Weak and numb feet in a man with knobby hands.
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Aged↗
Biomedical subjects
R A Conwit
Publications and source records attributed to R A Conwit.
Fatigue effects on motor unit activity during submaximal contractions.
OBJECTIVE: To examine motor unit changes during the development of fatigue in healthy subjects. DESIGN: Automated decomposition-enhanced spike-triggered averaging was used to characterize motor unit size and firing rate in the dominant vastus medialis during maintained contractions at 10% and 30% of maxima voluntary contraction (MVC). SETTING: Academic outpatient neuromuscular clinic. PARTICIPANTS: Healthy laboratory personnel. MAIN OUTCOME MEASURES: Surface electromyogram, surface-detected motor unit action potential amplitude (S-MUAP), mean firing rate, force (MVC), motor unit index. RESULTS: Surface electromyogram values and S-MUAP amplitudes increased during both 10% and 30% MVC fatiguing contractions, while mean firing rates decreased. A motor unit index, indicating the degree of motor unit pool activation, increased similarly to S-MUAP size, implying that new and larger units were recruited to maintain the contraction. Repeated contractions led to earlier motor unit changes and fatigue. CONCLUSION: During submaximal fatiguing contractions, additional motor units are activated to maintain strength. These changes begin early, within the first minute, particularly after a previous fatiguing effort.
Action Potentials↗
The relationship of motor unit size, firing rate and force.
OBJECTIVE: Using a clinical electromyographic (EMG) protocol, motor units were sampled from the quadriceps femoris during isometric contractions at fixed force levels to examine how average motor unit size and firing rate relate to force generation. METHODS: Mean firing rates (mFRs) and sizes (mean surface-detected motor unit action potential (mS-MUAP) area) of samples of active motor units were assessed at various force levels in 79 subjects. RESULTS: MS-MUAP size increased linearly with increased force generation, while mFR remained relatively constant up to 30% of a maximal force and increased appreciably only at higher force levels. A relationship was found between muscle force and mS-MUAP area (r2 = 0.67), mFR (r2 = 0.38), and the product of mS-MUAP area and mFR (mS-MUAP x mFR) (r2 = 0.70). CONCLUSIONS: The results support the hypothesis that motor units are recruited in an orderly manner during forceful contractions, and that in large muscles only at higher levels of contraction ( > 30% MVC) do mFRs increase appreciably. MS-MUAP and mFR can be assessed using clinical EMG techniques and they may provide a physiological basis for analyzing the role of motor units during muscle force generation.
Action Potentials↗
Firing rate analysis using decompostion-enhanced spike triggered averaging in the quadriceps femoris.
Electromyographic signals detected from the quadriceps femoris during various constant force contractions were decomposed to identify individual motor unit discharges and mean firing rates (FRs). Subject and group mean FRs were calculated for each force level. Mean FR values and FR variability increased with force. Individual, subject, and group mean FRs showed slight increases until 30% of maximum voluntary contraction and larger increases thereafter. Findings are discussed in relation to motor unit recruitment, frequency modulation, and fatigue.
Action Potentials↗
Ulnar nerve injury associated with removal of Norplant implants.
Improperly placed Norplant implants pose a risk to neurovascular structures at the time of removal. Appropriate attention to insertion and removal should minimize the chance of injury. However, when injuries do occur, a logical and systematic approach to evaluation of the injury can help predict the outcome and establish a plan of care for the patient.
Adult↗
Decomposition-enhanced spike-triggered averaging: contraction level effects.
Decomposition-enhanced spike-triggered averaging (DE-STA) was applied to the vastus medialis muscle to examine size distributions of surface-detected motor-unit action potentials (S-MUAPs) at various force levels. Using DE-STA, 15-20 S-MUAPs were identified during 5%, 10%, 20%, and 30% of maximum voluntary contraction. Average S-MUAPs showed increase in peak to peak (and negative peak) amplitude with force (In microV): 5% = 37.9 +/- 6.1 (16.6 +/- 2.5), 10% = 44.0 +/- 4.0 (20.4 +/- 1.8), 20% = 80.7 +/- 9.3 (41.3 +/- 4.5), and 30% = 102.5 +/- 10.3 (53.6 +/- 5.0). Test-retest variability of peak to peak (and negative peak amplitude) between repeated trials was 0.10 (0.14), 0.14 (0.14), 0.17 (0.15), and 0.21 (0.20) at 5%, 10%, 20%, and 30% respectively. A relationship was found between the S-MUAP amplitude and force (r2 = 0.78, df = 90, F = 160, P < 0.001). Increase in average S-MUAP amplitude with force suggests that STA performed only at low levels of contraction may result in a biased sampling and small average S-MUAP amplitudes.
Action Potentials↗
Mixed nerve action potentials in acquired demyelinating polyneuropathy.
Uncertainty about motor and sensory contributions in abnormal nerves has limited the use of mixed nerve action potentials (MNAPs). We recorded MNAPs in 21 patients with an acquired demyelinating neuropathy, 18 age-matched control subjects, and 10 patients with an axonal polyneuropathy. Bipolar and unipolar recordings from median and ulnar nerves were made above the elbow after electrical stimulation of the nerves at the wrist. Antidromic digital sensory action potentials and motor conduction velocity were also recorded for both nerves. In 19 median and 12 ulnar nerves from demyelinating polyneuropathy patients, compared with control subjects, MNAP amplitudes were significantly reduced (mean, 6 microV vs. 31 microV), MNAP velocities were mildly reduced (mean, 50 m/s vs. 62 m/s), motor conduction velocities were significantly reduced (mean, 33 m/s vs. 57 m/s), and MNAPs were significantly dispersed, with markedly prolonged rise times (mean 2.0 ms vs. 1.0 ms). Compared with the axonal polyneuropathy group, MNAP amplitudes from the median nerve were similarly reduced (mean, 8 microV vs. 9 microV), MNAP velocities were only slightly slower (mean, 52 m/s vs. 58 m/s), but the rise times were significantly prolonged (mean, 2.0 ms vs 1.2 ms). We conclude that, in acquired demyelinating neuropathies, the onset and, in some cases, the whole MNAP is from afferent fibers, which can be abnormally dispersed, and that, over the same segment MNAP velocity is less affected than motor conduction velocity.
Action Potentials↗
Sensorimotor neuropathy resembling CIDP in patients receiving FK506.
FK506 is an important immunosuppressant that has shown great promise in the treatment of autoimmune diseases. Approximately 5% of patients receiving FK506 develop major central nervous system toxicity, but the peripheral nerves are usually spared. During 1990-1991, some 1000 patients received liver transplants under FK506 immunosuppression. Of these, 3 patients developed severe multifocal demyelinating sensorimotor polyneuropathy 2-10 weeks after initiation of FK506 therapy. Improvement followed plasmapheresis or intravenous immunoglobulin (IVIG), suggesting an immune-mediated cause. Although autoimmune neuropathy has been previously reported in immune-deficient states such as Hodgkin's disease and AIDS, it is not an expected complication of immunosuppressive therapy. However, others have shown that this phenomenon can be produced in rats with cyclosporine A (CsA), whose effects on T-cell subsets are similar to those seen with FK506. These T-cell subset changes may have precipitated this dysimmune neuropathy in our patients.
Demyelinating Diseases↗
Relation of EEG alpha background to cognitive function, brain atrophy, and cerebral metabolism in Down's syndrome. Age-specific changes.
We studied 19 young adults (19 to 37 years old) and 9 older patients (42 to 66 years old) with Down's syndrome (DS) and a control group of 13 healthy adults (22 to 38 years old) to investigate the relation of electroencephalographic (EEG) alpha background to cognitive function and cerebral metabolism. Four of the older patients with DS had a history of mental deterioration, disorientation, and memory loss and were demented. Patients and control subjects had EEGs, psychometric testing, quantitative computed tomography, and positron emission tomography with fludeoxyglucose F 18. A "blinded" reader classified the EEGs into two groups--those with normal alpha background or those with abnormal background. All the control subjects, the 13 young adult patients with DS, and the 5 older patients with DS had normal EEG backgrounds. In comparison with the age-matched patients with DS with normal alpha background, older patients with DS with decreased alpha background had dementia, fewer visuospatial skills, decreased attention span, larger third ventricles, and a global decrease in cerebral glucose utilization with parietal hypometabolism. In the young patients with DS, the EEG background did not correlate with psychometric or positron emission tomographic findings, but the third ventricles were significantly larger in those with abnormal EEG background. The young patients with DS, with or without normal EEG background, had positron emission tomographic findings similar to those of the control subjects. The mechanism underlying the abnormal EEG background may be the neuropathologic changes of Alzheimer's disease in older patients with DS and may be cerebral immaturity in younger patients with DS.
Adult↗