Speaker
Saúl Ares
Description The segmentation clock is a population of coupled cellular oscillators in the vertebrate embryo that sets the rhythm for the formation of somites, the embryonic precursors of vertebrae and other segmented structures of the vertebrate adult. The oscillating cells of the segmentation clock are thought to possess noisy autonomous periods, which are synchronized by intercellular coupling. This intercellular coupling involves a complex cascade of events that introduces coupling delays. We have developed a generic description of the segmentation clock using an array of coupled phase oscillators, with coupling delays. This theory predicts that coupling strength and delays modulate the period of the segmentation clock and produce changes in segment length and cyclic gene expression patterns. A further prediction is that decreasing coupling delay time results in an instability of the segmentation clock. We have experimentally confirmed all this predictions in zebrafish embryos, and in doing so we have described the first mutants with a segmentation period differing from the wild type condition. Thus we have characterized a natural system where delayed coupling between oscillators plays a key role. In this talk I will present our delayed coupling theory of vertebrate segmentation. As an example of its most interesting technical aspects, we have developed a generic continuum approximation for systems of delay coupled oscillators, that shares features with the celebrated KPZ and Kuramoto-Sivashinsky models.
Speaker
Sergio Ciliberto
Description The study of the fluctuations in out of equilibrium systems is a subject of current interest ranging from the dynamical systems to the stochastic ones. In this talk we will discuss the problem of the fluctuations of the injected and dissipated power in these systems. There are only a few theoretical results, which may predict several statistical properties of these fluctuations. This is the case of the Fluctuation Theorem (FT), which has been first found in the context of dynamical systems and later extended to stochastic ones. The proof relies upon several hypotheses which merit to be tested experimentally. We will focus on stochastic systems dominated by thermal fluctuations and we will discuss them from the experimental point of view. We describe a rather general case, that is the work fluctuations of an oscillator in contact with a heat reservoir and driven out of equilibrium by an external force. We also consider the fluctuations of the heat dissipated towards the heat bath. The experimental analysis in the context of Fluctuation Theorem (FT) allows us to recall the general properties of this theorem and its importance for experimental systems. We discuss the finite time corrections that depend on the observable and show that for the total entropy production rate the FT is verified for all times. The role of non-linearities is considered by briefly describing the experimental results of a Brownian particle confined in a non double well potential and driven out of equilibrium by an external force. We finally describe the important consequences of FT and some useful applications to the analysis of experimental data.
Speaker
William Bialek
Description Synopsis: The Systems Biology Unit of The Centre de Regulació Genòmica (CRG) and the Departament de Física Fonamental, Universitat de Barcelona (U.B.) are pleased to invite you to a series of four lectures on biophysics and systems biology. Prof. Bialek will deliver two lectures in each institute, reviewing recent developments to which he and his colleagues have contributed. The course is aimed to foster exchanges between students and faculties of both institutes. Introduction: In this short lecture series Prof. Bialek will give some perspective on two very different approaches to the physics of biological systems. One approach is phenomenological, allow- ing ourselves to be driven by newly emerging data. The complementary approach is more abstract, in which we explore candidate principles from which the properties of these com- plex systems should be derivable. The course will be followed by a seminar session at the PRBB-GRC (Friday 28/01, 12:00-13:00). Lecture Plan: Thu 20/01, 11:00-13:00 Dimensionality reduction (UB) Mon 24/01, 11:00-13:00 Optimizing signal to noise ratio (UB) Wed 26/01, 11:00-13:00 Maximum entropy models for biological networks (CRG) Fri 28/01, 11:00-12:00 Optimizing information transmission (CRG) Fri 28/01, 12:00-13:00 Optimization and the physical limits to biological function (CRG)
Speaker
Clara Picallo
Description Understanding how materials deform and break is a subject of great technological importance. At the same time, it requires from the knowledge of the basic processes governing the phenomenon and hence it is very interesting from a fundamental physics perspective. Fracture and plasticity display intermittent scale invariant behavior. The presence of universal power law distributions in both temporal and spatial properties seems to suggest that they could be explained as some type of critical phenomenon. Therefore, simplified theoretical approaches based on fundamental concepts can help to capture the essential ingredients involved. In this sense, tools coming from statistical mechanics can help to deal with disorder, long range interactions and scaling laws. In the last decades several steps have been given in this direction and, to this aim, some simplified models have been developed and studied. In this talk we will make use of the Random Fuse Model, in which the vectorial mechanical response is replaced by an scalar electrical analogue. This extremely simple approach has become the cornerstone of this kind of models. Several results will be discussed in this context, ranging from the acoustic emission from brittle fracture to localization of damage, the roughness of the resulting fracture profiles and the scaling of the strain avalanches in ductile media.
Speaker
Annette Taylor