A modeling study predicts the presence of voltage gated Ca2+ channels on myelinated central axons

The objective of this current study was to investigate whether voltage gated Ca(2+) channels are present on axons of the adult rat optic nerve (RON). Simulations of axonal excitability using a Hodgkin-Huxley based one-compartment model incorporating I(Na), I(K) and leak currents were used to predict conditions under which the potential contribution of a Ca(2+) current to an evoked action potential could be measured. Under control conditions the inclusion of a high threshold Ca(2+) current (I(Ca)) in the model had a negligible effect on the action potential. Reducing I(K), by decreasing the value of g(K), elongated the repolarizing phase of the action potential, increasing its duration. Subsequent incorporation of I(Ca) in the model revealed a significant I(Ca) contribution to the repolarizing phase of the action potential. The simulation thus suggests that Ca(2+) channels may be present on RON axons, but that pharmacological intervention is required to unmask their presence. Experiments based on the simulations revealed that there was no significant contribution of I(Ca) to the control action potential. However, as predicted by the simulation, reducing the repolarizing effect of I(K) by adding the K(+) channel blocker 4-AP revealed a Ca(2+) component on the repolarizing phase of the action potential that was blocked by the Ca(2+) channel inhibitor nifedipine.     

有源箝位型三电平变频器功率器件损耗分析

针对传统中点箝位型三电平变频器运行过程中存在功率器件损耗分布不均匀问题,介绍了有源箝位型三电平拓扑的工作原理和基本调制策略,分析了有源箝位型三电平变频器的损耗特性。有源箝位型三电平拓扑使用双向功率器件取代传统中点箝位型三电平中的箝位二极管,通过选择恰当的零电压状态组合,可实现功率器件损耗均衡的目的。仿真结果表明,在各种典型工作状态下,相比于中点箝位型三电平拓扑,有源箝位型拓扑可更好地平衡功率器件内外管损耗,提升了变频器容量。     

基于服务质量的LTE增值业务下行跨层调度算法

针对LTE系统如何实现不同增值业务用户速率的问题,提出一种优化型比例公平(PF)算法。该算法在用户签约速率不能实现时,考虑信道条件、付费等级和满意度,同时通过引入业务服务质量(QoS)特征函数合理调度付费用户,从而实现各付费等级对应的用户速率。在Matlab环境中进行仿真,其中优化型算法在满意度、有效吞吐量方面优于传统PF算法。与PF算法相比,优化型算法使付费用户间满意度均值相差约26%,有效吞吐量均值提升约17%。仿真结果表明,优化型算法在保证多业务服务质量的前提下,可以实现各付费用户的感知平均速率,保障不同付费用户满意度,并提高系统有效吞吐量。     

A general-purpose coarse-grained molecular dynamics program

In this article, we describe a general-purpose coarse-grained molecular dynamics program COGNAC (COarse Grained molecular dynamics program by NAgoya Cooperation). COGNAC has been developed for general molecular dynamics simulation, especially for coarse-grained polymer chain models. COGNAC can deal with general molecular models, in which each molecule consists of coarse-grained atomic units connected by chemical bonds. The chemical bonds are specified by bonding potentials for the stretching, bending and twisting of the bonds, each of which are the functions of the position coordinates of the two, three and four atomic units. COGNAC can deal with both isotropic and anisotropic interactions between the non-bonded atomic units. As an example, the Gay-Berne potential is implemented. New potential functions can be added to the list of existing potential functions by users. COGNAC can do simulations for various situations such as under constant temperature, under constant pressure, under shear and elongational deformation, etc. Some new methods are implemented in COGNAC for modeling multiphase structures of polymer blends and block copolymers. A density biased Monte Carlo method and a density biased potential method can generate equilibrium chain configurations from the results of the self-consistent field calculations. Staggered reflective boundary conditions can generate interfacial structures with smaller system size compared with those of periodic boundary conditions.     

Microcomputer program for automated action potential waveform analysis

A program for action potential waveform analysis based on a PC compatible computer is described. Single or averaged action potentials are analyzed by obtaining its first derivative and using criteria which allow automatic measurement of several action potential components, including: depolarization rate, repolarization rate, amplitude, duration, resting membrane potential and afterhyperpolarization amplitude and slope. Data can be imported from pClamp (Axon Instruments) and exported to other software such as Excel, Sigmaplot and MatLab for example.     

Numerical integration in the reconstruction of cardiac action potentials using Hodgkin-Huxley-type models

A comparison between traditional numerical integration methods and a new hybrid integration method for the reconstruction of action potential activity is presented, using a mathematical model of the cardiac Purkinje fiber (MNT model). It is shown that the hybrid integration method reduces importantly the overall computation time required for solving the Hodgkin-Huxley differential equations describing membrane electrical events. To accomplish this, the particular form of the gating variable equations is exploited to reformulate the step-by-step computation. In this way, the time increment can be made much larger compared with traditional methods when the membrane potential changes slowly. A mathematical analysis of the hybrid integration method is presented also, together with a numerical verification of its performance both for the propagated and nonpropagated membrane action potential. It is shown that the local error, that is the error arising at each integration step, and the cumulative integration error are strictly controlled by the membrane potential offset. Using the MNT model, the nonpropagated cardiac Purkinje action potential can be reconstructed in real time with an accuracy of 1% for the potential and 5% for the time of occurrence of its main features. In reconstructing propagated events, the hybrid integration method allows computation time savings by a factor of 10 or more compared to accurate Runge-Kutta schemes.     

Spike detection, characterization, and discrimination using feature analysis software written in LabVIEW

Rapid and accurate discrimination of single units from extracellular recordings is a fundamental process for the analysis and interpretation of electrophysiological recordings. We present an algorithm that performs detection, characterization, discrimination, and analysis of action potentials from extracellular recording sessions. The program was entirely written in LabVIEW (National Instruments), and requires no external hardware devices or a priori information about action potential shapes. Waveform events are detected by scanning the digital record for voltages that exceed a user-adjustable trigger. Detected events are characterized to determine nine different time and voltage levels for each event. Various algebraic combinations of these waveform features are used as axis choices for 2-D Cartesian plots of events. The user selects axis choices that generate distinct clusters. Multiple clusters may be defined as action potentials by manually generating boundaries of arbitrary shape. Events defined as action potentials are validated by visual inspection of overlain waveforms. Stimulus-response relationships may be identified by selecting any recorded channel for comparison to continuous and average cycle histograms of binned unit data. The algorithm includes novel aspects of feature analysis and acquisition, including higher acquisition rates for electrophysiological data compared to other channels. The program confirms that electrophysiological data may be discriminated with high-speed and efficiency using algebraic combinations of waveform features derived from high-speed digital records.     

Revised formulation of the Hodgkin-Huxley representation of the sodium current in cardiac cells

The purpose of this paper is to revise the parameters of the Hodgkin-Huxley formulation for the Na+ current in ventricular myocardial cells. To this end we have assembled much of the recent voltage clamp data on cardiac preparations obtained with modern voltage clamp and patch clamp techniques. The selected activation and inactivation characteristics of the Na+ channel and other membrane parameters represent a good compromise between available experimental measurements and lead to a reasonable average representation of the cardiac Na+ membrane current. The resulting Na+ conductance changes during the action potential upstroke are much larger than in earlier models, so that the upstroke is much faster and the peak depolarization is close to the Na+ equilibrium potential. The firing threshold level is nearly constant for resting potentials in the range of -70 and -90 mV. The maximum rate of rise of the action potential displayed by the new model is quite comparable to experimental observations.     

一种基于网络编码的两用户自适应AF协同ARQ方案

提出了一种基于网络编码的两用户自适应放大前传AF协同ARQ(Automatic Repeat reQuest)方案。该方案选择当前信道条件较好的用户采用网络编码方式进行数据重传,能够在一定概率下节省一个重传时隙,并且能够通过协同选择分集的方式对抗衰落以降低误帧率,从而实现数据吞吐率的提升。仿真结果证明,该方案与非协同ARQ及普通AF协同ARQ相比均能够较大地提高各用户的传输吞吐率。     

A MOSFET-based model of a class 2 nerve membrane

We have constructed a nerve membrane using MOSFET circuitry, which can be a basic element of an FET-based neural system. Its mechanism of action potentials generation is designed to reproduce that of the Hodgkin-Huxley equations. The responses to singlet, doublet, repetitive pulse, and sustained stimuli are analyzed to show that it exhibits similar properties to the Hodgkin-Huxley equations; namely, 1) excitable dynamics with generation of action potentials, 2) the existence of a chaotic response to periodic stimuli, and 3) Class 2 excitability. It is known that Class 2 excitability is generated by an inverted Hopf bifurcation. We have applied Hopf bifurcation theory to our nerve membrane's system equations and have shown a routine for ascertaining whether a certain parameter set generates an inverted Hopf bifurcation.     

Optical diffraction intensity of computer-simulated muscle fibers

A computer program which calculates the Fraunhofer diffraction intensity of a simulated skeletal muscle fiber is described. The intensity calculation contains several adjustable parameters. Some of the parameters are related to the myofibrillar arrangement within the muscle fiber and the angle of incident light. The variations of the left and right first-order diffraction intensities with different incident angles are determined. Their profiles exhibit features related to Bragg reflections from planes formed by the myofibrils. The calculations provide the relationship between the observed intensity profile and the myofibrillar arrangement in a real muscle fiber.     

兴奋在有髓鞘神经纤维上传递速度的计算模型

神经系统的信号传递是学习、记忆、感觉、运动等各种脑正常功能的基础，因此它的理论也是神经精神病理学的基础。但是，到目前为止对于兴奋在有髓鞘神经纤维上传递速度的定量分析却很少。该文以有髓鞘神经纤维为研究对象，通过分析动作电位的传递机制，得出了可用跨膜电流的传递速度来等效动作电位的传递速度的结论。根据McNeal模型和CRRSS模型推导出跨膜电流的时间函数，最后给出了动作电位在有髓鞘神经纤维上传递速度的计算模型。     

Video placement and configuration of distributed video servers on cable TV networks

A large-scale, distributed video-on-demand (VOD) system allows geographically dispersed residential and business users to access video services, such as movies and other multimedia programs or documents on demand from video servers on a high-speed network. In this paper, we first demonstrate through analysis and simulation the need for a hierarchical architecture for the VOD distribution network.We then assume a hierarchical architecture, which fits the existing tree topology used in today's cable TV (CATV) hybrid fiber/coaxial (HFC) distribution networks. We develop a model for the video program placement, configuration, and performance evaluation of such systems. Our approach takes into account the user behavior, the fact that the user requests are transmitted over a shared channel before reaching the video server containing the requested program, the fact that the input/output (I/O) capacity of the video servers is the costlier resource, and finally the communication cost. In addition, our model employs batching of user requests at the video servers. We study the effect of batching on the performance of the video servers and on the quality of service (QoS) delivered to the user, and we contribute dynamic batching policies which improve server utilization, user QoS, and lower the servers' cost. The evaluation is based on an extensive analytical and simulation study.     

三电平NPC逆变器中点电压平衡混合调制策略

针对二极管中点钳位型(neutral point clamped,NPC)三电平逆变器直流侧电容电压不平衡问题,提出一种混合空间矢量脉宽调制技术(Space vector pulse width modulation,SVPWM)和虚拟空间矢量脉宽调制技术(Virtual space vector pulse widt...     

三电平脉冲整流器的仿真与分析

随着电力电子技术的发展,高功率因数,低谐波的PWM整流器也逐渐得到了应用。PWM整流器消除了传统整流电路（如二极管构成的不可控整流电路和晶闸管构成的相控整流电路）的谐波分量大、功率因数低且不可任意控制等缺点。当整流装置应用于高压场合时,需要使用高反压的功率开关管IGBT或将多个功率开关管串联使用,而三电平整流器[2]的每一个功率开关器件所承受的关断电压仅为直流侧电压的一半,因此在同等条件下直流母线电压可以提高一倍,故它比两电平更适合于高压、大功率应用场合。本实验以三电平中点箝位PWM整流器[2]为研究对象。     

基于隐马尔可夫模型的短波认知频率选择方法

针对短波频谱利用率低下及频率选择不够智能的局限性,提出一种基于隐马尔可夫模型(HMM)的短波认知频率选择方法。应用认知无线电原理,将短波传统用户作为主用户,将采用认知无线电技术的短波电台作为认知用户。首先,建立隐马尔可夫模型,结合频谱感知历史数据预测主用户信道状态;其次,在预测空闲的基础上估计信道参数;最后,根据估计的信道参数选择最优频率。仿真结果表明,所提方法能够准确预测传统短波用户信道状态,快速估计信道参数。在设定的仿真条件下,所提方法的成功传输率分别较HMM预测和能量感知随机信道选择方法有5.54%和10.56%的提升,能够选择最优信道。     

基于HH模型的心肌细胞电模型

提出了一种心肌细胞电生理模型的建模仿真方法。通过借助Hodgkin-Huxley模型对单个心肌细胞建立细胞膜的等效电路模型,利用四阶Rouge-Kutta算法,研究并分析了心肌细胞膜内外离子电流及电位差变化。然后用C语言完成了对细胞膜等效电路模型的编程,利用Matlab软件平台进行了计算机仿真,得到了心肌细胞电生理学模型在不同刺激下的仿真实验结果。     

Using Vmax to estimate changes in the sodium membrane conductance in cardiac cells

Relative changes in the sodium conductance of the resting cardiac cell membrane are often estimated from relative changes in the maximum rate of rise of the action potential (Vmax). This approach has given rise to some controversy and it has not been possible so far to test it directly on an experimental basis. We have examined here the validity of this estimation using three different Hodgkin-Huxley representations of the cardiac membrane sodium current. The two basic requirements are a constant membrane capacitance and a negligible relative value of the nonsodium membrane currents at the time of Vmax. It is shown further that the approach leads to a satisfactory estimation only when the latency of Vmax is kept constant and a correction factor for the sodium driving force is applied to Vmax measurements. This conclusion applies either to a nonpropagated action or to an action potential propagated at constant velocity, provided that the membrane is not too strongly depolarized. It is valid for a wide range of sodium equilibrium potentials and a range of maximum sodium conductances limited to about 50% of the nominal value.     

The muscle fatigue fiber model

The aim of this study was to integrate the effect of muscle fatigue into the skeletal muscle fiber model, which was previously developed in our laboratory and was based on the modified Huxley equation with the calcium activation taken into account. The cellular mechanisms such as (i) the attachment of myosin heads to the actin binding sites, (ii) the actin filament activation, (iii) the sensitivity of myofilaments to calcium ions and (iv) the calcium handling; which influence the force generated by the muscle fiber and are affected during fatigue, became the base stone for the development of the muscle fatigue fiber model.