利用单电极不对称双向脉冲刺激实现哺乳动物有髓神经纤维选择性兴奋的仿真研究

本研究的目的是要从理论上探讨利用单电极双向脉冲刺激实现哺乳动物神经纤维选择性刺激,(即当刺激一束神经时,不兴奋粗神经而兴奋细神经)的可能性.双向脉冲刺激可以降低刺激脉冲对神经纤维产生的电化学损伤.为研究哺乳动物有髓神经纤维的电特性,建立了一个基于简单的无穷大、各向同性的容积导体模型的仿真系统.利用该仿真系统,采用"不对称但电荷平衡"的双向脉冲刺激,计算了神经纤维的兴奋和阻断阈值与纤维直径、纤维-电极间距离的关系.结果表明:在距电极一定距离内采用该双向脉冲刺激模式确实可以实现哺乳动物有髓神经纤维的选择性兴奋.     

Proposal of a new method for narrowing and moving the stimulated region of cochlear implants: animal experiment and numerical analysis

We have proposed the tripolar electrode stimulation method (TESM) for narrowing the stimulation region and continuously moving the stimulation site for cochlear implants. The TESM stimulates the auditory nerve array using three adjacent electrodes which are selected among the electrodes of an electrode array within the lymphatic fluid. Current is emitted from each of the two lateral electrodes and received by the central electrode. The current received by the central electrode is made equal to the sum of the currents emitted from the lateral electrodes. In this paper, we evaluate whether or not TESM works according to a theory which is based on numerical analysis using an electrical equivalent circuit model of the auditory nerve fibers. In this simulation, the sums of the excited model fibers are compared to the compound action potentials (CAP's) which we obtained through animal experiments. To identify the main parameter while maintaining the amplitude of the CAP (the sum of the fired fibers), we assumed the presence of some parameters from the radial current density profile. In the case of the width value among the parameters being kept constant, the amplitude of the CAP was almost constant; thus, the number of the fired fibers was also almost constant. The width value equals the distance between the points at which the profile of the radial current density of the electrode array and the line of the radial threshold current density of the electrode array intersect. It is possible to determine the measure of the stimulation region or site by controlling the width value and the ratios of the currents emitted from the lateral electrodes. As a result, we succeeded in narrowing the stimulation region by controlling the sum of the currents emitted from the two lateral electrodes. Also we succeeded in continuously moving the stimulation site by modifying the currents emitted from the two lateral electrodes.     

Closed-loop control of ankle position using muscle afferentfeedback with functional neuromuscular stimulation

Describes a closed-loop functional neuromuscular stimulation system that uses afferent neural activity from muscle spindle fibers as feedback for controlling position of the ankle joint. Ankle extension against a load was effected by neural stimulation through a dual channel intrafascicular electrode of a fascicle of the tibial nerve that innervated the gastrocnemius muscle. Ankle joint angle was estimated from recordings of tibialis anterior and lateral gastrocnemius spindle fiber activity made with dual channel intrafascicular electrodes. Experiments were conducted in neurally intact anesthetized cats and in unanesthetized decerebrate cats to demonstrate the feasibility of this system. The system was able to reach and maintain a fixed target ankle position in the presence of a varying external moment ranging in magnitude between 7.3 and 22 N-cm opposing the action of the ankle extensor, as well as track a sinusoidal target ankle position up to a frequency of 1 Hz in the presence of a constant magnitude 22- or 37-N-cm external moment     

Selective activation of small motor axons by quasitrapezoidalcurrent pulses

We have found a method to activate electrically smaller nerve fibers without activating larger fibers in the same nerve trunk. The method takes advantage of the fact that action potentials are blocked with less membrane hyperpolarization in larger fibers than in smaller fibers. In our nerve stimulation system, quasitrapezoidal-shaped current pulses were delivered through a tripolar cuff electrode to effect differential block by membrane hyperpolarization. The quasitrapezoidal-shaped pulses with a square leading edge, a 350 microsecond(s) plateau, and an exponential trailing phase ensured the block of propagating action potentials and prevented the occurrence of anodal break excitation. The tripolar cuff electrode design restricted current flow inside the cuff and thus eliminated the undesired nerve stimulation due to a "virtual cathode." Experiments were performed on 13 cats. The cuff electrode was placed on the medial gastrocnemius nerve. Both compound and single fiber action potentials were recorded from L7 ventral root filaments. The results demonstrated that larger alpha motor axons could be blocked at lower current levels than smaller alpha motor axons, and that all alpha fibers could be blocked at lower current levels than gamma fibers. A statistical analysis indicated that the blocking threshold was correlated with the axonal conduction velocity or fiber diameter. This method could be used in physiological experiments and neural prostheses to achieve a small-to-large recruitment order in motor or sensory systems.     

Dipole distance for minimum threshold current to stimulate unmyelinated axons with microelectrodes

Excitation thresholds for long nerve or muscle fibers with two point sources parallel to the fiber axis depend on the dipole length. The aim of this study was to find the optimal interelectrode distance for the minimum stimulation current. For a specific electrode-fiber distance (z_el) dipole length is constrained by the energy efficacy of the electrodes requiring small interelectrode distances, and by rather low stimulation currents requiring large dipole distances. Far-field values for optimal dipole distance (approximately 1.4 *z_el) can be explained by the superposition of the positive parts of the activating functions for the monopolar elements of the dipole. A current redistribution effect in a target fiber close to the electrodes shifts the dipole length for threshold stimulation from the theoretical optimal activating function approach value towards greater dipole distances. Spike initiations in straight fibers and retinal ganglion cell axons are investigated.     

The transient subthreshold response of spherical and cylindricalcell models to extracellular stimulation

The effect of extracellular stimulation on excitable tissue is evaluated using analytical models. Primary emphasis is placed on the determination of the rate of rise of the membrane potential in response to subthreshold stimulation. Three models are studied: 1) a spherical cell in a uniform electric field, 2) an infinite cylindrical fiber with a point source stimulus, and 3) a finite length cable with sealed ends and a stimulus electrode at each end. Results show that the rate of rise of the transmembrane potential was more rapid than the step response of a space-clamped membrane for all geometries considered. The response of the cylindrical fiber to extracellular stimulation is compared to previously reported studies of the cylindrical fiber response to intracellular stimulation. It is found that the location of the stimulus has little effect on the infinite fiber response. For terminated cables, however, an accurate model of stimulus response must discriminate between intracellular and extracellular stimulation.     

Excitation of dorsal root fibers in spinal cord stimulation: atheoretical study

In epidural spinal cord stimulation it is likely not only that dorsal column fibers are activated, but also that dorsal root fibers will be involved as well. In this investigation a volume conductor model of the spinal cord was used and dorsal root fibers were modeled by an electrical network including fiber excitation. The effects of varying some geometric fiber characteristics, as well as the influence of the dorsal cerebrospinal fluid layer and the electrode configuration on the threshold stimulus for their excitation, were assessed. The threshold values were compared with those of dorsal column fibers. The results of this modeling study predict that, besides the well known influence of fiber diameter, the curvature of the dorsal root fibers and the angle between these fibers and the spinal cord axis are a major influence on their threshold values. Because of these effects, threshold stimuli of dorsal root fibers were relatively low as compared to dorsal column fibers. Excitation of the dorsal root fibers occurred near the entry point of the fibers     

Axon termination conditions for electrical stimulation

The cable model for electrical stimulation near the terminal of a passive fiber is derived for excitation by an arbitrary, time-varying, applied extracellular field. Unless the termination impedance is comparable to that of the mammalian node of Ranvier, the end-conditions require the longitudinal intracellular current at the fiber terminal to be negligibly small. This requirement substantially alters the membrane potential profile from that obtained with a fiber of infinite length. Stimulation near the end of a fiber may result in lower thresholds and may reverse the anodal/cathodal threshold ratio obtained with stimulation in the mid-portion of the fiber. Chronaxie for stimulation near the terminal may be much smaller than at a distance from the terminal and the strength-duration curve may be nonmonotonic. These differences may have significant implications for any application of electrical stimulation where fiber terminations may play a role in the excitatory process     

Threshold variability in fibers with field stimulation of excitablemembranes

The central focus of this report is the evolution of transmembrane potentials following initiation of a point-source field stimulus, particularly when the stimulus is short and the stimulating electrode is close to the fiber. The transmembrane voltage threshold in response to a point-source field stimulus was determined in a numerical model of a single unmyelinated fiber. Both nerve (Hodgkin-Huxley [1952]) and cardiac (Ebihara-Johnson [1980]) models of the fiber membrane were evaluated. A central question is whether it is possible to know in advance whether a stimulus of specific magnitude, duration, and location will result in a subsequent action potential. Such determination can be based on the membrane's “voltage threshold”. In contrast to the commonly held view, the voltage threshold was found to vary markedly depending on the duration and location of the field stimulus. Voltage thresholds ranged from about 8 mV above baseline to more than 100 mV above baseline, the higher thresholds occurring with shorter stimuli and electrode locations closer to the membrane. A related question is whether the passive membrane response can be used as a tool in determining whether a subsequent action potential is elicited. If the answer is affirmative, this finding can be very useful, since passive properties are linear and thereby much simpler to evaluate than active ones. The results show that the passive response tracks active responses long enough to be a good estimator of subsequent action potential development. Examples show that the evaluation of V_m at 0.2-0.5 msec after stimulus initiation, times chosen on the basis of membrane characteristics, was a better predictor of subsequent excitation than was either initial transmembrane current or V_m at the time when the stimulus ends. Most of the circumstances analyzed here with electric field stimulation also appear likely to be valid with magnetic field stimulation     

Measurements of the Uptake Area of Small-Size Electromyographic Electrodes

Extracellular action potentials from 76 different muscle fibers in the human brachial biceps were recorded, with a 14 lead multielectrode, each leading-off surface being 25 ?m in diameter. Volume conduction of these action potentials was calculated by representing the low-pass filter characteristics of the muscle tissue by a transfer function with one time constant and an attenuation factor. The radial decline of the action potentials was calculated in steps of 5 ?m, and the pickup radius of the electrode, defined as the distance at which the peak-to-peak amplitude of the action potentials declines to 200 ?V, was computed. The pickup radius for the 25 ?m diameter electrode, assuming an average action potential peak-to-peak amplitude of 6.2 mV was 292 Mm. With this uptake area, a fiber density in the brachial biceps of 1.37 fibers/uptake area (the average number of fibers belonging to one motor unit and included within the electrode uptake area) and a fiber radius of 28 ?m, the electrode "sees" 34 different motor units.     

Recording from a Single Motor Unit During Strong Effort

During strong voluntary effort it is rarely possible to identify the action potentials from single motor units. In large muscles the most selective recordings are obtained with bipolar wire electrodes. To elucidate this experimental finding we have calculated the extracellular field around a single muscle fiber from an intracellular muscle action potential. This model showed that the selectivity of a bipolar electrode is high provided: i) the diameter of the recording surfaces is less than half the diameter of the muscle fibers; ii) the center distance between the recording surfaces is of the same order or smaller than the diameter of the muscle fibers, and when iii) the center-line between the recording surfaces is oriented perpendicular to the direction of the muscle fibers.     

两种扩束透镜光纤的研制与模场分析

介绍了两种对光纤端面进行修饰以扩大模场直径（MFD）的方法（加热扩散法和熔接透镜光纤法）。利用刀片远场扫描法对单模光纤（SMF）、加热扩芯光纤（TECF）与熔接透镜光纤（GIF）这两种修饰后的高斯分布光纤进行了测量比较。验证了其模场直径分别为10．700μm,13．129μm和12．7l7μm。并针对实际问题作出修正，通过对单模光纤以及上述两种扩束透镜光纤的分析，得到了相当合理的结果。说明这两种端面修饰法确实能起到扩大模场直径同时又不破坏模式分布的作用。并且验证了修正后的刀片法用于测量修饰后的光纤的各参量是非常有利的。     

Estimation of fiber diameters in the spinal dorsal columns fromclinical data

Lack of human morphometric data regarding the largest nerve fibers in the dorsal columns (DCs) of the spinal cord has lead to the estimation of the diameters of these fibers from clinical data retrieved from patients with a new spinal cord stimulation (SCS) system. These patients indicated the perception threshold of stimulation induced paresthesia in various body segments, while the stimulation amplitude was increased. The fiber diameters were calculated with a computer model, developed to calculate the effects of SCS on spinal nerve fibers. This computer model consists of two parts: (1) a three-dimensional (3-D) volume conductor model of a spinal cord segment in which the potential distribution due to electrical stimulation is calculated and (2) an electrical equivalent cable model of myelinated nerve fiber, which uses the calculated potential field to determine the threshold stimulus needed for activation. It is shown that the largest fibers in the medial DCs are significantly smaller than the largest fibers in the lateral parts. This finding is in accordance with the fiber distribution in cat, derived from the corresponding propagation velocities. Moreover, it is shown that the mediolateral increase in fiber diameter is mainly confined to the lateral parts of the DCs. Implementation of this mediolateral fiber diameter distribution of the DCs in the computer model enables the prediction of the recruitment order of dermatomal paresthesias following increasing electrical stimulation amplitude     

The effect of externally applied electrical fields on myocardialtissue

The explanation of the response to electrical stimulus by macroscopic regions of cardiac tissue in terms of the behavior of membrane ion channels requires mathematical models that span the range of spatial scales from the single cardiac cell to the entire heart. This is accomplished by the bidomain model, which successfully characterizes the electrical properties of the heart and the effect of externally applied electric fields on myocardial tissue, Recently the bidomain model has been used to make several specific, testable predictions: (1) a 4-fold symmetric magnetic field pattern is associated with an expanding wave front, (2) a region of positive interstitial potential precedes an expanding wave front in the direction parallel to the myocardial fibers, (3) the rate of rise of an action potential depends on the direction of propagation in superfused tissue, (4) the wave front in superfused strands of tissue is curved (5) a “dog bone” shaped region of depolarization exists under a unipolar cathode, (6) depolarized regions along the fiber direction adjacent to a unipolar anode are responsible for anodal stimulation, (7) interactions between adjacent depolarized and hyperpolarized tissue cause anode-break and cathode-break stimulation, (8) reentry can be induced by successive stimulation using a single unipolar cathode, and (9) a mechanism for far field stimulation depends on fiber curvature. These predictions have been verified qualitatively in every case where they have been tested experimentally. In some cases, such as the mechanisms for reentry induction and far field stimulation, the necessary experiments have not yet been performed     

Fabry–PÉrot Cavity Based on a Suspended-Core Fiber for Strain and Temperature Measurement

A fiber-optic Fabry–PÉrot sensing structure based on the utilization of a suspended-core fiber is presented. The interferometric structure is formed when a small length of the suspended-core fiber is spliced to the end of a standard single-mode fiber. The interfering waves are generated by the refractive-index mismatches between the two fibers in the splice region and at the end of the suspended-core fiber. Thermal and strain responses of two different sensing heads associated with suspended-core fibers with three and four holes are characterized.      

极化保偏光纤结构对器件中电场强度的影响

针对可能的侧面抛磨的极化保偏光纤的电极结构，在提出相应的数学模型的基础上，分析了电极形状、被抛磨侧面与纤芯的距离、侧面抛磨后光纤两侧的厚度等结构参数对极化保偏光纤中电场强度及其分布产生的影响。由此分析得到了优化的极化光纤器件结构。     

Inversion of the current-distance relationship by transientdepolarization

The objective of this research was to develop a technique to excite selectively nerve fibers distant from an electrode without exciting nerve fibers close to the electrode. The shape of the stimulus current waveform was designed based on the nonlinear conductance properties of neuronal sodium channels. Models of mammalian peripheral myelinated axons and experimental measurements on cat sciatic nerve were used to determine the effects of subthreshold polarization on neural excitability and recruitment. Subthreshold membrane depolarization generated a transient decrease in neural excitability and thus an increase in the threshold for stimulation by a subsequent stimulus pulse. The decrease in excitability increased as the duration and amplitude of the subthreshold depolarization were increased, and the increase in threshold was greater for fibers close to the electrode. When a depolarizing stimulus pulse was applied immediately after the subthreshold depolarization, nerve fibers far from the electrode could be stimulated without stimulating fibers close to the electrode. Subthreshold depolarizing prepulses inverted the current-distance relationship and allowed selective stimulation of nerve fibers far from the electrode     

Electrical stimulation of cardiac tissue by a bipolar electrode ina conductive bath

A three-dimensional (3-D) computer simulation of the electrical stimulation of passive cardiac tissue from a bipolar electrode placed within a conductive bath is presented. Through the bidomain model, the syncytial and anisotropic properties of cardiac tissue are taken into account; tissues with equal anisotropy and no transverse coupling are also considered. The membrane is represented by a capacitor and passive resistor in parallel. Located within an isotropic bath, the bipolar electrode is oriented either perpendicular or parallel to the tissue surface. For anisotropic tissue with a small cathode-tissue separation, the tissue surface is highly depolarized under the cathode with the depolarization persisting a considerable distance from the electrode in the transverse fiber direction. Adjacent to this region in the longitudinal direction, areas of hyperpolarization exist. At large distances from the cathode, the tissue surface is hyperpolarized in all directions when the electrode axis is perpendicular to the tissue. In the parallel case, surface depolarization creates buried regions of hyperpolarization. For the perpendicular configuration, the ratio of the steady-state maximum depolarization to steady-state maximum hyperpolarization, an estimate of the ratio of anodal to cathodal threshold, decreases rapidly with increasing cathode-tissue separation. In the parallel case, the depth of the conductive bath significantly affected the transmembrane potential distribution in the tissue. The use of a 3-D model more realistically simulates real-life electrical stimulation (such as stimulation with an implantable pacemaker) and provides insight into the effect of the volume conductor adjacent to the tissue     

F:End filters: etalon on the beveled facet of a fiber with an outdiffused core

FiEnd (fiber end) etalon filters have been made by multilayer deposition directly on the beveled end facets of fibers. Both standard depressed-cladding single-mode fibers and beam-expanded fibers made by thermal diffusion of the core dopant of standard fibers were used. The fiber end facets were polished at appropriate bevel angles to eliminate back reflection. It was found that, if a standard fiber is used, the bandpass of the FiEnd etalon filter is asymmetric and skewed toward shorter wavelengths. It was also shown that to make reflection free FiEnd etalon filters on beveled fiber and facets with bandpasses smaller than about 5 nm, it is necessary to use fibers with mode field diameters larger than those of standard fibers.<>     

Optoelectronic properties of semiconductor lasers butt-coupled tooptical fibers

The authors have theoretically and experimentally examined the electrical noise properties of a semiconductor laser coupled to a single-mode fiber with a plane endface. It is found that the performance of semiconductor lasers butt-coupled to optical fibers critically depends on the axial position of the fiber. Measurements show that the influence of the fiber may change the output power as much as ±40% and the modulation bandwidth, determined by the relaxation frequency by as much as ±35% (for a bias 15% above the solitary threshold). Moreover, the pulse response and the longitudinal mode spectrum deteriorate if the laser-to-fiber distance departs from its optimal value. The authors propose a nonoptical scheme for maintaining the optimal laser-to-fiber distance so as to avoid these degrading effects