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A cylindrical multielectrode system specifically designed for intraspinal microstimulation was mechanically and electrically evaluated in the ventral horn of the feline lumbo-sacral spinal cord. Electrode insertions proved to be straight as evaluated from radiographs. Impedances were measured in situ and force recruitment curves from quadriceps muscles were collected over a wide range of stimulus parameters. For a given charge, higher current amplitudes produced greater forces than proportionally longer pulse durations, indicating that charge is not the sole indicator of evoked force in applications utilizing electrical stimulation. Overlap measurements for calculating current-distance constants were collected at a variety of current amplitudes, electrode pair separations, and pair orientations in the spinal grey matter. Forces obtained in the majority of these trials demonstrated an order effect, presumably due to asymmetric neuronal connectivity within the spinal cord. In the cases showing no order effect, the dorso-ventral electrode pair orientation yielded a higher average current-distance constant (278 /spl mu/A/mm/sup 2/) than either the medio-lateral or rostro-caudal electrode pair orientations (197 /spl mu/A/mm/sup 2/). Specifications of an array of cylindrical multielectrodes for use in future intraspinal microstimulation prostheses were updated.

Selective activation of muscle groups in the feline hindlimb by electrical stimulation of the ventral lumbo-sacral spinal cord was investigated. Spinal cord segments L5 to S1 were mapped using a penetrating tungsten needle electrode. Locations that produced isolated contraction of quadriceps, tibialis anterior or triceps surae/plantaris muscles when stimulated with a current of 40 /spl mu/A or less, and in which spread of activity to other muscles was not detected after increasing the stimulus to at least twice the threshold level, were defined as belonging to the target muscle's "activation pool." The quadriceps activation pool was found to extend from the beginning of L5 to the middle of L6. The tibialis anterior activation pool extended from the beginning of L6 to the middle of L7, and the triceps surae/plantaris activation pool extended from the caudal end of L6 to the beginning of S1. The three activation pools were located in Rexed motor lamina IX and their spatial organization was found to correspond well with that of the anatomically defined motor pools innervating the same muscles. The spatial and functional segregation of motor pools manifested at the spinal cord level can have direct applications in the areas of functional electrical stimulation and motor control.

The goal of the study was to provide specifications for a stimulating electrode array to be implanted in the lumbosacral spinal cord as part of a functional neuromuscular stimulation (FNS) system for control of lower extremity muscles in paralyzed individuals. Dual channel stimulation of the quadriceps activation pool in the feline ventral lumbo-sacral spinal cord was performed to measure electrode interactions and to explore the effect of various stimulation paradigms on muscle fatigue. There was no measurable overlap in the populations of motor neurons activated from 2 different electrodes for spacings /spl ges/1 mm with currents below 100 /spl mu/A. However, a statistically significant increase in the population of activated fibers due to current summation was observed when stimuli /spl ges/70 /spl mu/A. were simultaneously presented through pairs of electrodes within 3 mm of each other. Fatigue effects were studied with 3 paradigms: (1) stimuli were delivered through a single electrode, (2) stimuli were delivered through 2 electrodes with the stimulus to the second electrode presented during the refractory period of fibers stimulated by the first electrode, and (3) stimuli were interleaved between the 2 electrodes such that the stimulus to 1 electrode was presented midway between stimuli to the other electrode, and the rate of stimulation through a single electrode was half that used in the first 2 paradigms. During channel refractory and single channel stimulation did not differ from each other in the rate at which the muscle fatigued, in both cases the force decayed to 30% of its initial level within 2 min of the initiation of the stimulation regime, whereas the force with interleaved stimulation was still above the initial force at this time due to strong potentiation. Based on these results and on and activation pool dimensions obtained in an earlier study, preliminary specifications are presented for an electrode array to be implanted in the human spinal cord for functional neuromuscular stimulation.

This paper 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.

The limits of present electrode technology are being reached in current motor prostheses for restoring functional movement in paralyzed people. Improved devices require electrodes and stimulation methods that will activate muscles selectively and independently with less implanted hardware. A practical functional neuromuscular stimulation (FNS) system may need to employ extraneural, intraneural, epimysial, or intramuscular electrodes or a combination of these types. The limitations of current muscle electrodes and the anatomy of peripheral nerve innervation of muscle have pointed to stimulation of peripheral nerve trunks as a promising area for investigation. Attempts to use conventional (extraneural) peripheral nerve electrodes for selective activation of muscles in chronic applications have met with only limited success. Intraneural (intrafascicular) electrodes offer the advantages of greater selectivity and lower power requirements, but these may be offset by the difficulty of inserting delicate electrodes through the collagenous epineurium and perineurium while avoiding unacceptable amounts of trauma. Cuff electrodes require more power than intrafascicular ones but may provide more stable recruitment patterns over time, and the opportunity for retrieval and replacement. >

Using recordings of peripheral nerve activity made with carbon fiber intrafascicular electrodes, the authors compared the performance of three different recording techniques (single channel, differential, and dual channel) and four different unit classification methods (linear discriminant analysis, template matching, a novel time amplitude windowing technique, and neural networks) in terms of errors in waveform classification and artifact rejection. Dual channel recording provided uniformly superior unit separability, neural networks gave the lowest classification error rates, and template matching had the best artifact rejection performance. >

We have studied muscle recruitment with Teflon-insulated, 25 microns diameter, Pt-Ir intrafasicular electrodes implanted in nerves innervating the gastrocnemius and soleus muscles of cats. The purpose of this study was to measure the performance of these bipolar electrodes, which had been designed to optimize their ability to record unit activity from peripheral nerves, as stimulating electrodes. Recruitment curves identified the optimal stimulus configuration as a biphasic rectangular pulse, with an interphase separation of about 500 microseconds and a duration of about 50 microseconds. The current required for a half-maximal twitch contraction was on the order of 50 microA. Current and charge densities needed for stimulation were well below levels believed to be safe for the tissue and electrode materials involved. When the spinal reflex pathway was interrupted by crushing the nerve, the force produced by a given stimulus changed in some cases, but not in others, implying that the spinal reflex contribution was not the same in all the implants. We conclude that intrafascicular recording electrodes are also a potentially valuable technology for functional neuromuscular stimulation, and warrant further development.