2,508 research outputs found
Spatial Organization of the Cytoskeleton enhances Cargo Delivery to Specific Target Areas on the Plasma Membrane of Spherical Cells
Intracellular transport is vital for the proper functioning and survival of a
cell. Cargo (proteins, vesicles, organelles, etc.) is transferred from its
place of creation to its target locations via molecular motor assisted
transport along cytoskeletal filaments. The transport efficiency is strongly
affected by the spatial organization of the cytoskeleton, which constitutes an
inhomogeneous, complex network. In cells with a centrosome microtubules grow
radially from the central microtubule organizing center towards the cell
periphery whereas actin filaments form a dense meshwork, the actin cortex,
underneath the cell membrane with a broad range of orientations. The emerging
ballistic motion along filaments is frequently interrupted due to constricting
intersection nodes or cycles of detachment and reattachment processes in the
crowded cytoplasm. In order to investigate the efficiency of search strategies
established by the cell's specific spatial organization of the cytoskeleton we
formulate a random velocity model with intermittent arrest states. With
extensive computer simulations we analyze the dependence of the mean first
passage times for narrow escape problems on the structural characteristics of
the cytoskeleton, the motor properties and the fraction of time spent in each
state. We find that an inhomogeneous architecture with a small width of the
actin cortex constitutes an efficient intracellular search strategy.Comment: 14 pages, 9 figure
Discovery of a ~5 day characteristic timescale in the Kepler power spectrum of Zw 229-15
We present time series analyses of the full Kepler dataset of Zw 229-15. This
Kepler light curve --- with a baseline greater than three years, composed of
virtually continuous, evenly sampled 30-minute measurements --- is
unprecedented in its quality and precision. We utilize two methods of power
spectral analysis to investigate the optical variability and search for
evidence of a bend frequency associated with a characteristic optical
variability timescale. Each method yields similar results. The first
interpolates across data gaps to use the standard Fourier periodogram. The
second, using the CARMA-based time-domain modeling technique of Kelly et al.
(2014), does not need evenly-sampled data. Both methods find excess power at
high frequencies that may be due to Kepler instrumental effects. More
importantly both also show strong bends ({\Delta}{\alpha} ~ 2) at timescales of
~5 days, a feature similar to those seen in the X-ray PSDs of AGN but never
before in the optical. This observed ~5 day timescale may be associated with
one of several physical processes potentially responsible for the variability.
A plausible association could be made with light-crossing, dynamical or thermal
timescales, depending on the assumed value of the accretion disk size and on
unobserved disk parameters such as {\alpha} and H/R. This timescale is not
consistent with the viscous timescale, which would be years in a ~10^7 Solar
mass AGN such as Zw 229-15. However there must be a second bend on long (>~1
year) timescales, and that feature could be associated with the viscous
timescale.Comment: 10 pages, 5 figures, 1 table. To appear in the Astrophysical Journal,
Part
Alteration of cerebrovascular haemodynamic patterns due to atrial fibrillation: an in silico investigation
There has recently been growing evidence that atrial fibrillation (AF), the
most common cardiac arrhythmia, is independently associated with the risk of
dementia. This represents a very recent frontier with high social impact for
the number of individuals involved and for the expected increase in AF
incidence in the next 40 years. Although a number of potential haemodynamic
processes, such as microembolisms, altered cerebral blood flow, hypoperfusion
and microbleeds, arise as connecting links between the two pathologies, the
causal mechanisms are far from clear. An in silico approach is proposed that
combines in sequence two lumped-parameter schemes, for the cardiovascular
system and the cerebral circulation. The systemic arterial pressure is obtained
from the cardiovascular system and used as the input for the cerebral
circulation, with the aim of studying the role of AF on the cerebral
haemodynamics with respect to normal sinus rhythm (NSR), over a 5000 beat
recording. In particular, the alteration of the haemodynamic (pressure and
flowrate) patterns in the microcirculation during AF is analysed by means of
different statistical tools, from correlation coefficients to autocorrelation
functions, crossing times, extreme values analysis and multivariate linear
regression models. A remarkable signal alteration, such as a reduction in
signal correlation (NSR, about 3 s; AF, less than 1 s) and increased
probability (up to three to four times higher in AF than in NSR) of extreme
value events, emerges for the peripheral brain circulation. The described
scenario offers a number of plausible cause-effect mechanisms that might
explain the occurrence of critical events and the haemodynamic links relating
to AF and dementia.Comment: 13 pages, 9 Figures, 3 Table
Search for domain wall dark matter with atomic clocks on board global positioning system satellites
Cosmological observations indicate that 85% of all matter in the Universe is
dark matter (DM), yet its microscopic composition remains a mystery. One
hypothesis is that DM arises from ultralight quantum fields that form
macroscopic objects such as topological defects. Here we use GPS as a ~ 50,000
km aperture DM detector to search for such defects in the form of domain walls.
GPS navigation relies on precision timing signals furnished by atomic clocks
hosted on board GPS satellites. As the Earth moves through the galactic DM
halo, interactions with topological defects could cause atomic clock glitches
that propagate through the GPS satellite constellation at galactic velocities ~
300 km/s. Mining 16 years of archival GPS data, we find no evidence for DM in
the form of domain walls at our current sensitivity level. This allows us to
improve the limits on certain quadratic scalar couplings of domain wall DM to
standard model particles by several orders of magnitude.Comment: 7 pages (main text), and 12 pages for Supplementary Information. v3:
Update titl
Safety Margins for Flight Through Stochastic Gusts
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/140648/1/1.g000299.pd
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