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Biomedical subjects

J T Mortimer

Publications and source records attributed to J T Mortimer.

At least 37 records · Page 2Linked to original sources
PubMed1996

The effects of work intensity on adolescent mental health, achievement, and behavioral adjustment: new evidence from a prospective study.

This article examines the effects of work intensity on adolescent mental health, academic achievement, and behavioral adjustment. Questionnaire data were collected yearly from an initial panel of 1,000 randomly selected ninth graders (14-15 years old). Consistent with other studies, students who worked at higher intensity engaged in more alcohol use. The methodological strengths of this research (a representative panel studied prospectively over a 4-year period with minimal attrition and an analysis incorporating key control and lagged variables) provide strong evidence that adolescent work fosters alcohol use. The contention that work of high intensity has deleterious effects on mental health, academic achievement, and 2 other indicators of behavioral adjustment did not withstand our stringent tests. However, high school seniors who worked at moderate intensity (1-20 hours per week) had higher grades than both nonworkers and students who worked more hours per week.

Achievement↗
PubMed1995

Helpfulness and the development of competence in adolescence.

Using data from a representative panel of 1,000 Minnesota youth, this paper explores "helpfulness" in 2 spheres of adolescents' lives: the home and paid work settings. We examine the social structural conditions under which helpful behaviors are elicited, the interrelations of helpfulness and competence across 2 years of middle adolescence, and whether social circumstances moderate the effects of helpfulness on competence. Both boys' and girls' helpfulness in the home is responsive to family need. Furthermore, helpfulness at work and girls' competence are reciprocally related. We find evidence that the effects of helpfulness depend on the helper's motivations and the act's meaning, as shaped by the social context. Girls' competence is diminished by helpfulness in the home under conditions of poor father-daughter relationships and coercive maternal control.

Adolescent↗
PubMed1994

Long-term intramuscular electrical activation of the phrenic nerve: safety and reliability.

The safety and reliability of a system for long-term intramuscular electrical activation of the phrenic nerve was evaluated in seven dogs. In this system, electrodes are implanted bilaterally into the diaphragm without directly contacting the phrenic nerve using a laparoscope to direct placement. Five dogs underwent chronic bilateral intramuscular diaphragm stimulation (IDS) for 61 to 183 days at stimulus parameters selected to evoke at least 120% of the animal's basal ventilation. Two dogs maintained as controls did not undergo chronic stimulation. The safety and reliability of the system was evaluated in terms of tissue responses to the electrode, alterations in diaphragm muscle, pulmonary function, electrode reliability, and cardiac activation. (The efficacy of long-term intramuscular activation of the phrenic nerve is addressed in a companion paper.) No adverse responses to the electrode or stimulation were found. The histochemistry of chronically stimulated diaphragm suggested transformation towards type I (oxidative metabolism) muscle fibers. Two IDS electrodes dislodged out of a total of 32 IDS electrodes implanted. Both electrodes dislodged within seven days of implant. All IDS electrodes had stable and repeatable recruitment properties. No IDS electrode mechanical failures were found and no electrode corrosion was observed. We conclude from these experiments that intramuscular activation of the phrenic nerve will present a minimal risk to human patients who are good candidates for clinical studies using this technique.

Animals↗
PubMed1994

Long-term intramuscular electrical activation of the phrenic nerve: efficacy as a ventilatory prosthesis.

The efficacy of a system for long-term intramuscular activation of the phrenic nerve as a ventilatory prosthesis was evaluated in seven dogs. (The safety and reliability of this system is addressed in a companion paper.) Five dogs underwent chronic bilateral intramuscular diaphragm stimulation (IDS) for 61 to 183 days at stimulus parameters selected to evoke at least 120% of the animal's basal ventilation. Two dogs maintained as controls did not undergo chronic stimulation. The ability of IDS to provide long-term ventilation without diaphragm fatigue was evaluated in terms of the ventilatory capacity of IDS, the effects of chronic IDS on diaphragm contractile properties, and the phrenic nerve recruitment properties of chronic IDS electrodes. Hemidiaphragms with electrodes placed within 2 cm of the phrenic nerve trunk could be completely activated by 25 mA pulses having a 100 microsecond pulse width. The tidal volume evoked by IDS in this study was 167% (+/- 48 s.d.) of that required for full-time basal ventilation without diaphragm fatigue. Evoked tidal volume increased after 8 to 9 weeks of chronic IDS for stimulus pulse intervals longer than 50 mS. Electrode recruitment properties were stable for both active and passive implanted electrodes. We conclude from these studies that with properly placed electrodes IDS is capable of providing reliable full-time ventilatory support without fatiguing the diaphragm.

Animals↗
PubMed1993

Selective control of muscle activation with a multipolar nerve cuff electrode.

Acute experiments were performed on adult cats to study selective activation of medial gastrocnemius, soleus, tibialis anterior, and extensor digitorum longus with a cuff electrode. A spiral nerve cuff containing twelve "dot" electrodes was implanted around the sciatic nerve and evoked muscle twitch forces were recorded in six experiments. Spatially isolated "dot" electrodes in four geometries: monopolar, longitudinal tripolar, tripolar with four common anodes, and two parallel tripoles, were combined with transverse field steering current(s) from an anode(s) located 180 degrees around from the cathode(s) to activate different regions of the nerve trunk. To quantify the degree of selectivity, a selectivity index was defined as the ratio of the force in one muscle to the force in all four muscles in response to a particular stimulus. The selectivity index was used to construct recruitment curves for a muscle with the optimal degree of selectivity. Physiological responses were correlated with the anatomical structure of the sciatic nerve by identifying the nerve fascicles innervating the four muscles, and by determining the relative positions of the electrodes and the nerve fascicles. The results indicated that the use of transverse field steering current improved selectivity. We also found that tripoles with individual dot anodes were more selective than tripoles with four common dot anodes. Stimulation with two parallel tripoles was effective in activating selectively fascicles that could not be activated selectively with only a single tripole. The multipolar cuff proved an effective method to control selectively and progressively the force in muscles innervated by fascicles that were well defined at the level of the cuff.

Animals↗
PubMed1993

Management of the neurogenic bowel in patients with spinal cord injury.

Bowel dysfunction in patients with spinal cord injury has a significant impact on quality of life as well as causing morbidity and death. This article reviews the pathophysiologic features of the neurogenic bowel of patients with spinal cord injury. Also discussed are the clinical manifestations, current options for management, and newer approaches that address this chronic and debilitating problem.

Anus Diseases↗
PubMed1991

A study of the fatigue properties of small diameter wires used in intramuscular electrodes.

Single and multi-strand stainless steel and cobalt-nickel alloy wires, with strand diameters from 26 to 46 microns, were fatigue tested using a modified rotating bending test to determine what factors are most important in controlling fatigue life. The relation between cyclic strain and cyclic life was determined for each material by cyclically straining test specimens at various strain ranges and recording the number of cycles to failure. The results show that (a) the fatigue curves of the 316LVM, MP35N, DBS, and Syntacoben wires are very similar and have many of the same fatigue characteristics of specimens of large cross section. (b) Multi-stranded wires have the same average fatigue life as their individual constituent strands, but the variance of that life is smaller. (c) Deformities in the wire, which are created during the manufacturing, appear to have the effect of shortening the fatigue life of these small section wires. (d) Observation of wire fracture surfaces show a relatively small crack propagation zone and a large fast fracture zone suggesting that most of the fatigue life of these small wires is in the original crack formation, which creates a large stress concentration and quickly leads to wire failure. (e) The size of the wire cross sectional area is of secondary importance compared to the amplitude of the maximum cyclic strain of the individual strands in determining fatigue life of the cable. To maximize the fatigue life of electrodes in vivo, the highest fatigue life for a given bending radius of curvature is desired. This suggests wire strands should be manufactured at the smallest diameter possible (without introducing structural flaws) to maximize service life.

Electrodes, Implanted↗
PubMed1991

Alternate excitation of large and small axons with different stimulation waveforms: an application to muscle activation.

A method has been developed to activate separately and alternately two axon groups in an intact nerve trunk through a single-channel stimulation electrode. Two types of stimuli, quasitrapezoidal-shaped and narrow rectangular-shaped current pulses, were employed to activate smaller and larger axons, respectively. The technique was demonstrated on an alpha motor fibre population in cat peripheral nerve and was utilised to reduce muscle fatigue during repetitive stimulation. This method may have applications in physiological research and the design of neural prostheses.

Axons↗
PubMed1991

Selective activation of small motor axons by quasi-trapezoidal current pulses.

We have found a method to activate electrically smaller nerve fibers without activating larger fibers in the same nerve trunk. The method takes advantage of the fact that action potentials are blocked with less membrane hyperpolarization in larger fibers than in smaller fibers. In our nerve stimulation system, quasitrapezoidal-shaped current pulses were delivered through a tripolar cuff electrode to effect differential block by membrane hyperpolarization. The quasitrapezoidal-shaped pulses with a square leading edge, a 350 microsecond(s) plateau, and an exponential trailing phase ensured the block of propagating action potentials and prevented the occurrence of anodal break excitation. The tripolar cuff electrode design restricted current flow inside the cuff and thus eliminated the undesired nerve stimulation due to a "virtual cathode." Experiments were performed on 13 cats. The cuff electrode was placed on the medial gastrocnemius nerve. Both compound and single fiber action potentials were recorded from L7 ventral root filaments. The results demonstrated that larger alpha motor axons could be blocked at lower current levels than smaller alpha motor axons, and that all alpha fibers could be blocked at lower current levels than gamma fibers. A statistical analysis indicated that the blocking threshold was correlated with the axonal conduction velocity or fiber diameter. This method could be used in physiological experiments and neural prostheses to achieve a small-to-large recruitment order in motor or sensory systems.

Action Potentials↗
PubMed1991

A method to effect physiological recruitment order in electrically activated muscle.

A new stimulation method has been utilized to achieve physiological recruitment order of small-to-large motor units in electrically activated muscles. The use of quasitrapezoidal-shaped pulses and a tripolar cuff electrode made selective activation of small motor axons possible, thus recruiting slow-twitch, fatigue-resistant muscle units before fast-twitch, fatigable units in a heterogeneous muscle. Isometric contraction force from the medial gastrocnemius muscle was measured in five cats. The physiological recruitment order was evidenced by larger twitch widths at lower force levels and smaller twitch widths at higher force levels in the muscles tested. In addition, force modulation process was more gradual and fused contractions were obtained at lower stimulation frequencies when the new stimulation method was employed. Furthermore, muscles activated by the new method were more fatigue-resistant under repetitive activation at low force levels. This stimulation method is simpler to implement and has fewer adverse effects on the neuromuscular system than previous blocking methods. Therefore, it may have applications in future functional neuromuscular stimulation systems.

Animals↗
PubMed1990

Imbalanced biphasic electrical stimulation: muscle tissue damage.

The effects of imbalanced biphasic stimulation were studied on cat skeletal muscle to determine if greater charge densities can be safely used than with balanced or monophasic stimulation. The results of the study indicate that imbalanced biphasic stimulation can be tolerated safely by tissue at or below a net dc current density of 35 microA/mm2 and not safely tolerated at or above a net dc current of 50 microA/mm2. Monophasic stimulation has been shown to be safe at or below net dc current levels of 10 microA/mm2 and in these studies we found it was not safe at or above net dc current levels of 20 microA/mm2. Stimuli were applied to muscles via coiled wire intramuscular electrodes using a regulated current source. Since the safe average current density was higher for imbalanced biphasic stimulation than for monophasic stimulation, this suggests that: (a) pH change is not the primary reaction causing tissue damage and (b) the damaging electrochemical process that takes place during a cathodic stimulation pulse can be reversed by an anodic pulse having substantially less charge than its companion cathodic pulse. We conclude that greater cathodic charge densities can be safely employed with imbalanced biphasic stimulation than with either monophasic stimulation or balanced charge biphasic stimulation.

Animals↗
PubMed1990

A nerve cuff technique for selective excitation of peripheral nerve trunk regions.

Numerical modeling and experimental testing of a nerve cuff technique for selective stimulation of superficial peripheral nerve trunk regions is presented. Two basic electrode configurations ("snug" cuff monopolar and tripolar longitudinally aligned dots) have been considered. In addition, the feasibility of "steering" excitation into superficial nerve trunk regions using subthreshold levels of current flow from an electrode dot located on the opposite side of the nerve has been tested. Modeling objectives were to solve for the electric field that would be generated within a representative nerve trunk by each electrode configuration; and to use a simple nerve cable model to predict the effectiveness of each configuration in producing localized excitation. In three acute experiments on cat sciatic nerve the objective was to characterize the effectiveness of each electrode configuration in selectively activating only the medial gastrocnemius muscle. Modeling and experimentation both suggest that longitudinally aligned tripolar dot electrodes on the surface of a nerve trunk, and bounded by a layer of insulation (such as a nerve cuff), will restrict excitation to superficial nerve trunk regions more successfully than will monopolar dot electrodes. Excitation "steering" will improve the spatial selectivity of both monopolar and tripolar electrode configurations.

Animals↗
PubMed1990

Taking the laparoscope to the laboratory for ventilatory research.

Chronic ventilatory support by phrenic nerve stimulation has been accomplished, but potential permanent phrenic nerve injury has limited its therapeutic use. Prior work from the authors' laboratory has demonstrated that direct muscle stimulation can be safely performed for prolonged periods. Thus, in 1983, a laparoscopic technique was devised for the direct implantation of diaphragm pacing electrodes in the canine. During the subsequent four years, 16 animals (10 acute and 6 chronic) have undergone diaphragm pacing. Under general anesthesia, diaphragm pacing electrodes loaded into 19-gauge needles were implanted under direct laparoscopic visualization in both hemidiaphragms. In each animal, tidal volume, transdiaphragmatic pressure, arterial blood gas, impedence, cardiac activation, and histology of electrode placement site have been evaluated. Chronically paced animals were subjected to daily 8- to 24-hr per day pacing. The longest paced animal has been 179 days with continuous 24-hour per day pacing. In both acute and chronic experiments, maximal evoked tidal volume has approached 400 per cent of resting tidal volume. Cardiac ectopy has not occurred. This work has demonstrated the safety and feasibility of direct muscular diaphragm activation using the laparoscope. It has provided the basis for embarking upon a clinical study of this modality as a method of chronic ventilatory support. The laparoscopic technique of electrode placement may have a variety of other potential applications.

Animals↗
PubMed1989

Muscle plasticity: comparison of a 30-Hz burst with 10-Hz continuous stimulation.

The changes in the contractile properties induced by a 30-Hz phasic stimulation paradigm were measured and compared with the changes induced by a 10-Hz continuous stimulation paradigm. The study was performed on the tibialis anterior muscles of cats with one paradigm applied to one hindlimb muscle and the other to the contralateral limb. Both hindlimb muscles received the same number of stimuli in a day, making the average stimulation frequency 10 Hz. Two periods of daily stimulation were studied, 8 and 24 h/day. Muscles stimulated at 30 Hz produced greater overall tetanic tension and, during a prolonged stimulation test, exerted a greater mean tension than muscles stimulated at 10 Hz (50 and 32% increase for animals stimulated for 8 and 24 h/day, respectively). Muscle mass was least reduced and fewer pathological abnormalities were observed in the muscles stimulated at 30 Hz. There were no apparent differences in the histochemistry or biochemistry between muscles stimulated at 10 and 30 Hz, which could account for these differences in muscle properties. These results indicate the 30-Hz paradigm may be better suited than 10 Hz continuous stimulation for applications requiring sustained muscle tension such as correction of scoliosis or muscle conditioning for motor prostheses.

Animals↗
PubMed1986

Recruitment properties of monopolar and bipolar epimysial electrodes.

Muscle force was studied as a function of stimulus parameters, epimysial electrode position relative to nerve supply, and muscle length to provide insight into the properties of motor prostheses that employ epimysial electrodes. The results of the acute experiments indicated that the dependence of recruitment (force versus stimulus amplitude) on muscle length was minimal for a monopolar electrode positioned close to nerve entrance or 5 mm proximal to the motor point. The selectivity of stimulation (minimal activation of adjacent muscles) was best, and the recruitment rate the highest, for an electrode placed close to the nerve entrance. A bipolar pair of electrodes placed on the superficial surface of the muscle opposite to the nerve entrance gave better selectivity than the monopolar electrode at the low end of the recruitment range, and poorer selectivity at the high end. This electrode configuration required greater stimulus currents and exhibited a lower recruitment gain than was obtained for a monopolar electrode in the same position. Examination of tissue surrounding the electrode 30 days after implantation showed that the fibrous tissue encapsulating the electrode had been incorporated into the fascial layer. Slightly larger dependence on muscle length and lower selectivity of stimulation were measured after encapsulation than were measured in the acute experiments.

Animals↗
PubMed1986

Generation of unidirectionally propagating action potentials using a monopolar electrode cuff.

Unidirectionally propagating action potentials, which can be used to implement transmission failure on peripheral nerve through "collision block," have been generated electrically on cat myelinated peripheral nerve using a monopolar electrode cuff with the conductor positioned closest to the "arrest" end of the cuff. A single cathode located at least 5 mm from the arrest end resulted in unidirectional propagation with minimal current and charge injection. The range of stimulus current values that produced unidirectional propagation increased with increases in longitudinal asymmetry of cathode placement over the range of asymmetries tested (1.7:1 to 7:1). The stimulus current pulse that minimized charge injection was quasitrapezoidal in shape with a plateau pulse width of approximately 350 microseconds and an exponential trailing phase having a fall time (90%-10%) of approximately 600 microseconds These parameters were found to be independent of cuff geometry. Arrest efficiency was not degraded using a cuff of sufficient internal diameter to prevent nerve compression in chronic implantation. The critical current density within the extracellular space of the electrode cuff required to produce conduction failure at the arrest end was estimated to be 0.47 +/- 0.08 mA/mm2.

Action Potentials↗