Nerve-based stimulating electrodes provide the technology for advancing the function of motor system neural prostheses. The goal of this work was to measure and quantify the recruitment properties of a 12 contact spiral nerve cuff electrode. The cuff was implanted on the cat sciatic nerve trunk, which consists of at least four distinct motor fascicles, and the torque generated at the ankle joint by selective stimulation of the nerve was recorded in nine acute experiments. Comparisons of torques generated with the cuff to torques generated by selective stimulation of individual nerve branches indicated that the cuff allowed selective activation of individual nerve fascicles. Selectivity was dependent on the relative location of the electrode contacts and the nerve fascicles, as well as the size and relative spacing of neighboring fascicles. Selective stimulation of individual nerve fascicles allowed independent and graded control of dorsiflexion and plantarflexion torques in all nine experiments. Field steering currents improved selectivity as reflected by significant increases in the maximum torques that could be generated before spillover to other fascicles, significant increases in the difference between the current amplitude at spillover and the current amplitude at threshold, and significant increases in the slope of the current distance relationship.

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Quantification of recruitment properties of multiple contact cuff electrodes

Implantable bioelectronic interfaces for lost nerve functions

Advances in functional neuromuscular stimulation (FNS) have increased the need for nerve cuff designs that can control multiple motor functions through selective stimulation of selected populations of axons. This selectivity has proved to be difficult to achieve. Recent experiments suggest that it is possible to slowly reshape peripheral nerve without affecting its physiological function. Using computer simulations the authors have tested the hypothesis that changing the cross section of a nerve from a round to a flat configuration can significantly improve the selectivity of a nerve cuff. The authors' introduce a new index to estimate selectivity to evaluate the various designs. This index is based on the ability of a nerve electrode to stimulate a target axon without stimulating any other axons. The calculations involve a three-dimensional finite element model to represent the electrical properties of the nerve and cuff and the determination of the firing properties of individual axons. The selectivity rating was found to be significantly higher for the Flat Cuff than the Round Cuff. The result was valid with uniform or random distribution of axons and with a random distribution of fascicles diameters. Flattening of individual fascicles also improved the selectivity of the Flat Cuff but only when the number of contacts used was increased to maintain uniform contact density. Therefore, cuff designs that can reshape the nerve into flatter configurations should yield better cuff performance than the cylindrical cuffs but will require higher contact density.

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Selectivity of multiple-contact nerve cuff electrodes: a simulation analysis

The degree of spatial selectivity which can be obtained with longitudinal dot tripoles in an insulating cuff was quantified in terms of the overlap between fiber populations activated by different tripoles. Previous studies have failed to take into account the relative influences of transverse current and longitudinal current on position-selective activation, and furthermore have not controlled for the differing sensitivities of large and small nerve fibers to electrical stimuli. In this study, these factors were taken into account. Transverse current from an anode positioned opposite the stimulating cathode was found to improve spatial selectivity, and selectivity was enhanced when the ratio of transverse current to longitudinal current was increased. Large fibers were excited before small fibers, irrespective of fiber position, indicating a combination of position and size selectivity.

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Position-selective activation of peripheral nerve fibers with a cuff electrode

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.

Selective activation of muscle groups in the feline hindlimb through electrical microstimulation of the ventral lumbo-sacral spinal cord

Sequential stimulation during one muscle contraction of several compartments of a motor nerve, using multiple-electrodes, allows individual nerve-muscle compartments to be stimulated at fairly low frequencies. This provides time for recovery even during muscle contraction. However, the whole muscle is stimulated at near to its optimum fusion frequency, which provides smooth muscle contraction. This stimulation system imitates the natural activation of skeletal muscle. The new phrenic nerve stimulator described utilises the principle of sequential motor nerve stimulation. It also incorporates a sigh function. The sigh current recruits additional axons at certain intervals and thus creates and keeps available a reserve of conditioned muscle. Clinical advantages result: the conditioning phase after the beginning of long-term phrenic nerve stimulation for electroventilation is shortened and muscle fatigue is delayed. A need of increase of gas exchange can be answered by increasing tidal volume instead of respiration rate alone.

Neurophysiological and technical considerations for the design of an implantable phrenic nerve stimulator.

An electrical stimulation system for bilateral diaphragm pacing was constructed. The stimulator instrument comprises a transmitter and two implantable receivers and nerve electrodes. An optimised transcutaneous link system is described which utilises a radio frequency coupling to transmit both power and stimulus information to the receiver. The receiver unit was constructed in thick-film hybrid technology using bipolar and c.m.o.s. circuits and was packaged in a hermetically sealed metal case. The construction of the transmitter allows a high reliability, low power consumption and flexibility of the stimulus parameter control. A complete, bilateral phrenic nerve pacing system was implanted into a patient suffering from tetraplegia. It is concluded that electrical phrenic nerve stimulation to pace the diaphragm is capable of being used in clinical therapy.

G. A. Baer

Papers

Neurophysiological and technical considerations for the design of an implantable phrenic nerve stimulator.

Transcutaneous, dual channel phrenic nerve stimulator for diaphragm pacing