横向电场作用下外周神经的兴奋模型

外周神经的兴奋机理研究是以神经生理学的电缆方程为基础,方程中的激励因子只与外电场沿神经方向的纵向电场分量有关。但实验表明垂直于神经走向的横向电场也可以兴奋外周神经,因而传统电缆方程不能客观反映外周神经兴奋实际过程而需要改进。本文基于两阶段过程,建立了横向电场作用下外周神经的兴奋模型,表明了在横向电场作用下沿朗飞氏结径向的净内向电流导致了神经兴奋,从而阐述了外周神经在横向电场中的兴奋机理,改进了经典电缆方程。模型通过了离体实验验证。此改进的电缆方程可以描述任意电场激励下的外周神经兴奋。     

横向电场作用下外周神经兴奋特性的实验研究

传统的外周神经电缆方程只能描述纵向电场刺激下外周神经兴奋,实验发现在脉冲磁场诱导的横向电场作用下也可使神经兴奋,从而揭示出需要进一步对感应电场兴奋外周神经的机理进行研究。本文研究了横向电场作用下外周神经的兴奋特性,以在体的人体正中神经为例研究了横向电场作用下外周神经兴奋点的位置、刺激阈值、及刺激阈值与纤维半径的关系,以离体的蟾蜍坐骨神经为例研究了刺激阈值与作用时间的关系。实验结果验证了改进的电缆方程的有效性。实验研究成果有助于磁刺激技术的进一步发展与应用。     

一种神经纤维电磁感应刺激的模型

本文介绍了一个用以解释神经纤维电磁感应刺激物理特性的模型。用麦克斯韦方程来予测由电容通过刺激线圈放电时产生的感应电场的分布,用一个Hodgkin—Huxley电缆模型来描述神经纤维对这种感应电场的响应。在一定的电路参数(电阻、电容和初始电压)下,当给定线圈的位置、方向和形状以后,就可以用此模型     

磁刺激中深度感应电场分布的计算

磁刺激是利用变化磁场产生的感应电场作用于可兴奋人体组织的过程.根据磁刺激感应电场理论,计算8字形和四叶形线圈刺激深度感应电场的分布.结果表明通过线圈的电流方向直接影响感应电场的聚焦性.8字形线圈电流方向相反时用于刺激大脑皮层神经效果较好,而四叶形线圈电流方向左右相反,上下相同时,刺激外周神经纤维效果较好.     

血液在有横向电场的流道中的表观粘度

本文根据流体力学运动方程,电磁场Mazwell方程和描述血液特性的Casson方程,在一定近似条件下,从理论上分析了血液在横向电场中流动时的电粘性效应;导出了相应的表观粘度表达式;并设计了一个实验系统来观测血液在横向电场中流动的电粘性效应。实验结果和理论结果一致指出,在横向电场中,血液表观粘度与外加电压本身以及由此而引起的细胞定向的改变,电生化反应和红血球压积变化有关。当外加电压低于某值时,其值随外电压上升而增加;反之,其综合效应是血液表观粘度随电压上升而下降;实验还指出当电场的参量达到某一范围,血液凝聚过程发展迅速,这一结果为发展某些临床止血手段提供了依据。     

束内与外部神经刺激的模拟

模拟神经刺激被用于功能神经肌肉刺激植入系统中,控制协调骨骼肌群的运动,从而获得功能性的肢体运动,改善脊髓受伤病人的状况。为此研究运动神经的兴奋与肌张力之间的关系是非常必要的。本文介绍了一种神经刺激的模型,该模型以一种模拟的算法来描述肌肉的收缩及随后的等长肌张力。在这里采用多兴奋参数的慨率分布来描述神经纤维的兴奋。当神经内部在受刺激神经的容积传导模型中引入了神经的非均匀性及各向异性,并将神     

脑神经磁刺激技术的研究

生物组织磁刺激技术是一种无创伤生物刺激技术，在脑神经刺激以及深部神经刺激中较之传统电刺激具有明显优势，近１０几年来发展较迅速，本文概述了磁刺激技术的基本原理，介绍了脑部磁刺激常用的球形头模型和计算颅内感应电场分布的数学模型，分析了线圈设计中不同的结构，形状，大小，方向等对感应电场聚焦性的影响，比较了圆形（包括圆形的变形）线圈，八字形线圈，Ｓｌｉｎｋｙ线圈刺激脑部神经的基本特性，此外还简单描述了刺激     

脑神经磁刺激技术的研究

生物组织磁刺激技术是一种无创伤生物刺激技术，在脑神经刺激以及深部神经刺激中较之传统电刺激具有明显优势，近１０几年来发展较迅速。本文概述了磁刺激技术的基本原理，介绍了脑部磁刺激常用的球形头模型和计算颅内感应电场分布的数学模型，分析了线圈设计中不同的结构、形状、大小、方向等对感应电场聚焦性的影响，比较了圆形（包括圆形的变形）线圈、八字形线圈、Ｓｌｉｎｋｙ线圈刺激脑部神经的基本特性，此外还简单描述了刺激线圈的内部参数。     

基于COMSOL和NEURON的坐骨神经电刺激模型

为研究周围神经在接受电刺激时的兴奋规律,建立了坐骨神经电刺激的仿真模型。利用有限元仿真软件COMSOL对神经和卡肤电极建模并计算神经周围在外加刺激下的电场分布情况,将此电场分布信息导入到神经建模软件NEURON建立的神经模型中作为细胞外激励源,以分析电刺激下神经的电生理行为。在此模型上分析了刺激电极尺寸和间距、电刺激波形、环境温度和局部温度等参数对坐骨神经纤维兴奋性的影响,为神经功能电刺激的进一步研究及临床应用提供了理论基础。     

神经的建模研究方法

本文从神经膜结构、单个神经元和神经网络三个方面对神经系统的建模工作作了综述。在神经纤维的传导模型中,介绍了H-H方程模型、H-H方程模型的电路实现和神经膜的无电缆模型;在神经元的功能建模研究方面,介绍了电路实现的神经元功能模型和软件实现的神经元功能模型;在神经网络模型中,介绍了神经元模型、Hartline神经网络、Hopfield神经网络、Hamming神经网络和多层感知机模型。作者认为,神经建模的研究将是最终解释神经系统中复杂的功能特性的唯一最有效的方法。     

Transverse tripolar stimulation of peripheral nerve: a modelling study of spatial selectivity

Various anode-cathode configurations in a nerve cuff are modelled to predict their spatial selectivity characteristics for functional nerve stimulation. A 3D volume conductor model of a monofascicular nerve is used for the computation of stimulation-induced field potentials, whereas a cable model of myelinated nerve fibre is used for the calculation of the excitation thresholds of fibres. As well as the usual configurations (monopole, bipole, longitudinal tripole, ‘steering’ anode), a transverse tripolar configuration (central cathode) is examined. It is found that the transverse tripole is the only configuration giving convex recruitment contours and therefore maximises activation selectivity for a small (cylindrical) bundle of fibres in the periphery of a monofascicular nerve trunk. As the electrode configuration is changed to achieve greater selectivity, the threshold current increases. Therefore threshold currents for fibre excitation with a transverse tripole are relatively high. Inverse recruitment is less extreme than for the other configurations. The influences of several geometrical parameters and model conductivities of the transverse tripole on selectivity and threshold current are analysed. In chronic implantation, when electrodes are encapsulated by a layer of fibrous tissue, threshold currents are low, whereas the shape of the recruitment contours in transverse tripolar stimulation does not change.     

In vivo interactions between tungsten microneedles and peripheral nerves

Tungsten microneedles are currently used to insert neural electrodes into living peripheral nerves. However, the biomechanics underlying these procedures is not yet well characterized. For this reason, the aim of this work was to model the interactions between these microneedles and living peripheral nerves. A simple mathematical framework was especially provided to model both compression of the external layer of the nerve (epineurium) and the interactions resulting from penetration of the main shaft of the microneedle inside the living nerves. The instantaneous Young's modulus, compression force, the work needed to pierce the tissue, puncturing pressure, and the dynamic friction coefficient between the tungsten microneedles and living nerves were quantified starting from acute experiments, aiming to reproduce the physical environment of real implantations. Indeed, a better knowledge of the interactions between microneedles and peripheral nerves may be useful to improve the effectiveness of these insertion techniques, and could represent a key factor for designing robot-assisted procedures tailored for peripheral nerve insertion.     

Mapping location of excitation during magnetic stimulation: Effects of coil position

We determined the location of excitation for different positions of a round and butterfly coil duringin vitro magnetic stimulation of cut peripheral nerves. We analyzed the conditions under which excitation occurs, either at the termination or at the peak of the field gradients (first spatial derivative of the electric field). These results were then compared to predictions about the location of excitation sites from a theoretical model of magnetic stimulation of finite neuronal structures. Excitation along a straight nerve occurred at terminations when 1) a coil was positioned close to the end of a nerve (at least one diameter length from the end), 2) a nerve ended in a finite terminating impedance much greater than the axial resistance of the nerve, 3) the induced electric field was of sufficient magnitude, pointing in a direction away from the axis of a nerve. Excitation occurred at the negative peak of the field gradients along a nerve when 1) a coil was positioned far away from the ends of a nerve, 2) there were no geometric or volume conductor inhomogeneities around a nerve, and 3) it was of sufficient magnitude. Threshold strengths for excitation at terminations were significantly lower than that for field gradient excitation and comparable to that due to geometric and volume conductor inhomogeneities.     

The effect of increasing the innervation field sizes of nerves on their reflex response time in salamanders

1. A simple quantitative measure was sought which could describe the relationship between reflex coupling in the spinal cord of salamanders and the peripheral innervation fields of the nerves from which the reflexes were elicited.2. In decerebrate salamanders reflex responses were recorded between pairs of cut hind limb nerves. The latencies (S/R times) of these reflex responses were bilaterally symmetrical for a given pair of nerves and were shorter when the stimulated nerve of the pair had a large motor and sensory peripheral limb innervation field; this was especially obvious for reflexes between 15th and 17th segmental nerves.3. After cutting or crushing the 16th nerve in adult salamanders, the adjacent 15th and 17th nerves sprouted collaterally to innervate denervated skin and muscle. There was apparently complete recovery of normal tactile reflexes and walking movements within a month.4. The operation did not affect the reflex response (S/R) times for nerve combinations on the unoperated side, which were not significantly different from those of normal animals with similar sized peripheral nerve fields. The unoperated side therefore represented the preoperative condition.5. In animals where one or both the 15th and 17th nerves had increased its innervation field size, the S/R times between them were significantly shorter on the operated side when the nerve with the enlarged field was stimulated. The degree of shortening was greatest for nerves showing the largest increase in peripheral field area.6. The S/R times between the 15th and 17th nerves were similar to those measured in normal animals in which the peripheral fields were of similar size to the enlarged fields in the operated animals. In a few cases where the increase in field size was considerable, the S/R time between the 15th and 17th nerves became as short as that between the 15th and 16th nerves on the control side.7. After removal of the 15th nerve, the 14th nerve sprouted into the trunk skin and muscle previously innervated by the 15th nerve and the 16th nerve into denervated limb skin and muscle. In spite of the increased peripheral fields of both these nerves, there was no change in the S/R times between them, or between any other pair of limb nerves on the operated side.8. The decrease in the S/R times between the 15th and 17th nerves was only observed where the stimulated nerve had increased its peripheral limb innervation field. The possible causes and significance of this shortening reflex response times are discussed in the context of an apparently functionally appropriate adaptation in the spinal cord.     

Multielectrode spiral cuff for selective stimulation of nerve fibres.

In this paper we present the modelling, design, and initial experimental testing of a nerve cuff multielectrode system for selective stimulation of fibres in superficial peripheral nerve trunk regions which is capable of making a selective activation of multiple muscles. The developed multielectrode nerve cuff consists of 14 platinum stimulating electrodes embedded within a self-curling sheet of biocompatible insulation, exhibiting a spiral transverse cross-section. The spiral shape of the system is such that the number of stimulating electrodes which can be utilized depends on the diameter of the stimulated nerve. Nerves with a greater diameter automatically make use of more electrodes than thin ones. The development was based on results obtained by a histological examination of the peripheral nerves which were planned to be stimulated, and on models of excitation of myelinated nerve fibres. The modelling objectives were to determine the electric field that would be generated within a nerve trunk by a specific electrode. Moreover, the extent of initial excitation of the nerve fibres within the superficial region of the dog sciatic nerve elicited by a certain discrete stimulating electrode was predicted. For this purpose a calculation of activating function for six positions where the nerve fibres were supposed to lie within the longitudinally dissected sciatic nerve was performed. In two acute experiments on the sciatic nerve of the dog the objective was to characterize the effectiveness of the multielectrode system in monopolar selective stimulation of the superficial regions, innervating the gastrocnemius and tibialis anterior muscle. A selectivity preliminary tested by measuring the myoelectric activity of the gastrocnemius and tibialis anterior muscle after 2 months showed good results in both animals.     

Activity and distribution patterns of monkey pallidal neurons in response to peripheral nerve stimulation

The response patterns of pallidal neurons to electrical stimulation of the median and tibial nerves were examined in awake monkeys. Around 30% of the recorded neurons responded to the stimulation of either the median or tibial nerve, while only 6% responded to the stimulation of both nerves. The vast majority of these pallidal neurons displayed monophasic excitation or monophasic inhibition. The latency of the excitation was shorter than that of the inhibition. In each pallidal segment, the neurons responding to the median nerve stimulation (representing the forelimb) tended to be located ventral to those responding to the tibial nerve stimulation (representing the hindlimb). The present results indicate that the somatotopical arrangement in the globus pallidus can be outlined based on the neuronal responses to peripheral nerve stimulation.     

A new cable model formulation based on Green's theorem

We describe an alternative formulation of the cable equation to model excitation in a cylinder of cardiac fiber. The formulation uses Green's theorem to develop equations for the extracellular and intracellular potential on either side of the excitable membrane, the dynamics of which are described by a Hodgkin-Huxley type model, without assuming that the radial current is zero. These equations are discretized to yield a system of linear equations which are solved at each instant in time. We found no qualitative differences between this approach and the standard cable model for parameters within accepted physiological limits. When the cable diameter is of the same order as the length constant the new formulation takes into account the intracellular potential change in the radial direction and gives an accurate expression of the conduction velocity.     

Magnetic field excitation of peripheral nerves and the heart: a comparison of thresholds

Time-varying magnetic fields can theoretically excite the heart or peripheral nerves. Relative excitation thresholds of nerve and heart are compared using ellipsoidal representations of the human torso. Relative magnetic thresholds depend on the excitability of nerve and cardiac tissue, the geometric positions of anatomical features, stimulus waveform features and the direction and spatial distribution of the incident magnetic field. Minimum electric field thresholds for excitation of nerve and heart do not differ greatly. Nevertheless, nerve and heart magnetic thresholds may be disparate because of factors related to body geometry and the dependence of excitability on the stimulus waveform.