2,583,001 research outputs found
Complex patterns of subcellular cardiac alternans
Cardiac alternans, in which the membrane potential and the intracellular calcium concentration exhibit alternating durations and peak amplitudes at consecutive beats, constitute a precursor to fatal cardiac arrhythmia such as sudden cardiac death. A crucial question therefore concerns the onset of cardiac alternans. Typically, alternans are only reported when they are fully developed. Here, we present a modelling approach to explore recently discovered microscopic alternans, which represent one of the earliest manifestations of cardiac alternans. In this case, the regular periodic dynamics of the local intracellular calcium concentration is already unstable, while the whole-cell behaviour suggests a healthy cell state. In particular, we use our model to investigate the impact of calcium diffusion in both the cytosol and the sarcoplasmic reticulum on the formation of microscopic calcium alternans. We find that for dominant cytosolic coupling, calcium alternans emerge via the traditional period doubling bifurcation. In contrast, dominant luminal coupling leads to a novel route to calcium alternans through a saddle-node bifurcation at the network level. Combining semi-analytical and computational approaches, we compute areas of stability in parameter space and find that as we cross from stable to unstable regions, the emergent patterns of the intracellular calcium concentration change abruptly in a fashion that is highly dependent upon position along the stability boundary. Our results demonstrate that microscopic calcium alternans may possess a much richer dynamical repertoire than previously thought and further strengthen the role of luminal calcium in shaping cardiac calcium dynamics
Antagonistic Structural Patterns in Complex Networks
Identifying and explaining the structure of complex networks at different
scales has become an important problem across disciplines. At the mesoscale,
modular architecture has attracted most of the attention. At the macroscale,
other arrangements --e.g. nestedness or core-periphery-- have been studied in
parallel, but to a much lesser extent. However, empirical evidence increasingly
suggests that characterizing a network with a unique pattern typology may be
too simplistic, since a system can integrate properties from distinct
organizations at different scales. Here, we explore the relationship between
some of those organizational patterns: two at the mesoscale (modularity and
in-block nestedness); and one at the macroscale (nestedness). We analytically
show that nestedness can be used to provide approximate bounds for modularity,
with exact results in an idealized scenario. Specifically, we show that
nestedness and modularity are antagonistic. Furthermore, we evince that
in-block nestedness provides a parsimonious transition between nested and
modular networks, taking properties of both. Far from a mere theoretical
exercise, understanding the boundaries that discriminate each architecture is
fundamental, to the extent modularity and nestedness are known to place heavy
constraints on the stability of several dynamical processes, specially in
ecology.Comment: 7 pages, 4 figures and 1 supplemental information fil
Complex Networks Unveiling Spatial Patterns in Turbulence
Numerical and experimental turbulence simulations are nowadays reaching the
size of the so-called big data, thus requiring refined investigative tools for
appropriate statistical analyses and data mining. We present a new approach
based on the complex network theory, offering a powerful framework to explore
complex systems with a huge number of interacting elements. Although interest
on complex networks has been increasing in the last years, few recent studies
have been applied to turbulence. We propose an investigation starting from a
two-point correlation for the kinetic energy of a forced isotropic field
numerically solved. Among all the metrics analyzed, the degree centrality is
the most significant, suggesting the formation of spatial patterns which
coherently move with similar vorticity over the large eddy turnover time scale.
Pattern size can be quantified through a newly-introduced parameter (i.e.,
average physical distance) and varies from small to intermediate scales. The
network analysis allows a systematic identification of different spatial
regions, providing new insights into the spatial characterization of turbulent
flows. Based on present findings, the application to highly inhomogeneous flows
seems promising and deserves additional future investigation.Comment: 12 pages, 7 figures, 3 table
Complex patterns of local adaptation in teosinte
Populations of widely distributed species often encounter and adapt to
specific environmental conditions. However, comprehensive characterization of
the genetic basis of adaptation is demanding, requiring genome-wide genotype
data, multiple sampled populations, and a good understanding of population
structure. We have used environmental and high-density genotype data to
describe the genetic basis of local adaptation in 21 populations of teosinte,
the wild ancestor of maize. We found that altitude, dispersal events and
admixture among subspecies formed a complex hierarchical genetic structure
within teosinte. Patterns of linkage disequilibrium revealed four mega-base
scale inversions that segregated among populations and had altitudinal clines.
Based on patterns of differentiation and correlation with environmental
variation, inversions and nongenic regions play an important role in local
adaptation of teosinte. Further, we note that strongly differentiated
individual populations can bias the identification of adaptive loci. The role
of inversions in local adaptation has been predicted by theory and requires
attention as genome-wide data become available for additional plant species.
These results also suggest a potentially important role for noncoding
variation, especially in large plant genomes in which the gene space represents
a fraction of the entire genome
Species Abundance Patterns in Complex Evolutionary Dynamics
An analytic theory of species abundance patterns (SAPs) in biological
networks is presented. The theory is based on multispecies replicator dynamics
equivalent to the Lotka-Volterra equation, with diverse interspecies
interactions. Various SAPs observed in nature are derived from a single
parameter. The abundance distribution is formed like a widely observed
left-skewed lognormal distribution. As the model has a general form, the result
can be applied to similar patterns in other complex biological networks, e.g.
gene expression.Comment: 4 pages, 3 figures. Physical Review Letters, in pres
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