Stochastic resonance as a collective property of ion channel assemblies

By use of a stochastic generalization of the Hodgkin-Huxley model we investigate both the phenomena of stochastic resonance (SR) and coherence resonance (CR) in variable size patches of an excitable cell membrane. Our focus is on the challenge how internal noise stemming from individual ion channels does affect collective properties of the whole ensemble. We investigate both an unperturbed situation with no applied stimuli and one in which the membrane is stimulated externally by a periodic signal and additional external noise. For the nondriven case, we demonstrate the existence of an optimal size of the membrane patch for which the internal noise causes a most regular spike activity. This phenomenon shall be termed intrinsic CR. In presence of an applied periodic stimulus we demonstrate that the signal-to-noise ratio (SNR) exhibits SR vs. decreasing patch size, or vs. increasing internal noise strength, respectively. Moreover, we demonstrate that conventional SR vs. the external noise intensity occurs only for sufficiently large membrane patches, when the intensity of internal noise is below its optimal level. Thus, biological SR seemingly is rooted in the collective properties of large ion channel ensembles rather than in the individual stochastic dynamics of single ion channels.Comment: 9 pages, 2 figure

Effective rate equations for the over-damped motion in fluctuating potentials

We discuss physical and mathematical aspects of the over-damped motion of a Brownian particle in fluctuating potentials. It is shown that such a system can be described quantitatively by fluctuating rates if the potential fluctuations are slow compared to relaxation within the minima of the potential, and if the position of the minima does not fluctuate. Effective rates can be calculated; they describe the long-time dynamics of the system. Furthermore, we show the existence of a stationary solution of the Fokker-Planck equation that describes the motion within the fluctuating potential under some general conditions. We also show that a stationary solution of the rate equations with fluctuating rates exists.Comment: 18 pages, 2 figures, standard LaTeX2

Stochastic Resonance in Ion Channels Characterized by Information Theory

We identify a unifying measure for stochastic resonance (SR) in voltage dependent ion channels which comprises periodic (conventional), aperiodic and nonstationary SR. Within a simplest setting, the gating dynamics is governed by two-state conductance fluctuations, which switch at random time points between two values. The corresponding continuous time point process is analyzed by virtue of information theory. In pursuing this goal we evaluate for our dynamics the tau-information, the mutual information and the rate of information gain. As a main result we find an analytical formula for the rate of information gain that solely involves the probability of the two channel states and their noise averaged rates. For small voltage signals it simplifies to a handy expression. Our findings are applied to study SR in a potassium channel. We find that SR occurs only when the closed state is predominantly dwelled. Upon increasing the probability for the open channel state the application of an extra dose of noise monotonically deteriorates the rate of information gain, i.e., no SR behavior occurs.Comment: 10 pages, 2 figures, to appear in Phys. Rev.

Femoral Stem Displacement in a Patient Suffering Recurrent Dislocations After Hip Hemiarthroplasty: Case Report

Displacement of the femoral component during attempt to closed reduction of a dislocated hip arthroplasty is an exceptionally rare, catastrophic event, which renders operative management obligatory. We report the proximal migration of a femoral stem during attempt to closed reduction in a patient with recurrent postoperative dislocations after hip hemiarthroplasty, and describe successful management by conversion to a standard total hip arthroplasty, retaining the same stem in the existing cement mantle. This illustrative case is reported not only as an extremely rare event, but also to highlight and discuss pitfalls and efficient measures in the management of this complex issue

Optically levitated nanoparticle as a model system for stochastic bistable dynamics

Nano-mechanical resonators have gained an increasing importance in nanotechnology owing to their contributions to both fundamental and applied science. Yet, their small dimensions and mass raises some challenges as their dynamics gets dominated by nonlinearities that degrade their performance, for instance in sensing applications. Here, we report on the precise control of the nonlinear and stochastic bistable dynamics of a levitated nanoparticle in high vacuum. We demonstrate how it can lead to efficient signal amplification schemes, including stochastic resonance. This work contributes to showing the use of levitated nanoparticles as a model system for stochastic bistable dynamics, with applications to a wide variety of fields.inancial support from the ERC- QnanoMECA (Grant No. 64790), the Spanish Ministry of Economy and Competitiveness, under grant FIS2016-80293-R and through the ‘Severo Ochoa’ Programme for Centres of Excellence in R&D (SEV-2015-0522), Fundació Privada CELLEX and from the CERCA Programme/Generalitat de Catalunya. J.G. has been supported by H2020-MSCA-IF-2014 under REA grant Agreement No. 655369. L.R. acknowledges support from an ETH Marie Curie Cofund Fellowship

Neutralization Tests of a mN FEEP Thruster

Neutralization of the ion beam produced by a FEEP thruster is in principle a task similar to the well-known problem of neutralizing a traditional ion rocket.On the other hand, the neutralization of a small, micronewton thrust level FEEP poses several practical problems. The neutralizer has to produce a very low electron current, that must couple with the very fast ions exhausted by the thruster. As the envisaged use of FEEP is based on its fine throttling capabilities, the neutralizer has to be proportionately able to follow the variations of the ion current, with the same rapidity. A first experimental assessment of the problem has been recently performed at Centrospazio. A series of test has been carried out on a 3 cm emitter, together with two neutralizers, namely a plasma bridge hollow cathode and a hot filament. The hollow cathode, developed by SEP, was fed by gaseous cæsium. A thoriated tungsten filament was also tested. The thruster and neutralizer operational parameters have been recorded, including the coupling potential between the neutralizer and the ion beam. In order to measure this potential, the thruster assembly was electrically isolated from the vacuum facility and kept floating during the tests. This paper presents the experimental setup and the test results. A critical analysis of the data gathered is provided, aimed at the identification of the optimal technology for a flight model

E-learning, ethics and 'non-traditional' students: space to think aloud

This paper considers the piloting of an online learning component of a final-year social work degree ethics module at an inner-city English university. An Action Research approach was used to evaluate this pilot project and the paper illustrates how students were involved in developing and designing the teaching programme as part of the Action Research cycle. The paper explores theoretical aspects of e-learning pedagogy through an analysis of issues emerging during the planning and delivery of this pilot project. The cognitive and affective impact of both face-to-face and electronic learning is discussed. It is suggested that some students may experience e-learning as a safe space for thinking about ethical dilemmas. The challenges for teachers and learners are highlighted and the author argues that in order to support student learning, online teaching needs to be carefully designed and appropriately resource

The Victoria Climbié case: social work education for practice in children and families' work before and since

Based on their research into the Victoria Climbié Inquiry, the authors outline an analysis they undertook of literature which had made substantial comment on either the inquiry itself and/or the subsequent inquiry report. An overview of 18 publications is provided, with four categories of themes emerging. These themes are outlined and then connected with concerns the authors of the current paper identified in journal papers written 10 years or more ago. It is argued that the gap between recognition that society needs competent, well-trained and skilled social work and other professionals to safeguard the lives of children and families and understanding of what education, training and employment support mechanisms are necessary in order for workers to become and remain well-trained, skilled and effective, remains as wide as ever. The paper concludes with an outline of teaching approaches the current authors have adopted in pursuit of the kinds of learning opportunities they believe need to be put in place to improve professional practice in children and families' work. These include creative use of child abuse inquiry reports themselves, role plays and simulations and workshops designed to enhance critical reflection skills