J.P.A. Smit

Endoneural selective stimulating using wire-microelectrode arrays

In acute experiments eight 5- to 24-wire-microelectrode arrays were inserted into the common peroneal nerve of the rat, to investigate whether the electrodes could selectively stimulate motor units of the extensor digitorum longus (EDL) muscle. Twitch-force-recruitment curves were measured from the EDL for each array electrode. The curves were plotted on a double-logarithmic scale and parameterized by the low-force slope (which represents the power p in the power-law relationship of force F versus stimulus current I, or F approximately I(p)) and the threshold current. The slopes and threshold currents measured with array electrodes did not differ significantly from those obtained with randomly inserted single wire-microelectrodes. This indicates that, although involving a more invasive insertion procedure, electrode arrays provide neural contacts with low-force recruitment properties similar to those of single wires. Array results revealed partial blocking of neural conduction, similar to that reported with microneurographic insertion with single needles. The efficiency of the array was defined as the fraction of array electrodes selectively contacting a motor unit and evoking the corresponding threshold force. Efficiency thus expresses the practical value of the used electrode array in terms of the total number of distinct threshold forces that can be stimulated by selecting the appropriate electrodes. The eight arrays were capable of evoking threshold forces selectively with an average efficiency of 0.81 (or 81%).

Force-current relationships in intraneural stimulation: role of extraneural medium and motor fibre clustering.

Animal experiments and model simulations of monopolar, intrafascicular nerve stimulation are presented to study force-current relationships (recruitment curves). The conductivity of the extraneural medium is of prime importance to the resulting recruitment curves: an insulating extraneural medium generally leads to steeper curves with lower threshold currents than a well-conducting extraneural medium. Extensive statistical comparison of experimental and model results suggests the occurrence of clustering of α-motoneurons within the fascicle, manifesting itself mainly by an increased spread in threshold currents, as opposed to the situation where the fibres are distributed uniformly throughout the entire fascicle.

The influence of nerve fiber distribution on the recruitment behavior resulting from intraneural stimulation

Force recruitment cumes have been recorded for the rat extensor digitorum longus muscle (EDL) using monopolar stimulation with intraneural wire electrodes. The recruitment curves vary with the position of the electrode in the nerve This has implications when intraneural stimulation techniques are used to precisely control muscle force. To assess this effect, two parameters were extracted for each recruitment cuwe: the threshold current and the slnpe in the low-farce range. When these experimental parameters are compared with the same parameters resulting from simulated recruitment curves, it becomes clear that the low density of nodes of Ranvier and the degree of clustering of nerve fibers are of major importance for the recruitment behavior resulting from intraneural stimulation. It is also shown that the external medium conductivity σe, is an important parameter influencing the recruitment behavior.

Intraneural stimulation using wire-microelectrode arrays: analysis of force steps in recruitment curves

In acute experiments on six Wistar rats, a wire-microelectrode array was inserted into the common peroneal nerve. A 5-channel array and a 24-channel array were available. Each electrode in the array was used to generate a twitch contraction force recruitment curve for the extensor digitorum longus muscle. We constructed a histogram of the pooled force steps in all recruitment curves. From a comparison of this experimental histogram with one estimated from literature data, we found that the force steps encountered in our experiments are in the same range as those from the literature-based estimated distribution. Discrepancies between the experimental and the literature-based histogram might be ascribed to an approximation used in the estimated distribution. We conclude that force step histograms appear to provide a simple means for estimating motor unit twitch force distributions, and thus are of value in studies of intraneural selective stimulation.

The effect of spatial clustering of motor fibers in a nerve fascicle on force recruitment during electrical stimulation

Force recruitment curves have been determined using stimulation with an intraneural wire electrode. The recruitment curves vary with the position of the electrode in the fiber bundle. The results seem to support previous findings which indicated that the low density of nodes of Ranvier and a possible clustering of motor fibers within the fiber bundle might have a major impact on the efficacy of intraneural electrical stimulation.

A gate-array/multi-electrode device: circuit performance and interfacing

The Twente 128-fold 3-dimensional multi-micro electrode device for endoneural stimulation is controlled by a 4/spl times/4 mm CMOS gate array chip. The chip contains the electronic circuitry, i.e. eight 16-channel multiplexers, eight double current sources and six differential amplifiers. Control of the gate array circuitry by a PC is done by a PCI interface. We present (aspects of) the design and realization of the PCI interface, as well as the simulated and measured performance figures of the current sources and amplifiers.

Efficiency of endoneural stimulation with 5- to 24-fold multielectrodes

Optimal selective stimulation of nerve with endoneurally (intrafascicularly) inserted multi-microelectrodes means that each electrode activates, with its own threshold stimulation current, as few distinct motoneurons as possible, preferably only one, If the latter is the case, the efficiency of a multi-electrode is 100%. However, neighbouring electrodes may control the same motor fiber(s), as there are generally more fibers than electrodes and because the position of fibers is largely unknown. In that case, efficiency is less than 100%. This paper reports on experiments in rat peroneal nerve with 5- and 24-fold wire multi-microelectrode arrays, The threshold force of the twitch recruitment curve of the corresponding EDL muscle was used to monitor nerve activation, It was found that on average the threshold force efficiency was 0.48=48%, After re-inspection of the data, taking into account that neighbouring electrodes have a higher probability to activate the same motor units, in contrast to distant electrodes, the average efficiency even rises to 81%. For several reasons, threshold forces do not correspond to motor unit forces, implying that the threshold-force-efficiency can not be regarded as motor-unit-efficiency.

Intraneural stimulation using 2D wire-microelectrode arrays. II. Comparison with single-wire electrode results

A two-dimensional wire-microelectrode array was inserted into the peroneal nerve of the rat through an incision. For each of the electrodes in the array the corresponding twitch-force recruitment curve was recorded from the extensor digitorum longus muscle (EDL). The mean value and standard deviation of the threshold current were found to be not significantly different from those for single wire electrodes. This suggests that the incision does not introduce significant (additional) current leakage.

Intraneural stimulation using 2D wire-microelectrode arrays. I. Experimental results

A two-dimensional 24-channel wire-microelectrode array was inserted into the peroneal nerve of the rat during acute experiments. The electrodes in the array are on a regular grid of 6 by 4 electrodes; inter-electrode spacing is 120 /spl mu/m. For each of the electrodes in the array the corresponding twitch-force recruitment curve was recorded from the extensor digitorum longus muscle (EDL). A complete set of 24 recruitment curves is presented The shape of the recruitment curves varies among the electrodes in the array. This supports previous findings which suggest a different motor unit recruitment order for stimulating electrodes at different intraneural positions.