8,454 research outputs found
Why Microtubules run in Circles - Mechanical Hysteresis of the Tubulin Lattice
The fate of every eukaryotic cell subtly relies on the exceptional mechanical
properties of microtubules. Despite significant efforts, understanding their
unusual mechanics remains elusive. One persistent, unresolved mystery is the
formation of long-lived arcs and rings, e.g. in kinesin-driven gliding assays.
To elucidate their physical origin we develop a model of the inner workings of
the microtubule's lattice, based on recent experimental evidence for a
conformational switch of the tubulin dimer. We show that the microtubule
lattice itself coexists in discrete polymorphic states. Curved states can be
induced via a mechanical hysteresis involving torques and forces typical of few
molecular motors acting in unison. This lattice switch renders microtubules not
only virtually unbreakable under typical cellular forces, but moreover provides
them with a tunable response integrating mechanical and chemical stimuli.Comment: 5 pages, 4 Movies in the Supplemen
A Solvable Model for Polymorphic Dynamics of Biofilaments
We investigate an analytically tractable toy model for thermally induced
polymorphic dynamics of cooperatively rearranging biofilaments - like
microtubules. The proposed 4 -block model, which can be seen as a
coarse-grained approximation of the full polymorphic tube model, permits a
complete analytical treatment of all thermodynamic properties including
correlation functions and angular fourier mode distributions. Due to its
mathematical tractability the model straightforwardly leads to some physical
insights in recently discussed phenomena like the "length dependent persistence
length". We show that a polymorphic filament can disguise itself as a classical
worm like chain on small and on large scales and yet display distinct anomalous
tell-tale features indicating an inner switching dynamics on intermediate
length scales
The role of M cells and the long QT syndrome in cardiac arrhythmias: simulation studies of reentrant excitations using a detailed electrophysiological model
In this numerical study, we investigate the role of intrinsic heterogeneities
of cardiac tissue due to M cells in the generation and maintenance of reentrant
excitations using the detailed Luo-Rudy dynamic model. This model has been
extended to include a description of the long QT 3 syndrome, and is studied in
both one dimension, corresponding to a cable traversing the ventricular wall,
and two dimensions, representing a transmural slice. We focus on two possible
mechanisms for the generation of reentrant events. We first investigate if
early-after-depolarizations occurring in M cells can initiate reentry. We find
that, even for large values of the long QT strength, the electrotonic coupling
between neighboring cells prevents early-after-depolarizations from creating a
reentry. We then study whether M cell domains, with their slow repolarization,
can function as wave blocks for premature stimuli. We find that the inclusion
of an M cell domain can result in some cases in reentrant excitations and we
determine the lifetime of the reentry as a function of the size and geometry of
the domain and of the strength of the long QT syndrome
Large scale EPR correlations and cosmic gravitational waves
We study how quantum correlations survive at large scales in spite of their
exposition to stochastic backgrounds of gravitational waves. We consider
Einstein-Podolski-Rosen (EPR) correlations built up on the polarizations of
photon pairs and evaluate how they are affected by the cosmic gravitational
wave background (CGWB). We evaluate the quantum decoherence of the EPR
correlations in terms of a reduction of the violation of the Bell inequality as
written by Clauser, Horne, Shimony and Holt (CHSH). We show that this
decoherence remains small and that EPR correlations can in principle survive up
to the largest cosmic scales.Comment: 5 figure
Ultrashort light bullets described by the two-dimensional sine-Gordon equation
By using a reductive perturbation technique applied to a two-level model,
this study puts forward a generic two-dimensional sine-Gordon evolution
equation governing the propagation of femtosecond spatiotemporal optical
solitons in Kerr media beyond the slowly varying envelope approximation. Direct
numerical simulations show that, in contrast to the long-wave approximation, no
collapse occurs, and that robust (2+1)-dimensional ultrashort light bullets may
form from adequately chosen few-cycle input spatiotemporal wave forms. In
contrast to the case of quadratic nonlinearity, the light bullets oscillate in
both space and time and are therefore not steady-state lumps
Crunching Biofilament Rings
We discuss a curious example for the collective mechanical behavior of
coupled non-linear monomer units entrapped in a circular filament. Within a
simple model we elucidate how multistability of monomer units and exponentially
large degeneracy of the filament's ground state emerge as a collective feature
of the closed filament. Surprisingly, increasing the monomer frustration, i.e.,
the bending prestrain within the circular filament, leads to a conformational
softening of the system. The phenomenon, that we term polymorphic crunching, is
discussed and applied to a possible scenario for membrane tube deformation by
switchable dynamin or FtsZ filaments. We find an important role of cooperative
inter-unit interaction for efficient ring induced membrane fission
First Order Premelting Transition of Vortex Lattices
Vortex lattices in the high temperature superconductors undergo a first order
phase transition which has thus far been regarded as melting from a solid to a
liquid. We point out an alternative possibility of a two step process in which
there is a first order transition from an ordinary vortex lattice to a soft
vortex solid followed by another first order melting transition from the soft
vortex solid to a vortex liquid. We focus on the first step. This premelting
transition is induced by vacancy and interstitial vortex lines. We obtain good
agreement with the experimental transition temperature versus field, latent
heat, and magnetization jumps for YBCO and BSCCO.Comment: revised version replaces 9705092, 5 pages, Latex, 2 postscript
figures, defect line wandering is included, 2 step melting is propose
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