6,681 research outputs found
Nonlinear competition between asters and stripes in filament-motor-systems
A model for polar filaments interacting via molecular motor complexes is
investigated which exhibits bifurcations to spatial patterns. It is shown that
the homogeneous distribution of filaments, such as actin or microtubules, may
become either unstable with respect to an orientational instability of a finite
wave number or with respect to modulations of the filament density, where long
wavelength modes are amplified as well. Above threshold nonlinear interactions
select either stripe patterns or periodic asters. The existence and stability
ranges of each pattern close to threshold are predicted in terms of a weakly
nonlinear perturbation analysis, which is confirmed by numerical simulations of
the basic model equations. The two relevant parameters determining the
bifurcation scenario of the model can be related to the concentrations of the
active molecular motors and of the filaments respectively, which both could be
easily regulated by the cell.Comment: 13 pages, 7 figure
Wisconsin Sentence Modification: A View from the Trial Court
In Wisconsin, trial courts have discretion to modify a defendant\u27s criminal sentence if the defendant introduces a new factor. Published Wisconsin case law gives little guidance on what constitutes a new factor. The Wisconsin Supreme Court has declined to find a new factor present in every case it has published since defining new factor in 1978. Because of ambiguous and conflicting rulings, the standards for both prongs of the new factor definition remain unclear. This Comment attempts to shed light on the new factor requirement for sentence modification by examining Wisconsin trial court decisions on a limited sample of sentence modification motions. This study reveals that trial courts modified sentences in a variety of cases to effect their intent in sentencing the defendant when it has been frustrated, to respond to a change in a defendant\u27s health or family circumstances, or to reward a defendant for cooperating with the district attorney after sentencing. These grounds for sentence modification are offered in other states through statutory exceptions to the time limits governing sentence modification motions. This Comment concludes that Wisconsin\u27s sentence modification law, though frought with confusion at the appellate level, nonetheless provides defendants with a broader and more flexible form of post-con- viction relief than that provided in states where statues define the factors justifying a sentence modification
Lattice gas cellular automata model for rippling and aggregation in myxobacteria
A lattice-gas cellular automaton (LGCA) model is used to simulate rippling
and aggregation in myxobacteria. An efficient way of representing cells of
different cell size, shape and orientation is presented that may be easily
extended to model later stages of fruiting body formation. This LGCA model is
designed to investigate whether a refractory period, a minimum response time, a
maximum oscillation period and non-linear dependence of reversals of cells on
C-factor are necessary assumptions for rippling. It is shown that a refractory
period of 2-3 minutes, a minimum response time of up to 1 minute and no maximum
oscillation period best reproduce rippling in the experiments of {\it
Myxoccoccus xanthus}. Non-linear dependence of reversals on C-factor is
critical at high cell density. Quantitative simulations demonstrate that the
increase in wavelength of ripples when a culture is diluted with non-signaling
cells can be explained entirely by the decreased density of C-signaling cells.
This result further supports the hypothesis that levels of C-signaling
quantitatively depend on and modulate cell density. Analysis of the
interpenetrating high density waves shows the presence of a phase shift
analogous to the phase shift of interpenetrating solitons. Finally, a model for
swarming, aggregation and early fruiting body formation is presented
Use of Imaging Spectrometer Data and Multispectral Imagery for Improved Earthquake Response
Imaging and Applied Optics Technical Digest, 2012Multispectral imagery and imaging spectrometer data are used to develop prototype
map products for improved earthquake response. A tiered approach keyed to post-event
communications infrastructure is directed at providing critical information to emergency services personnel.This research is supported by the Science and Technology (S&T) Directorate, Department of Homeland Security (DHS). We gratefully acknowledge the participation of emergency responders and managers from the cities and counties of Monterey, Los Angeles, San Diego, and Riverside California. We also appreciate contributions during project definition stage and follow-ups by the California Emergency Management Agency (Cal EMA) and the Federal Emergency Management Agency (FEMA), the U.S. Geological Survey, and DHS. AVIRIS data were acquired by NASA/JPL. The LiDAR data were provided by the Association of Monterey Bay Area Governments, via a USGS grant through the American Reinvestment and Recovery Act of 2009. WV-2 data were provided by the National Geospatial Intelligence Agency (NGA) under the NextView imagery license agreement
Reversal of contractility as a signature of self-organization in cytoskeletal bundles.
Funder: FP7 People: Marie-Curie Actions; FundRef: http://dx.doi.org/10.13039/100011264; Grant(s): PCIG12-GA-2012-334053Bundles of cytoskeletal filaments and molecular motors generate motion in living cells, and have internal structures ranging from very organized to apparently disordered. The mechanisms powering the disordered structures are debated, and existing models predominantly predict that they are contractile. We reexamine this prediction through a theoretical treatment of the interplay between three well-characterized internal dynamical processes in cytoskeletal bundles: filament assembly and disassembly, the attachement-detachment dynamics of motors and that of crosslinking proteins. The resulting self-organization is easily understood in terms of motor and crosslink localization, and allows for an extensive control of the active bundle mechanics, including reversals of the filaments' apparent velocities and the possibility of generating extension instead of contraction. This reversal mirrors some recent experimental observations, and provides a robust criterion to experimentally elucidate the underpinnings of both actomyosin activity and the dynamics of microtubule/motor assemblies in vitro as well as in diverse intracellular structures ranging from contractile bundles to the mitotic spindle
Biomechanical Simulation of Electrode Migration for Deep Brain Stimulation
International audienceDeep Brain Stimulation is a modern surgical technique for treating patients who suffer from affective or motion disorders such as Parkinson's disease. The efficiency of the procedure relies heavily on the accuracy of the placement of a micro-electrode which sends electrical pulses to a specific part of the brain that controls motion and affective symptoms. However, targeting this small anatomical structure is rendered difficult due to a series of brain shifts that take place during and after the procedure. This paper introduces a biomechanical simulation of the intra and postoperative stages of the procedure in order to determine lead deformation and electrode migration due to brain shift. To achieve this goal, we propose a global approach, which accounts for brain deformation but also for the numerous interactions that take place during the procedure (contacts between the brain and the inner part of the skull and falx cerebri, effect of the cerebro-spinal fluid, and biomechanical interactions between the brain and the electrodes and cannula used during the procedure). Preliminary results show a good correlation between our simulations and various results reported in the literature
Tariff-Mediated Network Effects versus Strategic Discounting: Evidence from German Mobile Telecommunications
Mobile telecommunication operators routinely charge subscribers lower prices for calls on their own network than for calls to other networks (on-net discounts). Studies on tariff-mediated network effects suggest this is due to large operators using on-net discounts to damage smaller rivals. Alternatively, research on strategic discounting suggests small operators use on-net discounts to advertise with low on-net prices. We test the relative strength of these effects using data on tariff setting in German mobile telecommunications between 2001 and 2009. We find that large operators are more likely to offer tariffs with on-net discounts but there is no consistently significant difference in the magnitude of discounts. Our results suggest that tariff-mediated network effects are the main cause of on-net discounts
Collective Effects in Models for Interacting Molecular Motors and Motor-Microtubule Mixtures
Three problems in the statistical mechanics of models for an assembly of
molecular motors interacting with cytoskeletal filaments are reviewed. First, a
description of the hydrodynamical behaviour of density-density correlations in
fluctuating ratchet models for interacting molecular motors is outlined.
Numerical evidence indicates that the scaling properties of dynamical behavior
in such models belong to the KPZ universality class. Second, the generalization
of such models to include boundary injection and removal of motors is provided.
In common with known results for the asymmetric exclusion processes,
simulations indicate that such models exhibit sharp boundary driven phase
transitions in the thermodynamic limit. In the third part of this paper, recent
progress towards a continuum description of pattern formation in mixtures of
motors and microtubules is described, and a non-equilibrium ``phase-diagram''
for such systems discussed.Comment: Proc. Int. Workshop on "Common Trends in Traffic Systems", Kanpur,
India, Feb 2006; to be published in Physica
Dense active matter model of motion patterns in confluent cell monolayers
Epithelial cell monolayers show remarkable displacement and velocity
correlations over distances of ten or more cell sizes that are reminiscent of
supercooled liquids and active nematics. We show that many observed features
can be described within the framework of dense active matter, and argue that
persistent uncoordinated cell motility coupled to the collective elastic modes
of the cell sheet is sufficient to produce swirl-like correlations. We obtain
this result using both continuum active linear elasticity and a normal modes
formalism, and validate analytical predictions with numerical simulations of
two agent-based cell models, soft elastic particles and the self-propelled
Voronoi model together with in-vitro experiments of confluent corneal
epithelial cell sheets. Simulations and normal mode analysis perfectly match
when tissue-level reorganisation occurs on times longer than the persistence
time of cell motility. Our analytical model quantitatively matches measured
velocity correlation functions over more than a decade with a single fitting
parameter.Comment: updated version accepted for publication in Nat. Com
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