Oscillatory Nature (oscillatory + nature)

Distribution by Scientific Domains

Selected Abstracts

Oscillatory nature of human basal ganglia activity: Relationship to the pathophysiology of Parkinson's disease

Peter Brown MD
Abstract Alterations of basal ganglia physiology in parkinsonism may consist of two elements, an increase in the firing rate of neurones and a change in the pattern of synchronisation of discharges between neurones. Recent findings suggest the presence of two principal modes of synchronised activity within the human subthalamo-pallidal-thalamo-cortical circuit, at <30 Hz and >60 Hz. These oscillations are dynamically and systematically modulated by task, thereby suggesting a functional role in movement. More importantly, the two frequency modes are inversely affected by movement, consistent with opposing actions, and differentially expressed according to the prevailing level of dopaminergic activity. It is argued that the balance between these modes determines the effects of basal ganglia-thalamocortical projections on the motor areas of the cortex. The lower frequency oscillations facilitate slow idling rhythms in the motor areas of the cortex, whereas synchronisation at high frequency restores dynamic task-related cortical ensemble activity in the gamma band. 2002 Movement Disorder Society [source]

Surface deformation due to loading of a layered elastic half-space: a rapid numerical kernel based on a circular loading element

E. Pan
SUMMARY This study is motivated by a desire to develop a fast numerical algorithm for computing the surface deformation field induced by surface pressure loading on a layered, isotropic, elastic half-space. The approach that we pursue here is based on a circular loading element. That is, an arbitrary surface pressure field applied within a finite surface domain will be represented by a large number of circular loading elements, all with the same radius, in which the applied downwards pressure (normal stress) is piecewise uniform: that is, the load within each individual circle is laterally uniform. The key practical requirement associated with this approach is that we need to be able to solve for the displacement field due to a single circular load, at very large numbers of points (or ,stations'), at very low computational cost. This elemental problem is axisymmetric, and so the displacement vector field consists of radial and vertical components both of which are functions only of the radial coordinate r. We achieve high computational speeds using a novel two-stage approach that we call the sparse evaluation and massive interpolation (SEMI) method. First, we use a high accuracy but computationally expensive method to compute the displacement vectors at a limited number of r values (called control points or knots), and then we use a variety of fast interpolation methods to determine the displacements at much larger numbers of intervening points. The accurate solutions achieved at the control points are framed in terms of cylindrical vector functions, Hankel transforms and propagator matrices. Adaptive Gauss quadrature is used to handle the oscillatory nature of the integrands in an optimal manner. To extend these exact solutions via interpolation we divide the r -axis into three zones, and employ a different interpolation algorithm in each zone. The magnitude of the errors associated with the interpolation is controlled by the number, M, of control points. For M= 54, the maximum RMS relative error associated with the SEMI method is less than 0.2 per cent, and it is possible to evaluate the displacement field at 100 000 stations about 1200 times faster than if the direct (exact) solution was evaluated at each station; for M= 99 which corresponds to a maximum RMS relative error less than 0.03 per cent, the SEMI method is about 700 times faster than the direct solution. [source]

Partition of unity enrichment for bimaterial interface cracks

N. Sukumar
Abstract Partition of unity enrichment techniques are developed for bimaterial interface cracks. A discontinuous function and the two-dimensional near-tip asymptotic displacement functions are added to the finite element approximation using the framework of partition of unity. This enables the domain to be modelled by finite elements without explicitly meshing the crack surfaces. The crack-tip enrichment functions are chosen as those that span the asymptotic displacement fields for an interfacial crack. The concept of partition of unity facilitates the incorporation of the oscillatory nature of the singularity within a conforming finite element approximation. The mixed-mode (complex) stress intensity factors for bimaterial interfacial cracks are numerically evaluated using the domain form of the interaction integral. Good agreement between the numerical results and the reference solutions for benchmark interfacial crack problems is realized. Copyright 2004 John Wiley & Sons, Ltd. [source]

A numerical study of an unsteady laminar flow in a doubly constricted 3D vessel

B. V. Rathish Kumar
Abstract Unsteady flow dynamics in doubly constricted 3D vessels have been investigated under pulsatile flow conditions for a full cycle of period T. The coupled non-linear partial differential equations governing the mass and momentum of a viscous incompressible fluid has been numerically analyzed by a time accurate Finite Volume Scheme in an implicit Euler time marching setting. Roe's flux difference splitting of non-linear terms and the pseudo-compressibility technique employed in the current numerical scheme makes it robust both in space and time. Computational experiments are carried out to assess the influence of Reynolds' number and the spacing between two mild constrictions on the pressure drop across the constrictions. The study reveals that the pressure drop across a series of mild constrictions can get physiologically critical and is also found to be sensitive both to the spacing between the constrictions and the oscillatory nature of the inflow profile. The flow separation zone on the downstream constriction is seen to detach from the diverging wall of the constriction leading to vortex shedding with 3D features earlier than that on the wall in the spacing between the two constrictions. Copyright 2002 John Wiley & Sons, Ltd. [source]

NaCl-induced changes in cytosolic free Ca2+ in Arabidopsis thaliana are heterogeneous and modified by external ionic composition

ABSTRACT Increases in cytosolic free Ca2+ ([Ca2+]cyt) are common to many stress-activated signalling pathways, including the response to saline environments. We have investigated the nature of NaCl-induced [Ca2+]cyt signals in whole Arabidopsis thaliana seedlings using aequorin. We found that NaCl-induced increases in [Ca2+]cyt are heterogeneous and mainly restricted to the root. Both the concentration of NaCl and the composition of the solution bathing the root have profound effects on the magnitude and dynamics of NaCl-induced increases in [Ca2+]cyt. Alteration of external K+ concentration caused changes in the temporal and spatial pattern of [Ca2+]cyt increase, providing evidence for Na+ -induced Ca2+ influx across the plasma membrane. The effects of various pharmacological agents on NaCl-induced increases in [Ca2+]cyt indicate that NaCl may induce influx of Ca2+ through both plasma membrane and intracellular Ca2+ -permeable channels. Analysis of spatiotemporal [Ca2+]cyt dynamics using photon-counting imaging revealed additional levels of complexity in the [Ca2+]cyt signal that may reflect the oscillatory nature of NaCl-induced changes in single cells. [source]