fluctuations observed in muscles close to the stall
force.Comment: Accepted for publication in Physical Review Letter
Mechanics of collective unfolding
Mechanically induced unfolding of passive crosslinkers is a fundamental
biological phenomenon encountered across the scales from individual
macro-molecules to cytoskeletal actin networks. In this paper we study a
conceptual model of athermal load-induced unfolding and use a minimalistic
setting allowing one to emphasize the role of long-range interactions while
maintaining full analytical transparency. Our model can be viewed as a
description of a parallel bundle of N bistable units confined between two
shared rigid backbones that are loaded through a series spring. We show that
the ground states in this model correspond to synchronized, single phase
configurations where all individual units are either folded or unfolded. We
then study the fine structure of the wiggly energy landscape along the reaction
coordinate linking the two coherent states and describing the optimal mechanism
of cooperative unfolding. Quite remarkably, our study shows the fundamental
difference in the size and structure of the folding-unfolding energy barriers
in the hard (fixed displacements) and soft (fixed forces) loading devices which
persists in the continuum limit. We argue that both, the synchronization and
the non-equivalence of the mechanical responses in hard and soft devices, have
their origin in the dominance of long-range interactions. We then apply our
minimal model to skeletal muscles where the power-stroke in acto-myosin
crossbridges can be interpreted as passive folding. A quantitative analysis of
the muscle model shows that the relative rigidity of myosin backbone provides
the long-range interaction mechanism allowing the system to effectively
synchronize the power-stroke in individual crossbridges even in the presence of
thermal fluctuations. In view of the prototypical nature of the proposed model,
our general conclusions pertain to a variety of other biological systems where
elastic interactions are mediated by effective backbones
Cooperative folding of muscle myosins: I. Mechanical model
Mechanically induced folding of passive cross-linkers is a fundamental biological phenomenon. A typical example is a conformational change in myosin II responsible for the power-stroke in skeletal muscles. In this paper we present an athermal perspective on such folding by analyzing the simplest purely mechanical prototype: a parallel bundle of bi-stable units attached to a common backbone. We show that in this analytically transparent model, characterized by a rugged energy landscape, the ground states are always highly coherent, single-phase configurations. We argue that such cooperative behavior, ensuring collective conformational change, is due to the dominance of long- range interactions making the system non-additive. The detailed predictions of our model are in agreement with experimentally observed non-equivalence of fast force recovery in skeletal muscles loaded in soft and hard devices. Some features displayed by the model are also recognizable in the behavior of other biological systems with passive multi-stability and long-range interactions including detaching adhesive binders and pulled RNA/DNA hairpins
Lâattitude des artisans gallo-romains Ă lâĂ©gard du travail manuel. Ătude de lâiconographie lapidaire funĂ©raire
Cet article se propose dâĂ©tudier lâattitude des artisans gallo-romains Ă lâĂ©gard du travail manuel Ă partir du mode de reprĂ©sentation adoptĂ© sur leurs monuments funĂ©raires. Deux postures peuvent ĂȘtre ainsi mises en Ă©vidence : celle du travailleur manuel et celle de lâentrepreneur. Lâanalyse de la documentation iconographique nĂ©cessite la prise en compte de plusieurs critĂšres, comme la position des diffĂ©rentes images sur le monument, leur Ă©chelle, le nombre de personnages figurĂ©s et leurs vĂȘtements.In this article we study the attitude of Gallo-Roman craftsmen towards manual work from the way they were depicted on their funerary monuments. Two postures stand out: that of the craftsman and of the contractor. Several criteria have to be taken into account for the study of the iconographic records: the position of the images on the monument, their scale, the number of craftsmen depicted and what they are wearing
Laurens E. Tacoma, Moving Romans: Migration to Rome in the Principate
Marie-Sophie Caruel ArticulĂ© autour de huit chapitres, lâouvrage de L.E. Tacoma sur les migrations romaines aborde les questions liĂ©es au genre aux chapitres sur la famille (chap. 4) et sur le travail (chap. 6). La place faite aux femmes demeure donc relativement modeste et assez conventionnelle mĂȘme si les apports sur la question ne sauraient ĂȘtre qualifiĂ©s de nĂ©gligeables pour un sujet longtemps en marge des Ă©tudes sur la mobilitĂ©. Le cadre thĂ©orique proposĂ© dans cet ouvrage est celui dâune..
Thermodynamical framework for modeling chemo-mechanical coupling in muscle contraction â Formulation and preliminary results
International audienceWe propose a multiscale model of cardiac contraction in which the molecular motors at the origin of the contractile process are considered as multistable mechanical entities endowed with internal degrees of freedom of both mechanical and chemical nature. This model provides a thermodynamical basis for modeling the complex interplay of chemical and mechanical phenomena at the sub-cellular level. Important motivations for this work include the ability to represent the experimentally observed physiological characteristics of the contractile apparatus such as (i) the passive quick force recovery mechanism, (ii) the relation between the contraction velocity and the applied force and (iii) the so called Lymn-Taylor cycle describing the metabolism.Nous proposons un modÚle multi-échelle de la contraction cardiaque dans lequel les moteurs moléculaires à l'origine du processus contractile sont représentés par des élé-ments mécaniques multistables paramétrés à la fois par des degrés de liberté géométriques et par des états chimiques. Ce modÚle permet de poser les fondements thermody-namiques permettant de décrire l'interaction complexe entre les phénomÚnes mécaniques et chimiques a l'échelle sub-cellulaire. Ce travail a pour objet de représenter les car-actéristiques physiologiques du dispositif contractile observées expérimentalement et en particulier (i) le mécanisme passif de récupération rapide de force, (ii) la relation entre la vitesse de contraction et la charge appliquée et (iii) le cycle dit de Lymn-Taylor décrivant l'activité métabolique. Abstract : We propose a multiscale model of cardiac contraction in which the molecular motors at the origin of the contractile process are considered as multistable mechanical entities endowed with internal degrees of freedom of both mechanical and chemical nature. This model provides a thermodynamical basis for modeling the complex interplay of chemical and mechanical phenomena at the sub-cellular level. Important motivations for this work include the ability to represent the experimentally observed physiological characteristics of the contractile apparatus such as (i) the passive quick force recovery mechanism, (ii) the relation between the contraction velocity and the applied force and (iii) the so called Lymn-Taylor cycle describing the metabolism
Cooperative folding of muscle myosins: I. Mechanical model
Mechanically induced folding of passive cross-linkers is a fundamental biological phenomenon. A typical example is a conformational change in myosin II responsible for the power-stroke in skeletal muscles. In this paper we present an athermal perspective on such folding by analyzing the simplest purely mechanical prototype: a parallel bundle of bi-stable units attached to a common backbone. We show that in this analytically transparent model, characterized by a rugged energy landscape, the ground states are always highly coherent, single-phase configurations. We argue that such cooperative behavior, ensuring collective conformational change, is due to the dominance of long- range interactions making the system non-additive. The detailed predictions of our model are in agreement with experimentally observed non-equivalence of fast force recovery in skeletal muscles loaded in soft and hard devices. Some features displayed by the model are also recognizable in the behavior of other biological systems with passive multi-stability and long-range interactions including detaching adhesive binders and pulled RNA/DNA hairpins
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