Showing papers by "Warren M. Grill published in 2000"

Selective microstimulation of central nervous system neurons.

Abstract: The goal of this study was to identify stimulus parameters and electrode geometries that were effective in selectively stimulating targeted neuronal populations within the central nervous system (CNS). Cable models of neurons that included an axon, initial segment, soma, and branching dendritic tree, with geometries and membrane dynamics derived from mammalian motoneurons, were used to study excitation with extracellular electrodes. The models reproduced a wide range of experimentally documented excitation patterns including current-distance and strength-duration relationships. Evaluation of different stimulus paradigms was performed using populations of fifty cells and fifty fibers of passage randomly positioned about an extracellular electrode(s). Monophasic cathodic or anodic stimuli enabled selective stimulation of fibers over cells or cells over fibers, respectively. However, when a symmetrical charge-balancing stimulus phase was incorporated, selectivity was greatly diminished. An anodic first, cathodic second asymmetrical biphasic stimulus enabled selective stimulation of fibers, while a cathodic first, anodic second asymmetrical biphasic stimulus enabled selective stimulation of cells. These novel waveforms provided enhanced selectivity while preserving charge balancing as is required to minimize the risk of electrode corrosion and tissue injury. Furthermore, the models developed in this study can predict the effectiveness of electrode geometries and stimulus parameters for selective activation of specific neuronal populations, and in turn represent useful tools for the design of electrodes and stimulus waveforms for use in CNS neural prosthetic devices. © 2000 Biomedical Engineering Society. PAC00: 8717Nn, 8719La, 8719Nn, 8717Aa

Neural and connective tissue response to long‐term implantation of multiple contact nerve cuff electrodes

Abstract: The objective of this study was to characterize the tissue response to multiple contact spiral nerve cuff electrodes implanted on the sciatic nerve of seven cats for 28-34 weeks. The cuffs were surrounded by fibrous tissue encapsulation consisting of foreign body cells, collagen, and fibroblasts. Focal areas of abnormal neural morphology including perineurial thickening, endoneurial fibrosis, thinly myelinated axons, and focal reduction in the density of myelinated axons were noted in five of seven nerves. In three implants, the percutaneous lead cable was destroyed by the animal pulling on the external leads. Morphological changes were observed in two of three nerves from implants sustaining no known animal induced trauma (group A), and in three of four nerves from implants damaged by the animal pulling at the leads (group B). All nerves appeared normal 2 cm proximal to the cuff. At the cuff level, small regions of one fascicle in each of two nerves (both group B) exhibited abnormalities, while the proximal and distal sections of both nerves were normal. Distal to the cuff, small regions of seven fascicles distributed among three nerves (two group A, one group B) exhibited abnormalities. These nerves were normal at the cuff level but exhibited abnormalities in individual nerve branches distal to the cuff. The incidence and characteristics of the morphological abnormalities at the cuff level are consistent with those observed in previous studies of nerve cuff electrodes, and support the hypothesis that spiral cuff electrodes can be implanted with an internal diameter less than that of the nerve and expand to accommodate the nerve without compression The pattern of morphological abnormalities indicated that mechanical trauma had occurred at some time in the past, and the distribution suggested animal intervention and the lead cable as possible causes.

Modelling the effects of electric fields on nerve fibres: Influence of the myelin sheath

Abstract: The excitation and conduction properties of computer-based cable models of mammalian motor nerve fibres, incorporating three different myelin representations, are compared. The three myelin representations are a perfectly insulating single cable (model A), a finite impedance single cable (model B) and a finite impedance double cable (model C). Extracellular stimulation of the three models is used to study their strength-duration and current-distance (I-X) relationships, conduction velocity (CV) and action potential shape. All three models have a chronaxie time that is within the experimental range. Models B and C have increased threshold currents compared with model A, but each model has slope to the I-X relationship that matches experimental results. Model B has a CV that matches experimental data, whereas the CV of models A and C are above and below the experimental range, respectively. Model C is able to produce a depolarising afterpotential (DAP), whereas models A and B exhibit hyperpolarising afterpotentials. Models A and B are determined to be the preferred models when low-frequency stimulation (< approximately 25 Hz) is used, owing to their efficiency and accurate excitation and conduction properties. For high frequency stimulation (approximately 25 Hz and greater), model C, with its ability to produce a DAP, is necessary accurately to simulate excitation behaviour.

Neuroprosthetic applications of electrical stimulation.

Abstract: Neural prostheses are a developing technology that use electrical activation of the nervous system to restore function to individuals with neurological impairment. Neural prostheses function by electrical initiation of action potentials in nerve fibers that carry the signal to an endpoint where chemical neurotransmitters are released, either to affect an end organ or another neuron. Thus, in principle, any end organ under neural control is a candidate for neural prosthetic control. Applications have included stimulation in both the sensory and motor systems and range in scope from experimental trials with single individuals to commercially available devices. Outcomes of motor system neural prostheses include restoration of hand grasp and release in quadriplegia, restoration of standing and stepping in paraplegia, restoration of bladder function (continence, micturition) following spinal cord injury, and electrophrenic respiration in high-level quadriplegia. Neural prostheses restore function and provide greater independence to individuals with disability.

Electrical activation of spinal neural circuits: application to motor-system neural prostheses.

Abstract: Present motor-system neural prostheses use electrical activation of last-order (motor) neurons to restore function. We are pursuing a new approach: restoration of function by electrical activation of higher-order interneurons. Our hypothesis is that electrical activation of spinal neural circuits, rather than direct activation of last-order motoneurons, will simplify generation of complex motor behaviors. We review two approaches to control bladder function and to control skeletal motor function: intraspinal microstimulation for direct activation of spinal neurons and peripheral afferent stimulation for indirect, synaptic activation of spinal neurons. The results demonstrate that electrical activation of spinal neural circuits allows generation of complex motor behaviors including micturition and organized multi-joint motor responses with a single electrode. Electrical activation of spinal neural circuits, and generation of the complex functions they subserve, holds great promise to advance the function of motor system neural prostheses.

Endpoint force patterns evoked by intraspinal stimulation-ipsilateral and contralateral responses

Abstract: We studied the mapping and structure of endpoint forces produced by microstimulation of the cat spinal cord. The forces evoked by microstimulation varied in magnitude and direction as a function of limb configuration. At some stimulation sites, the force patterns exhibited a point of convergence where the net endpoint force was zero. Ipsilateral stimulation in the dorsal aspect of the cord evoked flexion responses that exhibited a convergent point, while extension responses evoked by ipsilateral stimulation were not convergent. Conversely, contralateral stimulation in the dorsal aspect of the cord evoked extension responses that exhibited a convergent point, while flexion responses evoked by contralateral stimulation were not convergent. Stimulation in the ventral aspects of the cord, as well as intramuscular stimulation of single muscles did not produce convergent force patterns. The results demonstrate that stimulation in the dorsal and intermediate aspects of the spinal cord can generate organized, convergent force patterns, while stimulation in the ventral aspect of the cord can not. These results suggest that electrical activation of higher-order neurons may be used to coordinate the activation of the multiple muscles required in multi-joint movements.

Current density and electric field analysis of microelectrodes using finite element modeling

Abstract: Geometrically and electrically accurate finite element models of microelectrodes used in microstimulation of the central nervous system were created to examine the current density along the surface of the electrode as well as the electric field generated within the tissue medium. Sharp tipped microelectrodes (tip radius=15 /spl mu/m; cone angle=10/spl deg/) were examined with electrode surface areas that ranged from 250-1000 /spl mu/m/sup 2/. Analysis of these models showed that the current density was concentrated at the tip of the microelectrode, and increasing the surface area of the electrode has little effect on reducing the peak current density. The models also showed that as the surface area of the microelectrode was increased, the gradient of the electric field within the tissue decreased, and the modeling of the electric field generated by a sharp tipped microelectrode by a theoretical point source electrode was valid only for distances greater than 20 /spl mu/m from the electrode tip. The results of this study have important implications for electrochemical reactions at the electrode surface, and for modeling of neural excitation with metal microelectrodes.

Influence of the myelin sheath on excitation properties of nerve fibers

Abstract: The excitation and conduction properties of computer-based cable models of mammalian motor nerve fibers incorporating three different myelin representations were compared. The three myelin representations were a perfectly insulating single cable (model A), a finite impedance single cable (model B), and a finite impedance double cable (model C). Extracellular stimulation of the three models was used to study strength-duration, current-distance (I-X), conduction velocity (CV) properties. All three models had a chronaxie time (Tch) that was within the experimental range. Models B and C had increased threshold current as compared to model A, but each model had a slope to the I-X relationship that matched experimental results. Model B had a CV that matched experimental data while the CV of models A and C were above and below the experimental range, respectively. These results indicate that the presence of a finite impedance myelin sheath does influence the excitation properties of nerve fiber models and must be considered when using models for design of neural stimulation paradigms.

Modeling of mammalian myelinated nerve with stochastic sodium ionic channels

Abstract: This paper presents the excitation properties of mammalian myelinated nerve fibers with the nodes of Ranvier represented with stochastic sodium channels. The hybrid multi-compartment cable model developed here consisted of twenty one nodes: five central "stochastic nodes" including sodium ionic channels obeying a Markov process, and otherwise conventional deterministic nodes. The strength duration relationship and the threshold current versus the electrode-to-fiber distance were investigated to see whether the neural excitability differs from that of the conventional model possessing twenty one deterministic nodes. The stochastic nodes created variability in the fiber response, including changes in the latency of action potential generation and failures to fire on some trials. The influence of the stochastic nodes on threshold was dependent on the duration of the stimulus. At short durations the threshold of the stochastic model was greater than that of the deterministic model, while at longer durations the threshold of the stochastic model was less than that of the deterministic model. The differences between the stochastic and deterministic models were only weakly dependent on the electrode-to-fiber distance. The hybrid-cable model is computationally efficient and demonstrates the role of the stochastic properties of ionic channels in neural excitation.