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Biomedical subjects

K C McGill

Publications and source records attributed to K C McGill.

At least 19 recordsLinked to original sources
PubMed1999

The contribution of the interosseous muscles to the hypothenar compound muscle action potential.

The contributions of the various ulnar-innervated muscles of the hand to the hypothenar compound muscle action potential (CMAP) were estimated by directly stimulating individual muscles and by analyzing CMAP shape changes resulting from manipulations that changed individual muscle lengths. The results show that the first peak of the negative phase of the hypothenar CMAP comes from the hypothenar muscles, but that the second peak is due to a large volume-conducted potential from the interosseous muscles. The interosseous contribution affects both the amplitude and the area of the CMAP, and makes these parameters sensitive to changes in the configuration of the fingers and the temperature gradient in the hand. To reduce the interosseous contribution, a "balanced reference" consisting of two reference electrodes, one over each tendon, is proposed.

Action Potentials↗
PubMed1998

The physiological origin of the slow afterwave in muscle action potentials.

OBJECTIVE: Both intramuscularly-recorded motor unit action potentials (MUAPs) and surface recorded MUAPs and compound muscle action potentials (CMAPs) have slow afterwaves which can contribute as much as half their measured duration. This study tested the hypothesis that the slow afterwave has its physiological origin in the negative afterpotential of the muscle fiber intracellular action potential (IAP). METHODS: We investigated the slow afterwave in MUAPs and CMAPs from brachial biceps, tibialis anterior, first dorsal interosseous, thenar and hypothenar muscles in 15 normal subjects, and using computer simulations. RESULTS: The slow afterwaves did not match the time constant of the amplifier's high-pass filter, and so were not filtering artifacts. They lasted long after propagation had terminated at the muscle/tendon junction, and so were not due to the temporal or spatial dispersion of propagating single-fiber potentials. Their amplitude and polarity varied with the recording site as predicted by computer simulations that modeled the IAP as having a negative afterpotential. They also changed with double-pulse stimulation and decreasing temperature in ways consistent with the results of intracellular studies of the IAP negative afterpotential. CONCLUSIONS: The presented results support our hypothesis that the slow afterwave is a manifestation of the IAP negative afterpotential.

Action Potentials↗
PubMed1996

Anatomical and electrophysiological determinants of the human thenar compound muscle action potential.

Clinical interpretation of the compound muscle action potential (CMAP) requires a precise understanding of its underlying mechanisms. We recorded normal thenar CMAP5 and motor unit action potentials using different electrode configurations and different thumb positions. Computer simulations show that the CMAP has four parts: rising edge, negative phase, positive phase, and tail which correspond to four distinct stages of electrical activity in the muscle: initiation at the end-plate, propagation, termination at the muscle/tendon junctions, and slow repolarization. The shapes of volume-conducted signals recorded beyond the muscle are also explained by these four stages. Changes in CMAP shape associated with thumb abduction are due to changes in termination times resulting from changes in muscle-fiber lengths. These findings demonstrate that the negative and positive phases of the CMAP are due to different mechanisms, and that anatomical factors, particularly muscle-fiber lengths, play an important role in determining CMAP shape.

Action Potentials↗
PubMed1995

Action potentials of curved nerves in finite limbs.

Previous simulations of volume-conducted nerve-fiber action-potentials have modeled the limb as semi-infinite or circularly cylindrical, and the fibers as straight lines parallel to the limb surface. The geometry of actual nerves and limbs, however, can be considerably more complicated. This paper presents a general method for computing the potentials of fibers with arbitrary paths in arbitrary finite limbs. It involves computing the propagating point-source response (PPSR), which is the potential arising from a single point source (dipole or tripole) travelling along the fiber. The PPSR can be applied to fibers of different conduction velocities by simple dilation or compression. The method is illustrated for oblique and spiralling nerve fibers. Potentials from oblique fibers are shown to be different for orthodromic and antidromic propagation. Such results show that the straight-line models are not always adequate for nerves with anatomical amounts of curvature.

Action Potentials↗
PubMed1993

The nerve gap dilemma: a comparison of nerves repaired end to end under tension with nerve grafts in a primate model.

The objective of this study was to compare, in a clinically relevant primate model, axon regeneration after epineurial repair under tension (15 mm gap) with interfascicular nerve grafts with the use of either standard microsuture techniques or a new interfascicular nerve graft technique termed fascicular tubulization that uses a hypoantigenic collagen membrane formed into a tube to approximate nerve ends. Electrophysiologic analysis demonstrated that the percentage of proximal axons that conducted across the repair site was greater in those nerves repaired under tension with epineurially placed sutures than in either of the tensionless repairs involving interfascicular graft techniques. The mean diameters of the regenerated axons repaired under tension with epineurial sutures were greater than those of the nerves repaired with interfascicular grafts, although the difference was not statistically significant. Interfascicular nerve grafting with tubulization using the current collagen tube resulted in regeneration equal to the sutured interfascicular nerve grafts. For modest defects (perhaps up to 3 to 4 cm in the adult), it seems advantageous to accept the modest tension associated with an epineurial repair rather than to use an autograft (or artificial graft) to achieve a tension-free repair.

Anastomosis, Surgical↗
PubMed1991

A comparison of turns analysis and motor unit analysis in electromyography.

We compared the results of turns analysis and motor unit analysis on 4056 electromyographic interference patterns (IPs) from normal subjects and patients with neuromuscular disorders. The motor unit analysis involved decomposing the IPs into their component motor unit action potentials (MUAPs) using automatic decomposition electromyography (ADEMG). We checked the accuracy of the decompositions by attempting to reconstruct some of the IPs from their identified MUAPs using computer simulations. The simulations revealed that ADEMG typically identified more than 60% (but not all) of the MUAPs in a given IP. Both turns and MUAP properties showed regular and related changes with force, age, muscle, and recording electrode type. The number of turns in each IP was highly correlated with the number of active MUAPs (r = 0.65), the mean MUAP firing rate (r = 0.72), the mean number of turns per MUAP (r = 0.34), and the product of these 3 properties (r = 0.83). The mean amplitude change per turn was highly correlated with the mean MUAP amplitude (r = 0.82), but also depended on the number of turns per MUAP. Due to the lack of a one-to-one relationship between the turns analysis properties and the MUAP properties, the turns analysis properties by themselves did not provide sufficient information to infer unambiguous physiological information about motor unit morphology or firing behavior.

Action Potentials↗
PubMed1991

A comparison of suture and tubulization nerve repair techniques in a primate.

This study compared standard methods of nerve repair, epineurial or perineurial sutures with a technique termed fascicular tubulization using a biodegradable polyglycolic acid tube in a nonhuman primate model. Electrophysiologic analysis demonstrated that the percentage of proximal axons that conducted across the repair site did not significantly differ among the three techniques while epineurial suture repairs were associated with significantly longer conduction delays across the repair site compared with the other two techniques. Even though fascicular tubulization using the current polyglycolic acid tube resulted in regeneration equal to the currently perceived best suture repair technique, associated technical problems with the current tube design indicate that this fascicular tubulization technique cannot, at present, be considered as an alternative to present clinically used nerve suture techniques.

Animals↗
PubMed1990

Triphasic behavioral response of motor units to submaximal fatiguing exercise.

We have measured the firing rate and amplitude of 4551 motor unit action potentials (MUAPs) recorded with concentric needle electrodes from the brachial biceps muscles of 10 healthy young adults before, during, and after 45 minutes of intermittent isometric exercise at 20% of maximum voluntary contraction (MVC), using an automatic method for decomposition of electromyographic activity (ADEMG). During and after exercise, MUAPs derived from contractions of 30% MVC showed progressive increase in mean firing rate (P less than or equal to .01) and amplitude (P less than or equal to .05). The firing rate increase preceded the rise in mean amplitude, and was evident prior to the development of fatigue, defined as reduction of MVC. Analysis of individual potentials revealed that the increase in firing rate and in amplitude reflected different MUAP subpopulations. A short-term (less than 1 minute) reduction in MUAP firing rates (P less than or equal to .05) was also observed at the onset of each test contraction. These findings suggest that motor units exhibit a triphasic behavioral response to prolonged submaximal exercise: (1) short-term decline and stabilization of onset firing rates, followed by (2) gradual and progressive increase in firing rates and firing variability, and then by (3) recruitment of additional (larger) motor units. The (2) and (3) components presumably compensate for loss of force-generating capacity in the exercising muscle, and give rise jointly to the well-known increase in total surface EMG which accompanies muscle fatigue.

Action Potentials↗
PubMed1989

Motor unit firing rates and firing rate variability in the detection of neuromuscular disorders.

We have used automatic decomposition electromyography (ADEMG) to study 41 muscles in 29 patients with well-defined peripheral and central motor disorders. In motor neuron diseases motor unit action potentials (MUAPs) showed increased amplitudes, firing rates and firing variability. Relatively large MUAPs sometimes were not identified by the computer program if they lacked sufficient high-frequency signal content, or were too variable in shape. In myopathies the MUAPs showed reduced amplitudes, durations and turns, and sometimes dramatic increases in firing rates. Also, the mean number of MUAPs per recording site was often increased, indicating excessive recruitment. In polymyositis (the best studied myopathy) the nature and magnitude of the MUAP shape and firing abnormalities were usually similar at different levels of contractile force, suggesting that motor units are affected without regard to recruitment order. In upper motor neuron paresis (multiple sclerosis), the shape properties of the MUAPs were normal, but mean firing rates were reduced, and firing variability increased. These findings confirm many of the traditional criteria for distinguishing neurogenic from myopathic disease electrophysiologically at the level of the individual MUAP. In addition, they demonstrate the potential diagnostic sensitivity of MUAP firing rate measurements for detecting neuromuscular dysfunction, and for differentiating between some cases of central and peripheral paresis, but not for distinguishing peripheral neurogenic from myopathic weakness, since firing rates tend to increase in both. Increased firing rate variability may be a marker of central or peripheral neurogenic weakness.

Action Potentials↗
PubMed1988

Age effects on properties of motor unit action potentials: ADEMG analysis.

We have measured the configurational and firing properties of 13,206 motor unit action potentials (MUAPs) from the brachial biceps, brachial triceps, and anterior tibial muscles in 10 young (20-40 years), 10 middle-aged (40-60 years), and 10 elderly (60-80 years) normal individuals, using an automatic method for decomposition of the electromyographic (EMG) interference pattern (ADEMG). Recording were made during stable isometric contractions at threshold, 10%, and 30% of maximum voluntary contraction using standard concentric needle electrodes. At supra-threshold forces, an average of 5.9 simultaneously active MUAPs were identified at each recording site. Mean amplitudes, durations, and numbers of turns all increased linearly with age in both low-threshold and high-threshold MUAPs (p less than 0.01), suggesting an ongoing process of progressive denervation and compensatory reinnervation. Mean MUAP firing rates decreased with age (p = 0.01) when force was measured proportionately, but not when measured absolutely. In a subgroup of 12 age-matched gender pairs, men had larger mean MUAP amplitudes, rise rates, and numbers of turns (p less than 0.05), probably reflecting larger muscle fiber diameters. These findings amplify previous observations from traditional analysis of lowest-threshold single MUAPs; establish a base of normative adult data for ADEMG; and further validate the clinical applicability of rapid, automatic EMG decomposition.

Action Potentials↗
PubMed1988

AAEE minimonograph #29: automatic quantitative electromyography.

The present status of different computerized methods of automatic quantitative electromyography are reviewed. Interference pattern methods-turns analysis, spectral analysis-are efficient, but the results usually cannot be directly related to the physiological properties of the motor units. Integration analysis does not currently have a major role in diagnostic electromyography. Traditional measurement of single motor unit action potentials during weak contraction can be facilitated and made more objective with computer assistance, but only the lowest-threshold motor units in the muscle are amenable to study. A new class of methodologies under development permit the decomposition of interference patterns into their constituent motor unit action potentials for measurement of configurational and behavioral properties. Patient data from these various methods can be statistically compared with normative data bases available on-line in computerized electromyographs. Both quantitative and quantitative electromyography have applications in the neuromuscular electrodiagnostic examination.

Electromyography↗
PubMed1988

Properties of motor unit action potentials recorded with concentric and monopolar needle electrodes: ADEMG analysis.

We compared the configurational and firing properties of 7270 motor unit action potentials (MUAPs) recorded with either concentric (CNE) or monopolar (MNE) needle electrodes from the brachial biceps and anterior tibial muscles of 10 healthy young adults (mean age 27 +/- 4.5 years) using automatic decomposition electromyography (ADEMG). In both muscles, mean MUAP amplitude, rise rate, and number of turns were significantly greater when recorded with MNE (paired t-test, P less than 0.001 in each case). Similar findings were observed at all three tested levels of isometric contractile force: threshold, 10% of maximum voluntary contraction (MVC), and 30% MVC. In contrast, there was no significant difference between electrode types on measurements of mean MUAP duration or firing rate (P greater than 0.05 in each case). These findings indicate that it is acceptable to generalize normative data on MUAP duration and firing rate from one electrode type to another, but that measures of MUAP amplitude and complexity require independent normative databases.

Action Potentials↗
PubMed1988

Influence of contractile force on properties of motor unit action potentials: ADEMG analysis.

We have used automatic decomposition electromyography (ADEMG) to measure the configurational and firing properties of 13,206 motor unit action potentials (MUAPs) in the brachial biceps, brachial triceps and anterior tibial muscles of 30 healthy adults (22 men, 8 women; mean age 48.6 +/- 16.9 years, range 20-76) at three levels of isometric contractile force: threshold, 10% of maximum voluntary contraction (MVC), and 30% MVC. In all muscles, the increment in contractile force from threshold to 10% MVC was associated with a significant (P less than 0.05, paired t-test) increase in mean MUAP firing rate and number of turns per MUAP. The increment from 10% to 30% MVC led to highly significant (P less than 0.005) increase in mean firing rate, number of turns, amplitude and rise rate. Each force increment was associated with an increase in the number of simultaneously-active MUAPs per recording site; and with a significant decrease in mean MUAP duration in all muscles, due to noise-dependency of the duration measurement. Quantitatively, the changes in MUAP properties with force were comparable to or exceeded the effects of age, gender differences, or intermuscular variability. Test-retest measurements 2 years apart in a subgroup of young adults showed good correspondence of mean MUAP properties with force standardization. These results demonstrate that contractile force is a major determinant of MUAP shape and behavior properties, and so must be precisely measured or controlled in clinical EMG studies.

Action Potentials↗
PubMed1985

Electrical properties of commercial concentric EMG electrodes.

Five electrical characteristics--impedance, broadband noise generation, line interference sensitivity, signal distortion, and common-mode conversion--were measured in five electromyographic (EMG) concentric needle electrodes (CNEs) from each of six commercial manufacturers. Untreated CNEs showed considerable variation in impedance and broadband noise characteristics, both within and among manufacturers. Electrolytic treatment reduced impedances by a factor between 1.5 and 4.0, and lessened within-manufacturer variability. Average post-treatment impedances at 100 Hz ranged from 31 to 436 kOhms, reflecting in part the range of core surface areas. Treatment also reduced the broadband noise to the level of the instrumentation noise for all but the highest impedance CNEs. Distortion and common-mode conversion were negligible for the lowest impedance CNEs. Line interference from a nearby power cord was completely suppressed only by those CNEs with fully shielded cables, and then only when the electromyographer also was grounded; there was no measurable benefit when the shield was driven, as opposed to grounded. We conclude that there are consistent differences in the properties of CNEs from different manufacturers, reflecting differences in materials, design, and construction; electrolytic treatment temporarily improves the performance of all CNEs, and tends to lessen the differences among them, most likely through a surface electrochemical effect; and both active and passive cable shields are effective in reducing extraneous line interference.

Electrochemistry↗
PubMed1985

Automatic decomposition electromyography (ADEMG): validation and normative data in brachial biceps.

We describe a new, automatic method (ADEMG) for decomposing EMG interference patterns into their constituent motor unit action potentials (MUAPs), and quantitating the configurational and firing properties of the MUAPs. ADEMG is fast (90 sec analysis time for a 10 sec EMG epoch) and efficient (33-98% of MUAP occurrences correctly identified) because of 4 signal-processing innovations designated digital prefiltering, high-resolution wave form alignment, firing-time analysis and interference-cancellation averaging. Validation experiments are described involving recruitment/derecruitment of low-threshold MUAPs, and single-fiber-triggered averaging. Normative data are presented for 2000 MUAPs from brachial biceps (mean 9.7 MUAPs per site) at 3 levels of isometric contraction (7%, 15% and 30% MVC) and contrasted with normal findings obtained using traditional, low-threshold MUAP analysis. The main advantages of ADEMG are speed of data acquisition and processing in the clinical setting; ability to analyze both low- and high-threshold MUAPs during moderately strong muscular contractions; and MUAP firing-rate information.

Action Potentials↗