1,367 research outputs found
Channel Density Regulation of Firing Patterns in a Cortical Neuron Model
AbstractModifying the density and distribution of ion channels in a neuron (by natural up- and downregulation or by pharmacological intervention or by spontaneous mutations) changes its activity pattern. In this investigation we analyzed how the impulse patterns are regulated by the density of voltage-gated channels in a neuron model based on voltage-clamp measurements of hippocampal interneurons. At least three distinct oscillatory patterns, associated with three distinct regions in the Na-K channel density plane, were found. A stability analysis showed that the different regions are characterized by saddle-node, double-orbit, and Hopf-bifurcation threshold dynamics, respectively. Single, strongly graded action potentials occur in an area outside the oscillatory regions, but less graded action potentials occur together with repetitive firing over a considerable range of channel densities. The relationship found here between channel densities and oscillatory behavior may partly explain the difference between the principal spiking patterns previously described for crab axons (class 1 and 2) and cortical neurons (regular firing and fast spiking)
Conjunctors and their residual implicators: characterizations and construction methods
In many practical applications of fuzzy logic it seems clear that one needs more flexibility
in the choice of the conjunction: in particular, the associativity and the commutativity of
a conjunction may be removed. Motivated by these considerations, we present several classes
of conjunctors, i.e. binary operations on that are used to extend the boolean conjunction
from to , and characterize their respective residual implicators. We establish
hence a one-to-one correspondence between construction methods for conjunctors and construction
methods for residual implicators. Moreover, we introduce some construction methods directly in the class
of residual implicators, and, by using a deresiduation procedure, we obtain new conjunctors
Cancer Therapy Targeting the HER2-PI3K Pathway: Potential Impact on the Heart
The HER2-PI3K pathway is the one of the most mutated pathways in cancer. Several drugs targeting the major kinases of this pathway have been approved by the Food and Drug Administration and many are being tested in clinical trials for the treatment of various cancers. However, the HER2-PI3K pathway is also pivotal for maintaining the physiological function of the heart, especially in the presence of cardiac stress. Clinical studies have shown that in patients treated with doxorubicin concurrently with Trastuzumab, a monoclonal antibody that blocks the HER2 receptor, the New York Heart Association class III/IV heart failure was significantly increased compared to those who were treated with doxorubicin alone (16 vs. 3%). Studies in transgenic mice have also shown that other key kinases of this pathway, such as PI3Kα, PDK1, Akt, and mTOR, are important for protecting the heart from ischemia-reperfusion and aortic stenosis induced cardiac dysfunction. Studies, however, have also shown that inhibition of PI3Kγ improve cardiac function of a failing heart. In addition, results from transgenic mouse models are not always consistent with the outcome of the pharmacological inhibition of this pathway. Here, we will review these findings and discuss how we can address the cardiac side-effects caused by inhibition of this important pathway in both cancer and cardiac biology
Copulas constructed from horizontal sections
In analogy with the study of copulas whose diagonal sections have
been fixed, we study the set of copulas for which a horizontal section has been given. We first show that this set is not empty, by explicitly writing one such copula, which we call \textit{horizontal copula}. Then we find the copulas that
bound both below and above the set . Finally we determine the expressions for Kendall's tau and Spearman's rho for the horizontal and the bounding copulas
Distorted Copulas: Constructions and Tail Dependence
Given a copula C, we examine under which conditions on an order isomorphism ψ of [0, 1] the distortion C ψ: [0, 1]2 → [0, 1], C ψ(x, y) = ψ{C[ψ−1(x), ψ−1(y)]} is again a copula. In particular, when the copula C is totally positive of order 2, we give a sufficient condition on ψ that ensures that any distortion of C by means of ψ is again a copula. The presented results allow us to introduce in a more flexible way families of copulas exhibiting different behavior in the tails
A naked-eye triple system with a nonaccreting black hole in the inner binary
Several dozen optical echelle spectra demonstrate that HR 6819 is a
hierarchical triple. A classical Be star is in a wide orbit with an
unconstrained period around an inner 40 d binary consisting of a B3 III star
and an unseen companion in a circular orbit. The radial-velocity semi-amplitude
of 61.3 km/s of the inner star and its minimum (probable) mass of 5.0 Msun (6.3
+- 0.7 Msun) imply a mass of the unseen object of >= 4.2 Msun (>= 5.0 +- 0.4
Msun), that is, a black hole (BH). The spectroscopic time series is stunningly
similar to observations of LB-1. A similar triple-star architecture of LB-1
would reduce the mass of the BH in LB-1 from ~70 Msun to a level more typical
of Galactic stellar remnant BHs. The BH in HR 6819 probably is the closest
known BH to the Sun, and together with LB-1, suggests a population of quiet
BHs. Its embedment in a hierarchical triple structure may be of interest for
models of merging double BHs or BH + neutron star binaries. Other triple stars
with an outer Be star but without BH are identified; through stripping, such
systems may become a source of single Be stars.Comment: Accepted as Letter to the Editor for A&
Identifying stellar streams in the 1st RAVE public data release
We searched for and detected stellar streams or moving groups in the solar
neighbourhood, using the data provided by the 1st RAVE public data release.
This analysis is based on distances to RAVE stars estimated from a
color-magnitude relation that was calibrated on Hipparcos stars. Our final
sample consists of 7015 stars selected to be within 500 pc of the Sun and to
have distance errors better than 25%. Together with radial velocities from RAVE
and proper motions from various data bases, there are estimates for all 6
phase-space coordinates of the stars in the sample. We characterize the orbits
of these stars through suitable proxies for their angular momentum and
eccentricity, and compare the observed distribution to the expectations from a
smooth distribution. On this basis we identify at least four "phase space
overdensities" of stars on very similar orbits in the Solar neighbourhood. We
estimate the statistical significance of these overdensities by Monte Carlo
simulations. Three of them have been identified previously: the Sirius and
Hercules moving group and a stream found independently in 2006 by Arifyanto and
Fuchs and Helmi et al. In addition, we have found a new stream candidate on a
quite radial orbit, suggesting an origin external to the Milky Way's disk.
Also, there is evidence for the Arcturus stream and the Hyades-Pleiades moving
group in the sample. This analysis, using only a minute fraction of the final
RAVE data set, shows the power of this experiment to probe the phase-space
substructure of stars around the Sun.Comment: accepted for publication in Ap
On the diffraction pattern of C60 peapods
We present detailed calculations of the diffraction pattern of a powder of
bundles of C peapods. The influence of all pertinent structural
parameters of the bundles on the diffraction diagram is discussed, which should
lead to a better interpretation of X-ray and neutron diffraction diagrams. We
illustrate our formalism for X-ray scattering experiments performed on peapod
samples synthesized from 2 different technics, which present different
structural parameters. We propose and test different criteria to solve the
difficult problem of the filling rate determination.Comment: Sumitted 19 May 200
Using spectroscopic data to disentangle stellar population properties
It is well known that, when analyzed in the light of current synthesis model predictions, variations in the physical properties of single stellar populations (e.g. age, metallicity, initial mass function, element abundance ratios) may have a similar effect in their integrated spectral energy distributions. The confusion is even worsened when more realistic scenarios, i.e. composite star formation histories, are considered. This is, in fact, one of the major problems when facing the study of stellar populations in star clusters and galaxies. Typically, the observational efforts have aimed to find the most appropriate spectroscopic indicators in order to avoid, as far as possible, degeneracies in the parameter space. However, from a practical point of view, the most suited observables are not, necessarily, those that provide more orthogonality in that parameter space, but those that give the best balance between parameter degeneracy and sensitivity to signal-to-noise ratio per Angstrom, S/N(Angstrom). In order to achieve the minimum combined total error in the derived physical parameters, this work discusses how the functional dependence of typical line-strength indices and colors on S/N(Angstrom) allows to define a suitability parameter which helps to obtain more realistic combinations of spectroscopic data. As an example, we discuss in more detail the problem of breaking the well known age-metallicity degeneracy in relatively old stellar populations, comparing the suitability of different spectroscopic diagrams for a simple stellar population of solar metallicity and of 12 Gyr in age
Bis(triphenylguanidinium) tetrachloridocuprate(II)
The structure of the title compound, (C19H18N3)2[CuCl4], consists of square-planar [CuCl4]2− anions and triphenylguanidinium cations. The CuII ion occupies a crystallographic inversion centre. In the cation, the dihedral angles between the phenyl rings and the plane defined by the central guanidinium fragment are in the range 51.9 (4)–64.4 (3)°. N—H⋯Cl hydrogen bonds assemble the ions into infinite chains running along the b axis
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