Computational convergence of the path integral for real dendritic morphologies

Neurons are characterised by a morphological structure unique amongst biological cells, the core of which is the dendritic tree. The vast number of dendritic geometries, combined with heterogeneous properties of the cell membrane, continue to challenge scientists in predicting neuronal input-output relationships, even in the case of sub-threshold dendritic currents. The Green’s function obtained for a given dendritic geometry provides this functional relationship for passive or quasi-active dendrites and can be constructed by a sum-over-trips approach based on a path integral formalism. In this paper, we introduce a number of efficient algorithms for realisation of the sum-over-trips framework and investigate the convergence of these algorithms on different dendritic geometries. We demonstrate that the convergence of the trip sampling methods strongly depends on dendritic morphology as well as the biophysical properties of the cell membrane. For real morphologies, the number of trips to guarantee a small convergence error might become very large and strongly affect computational efficiency. As an alternative, we introduce a highly-efficient matrix method which can be applied to arbitrary branching structures

Crustal seismic velocity responds to a magmatic intrusion and seasonal loading in Iceland's Northern Volcanic Zone.

Seismic noise interferometry is an exciting technique for studying volcanoes, providing a continuous measurement of seismic velocity changes (dv/v), which are sensitive to magmatic processes that affect the surrounding crust. However, understanding the exact mechanisms causing changes in dv/v is often difficult. We present dv/v measurements over 10 years in central Iceland, measured using single-station cross-component correlation functions from 51 instruments across a range of frequency bands. We observe a linear correlation between changes in dv/v and volumetric strain at stations in regions of both compression and dilatation associated with the 2014 Bárðarbunga-Holuhraun dike intrusion. Furthermore, a clear seasonal cycle in dv/v is modeled as resulting from elastic and poroelastic responses to changing snow thickness, atmospheric pressure, and groundwater level. This study comprehensively explains variations in dv/v arising from diverse crustal stresses and highlights the importance of deformation modeling when interpreting dv/v, with implications for volcano and environmental monitoring worldwide

Random Field and Random Anisotropy Effects in Defect-Free Three-Dimensional XY Models

Monte Carlo simulations have been used to study a vortex-free XY ferromagnet with a random field or a random anisotropy on simple cubic lattices. In the random field case, which can be related to a charge-density wave pinned by random point defects, it is found that long-range order is destroyed even for weak randomness. In the random anisotropy case, which can be related to a randomly pinned spin-density wave, the long-range order is not destroyed and the correlation length is finite. In both cases there are many local minima of the free energy separated by high entropy barriers. Our results for the random field case are consistent with the existence of a Bragg glass phase of the type discussed by Emig, Bogner and Nattermann.Comment: 10 pages, including 2 figures, extensively revise

Inverse analysis on two geotechnical works : a tunnel and a cavern

One of the major difficulties for geotechnical engineers during project phase is to estimate in a reliable way the mechanical parameters values of the adopted constitutive model. In project phase, they can be evaluated by laboratory and in situ tests. But, these tests lead to uncertainties due to the soil reworking and to local character of the test which is not representative of the soil mass. Moreover for in situ tests interpretation difficulties exist due to the non homogeneous character of the strain and stress fields applied to the soil mass. In order to reduce these uncertainties, geotechnical engineers can use inverse analysis processes during construction. This article shows the application of two of these processes (a deterministic and a probabilistic method) on convergence leveling measurements realized during the excavation of the Bois de Peu tunnel (France). Moreover, these two processes are also applied on displacements measured by inclinometers during the excavation of the hydroelectric powerhouse cavern Venda Nova II (Portugal). The two inverse analysis methods are coupled with two geotechnical software (CESAR-LCPC and FLAC3D) to identify soil parameters. Numerical and experimental results are compared.(undefined

Stocking activities for the Arctic charr in Lake Geneva: Genetic effects in space and time

Artificial stocking practices are widely used by resource managers worldwide, in order to sustain fish populations exploited by both recreational and commercial activities, but their benefits are controversial. Former practices involved exotic strains, although current programs rather consider artificial breeding of local fishes (supportive breeding). Understanding the complex genetic effects of these management strategies is an importan t challenge with economic and conservation implications, especially in the context of population declines. In this study, we focus on the declining Arctic charr (Salvelinus alpinus) population from Lake Geneva (Switzerland and France), which has initially been restocked with allochtonous fishes in the early eighties, followed by supportive breeding. In this context, we conducted a genetic survey to document the evolution of the genetic diversity and structure throughout the last 50 years, before and after the initiation of hatchery supplementation, using contemporary and historical samples. We show that the introduction of exotic fishes was associated with a genetic bottleneck in the 1980–1990s, a break of Hardy–Weinberg Equilibrium (HWE), a reduction in genetic diversity, an increase in genetic structure among spawning sites, and a change in their genetic composition. Together with better environmental conditions, three decades of subsequent supportive breeding using local fishes allowed to re-establish HWE and the initial levels of genetic variation. However, current spawning sites have not fully recovered their original genetic composition and were extensively h omogenized across the lake. Our study demonstrates the drastic genetic consequ e nces of different restocking tactics in a comprehensive spatiotemporal framewo rk and suggests that genetic alteration by nonlocal stocking may be partly reversible through supportive breeding. We recommend that conservation-based programs consider local diversity and implement adequate protocols to limit the genetic homogenization of this Arctic charr population

Dispelling the myths of online education: learning via the information superhighway

There continues to be a perception that online education is inferior to traditional education. In the U.S. online learning is more developed than in the U.K. This paper provides insights into a U.S. provision and takes a close look at what are perceived as weaknesses of on line learning and argues that these are not necessarily inherent weaknesses of this form of educational delivery. Then, results of two major studies, undertaken in the U.S. are provided comparing the effectiveness of online education to traditional education as perceived by current MBA students and past graduates. Results of these studies suggest that students of MBA modules and MBA graduates perceive the quality and effectiveness of online education to be similar to, if not higher than, the quality and effectiveness of traditional modules and programmes

Numerical analysis of the venda nova II hydroelectrical complex main caverns and backanalysis of the geomechanical parameters

Neste trabalho é apresentado um modelo numérico 3D para as cavernas principais do complexo da Venda Nova II considerando as diferentes fases de escavação. Os parâmetros geomecânicos do maciço granítico, para os modelos, foram calculados utilizando o software GEOPAT. Este programa é um sistema baseado em conhecimento que permite obter os parâmetros geomecânicos para a modelação de obras subterrâneas em maciços graníticos. Os resultados do modelo são comparados com os deslocamentos observados por intermédio de extensómetros ao longo das várias fases construtivas. Nesta comparação são utilizadas diversas ferramentas estatísticas para a validação dos resultados obtidos. Finalmente, é realizada a retroanálise de parâmetros geomecânicos do maciço rochoso utilizando um algoritmo de optimização inovador. Os resultados obtidos permitiram concluir que a estimativa inicial dos parâmetros apresenta uma precisão bastante aceitável e que o algoritmo possui uma boa robustez e eficiência.In this work a 3D numerical model for the main caverns of the Venda Nova II complex which considers the different excavation stages is presented. The geomechanical parameters for the numerical models were computed using GEOPAT. This software is a knowledge based system which allows evaluating geomechanical parameters for the modelling of underground works built in granite rock masses. The results are compared with the observed displacements measured with extensometers along the different construction stages. In this comparison several statistical tools are used for the validation of the obtained results. Finally, the backanalysis of geomechanical parameters of the interested rock mass is performed using an innovative optimization algorithm. The results show that the initial evaluation of the parameters presents a very acceptable accuracy and that the algorithm posses good robustness and efficiency.Fundação para a Ciência e a Tecnologia (FCT) - Projecto POCI/ECM/57495/2004, intitulado Geotechnical Risk in Tunnels for High Speed Trains.Faculdade de Ciências e Tecnologia da Universidade de Coimbra (FCT-UC

Numerical analysis of the Venda Nova II powerhouse complex

In the North of Portugal a hydroelectric scheme called Venda Nova II was recently built in order to optimize the resources of the reservoirs created by Venda Nova and Salamonde dams. The scheme, almost fully composed by underground structures and built in a predominantly granite rock mass, include several tunnels with a total length of about 7.5km, inclined and vertical shafts with a total length of 750m and two caverns which compose the powerhouse complex. The complex consists of two caverns interconnected by two galleries at a dept of about 350m. For this complex, 2D and 3D numerical models were developed considering the different construction stages. The geomechanical parameters of the granite formation for the numerical models were obtained using GEOPAT. This software is a knowledge based system which allows obtaining geomechanical parameters for underground structures modelling in granite formations. The 2D model was developed in the Phases2 software while the 3D model in FLAC3D. In this paper results of these models are analysed. Some comparisons are carried out between the models results and the monitored data. The numerical results show in general a good agreement with the monitored ones.Fundação para a Ciência e a Tecnologia (FCT) - projecto POCI/ECM/57495/2004 - Geotechnical Risk in Tunnels for High Speed Trains

Fermi Surface as the Driving Mechanism for Helical Antiferromagnetic Ordering in Gd-Y Alloys

The first direct experimental evidence for the Fermi surface (FS) driving the helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported. The presence of a FS sheet capable of nesting is revealed, and the nesting vector associated with the sheet is found to be in excellent agreement with the periodicity of the helical ordering.Comment: 4 pages, 4 figure

Modeling oscillatory Microtubule--Polymerization

Polymerization of microtubules is ubiquitous in biological cells and under certain conditions it becomes oscillatory in time. Here simple reaction models are analyzed that capture such oscillations as well as the length distribution of microtubules. We assume reaction conditions that are stationary over many oscillation periods, and it is a Hopf bifurcation that leads to a persistent oscillatory microtubule polymerization in these models. Analytical expressions are derived for the threshold of the bifurcation and the oscillation frequency in terms of reaction rates as well as typical trends of their parameter dependence are presented. Both, a catastrophe rate that depends on the density of {\it guanosine triphosphate} (GTP) liganded tubulin dimers and a delay reaction, such as the depolymerization of shrinking microtubules or the decay of oligomers, support oscillations. For a tubulin dimer concentration below the threshold oscillatory microtubule polymerization occurs transiently on the route to a stationary state, as shown by numerical solutions of the model equations. Close to threshold a so--called amplitude equation is derived and it is shown that the bifurcation to microtubule oscillations is supercritical.Comment: 21 pages and 12 figure