Counted cycles method to measure the block inception time of kiloHertz frequency mammalian motor nerve block.

BACKGROUND: Kilohertz frequency alternating currents (KHFAC) produce rapid nerve conduction block of mammalian peripheral nerves and have potential clinical applications in reducing nerve hyperactivity. However, there are no experimental measurements of the block inception time (BIT) for the complete block of mammalian motor axons, i.e. the time from the start of delivery of the KHFAC to the axons reaching a fully blocked state. NEW METHOD: A "counted cycles" method (CCM) was designed to exploit characteristics of the onset response, which is typical of KHFAC block, to measure the BIT with a millisecond time resolution. Randomized and repeated experiments were conducted in an in-vivo rodent model, using trains of KHFAC over a range of complete cycle counts at three frequencies (10, 20, and 40 kHz). RESULTS: Complete motor nerve conduction block was obtained in the rat sciatic nerve (N = 4) with an average BIT range of 5 ms-10 ms. The fastest BIT measured was 2.5 ms-5 ms. There was no statistical difference between the block inception times for the three frequencies tested. COMPARISON WITH EXISTING METHODS: There are no comparable methods to measure the KHFAC BIT. CONCLUSION: The KHFAC BIT is faster than previously estimated. KHFAC motor nerve block is established in milliseconds. These results may assist in the design of methods to eliminate the onset response produced by KHFAC nerve block.

Reduction of the onset response in kilohertz frequency alternating current nerve block with amplitude ramps from non-zero amplitudes.

BACKGROUND: Kilohertz frequency alternating current (KHFAC) waveforms reversibly block conduction in mammalian peripheral nerves. The initiation of the KHFAC produces nerve activation, called the onset response, before complete block occurs. An amplitude ramp, starting from zero amplitude, is ineffective in eliminating this onset activity. We postulated that initiating the ramp from a non-zero amplitude would produce a different effect on the onset. METHODS: Experiments were conducted in an in vivo rat model. KHFAC was applied at supra block threshold amplitudes and then reduced to a lower sub block amplitude (25, 50, 75 and 90% of the block threshold amplitude). The amplitude was then increased again to the original supra block threshold amplitude with an amplitude ramp. This ramp time was varied for each of the amplitude levels tested. RESULTS: The amplitude ramp was successful in eliminating a second onset. This was always possible for the ramps up from 75 and 90% block threshold amplitude, usually from 50% but never from 25% of the block threshold amplitude. CONCLUSIONS: This maneuver can potentially be used to initiate complete nerve block, transition to partial block and then resume complete block without producing further onset responses.

VTC4 is a bifunctional enzyme that affects myoinositol and ascorbate biosynthesis in plants.

Myoinositol synthesis and catabolism are crucial in many multiceullar eukaryotes for the production of phosphatidylinositol signaling molecules, glycerophosphoinositide membrane anchors, cell wall pectic noncellulosic polysaccharides, and several other molecules including ascorbate. Myoinositol monophosphatase (IMP) is a major enzyme required for the synthesis of myoinositol and the breakdown of myoinositol (1,4,5)trisphosphate, a potent second messenger involved in many biological activities. It has been shown that the VTC4 enzyme from kiwifruit (Actinidia deliciosa) has similarity to IMP and can hydrolyze l-galactose 1-phosphate (l-Gal 1-P), suggesting that this enzyme may be bifunctional and linked with two potential pathways of plant ascorbate synthesis. We describe here the kinetic comparison of the Arabidopsis (Arabidopsis thaliana) recombinant VTC4 with d-myoinositol 3-phosphate (d-Ins 3-P) and l-Gal 1-P. Purified VTC4 has only a small difference in the V(max)/K(m) for l-Gal 1-P as compared with d-Ins 3-P and can utilize other related substrates. Inhibition by either Ca(2+) or Li(+), known to disrupt cell signaling, was the same with both l-Gal 1-P and d-Ins 3-P. To determine whether the VTC4 gene impacts myoinositol synthesis in Arabidopsis, we isolated T-DNA knockout lines of VTC4 that exhibit small perturbations in abscisic acid, salt, and cold responses. Analysis of metabolite levels in vtc4 mutants showed that less myoinositol and ascorbate accumulate in these mutants. Therefore, VTC4 is a bifunctional enzyme that impacts both myoinositol and ascorbate synthesis pathways.

Effects of ramped amplitude waveforms on the onset response of high-frequency mammalian nerve block.

Though high-frequency alternating current (HFAC) can block nerve conduction, the block is invariably preceded by an onset response which is a period of repetitive nerve firing. We tested the hypothesis that slowly ramping up the amplitude of the HFAC waveform could produce block without this initial onset response. Computer simulations were performed, using the McIntyre-Richardson-Grill (MRG) model of myelinated mammalian axon. A ramped-amplitude HFAC was applied to axons of diameters ranging from 7.3 microm to 16 microm and at frequencies ranging from 3125 Hz to 40 kHz. The ramped-amplitude HFAC was also investigated in vivo in preparations of rat sciatic nerve. Sinusoidal voltage-regulated waveforms, at frequencies between 10 kHz and 30 kHz, were applied with initial amplitudes of 0 V, linearly increasing with time to 10 V. Ramp durations ranged from 0 s to 60 s. In both the MRG model simulations and the experiments, ramping the HFAC waveform did not eliminate the onset response. In the rat experiments, the peak amplitude of the onset response was lessened by ramping the amplitude, but both the onset response duration and the amount of onset activity as measured by the force-time integral were increased.

Selective block of external anal sphincter activation during electrical stimulation of the sacral anterior roots in a canine model.

Our aim was to electrically activate small diameter parasympathetic fibres in the sacral anterior roots, without activating the larger somatic fibres to the external anal sphincter (EAS). Electrodes were implanted on selected roots in five adult dogs. Pressures were recorded from the rectum and EAS. Quasitrapezoidal (Qzt) pulses for selective activation of smaller axons and narrow rectangular (Rct) pulses to activate all fibres were applied. Sphincter block was defined as [(P(max) - P(min))/P(max)] x 100%. Roots were also tested with 20 Hz trains. In three animals, evacuation of bowel contents was recorded with artificial fecal material. Stimulation with Qzt pulses showed decrease in sphincter recruitment with increasing pulse amplitudes, indicating propagation arrest in the large fibres. The average sphincter suppression was 94.1% in 16 roots implanted. With Qzt pulse trains, the average evoked sphincter pressure was significantly lower than Rct pulses. Evoked rectal pressures were not significantly different. The mean mass of expelled bowel contents of 51.1 g by Qzt trains was significantly higher than that of 14.8 g expelled by Rct trains. Our results demonstrate that this selective stimuli can activate small diameter fibres innervating the distal bowel and result in significant evacuation of rectal contents.

Nerve conduction block utilising high-frequency alternating current.

High-frequency alternating current (AC) waveforms have been shown to produce a quickly reversible nerve block in animal models, but the parameters and mechanism of this block are not well understood. A frog sciatic nerve/gastrocnemius muscle preparation was used to examine the parameters for nerve conduction block in vivo, and a computer simulation of the nerve membrane was used to identify the mechanism for block. The results indicated that a 100% block of motor activity can be accomplished with a variety of waveform parameters, including sinusoidal and rectangular waveforms at frequencies from 2 kHz to 20 kHz. A complete and reversible block was achieved in 34 out of 34 nerve preparations tested. The most efficient waveform for conduction block was a 3-5 kHz constant-current biphasic sinusoid, where block could be achieved with stimulus levels as low as 0.01 microCphase(-1). It was demonstrated that the block was not produced indirectly through fatigue. Computer simulation of high-frequency AC demonstrated a steady-state depolarisation of the nerve membrane, and it is hypothesised that the conduction block was due to this tonic depolarisation. The precise relationship between the steady-state depolarisation and the conduction block requires further analysis. The results of this study demonstrated that high-frequency AC can be used to produce a fast-acting, and quickly reversible nerve conduction block that may have multiple applications in the treatment of unwanted neural activity.

High-frequency nerve conduction block.

High frequency alternating current waveforms have been shown to produce a rapidly reversible nerve block in animal models, but the parameters and mechanism of this block are not well understood. A frog sciatic nerve/gastrocnemius muscle preparation was used to examine the parameters for nerve conduction block in vivo. A complete and reversible nerve block was achieved in all preparations. The results indicate that a 100% block of motor activity can be accomplished with a variety of waveform parameters, including sinusoidal and rectangular waveforms at frequencies from 2 kHz to 20 kHz. The most efficient waveform for conduction block was a 3-5 kHz constant-current biphasic sinusoid. It was demonstrated that the block is not produced indirectly through fatigue.

A reusable, self-adhesive electrode for intraoperative stimulation in the lower limbs.

A suction-based stimulating electrode was designed and fabricated to allow intraoperative testing of lower-limb muscles during routinely scheduled surgical procedures. The suction device can adhere to a small exposure of muscle surface with reproducible contact forces and can maintain its geometric relationship to the underlying tissue for sufficient time to grade the resulting muscle contraction before removal and repositioning. When operated with a 10-cc syringe, the device can generate between 0 and 23 N of contact force; correlation between measured contact forces and those analytically predicted was 0.989. Preliminary animal testing indicates that the reusable device maintains its position over the nerve entry point even during vigorous active contractions of the stimulated muscle. Thus, it may be a valuable useful tool for locating the optimal site for a permanent electrode for functional electrical stimulation (FES) applications, as well as an ideal means of providing accurate and repeatable stimulation in various locations.

Urethral pressure profiles in the female canine implanted with sacral anterior nerve root electrodes.

The purpose of this work was to study the pressure distribution along the urethra in female canines with and without electrical stimulation of the sacral anterior roots innervating the bladder and urethra. Urethral pressure profiles were recorded in two orientations, dorsal and ventral, with microtransducer catheters. Two pulse types were applied at 1 Hz, 500 micros quasitrapezoidal pulses to selectively activate the small axons and 100 micros rectangular pulses. Four parameters were measured from each profile; maximum urethral pressure (MUP), bladder pressure (Pv), functional urethral length (FUL), and the position of the maximum peak from the bladder neck (PMP). Two derived measures, the estimated maximum urethral closing pressure (UCP) and the position of the maximum as a percentage of the FUL (PM%) were calculated. There were highly significant differences (P<0.01) in the value and position of the estimated UCP in the two orientations of the transducers. The highest pressures were recorded in the ventral orientation near the terminal portions of the urethra. Principal sphincter activity during electrical stimulation of the ventral sacral roots was also confined to this part. Selective small fiber activation did not result in any significant increase in this peak pressure from passive values.

Implanted stimulators for restoration of function in spinal cord injury.

Neuroprostheses that electrically stimulate paralyzed muscles provide functional enhancements for individuals with spinal cord injury and stroke such as standing and stepping, reaching and grasping, and bladder and bowel function. For chronic applications, implanted neuroprostheses lead to reliable, low-maintenance and patient-acceptable systems. The advantages of such systems are discussed followed by a generic description of implantable stimulators. Features of current first and second generation neuroprostheses developed at our centre are discussed followed by our experience in the application of these devices in the rehabilitation of individuals with spinal cord injury.

Surgical technique for installing an eight-channel neuroprosthesis for standing.

A standardized surgical procedure to implant an eight-channel functional neuromuscular stimulation system in the lower extremities for standing, exercise, and transfers for individuals with spinal cord injury has been developed. The implanted components include: (1) one eight-channel receiver-stimulator, (2) epimysial electrodes, (3) intramuscular electrodes, and (4) inline connectors. The development process included identifying the target muscle set for electrode placement and the corresponding surgical approaches, determining the stages of the surgical procedure, and assessing the effectiveness and stability of the implanted neuroprosthesis. The bilateral muscle set consists of the vastus lateralis, the gluteus maximus, the semimembranosus, and the erector spinae. Surgical approaches to the nerve entry points were developed through a series of cadaveric studies and intraoperative tests. Electrode placement is related to bony landmarks and based on standard orthopaedic approaches. The components of the neuroprosthesis are installed in one surgical session, with three stages. This procedure has been applied successfully in seven individuals, resulting in strong, isolated stimulated contractions adequate to raise and lower the body, maintain standing with a walker, and perform pivot transfers. The standardized surgical procedure is repeatable and teachable and will be used in upcoming multicenter clinical trials of the implanted neuroprosthesis.

Structured sleeve for repair of implantable in-line connectors.

A structured miniature repair sleeve has been designed for implantable in-line connectors that develop small current leaks post-implant. The repair sleeve has been successfully utilised in one subject following the development of current leakage in connectors on an implanted joint angle sensor.

Implantable transducer for two-degree of freedom joint angle sensing.

An implantable joint angle transducer (IJAT) was developed to provide command-control and feedback-control information for chronic use with functional neuromuscular stimulation (FNS) neuroprostheses. The IJAT uses Hall effect sensors to transduce joint angle. A titanium encapsulated array of Hall effect sensors and support circuitry is surgically implanted in one bone, and a similarly encapsulated permanent magnet in an opposing bone, across a joint. The IJAT provides consistent, reliable, high quality signals that reflect joint movement from midsized two-degree-of-freedom joints. IJAT's were implanted using a chronic in vivo dog model to demonstrate the feasibility of implantation and periodic measurement techniques, and to validate modeling techniques used for prediction of function and calibration. The flexion resolution ranged from 0.4 to 3.0 degrees over a range of 115 degrees. The maximum deviation from a linear response was 9 degrees. The resolution and linearity depend on several transducer and joint geometry parameters, and can be predicted prior to implantation and calibrated after implantation. The results of this study 1) defined the most appropriate hermetic capsule designs for the IJAT sensor and magnet, 2) defined the best orientation of the magnetic field to optimize device function, 3) provided a computer model of the IJAT to aid in placement, calibration, and evaluation of the device, 4) verified the surgical techniques used to implant the device, and 5) verified the long-term functionality and the biocompatibility of the device.

Variations in innervation of the flexor digitorum profundus muscle.

The composite flexor digitorum profundus muscle has a dual nerve supply from the ulnar nerve (UN) and the anterior interosseous nerve (AIN) but anatomic data regarding the territories of these 2 nerves are limited. In this study, muscles from 20 cadaver forearms were dissected microscopically. The motor nerves were followed to their terminations on individual muscle bellies and the innervation domains mapped. In 75% of cases the AIN supplied the index and middle fingers and the UN supplied the middle, ring, and little fingers; thus, the middle finger had dual innervation. In 20% of cases the AIN went to the index and middle fingers and the UN went to the ring and little fingers. In 5% of cases the AIN went to the index finger and the UN went to the middle, ring, and little fingers. The motor entry points were normalized to the forearm length. The entry points of the UN and AIN branches were at 15% and 30% of forearm length, respectively, distal to the medial epicondyle.

Bladder and urethral pressures evoked by microstimulation of the sacral spinal cord in cats.

Experiments were conducted to measure the bladder and urethral pressures evoked by intraspinal microstimulation of the sacral segments (S1-S2) in neurologically intact, chloralose anesthetized adult male cats. The bladder pressure was measured with a superpubic catheter and the urethral pressure was measured simultaneously at the level of the urethral sphincter and at the level of the penis using a two-element micromanometer. Intraspinal stimuli (typically 1 s, 20 Hz, 100 microA, 100 microseconds) were applied with activated iridium microwire electrodes in ipsilateral segments and intersegmental boundaries with a 250 micrometer mediolateral resolution and a 200 micrometer dorsoventral resolution. Increases in bladder pressures were generated by microstimulation in the intermediolateral region, in the lateral and ventrolateral ventral horn, and around the central canal. Simultaneous increases in urethral pressure were evoked by microstimulation in the ventrolateral ventral horn, but not at the other locations. Small reductions in urethral pressure (<10 cm H(2)O) were evoked at locations in the intermediate laminae and around the central canal. The magnitude of these pressure reductions was weakly dependent on the stimulus parameters. Stimulation around the central canal produced bladder contractions with either no change or a reduction in urethral pressure and voiding of small amounts of fluid. These results demonstrate that regions are present in the spinal intact anesthetized cat where microstimulation generates selective contraction of the bladder without increases in urethral pressure and that regions are present where microstimulation generates small reductions in urethral pressure.

The function of the finger intrinsic muscles in response to electrical stimulation.

The actions of the dorsal interosseous, volar interosseous, and lumbrical muscles were investigated using applied electrical stimulation and recording the moments that were generated across the metacarpophalangeal joint in flexion/extension and abduction/adduction, the proximal interphalangeal joint in flexion/extension, and the distal interphalangeal joint in flexion/extension. These measurements were made isometrically at various joint angles and levels of stimulation with both able bodied subjects and persons who had sustained tetraplegia. It was determined that the dorsal interossei, including the first, were strong abductors of the fingers and generated a significant moment in metacarpophalangeal (MP) joint flexion and interphalangeal (IP) joint extension. The volar interossei were the primary adductors of the fingers, as well as providing a significant moment in MP joint flexion and IP joint extension. The lumbrical muscles were found to be MP joint flexors and IP joint extensors, although the moments that were generated were on average 70% lower than the interossei. The role of the lumbricals as finger abductors or adductors could not be determined from the data. This information on the actions and moment generating capabilities of the intrinsic muscles led to the incorporation of the interossei into electrically induced hand grasp provided by an implanted neuroprosthesis. The evaluation of the intrinsic muscles in the neuroprosthesis was accomplished by recording the moment generating capabilities of these muscles across each of the joints of the finger. These muscles were capable of generating moments that were 80-90% of the average attained by the able bodied subjects, and have provided a substantial improvement to the electrically induced hand grasp.

Functional anatomy of the male feline urethra: morphological and physiological correlations.

Anatomical and histological methods were combined with measurements of the urethral pressure profile (UPP) to investigate the functional aspects of the urethra in male cats. A silicone rubber catheter with two microdiaphragm pressure transducers was used to measure the UPP. Gross anatomy and ultrastructure of the urethra at each segment were examined and correlated with the pressure profile data. The preprostatic urethra was composed of three layers of smooth muscle, while distal to the prostate striated muscle became predominant. Increased baseline pressure and rapid fluctuations in pressure in the postprostatic urethra and bulbourethra resulted from the function of periurethral striated musculature. The UPP was affected by the bladder pressure, repetition of the measurement, the sensor orientation in the urethra, and the type of measurement catheter. Well controlled high fidelity measurements enabled a clear correlation to be established between the features of the UPP and the anatomy of the urethra and surrounding musculature. Additionally, observations on the ultrastructural and microscopic anatomy of the urethra extend a previous description of the pelvic urethra.

Functional conditions of micturition induced by selective sacral anterior root stimulation: experimental results in a canine animal model.

Electrical stimulation of the sacral anterior roots using conventional rectangular current pulses results in a simultaneous contraction of the urinary bladder and the striated urethral sphincter. Using a tripolar nerve cuff electrode with quasitrapezoidal current pulses and appropriate stimulation parameters, hyperpolarization of the nerve-fiber cell membrane under the anode of the stimulating electrode can reversibly arrest action potential propagation in large myelinated nerve fibers, innervating the striated urethral sphincter, while leaving action potential propagation unaffected in small nonmyelinated nerve fibers innervating the urinary bladder smooth muscle (anodal arrest). Using this technique in 19 female mongrel dogs, we studied the effect of bladder filling, level of anesthesia, and sacral deafferentation on bladder pressure, urethral pressure, and urinary flow. Effective micturition could be induced only after complete dorsal rhizotomy, abolishing reflex contraction of the striated urethral sphincter, when blocking quasitrapezoidal current pulses were used for stimulation. Stimulation with rectangular current pulses directly induced a rise in distal urethral pressure, preventing micturition during stimulation.

Implantation of a 16-channel functional electrical stimulation walking system.

A 16-channel electrical stimulation system was implanted in a 39-year-old patient with T10 paraplegia to restore sit to stand, walking, and exercise functions. System implantation required two surgical sessions. In the first session, the posterior muscle set consisting of bilateral semimembranosus, adductor magnus, and gluteus maximus muscles were exposed and epimysial electrodes sutured at the point of greatest muscle contraction. Closed double helix intramuscular electrodes were implanted in the erector spinae. Two weeks later, epimysial electrodes were attached to the eight anterior muscles consisting of the tibialis anterior, sartorius, tensor fasciae latae, and vastus lateralis with all 16 electrode leads passed to the anterior abdominal wall. The electrodes were connected to two eight-channel stimulators placed in the iliac fossae, and the system was checked by activating the individual muscles. The implanted stimulators received stimulation instructions and power via a radio frequency link to an external control. Stimulation patterns for standing, walking, sitting, and exercise functions were chosen from a preprogrammed menu via a finger key pad. After 3 weeks of restricted patient activity, all electrodes stimulated either the target muscle or had an acceptable spillover pattern. The patient is undergoing a 16-week rehabilitation course of stimulated exercises gradually increasing in intensity. At the conclusion, the goal is to discharge the patient with the system for spontaneous use. Although long term followup is required to determine system reliability, preliminary clinical results indicate that targeted, repeatable, functional muscle contractions in the lower extremity can be achieved with a system consisting of epimysial electrodes.