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Biomedical subjects

A McComas

Publications and source records attributed to A McComas.

11 recordsLinked to original sources
PubMed2001

Depletion and sizes of motor units in spinal muscular atrophy.

Motor unit number estimation (MUNE) was applied to the biceps brachii muscles of 13 young patients (age 5--24 years) with spinal muscular atrophy (SMA) and the results compared with those of healthy control subjects matched for age and gender. In the SMA patients, all motor unit (MU) estimates fell below the control range, and there was good correspondence between the values for the two arms in the same subject. No correlation could be found between the MUNEs and the severity of the weakness. This unexpected result was attributed to the presence of small and normal-sized MUs in the muscles of patients, in addition to MUs that appeared to be considerably enlarged. The threefold mean increase in MU potential size was insufficient to compensate for the MU loss. In addition, the study confirmed that there are, on average, approximately 130 MUs in the healthy biceps brachii muscle.

Adolescent↗
PubMed2000

Developmental apraxia arising from neonatal brachial plexus palsy.

OBJECTIVE: To determine whether motor unit activation is impaired in patients with persisting disability arising from neonatal brachial plexus palsy (NBPP). BACKGROUND: In NBPP patients, the authors previously found more extensive muscle reinnervation than might have been anticipated from the clinical examination. METHODS: Motor skills were tested in a group of nine boys and seven girls with prior NBPP, who then underwent physiologic investigation of proximal and distal muscles in their affected and unaffected arms. The latter tests comprised measurements of maximal evoked muscle compound action potential (M-wave) amplitude, maximal voluntary torque, twitch torque, and twitch interpolation. A group of 17 children of similar ages served as control subjects. RESULTS: In the NBPP group, motor skills were diminished and voluntary torque was reduced relative to M-wave amplitude and twitch torque. Moreover, interpolated twitches could be demonstrated in some NBPP patients but not in control subjects. CONCLUSION: Persisting disability in NBPP patients is due, at least in part, to impaired motor unit activation. The authors suggest that the impairment is a form of developmental apraxia caused by defective motor programming in early infancy.

Action Potentials↗
PubMed1997

Loss of twitch torque following muscle compression.

With the elbow flexed, compression of the human biceps brachii has been found to reduce twitch torque, with an approximately linear relationship being observed between the loss of torque and the applied pressure (up to 45 kPa). The decline in torque could no longer be demonstrated when the biceps muscle was stretched, by extending the elbow from a flexed position. The loss of torque in the flexed position appeared to be due to an inability of muscle sarcomeres to bulge sufficiently to take up the series elasticity at the fiber ends.

Adult↗
PubMed1992

Transient hyperpolarization of non-contracting muscle fibres in anaesthetized rats.

1. Following laminectomy, the L5 ventral roots of anaesthetized rats were split and approximately half of the nerve fibres were stimulated at 40 Hz. Resting membrane potentials were then measured in previously contracting and non-contracting soleus muscle fibres. 2. In the non-contracting soleus fibres there was a post-tetanic increase in mean resting potential (from -82.2 +/- 7.1 (S.D.) to -91.6 +/- 8.7 mV) which was similar to that in contracting fibres (from -83.02 +/- 6.1 to -91.3 +/- 7.3 mV). In both types of fibres the hyperpolarizing responses (HRs) were evidently due to increased sodium pump activity since they could be abolished by the addition of ouabain (1 x 10(-4) M) to the bathing fluid. 3. The beta-adrenergic antagonist, propranolol, completely suppressed HRs in the non-contracting fibres and produced moderate reductions in the contracting ones. The alpha-adrenergic blocking agent, phentolamine, had no effect on the contracting fibres and only a modest, inhibitory, one on the non-contracting fibres. 4. On the basis of the above drug actions it appeared that catecholamines were necessary for the full development of HRs in contracting and non-contracting fibres; noradrenaline, released from intramuscular sympathetic nerve endings, may have been involved. 5. The increased sodium pump activity in the non-contracting fibres would serve to moderate the rise in interstitial [K+] caused by K+ efflux from the contracting fibres. By preventing passive depolarization, due to the rise in interstitial [K+], the sodium pump would also maintain the availability of non-contracting fibres for subsequent recruitment.

Animals↗
PubMed1991

The extent and time course of motoneuron involvement in amyotrophic lateral sclerosis.

The numbers and relative sizes of motor units have been estimated in 373 muscles of 123 patients with ALS: 74 of the muscles were examined on more than one occasion. The median duration between the onset of symptoms and the initial examination was 12 months; by this time, approximately 90% of the tested muscles showed losses of motor units. The evoked motor unit potentials continued to enlarge in most, but not all, muscles as the disease progressed. Once a muscle became affected by the disease process, the average time-course was such that the motor unit population halved in each 6-month period of the first year and diminished more slowly thereafter. A small proportion of patients was encountered in whom the disease progressed much more slowly and there were occasional large fluctuations in the motor unit estimates suggestive of reversible motoneuron dysfunction.

Adult↗
PubMed1987

Effects of impulse blockade on the contractile properties of rat skeletal muscle.

1. Pressure was used to produce a localized block of impulse conduction in motor nerve fibres supplying rat plantaris and soleus muscles. After 1 week of inactivity both muscles exhibited marked atrophy of type I and type II muscle fibres. 2. In treated plantaris and soleus muscles the specific tetanic tensions were reduced to approximately half those of untreated control muscles; the specific twitch tensions were unchanged. The fall in specific tetanic tension in the nerve-blocked muscles was not associated with impaired excitation of muscle fibres during repetitive stimulation. 3. In nerve-blocked plantaris muscles the contraction and half-relaxation times became significantly prolonged whereas no change could be demonstrated in similarly treated soleus muscles. In both muscles the maximum rates of tetanic tension development increased following nerve block. 4. It is concluded that, in the rat, most of the changes in contractile properties and in muscle fibre size which follow denervation may be attributed to disuse rather than to interruption of axoplasmic transport.

Animals↗
PubMed1985

Influence of human muscle length on fatigue.

The effect of muscle length on susceptibility to fatigue has been examined in human ankle dorsiflexor muscles. The fatiguing procedure consisted of either indirect tetanic stimulation at 20 Hz or maximal voluntary contraction; each procedure lasted 90 s. The amplitude of the evoked muscle compound action potential (M-wave) increased during the first 30 s or so of the tetanic fatiguing procedure and then decreased. The torque developed by the dorsiflexor muscles declined throughout the period of tetanization. A significantly greater reduction in twitch and tetanic torque was found after the fatiguing procedure had been conducted at the optimum muscle length rather than with the muscle in a shortened position. Relaxation after tetanic stimulation was slower after fatigue had been induced at the optimum muscle length. It is concluded that muscle fatigue is related to the number of actin-myosin cross-bridge interactions and is unlikely to be accounted for solely on the basis of changes in the ionic composition of the transverse tubular fluid.

Adult↗
PubMed1983

Absence of fibrillation after muscle injury.

The possibility was examined that fibrillation potentials may occur in those parts of muscle fibers which become functionally denervated following segmental necrosis. Focal necrotic lesions were induced in rat semitendinosus muscles by crushing, ligating, or cutting muscle fibers. When the treated muscles were examined microscopically and recordings made with intracellular or extracellular electrodes, fibrillations were found to be either completely absent or extremely infrequent. Fibrillation potentials were also absent in the brachioradialis muscle of a human subject after myotomy. In contrast to these negative results, surgical denervation of the rat semitendinosus induced fibrillation activity within 3 days; when fully developed, fibrillations occurred in approximately one-half of the fibers at any time. The findings are considered to have significance for an understanding of the pathogenesis of those "myopathic" disorders in which fibrillations are found.

Action Potentials↗
PubMed1975

Patterns of motoneuron dysfunction and recovery.

Electrophysiological studies have been carried out on five patients with neuropathies of different etiologies. In each patient serial estimates were made of the numbers of functioning motor units in various muscles. It was found that the intensity of the neuropathic process and the rate of recovery differed in a consistent way among the motoneuron pools investigated. The lesion was more severe in extensor digitorum brevis neurons than in thenar neurons, while the hypothenar ones were least affected. A stage of partial synaptic failure has been recognized in which a motoneuron appears to be no longer able to excite a muscle fiber, but still capable of maintaining certain trophic activities. By comparing the number of functioning motor units with the size of the maximum evoked muscle response it has been possible to detect the adoption of denervated muscle fibers by axonal sprouts from 'healthy' surviving neurons (collateral reinnervation). Lastly, in some muscles it appears that the adopted muscle fibers may subsequently be recaptured by the original motoneurons following recovery of the latter from the neurotoxic insult.

Adult↗