3,507 research outputs found
Deformation of crosslinked semiflexible polymer networks
Full text linkNetworks of filamentous proteins play a crucial role in cell mechanics. These
cytoskeletal networks, together with various crosslinking and other associated
proteins largely determine the (visco)elastic response of cells. In this letter
we study a model system of crosslinked, stiff filaments in order to explore the
connection between the microstructure under strain and the macroscopic response
of cytoskeletal networks. We find two distinct regimes as a function primarily
of crosslink density and filament rigidity: one characterized by affine
deformation and one by non-affine deformation. We characterize the crossover
between these two.Comment: Typos fixed and some technical details clarified. To appear in Phys.
Rev. Let
Do not ask whether they have a cognitive map, but how they find their way about.
Get PDFfind their way abou
Pavlov and Associationism
Get PDFLa contribución de Pavlov a la psicologÃa experimental fue inventar una técnica que le permitió acometer una serie prolongada y sistemática de experimentos bien controlados que desvelaron muchos, si no la mayorÃa, de los fenómenos de lo que justamente se llama condicionamiento pavloviano. Los psicólogos angloparlantes tardarÃan 30 años o más en igualar ese logro. Por supuesto, se han dado nuevos desarrollos y descubrimientos desde su época. Se comentan dos ejemplos: el importante papel de la asociabilidad variable o la atención incluso en el condicionamiento simple y la aplicación rigurosa de la teorÃa del aprendizaje asociativo a la conducta de los seres humanos adultos.Pavlov’s contribution to experimental psychology was to invent a technique that allowed him to undertake a prolonged and systematic series of well-controlled experiments that, astonishingly enough, uncovered many if not most of the phenomena of what is rightly called Pavlovian conditioning. It was not for another 30 years or more that Englishspeaking psychologists began to match that achievement. Of course there have been new developments and discoveries since his time. Two examples are discussed: the important role of variable associability or attention even in simple conditioning, and the rigorous application of associative learning theory to the behavior of adult humans
Non-equilibrium mechanics and dynamics of motor activated gels
Full text linkThe mechanics of cells is strongly affected by molecular motors that generate
forces in the cellular cytoskeleton. We develop a model for cytoskeletal
networks driven out of equilibrium by molecular motors exerting transient
contractile stresses. Using this model we show how motor activity can
dramatically increase the network's bulk elastic moduli. We also show how motor
binding kinetics naturally leads to enhanced low-frequency stress fluctuations
that result in non-equilibrium diffusive motion within an elastic network, as
seen in recent \emph{in vitro} and \emph{in vivo} experiments.Comment: 21 pages, 8 figure
‘Escritores de envergadura’:Gendering bestsellers in Claudia Piñeiro’s ‘La muerte y la canoa’ and ‘Bendito aire de Buenos Aires’
Get PDFActive biopolymer networks generate scale-free but euclidean clusters
Get PDFWe report analytical and numerical modelling of active elastic networks,
motivated by experiments on crosslinked actin networks contracted by myosin
motors. Within a broad range of parameters, the motor-driven collapse of active
elastic networks leads to a critical state. We show that this state is
qualitatively different from that of the random percolation model.
Intriguingly, it possesses both euclidean and scale-free structure with Fisher
exponent smaller than . Remarkably, an indistinguishable Fisher exponent and
the same euclidean structure is obtained at the critical point of the random
percolation model after absorbing all enclaves into their surrounding clusters.
We propose that in the experiment the enclaves are absorbed due to steric
interactions of network elements. We model the network collapse, taking into
account the steric interactions. The model shows how the system robustly drives
itself towards the critical point of the random percolation model with absorbed
enclaves, in agreement with the experiment.Comment: 6 pages, 7 figure
The social practice of rescue: the safety implications of acute illness trajectories and patient categorisation in medical and maternity settings.
Full text linkThe normative position in acute hospital care when a patient is seriously ill is to resuscitate and rescue. However, a number of UK and international reports have highlighted problems with the lack of timely recognition, treatment and referral of patients whose condition is deteriorating while being cared for on hospital wards. This article explores the social practice of rescue, and the structural and cultural influences that guide the categorisation and ordering of acutely ill patients in different hospital settings. We draw on Strauss et al.'s notion of the patient trajectory and link this with the impact of categorisation practices, thus extending insights beyond those gained from emergency department triage to care management processes further downstream on the hospital ward. Using ethnographic data collected from medical wards and maternity care settings in two UK inner city hospitals, we explore how differences in population, cultural norms, categorisation work and trajectories of clinical deterioration interlink and influence patient safety. An analysis of the variation in findings between care settings and patient groups enables us to consider socio-political influences and the specifics of how staff manage trade-offs linked to the enactment of core values such as safety and equity in practice
Mechanics and force transmission in soft composites of rods in elastic gels
Get PDFWe report detailed theoretical investigations of the micro-mechanics and bulk
elastic properties of composites consisting of randomly distributed stiff
fibers embedded in an elastic matrix in two and three dimensions. Recent
experiments published in Physical Review Letters [102, 188303 (2009)] have
suggested that the inclusion of stiff microtubules in a softer, nearly
incompressible biopolymer matrix can lead to emergent compressibility. This can
be understood in terms of the enhancement of the compressibility of the
composite relative to its shear compliance as a result of the addition of stiff
rod-like inclusions. We show that the Poisson's ratio of such a composite
evolves with increasing rod density towards a particular value, or {\em fixed
point}, independent of the material properties of the matrix, so long as it has
a finite initial compressibility. This fixed point is in three
dimensions and in two dimensions. Our results suggest an important
role for stiff filaments such as microtubules and stress fibers in cell
mechanics. At the same time, our work has a wider elasticity context, with
potential applications to composite elastic media with a wide separation of
scales in stiffness of its constituents such as carbon nanotube-polymer
composites, which have been shown to have highly tunable mechanics.Comment: 10 pages, 8 figure
Actively stressed marginal networks
Get PDFWe study the effects of motor-generated stresses in disordered three
dimensional fiber networks using a combination of a mean-field, effective
medium theory, scaling analysis and a computational model. We find that motor
activity controls the elasticity in an anomalous fashion close to the point of
marginal stability by coupling to critical network fluctuations. We also show
that motor stresses can stabilize initially floppy networks, extending the
range of critical behavior to a broad regime of network connectivities below
the marginal point. Away from this regime, or at high stress, motors give rise
to a linear increase in stiffness with stress. Finally, we demonstrate that our
results are captured by a simple, constitutive scaling relation highlighting
the important role of non-affine strain fluctuations as a susceptibility to
motor stress.Comment: 8 pages, 4 figure
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