PubMed Health⌕ Search

Biomedical subjects

J N Howell

Publications and source records attributed to J N Howell.

At least 19 recordsLinked to original sources
PubMed2001

Can nurses govern in a government agency?

A shared governance model was implemented within the nursing service of the Durham Veterans Affairs Medical Center in 1997. In 1999, members of the Nursing Quality/Staff Education/Research Council examined facility nurses' perceptions of governance to determine the degree to which the model had been implemented. Results of the study indicate that although imperatives at the organizational level were incompatible with three aspects of shared governance, there were three shared governance dimensions pertaining to basic nursing practice that reached or surpassed governance thresholds. This study highlights the potential of nursing management innovation within highly bureaucratic organizations.

Academic Medical Centers↗
PubMed2001

In vivo measurement of fascicle length and pennation angle of the human biceps femoris muscle.

The purpose of this investigation was to measure in vivo fascicle length (L(fas)) and pennation angle (PA) of the long head of the biceps femoris muscle (BF(lh) m.) at different hip and knee angles while the muscle was relaxed using ultrasonography (US). Data were collected from 18 healthy females (23.0 +/- 1.8 years). To validate the measurements of the L(fas) and PA, the US measurements on cadavers (n = 5), embalmed with the hip and knee in anatomical position, were compared to actual fiber lengths and PA from the dissected muscles. US images from volunteer subjects were recorded when the hip joint was positioned at 0 degrees, 45 degrees and 90 degrees of hip flexion, and at each hip position, the knee joint was placed at 0 degrees, 45 degrees and 90 degrees from full extension. The images were digitized to measure L(fas) and PA. Results showed no significant differences between US and direct measurements of the L(fas) and PA on the cadaver BF(lh) m. (p > 0.05). L(fas) and PA changed significantly between the different hip and knee positions in volunteer subjects (p < 0.05). Changes in the L(fas) and PA are more sensitive to changes in hip position with the knee position constant than to changes in knee position with the hip position constant. This difference may be related to the larger muscle moment arm at the hip resulting in greater excursion of the muscle with changing hip position. Based on the changes in L(fas) with changing joint positions, BF(lh) sarcomere length was estimated to occupy a portion of the ascending limb, the plateau and descending limb of the length-tension relationship over the range of motion studied. US scanning is valid and reliable for measuring L(fas) and PA of the BF(lh) m. An understanding of the dynamic nature of muscle architecture will assist in determining effective and efficient clinical evaluation and rehabilitation techniques.

Adult↗
PubMed2000

Discharge behaviour of single motor units during maximal voluntary contractions of a human toe extensor.

1. While it is known that the average firing rate of a population of motoneurones declines with time during a maximal voluntary contraction, at least for many muscles, it is not known how the firing patterns of individual motoneurones adapt with fatigue. To address this issue we used tungsten microelectrodes to record spike trains (mean +/- s.e.m., 183 +/- 27 spikes per train; range, 100-782 spikes) from 26 single motor units in extensor hallucis longus during sustained (60-180 s) maximal dorsiflexions of the big toe in seven human subjects. 2. Long spike trains were recorded from 13 units during the first 30 s of a maximal voluntary contraction (mean train duration, 9.6 +/- 1.2 s; range, 3.6-21.9 s) and from 13 units after 30 s (mean train duration, 16.6 +/- 3.7 s; range, 7.1-58.1 s). Maximal isometric force generated by the big toe declined to 78.3 +/- 6.3 % of its control level by 60-90 s and to 39.5 +/- 1.4 % of control by 120-150 s. Despite this substantial fatigue, mean firing rates did not change significantly over time, declining only slightly from 15.8 +/- 0.7 Hz in the first 30 s to 14.0 +/- 0.5 Hz by 60-90 s and 13.6 +/- 0.3 Hz by 120-150 s. 3. To assess fatigue-related adaptation in discharge frequency and variability of individual motor units, each spike train was divided into 2-15 equal segments containing at least 50 interspike intervals. Discharge variability was measured from the coefficient of variation (s.d. /mean) in the interspike intervals, with the s.d. being calculated using a floating mean of 19 consecutive intervals. Adaptation was computed as the average change in firing rate or variability that would occur for each 1 s of activity. There were no systematic changes in either firing rate or variability with time. 4. We conclude that single motoneurones supplying the extensor hallucis longus, a muscle comprised primarily of slow twitch muscle units, show little adaptation in firing with fatigue, suggesting that a progressive reduction in firing rate is not an invariable consequence of the fatigue associated with sustained maximal voluntary contractions.

Action Potentials↗
PubMed1998

Relationship between muscle swelling and stiffness after eccentric exercise.

PURPOSE: The time courses of muscle compartmental swelling and passive stiffness change were measured to determine whether muscle compartmental swelling accounted for increased stiffness. METHODS: Eleven untrained female college students exercised eccentrically by lowering a weight with their elbow flexors. Measurements of muscle compartment volume, stiffness, relaxed elbow angle, circumference, and pain were recorded before exercise, immediately after exercise, and 1-5, 7, 9, and 11 d after exercise. Muscle compartment volume was calculated from cross-sectional ultrasound images taken along the upper arm. Stiffness was measured using a device that extended the elbow and recorded the torque required to hold the forearm at successive angles. RESULTS: Elbow flexor volume increased gradually to peak on the fourth day (26.1 +/- 4.3%, P < 0.05) and then decreased to baseline values over days 7-11. Stiffness increased immediately after exercise (59.9 +/- 14.1%, P < 0.05) and remained at or above this level until decreasing to pre-exercise levels over days 7-11. CONCLUSIONS: This suggests that muscle swelling does not account for the sudden increase in stiffness of the elbow flexor muscles within the first 48 h after exercise but may play a role in the subsequent time course of stiffness.

Adult↗
PubMed1995

Intermittent pneumatic compression effect on eccentric exercise-induced swelling, stiffness, and strength loss.

OBJECTIVE: The purpose was to determine if intermittent pneumatic compression (IPC) affects muscle swelling, stiffness, and strength loss resulting from eccentric exercise-induced injury of the elbow flexors. We hypothesized that the compression would decrease swelling and stiffness. DESIGN: Repeated measures design with a before-after trial comparison within each day. SETTING: Conducted at a university Somatic Dysfunction Laboratory. SUBJECTS: Twenty-two college women students were studied. They had not been lifting weights or otherwise participating in regular arm exercise for the 6 months before the study. They had no history of upper extremity injury or cardiovascular disease. INTERVENTIONS: Subjects performed one bout of eccentric exercise at a high load to induce elbow flexor muscle injury. Uniform IPC was applied on the day of exercise and daily for 5 days at 60mmHg, 40 seconds inflation, 20 deflation for 20 minutes. MAIN OUTCOME MEASURES: Measurements of arm circumference, stiffness, and isometric strength were recorded before exercise, then before and after IPC for 5 days after exercise. Passive muscle stiffness was measured on a device that extends the elbow stepwise and records the torque required to hold the forearm at each elbow angle. RESULTS: Circumference and stiffness increased and strength decreased during the 5 days post-exercise (p < .05). IPC significantly decreased circumference and stiffness most notably on days 2 and 3 after exercise (p < .05). The strength loss was not affected by IPC. CONCLUSION: IPC is effective in temporarily decreasing the swelling and stiffness after exercise-induced muscle injury.

Adult↗
PubMed1995

Motor unit activity during isometric and concentric-eccentric contractions of the human first dorsal interosseus muscle.

1. Motor unit activity was recorded with intramuscular fine wire electrodes during isometric, concentric, and eccentric activity in the human first dorsal interosseus muscle. Twenty-one units from 11 subjects were sampled. 2. During isotonic cycles of shortening and lengthening, 18 of 21 units were recruited during the concentric phase, increased their discharge rates as the concentric movement progressed, then decreased their discharge rate during the eccentric phase, and were derecruited. 3. A different pattern of recruitment was observed in recordings from three units. These units were recruited during the eccentric phase, at a time when other units were decreasing their discharge rate or being derecruited. In two of the units selectively recruited during the eccentric phase, it was possible to determine their isometric thresholds, which were higher than those of units exhibiting the more common pattern of recruitment. 4. For two of the three units exhibiting selective recruitment during eccentric contraction, the unit was recorded simultaneously with different pairs of recording wires separated by 5-10 mm. Each discharge of these units was detected by both electrodes, making it unlikely that movement artifact was responsible for the initiation or cessation of discharge. 5. The recruitment patterns observed suggest that changes in the type or distribution of synaptic inputs to motoneurons during movement can, in some instances, override pre- and postsynaptic factors that shape recruitment order in isometric conditions.

Adolescent↗
PubMed1993

Muscle stiffness, strength loss, swelling and soreness following exercise-induced injury in humans.

1. In order to study injury-related changes in muscle stiffness, injury to the elbow flexors of thirteen human subjects was induced by a regimen of eccentric exercise. 2. Passive stiffness over an intermediate range of elbow angles was measured with a device which held the relaxed arm of the subject in the horizontal plane and stepped it through the range of elbow angles from 90 deg to near full extension at 180 deg. The relation between static torque and elbow angle was quite linear over the first 50 deg and was taken as stiffness. 3. Stiffness over this range of angles more than doubled immediately after exercise and remained elevated for about 4 days, and may result from low level myofibrillar activation induced by muscle stretch. 4. Arm swelling was biphasic; arm circumference increased by about 3% immediately after exercise, fell back toward normal, then increased by as much as 9% and remained elevated for as long as 9 days. 5. Ultrasound imaging showed most of the swelling immediately following the exercise to be localized to the flexor muscle compartment; subsequent swelling involved other tissue compartments as well. 6. Muscle strength declined by almost 40% after the exercise and recovery was only slight 10 days later; the half-time of recovery appeared to be as long as 5-6 weeks.

Adult↗
PubMed1988

Relationship between sensitivity of cells from patients with hereditary cutaneous malignant melanoma to killing and mutations by 4-nitroquinoline 1-oxide and adduct formation.

Hereditary cutaneous malignant melanoma (HCMM) is an autosomal dominant disease and develops mostly in patients with the hereditary dysplastic nevus syndrome (DNS). A previous study from this laboratory showed that fibroblasts derived from skin biopsies of four HCMM/DNS patients and one DNS patient were significantly more sensitive to the cytotoxic and mutagenic effects of 4-nitroquinoline 1-oxide (4-NQO) than three fibroblast cell lines from normal newborn males and one from a 45-yr-old normal donor. The survival curves of the HCMM/DNS or DNS cell lines fell into two groups, one with a slope approximately twice as steep as the controls and one with a slope approximately 3 times as steep. This marked difference in survival curves disappeared if the same cell lines were exposed to 4-hydroxyaminoquinoline 1-oxide, a partially reduced metabolite of the parent compound, suggesting that the difference in sensitivity reflected a difference in the cells' ability to metabolize the parent compound into a more reactive form. To see if such increased sensitivity to the cytotoxic effect of 4-NQO was an identifying characteristic of cells from HCMM/DNS and DNS patients and whether cells derived from unaffected members of such families were also more sensitive to 4-NQO than the controls, the survival curves of a series of 40 coded cell lines, derived from patients, family members, or normal donors of various ages, were compared in a blind study. The slopes of the survival curves of all the HCMM/DNS and DNS cell lines proved to be 2- to 3-fold steeper than those of the four previously studied control cell lines, bringing the total to nine of nine and eight of eight, respectively. Ten of the 28 coded cell lines from nonaffected family members or normal, age-matched donors proved to be as resistant as the four previously studied control cell lines and another foreskin-derived control cell line, bringing the total of relatively resistant cell lines to 15 of 33. However, the rest were just as sensitive as the cells from the patients. Mutagenesis studies showed that increased sensitivity to killing by 4-NQO was accompanied by increased sensitivity to mutation induction. Relatively sensitive and relatively resistant cell lines were treated with tritiated 4-NQO and assayed for survival and for the number of 4-NQO residues bound to DNA. The relationship between the number of adducts formed and cell survival was linear, with the relatively sensitive cell lines exhibiting the higher level of residues.(ABSTRACT TRUNCATED AT 400 WORDS)

4-Nitroquinoline-1-oxide↗
PubMed1987

Evidence for t-tubular conduction failure in frog skeletal muscle induced by elevated extracellular calcium concentration.

In order to investigate the mechanism by which elevated extracellular calcium ions decrease tetanus tension in frog skeletal muscle, we made mechanical, electrophysiological and photographic measurements on single fibres or small bundles of fibres. Three lines of evidence point to t-tubular conduction failure as the primary mechanism of action of high calcium. They are a decrease in amplitude of the late afterpotential, attenuation or elimination of the notch and hump configuration of the early afterpotential, and the appearance of wavy myofibrils in the axial core of fibres during tetanus. These effects are fully reversible and are shared by other bivalent cations. High calcium concentration causes a change in the time course of the early afterpotential but does not alter the passive cell membrane characteristics, as reflected by the time course of decay of applied hyperpolarizing pulses.

Action Potentials↗
PubMed1987

Effects of repetitive activity, ruthenium red, and elevated extracellular calcium on frog skeletal muscle: implications for t-tubule conduction.

In this report we review evidence that indicates that experimental elevation of t-tubular calcium can lead to failure of action potential propagation within the t system and we present some new evidence suggesting that t-tubular calcium concentration may rise during repetitive activity. The evidence for t-tubular conduction failure consists of comparisons of the effects of high calcium and of ruthenium red on excitation and excitation-contraction coupling as well as morphological observations of wavy myofibrils in the axial core of fibers contracting tetanically in solutions containing elevated calcium concentrations. Evidence for elevation of t-tubular calcium concentration during repetitive activity comes from the following. During twitches, the early, large birefringence signal and force development are delayed in onset if the extracellular calcium and (or) potassium concentrations are above normal or if the fiber has been stimulated tetanically just prior to the test twitch. The delays that occur in twitches following tetanic contractions are attenuated when the extracellular and, therefore, the t-tubular calcium concentration is buffered with citrate.

Action Potentials↗
PubMed1985

An electromyographic study of elbow motion during postexercise muscle soreness.

Postexercise muscle soreness was induced in the elbow flexors of human volunteer subjects through the use of a regimen of eccentric contractions. Physical examination before and 48 h after the exercise included measurements of range of elbow motion at the elbow and of arm diameter. Electromyographic (EMG) observations, utilizing fine wire electrodes introduced into each of the elbow flexors, were carried out before and 48 h after the exercise. These observations included resting EMG levels, EMG activity under various isometric loads, and activity during active and passive extensions. The soreness induced was accompanied by a reduction in resting elbow angle while the subjects stood with arms relaxed at their sides, decreased range of both flexion and extension of the elbow, and swelling of the arm. EMG measurements showed no increase in EMG activity as the sore arms were extended passively at the elbow, indicating that the extra resistance to extension associated with the soreness was not a result of stretch receptor-induced activity in the flexors. The results rule out the possibility that neuromuscular activity is responsible for the restriction of motion and are consistent with the idea that edematous changes within the perimuscular connective tissue alter the elastic behavior of the muscles and cause restriction of motion.

Adult↗
PubMed1984

Ruthenium red: differential effects on excitation and excitation-contraction coupling in frog skeletal muscle.

The effects of purified ruthenium red (RR6+) on action potentials, resting potentials, twitches, tetani and potassium contractures in fibre bundles dissected from frog leg muscles were compared to the effects of supranormal concentrations of Ca2+. The effects of RR6+ include a shift of action potential threshold and reduction in rates of depolarization, repolarization and conduction velocity. Resting potentials and action potential overshoots are unaffected by RR6+ in concentrations of 10 microM or less. Higher RR6+ concentrations cause membrane depolarization. Low concentrations of RR6+ cause twitch potentiation but do not affect tetanus tension. Unlike Ca2+, RR6+ has no effect on the mechanical activation curve. However, crude preparations of RR6+ contain an unidentified contaminant which does shift the mechanical activation curve to the right, i.e. it increases mechanical threshold. RR6+ itself appears to be largely excluded from the t-system.

Action Potentials↗
PubMed1984

Fibroblasts from patients with hereditary cutaneous malignant melanoma are abnormally sensitive to the mutagenic effect of simulated sunlight and 4-nitroquinoline 1-oxide.

Because of a possible etiologic link between mutations and carcinogenesis, we compared fibroblasts derived from skin biopsies of several patients with hereditary cutaneous malignant melanoma and the dysplastic nevus syndrome for sensitivity to the mutagenic and/or cytotoxic effect of broad-spectrum simulated sunlight and of a UV mimetic carcinogen, 4-nitroquinoline 1-oxide (4NQO). The genetic marker was resistance to 6-thioguanine; loss of colony-forming ability was the assay for cytotoxicity. All five strains tested were more sensitive than normal to the killing effect of 4NQO (slopes of survival curves were 2- to 3-fold steeper), but only one strain was hypersensitive to killing by Sun Lamp radiation. Two strains were tested for mutagenicity. The response of each to the mutagenic action of these agents corresponded to its response to cell killing. Both strains were hypermutable after exposure to 4NQO, but only one showed a higher than normal frequency of mutants induced by simulated sunlight. The finding that nonmalignant fibroblasts from patients with a hereditary variant of malignant melanoma are abnormally susceptible to carcinogen-induced mutations suggests that hypersensitivity to mutagens contributes to risk of melanoma in patients. It also supports the somatic cell mutation hypothesis for the origin of cancer.

4-Nitroquinoline-1-oxide↗
PubMed1982

Distribution of physiological adult lactase phenotypes, lactose absorber and malabsorber, in Germany.

A total of 1805 apparently healthy, adult and adolescent Germans (1572 males and 233 females with a mean age of 20.3 years) were examined for lactose absorption capacity employing a field version of the breath hydrogen (H2) test. The diagnostic parameter, maximal change of breath hydrogen concentration 120 or 150 min after a load of 50 g lactose, showed a bimodal distribution, separating lactose absorbers (n = 1537, 85.2%) and lactose malabsorbers (n = 268, 14.8%). The distribution of the adult lactase phenotypes was independent of age, sex, and educational status. The incidence of gastrointestinal symptoms after lactose administration demonstrated the incongruity of lactose malabsorption and lactose intolerance. In addition to grouping by residence, the probands were classified according to the birthplaces of their grandparents in order to reconstruct the distribution pattern of the lactase phenotypes prior to World War I, a period of relative population stability. Considerable differences in the frequency of lactose malabsorption were found in regions corresponding to traditional ethnic groups within the German population: northwest Germany 6-9%, west and south 13-14%, southwest 23%, east (including formerly German territories east of rivers Oder and Neisse) 22%. These differences are discussed with reference to population history. The present fairly even distribution of the lactase phenotypes in West Germany is the result of internal migrations at the end of World War II.

Adolescent↗
PubMed1982

The interaction between ruthenium red and the isolated sarcoplasmic reticulum.

The multivalent anions, ATP and oxalate, present at 5mM concentrations in incubation mixtures with isolated sarcoplasmic reticulum (SR) preparations, reduce the binding of ruthenium red (RR) to the SR. When oxalate is omitted from the incubation mixture and ATP is used at reduced concentrations, it is possible to observe an inhibitory effect of RR on calcium uptake by the SR and on ATPase activity. However, this inhibition is only partial and it remains clear that calcium transport in the SR is much less susceptible to inhibition by RR than is calcium transport in mitochondria. The effect of multivalent anions in suppressing the effect of RR, a hexavalent cation, is probably due to the formation of soluble complexes with RR.

Adenosine Triphosphate↗