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A J McComas

Publications and source records attributed to A J McComas.

At least 19 recordsLinked to original sources
PubMed1999

Denervation and reinnervation in congenital brachial palsy.

Motor unit number estimation (MUNE) was shown to be useful in assessing the neurophysiological status of 18 subjects with congenital brachial palsy. This was especially so since conventional M-wave measurements may give misleading impressions as to the extent of motor axon regeneration. In most subjects the involvement of sensory nerve fibers indicated that the traumatic lesions included postganglionic segments of the fibers, with or without preganglionic damage. In a minority the lesions were purely preganglionic. Digital sensory nerve involvement was more in a mediolateral direction, consistent with greater damage to the uppermost elements in the brachial plexus. In 5 individuals, MUNE and sensory testing showed that there had been trauma to the supposedly unaffected arm. Discrepancies between sensory and motor results suggested that reinnervation of the biceps brachii muscle was greater than that of the intrinsic muscles of the hand. In one subject examined serially, reinnervation of the hand muscles was detected by 10 months and continued in the hypothenar muscles for the next 6 years.

Action Potentials↗
PubMed1999

The RULER model. Is this how the somatosensory cortex works?

Despite a wealth of information, it is still not known how neurones in the different neocortical layers interact to produce a conscious perception. We now put forward a model for the somatosensory cortex in which a touch is perceived whenever superficial cortical pyramidal cells (in layers II and III) are made to discharge by a recurrent input from deep pyramidal neurones (in layer V). The superficial cells act as biological amplifiers and the number discharging will depend both on the strength of the message from the thalamus and on the variable background depolarisation of their apical dendrites. The recurrent volley arises in the layer V neurones at the end of an IPSP (inhibitory postsynaptic potential), which itself follows an excitatory response induced by the incoming thalamic signal; the IPSP is generated by local basket cells. The duration of the initial excitation--IPSP--late excitation sequence corresponds to a time chunk, that is, the period over which neural activity is integrated to produce a perception. During the time chunk, the superficial cortical pyramids, unlike the deeper ones, can accumulate information as subthreshold excitatory postsynaptic potentials (EPSPs). The relative time at which the information arrived in the cortex is roughly coded by the gradient of EPSPs among cells in an axis perpendicular to the cortical surface. Although developed for the somatosensory cortex, the basic features of the model may well apply to other sensory receiving areas of the cortex.

Attention↗
PubMed1999

The world of touch--from evoked potentials to conscious perception.

Microelectrode recordings have enabled several maps of the body surface to be recognized in the mammalian somatosensory cortex. The maps appear to represent increasingly complex levels of analysis of the sensory message. At present the prevailing opinion is that the different components of the ERPs (event related potentials) represent sequential steps in such an analysis, and such an interpretation is supported by the enhancement of the ERPs when attention is paid to a somatic stimulus. However, there are a number of critical observations which are inconsistent with this view and suggest that the ERP enhancement may be an epiphenomenon. An alternative explanation for the ERPs is that they reflect discharges from the non-specific thalamic nuclei, and are essentially similar to the long latency responses which can be recorded from the cortex during sleep or anaesthesia. Lastly, a hypothesis is proposed for the neuronal events in the somatosensory cortex which culminate in a conscious perception. In this "RULER" model, the deep pyramidal neurones read out the sensory information which has been retained in the apical dendrites of more superficial cells, and do so at the end of successive "time-chunks".

Animals↗
PubMed1998

1998 ISEK Congress Keynote Lecture: Motor units: how many, how large, what kind? International Society of Electrophysiology and Kinesiology.

There are now at least nine methods for motor unit number estimation (MUNE) in living human muscles. All methods are based on the comparison of an average single motor unit potential (or twitch) with the response of the whole muscle. Such estimations have been performed for proximal and distal muscles of the arm and leg in healthy subjects and in patients with various neuromuscular disorders. In healthy subjects there is a loss of motor units which is most evident in distal muscles and after the age of 60 years. Substantial losses of motor units have been measured in patients with ALS, post-polio symptoms, and diabetic peripheral neuropathies. In contrast, normal MUNEs have been found in approximately half of patients with persisting obstetric brachial palsies. The sizes of motor units show considerable variations within the same muscle and also between muscles; very large units are usually present in severe partial denervation. Although many motor unit properties are largely governed by motoneurons, some exhibit less plasticity in humans than in other mammals.

Adolescent↗
PubMed1998

Oro-facial muscles: internal structure, function and ageing.

Structure and function are reviewed in the masticatory muscles and in the muscles of the lower face and tongue. The enormous strength of jaw closure is in large part due to the pinnated arrangement of the muscle fibres in the masseter. This muscle, like other masticatory muscles, is unusual in that the cell bodies of the muscle spindle afferents lie in the brain stem rather than in an external ganglion; spindles are absent in the lower facial muscles. Although few data are available, the numbers of motor units in the masticatory muscles, and probably in the lower facial muscles also, appear to be much greater than in limb muscles. The motor units in the facial and tongue muscles are largely composed of histochemical type II ('fast-twitch') fibres, but in the masticatory muscles there are substantial numbers of fibres intermediate between type I ('slow twitch') and type II, and fibre type grouping is present. In comparison with limb muscles, there is little information on ageing changes in oro-facial muscles. The masticatory muscles do, however, show some atrophy and loss of X-ray density, while motor unit twitches are prolonged. Strength is reduced in the tongue and masticatory muscles. It is known that limb muscle properties are largely governed by their innervation, both through the pattern and amount of impulse activity, and the delivery of trophic messengers; the situation for oro-facial muscles is unclear. The structural and functional differences between the two types of muscle indicate the need for conducting ageing studies on the oro-facial muscles, rather than relying on extrapolations from limb muscles.

Aged↗
PubMed1997

Depression of human electromyographic activity by fatigue of a synergistic muscle.

In human volunteers, lateral gastrocnemius muscles were stimulated electrically under ischemic conditions so as to produce fatigue. Recordings of electromyographic (EMG) activity were then made from those muscles and simultaneously from untreated medial gastrocnemius muscles during maximal voluntary efforts. In the lateral gastrocnemius the mean amount of EMG activity declined by 52% and was associated with a 35% reduction in the mean amplitude of the M wave (muscle compound action potential) and an insignificant change in M-wave area. In the medial gastrocnemius the EMG was also diminished, by 29%, but there were no significant changes in M-wave amplitude or area. The findings in the medial gastrocnemius are consistent with the existence of an inhibitory reflex effect which originates in the fatigued lateral gastrocnemius muscle and serves to depress excitation in motoneurons supplying that muscle and also in those innervating synergists. The inhibitory effect appears to be long-lasting, in that a significant reduction of the EMG could still be demonstrated 10 min after release of the arterial cuff.

Adolescent↗
PubMed1997

Early and late losses of motor units after poliomyelitis.

Motor unit number estimation was employed to assess muscle innervation in 76 patients with prior poliomyelitis. Of the 68 patients who were < 70 years of age, new musculoskeletal symptoms had appeared in all but four; the mean latent interval was 38.0 +/- 10.1 years. As expected, there was a high incidence of muscles exhibiting denervation in previously affected limbs (87%). However, the incidence in supposedly unaffected limbs was also high (65%). Significant differences in the degree of denervation were found between muscles of the same hands and feet. Judged on the basis of their potential amplitudes, the surviving motor units in partially denervated muscles tended to be enlarged. The enlargement was proportional to the extent of the denervation and was comparable to that found in amyotrophic lateral sclerosis. In some muscles, possibly those innervated by failing motor neurons, motor-unit enlargement was not present. Needle examination confirmed the high incidences of denervation in affected and allegedly unaffected limbs. Of the 188 muscles with EMG features of chronic denervation, only nine exhibited fibrillations or positive sharp waves (4.8%). Ninety-five muscles of 18 patients were studied a second time after an interval of 2 years. Overall, there was a 13.4% reduction in motor-unit number and a 18.4% diminution in M-wave amplitude (P < 0.001). The rate of motor-unit loss was twice that occurring in healthy subjects aged > 60 years. Analysis of individual patients indicated that some were deteriorating more rapidly than others. These studies confirm that denervation progresses in patients with prior poliomyelitis in both clinically affected and unaffected muscles, and indicate that this progression is more rapid than that occurring in normal ageing.

Adult↗
PubMed1996

Late depression of muscle excitability in humans after fatiguing stimulation.

1. Changes in muscle excitation and in isometric twitch force have been studied for up to 8 h after fatiguing stimulation of the human biceps brachii. 2. Within 10 s of a cessation of the 20 Hz fatiguing tetanus, the amplitudes of the M waves (muscle compound action potentials) had returned to control values, whereas the twitch forces were reduced in all subjects. The M waves then decreased in amplitude over the next 3 h, reaching a mean value that was 42.4 +/- 18.6% of control levels (means +/- S.E.M.; P < 0.001). 3. By 8 h, the mean M wave amplitude had recovered to 93.8 +/- 33.3% of control levels, while the corresponding mean twitch force was 104.1 +/- 36.9%. 4. The cellular mechanism responsible for the depression of the M wave is presently unknown, but it is likely to be postsynaptic and may involve Na+ channels.

Action Potentials↗
PubMed1996

Non-conscious choice in cutaneous backward masking.

A simple but effective method has been devised to explore backward masking in cutaneous sensation. In this method a weak electrical shock to the palm (test stimulus) is followed by a stronger shock to the same site (masking stimulus). By combining this stimulation strategy with a choice reaction time paradigm, it has been possible to show that the nervous system responds to the test stimuli even though they are not consciously perceived. Further, the distribution of errors in the different types of trial suggests that, under these experimental conditions, the identification of the stimulus and the choice of response may also be made at a non-conscious level. A tentative scheme for the timing of the various neural events is proposed.

Adult↗
PubMed1996

Potentiation and depression of the M wave in human biceps brachii.

1. The effects of repeated excitation on the compound action potential, or M wave, of mammalian muscle fibres have been investigated in the human biceps brachii. 2. During continuous indirect stimulation at 10 and 20 Hz the mean voltage-time area of the M wave doubled within the first minute, while the mean peak-to-peak amplitude increased by approximately half. The enlargement of the M wave was sustained during stimulation at 10 Hz but not at 20 Hz. Stimulation at 3 Hz caused a small increase which was significant for M wave amplitude only. 3. When the 20 Hz stimulation was performed under ischaemic conditions, the M wave first enlarged and then gradually declined. After 20 Hz stimulation was discontinued, the M wave increased in size; in the ischaemic experiments the release of the cuff produced a further, rapid augmentation. In both the ischaemic and non-ischaemic experiments, the amplitudes and areas of the M waves during the recovery period became significantly larger than the resting values (range, 15-60% at the endplate zone). 4. The mean muscle fibre impulse conduction velocity decreased to less than half the resting value during 20 Hz stimulation, with or without ischaemia, and then increased above the resting value during recovery. 5. On the basis of previous experiments in animals, the augmentation of the M wave was attributed to enhanced electrogenic Na(+)-K+ pumping, and the biceps brachii appeared to be an excellent preparation for studying the time course of this enhancement.

Action Potentials↗
PubMed1995

Motor unit estimation: anxieties and achievements.

The history of motor unit number estimation (MUNE) is given, together with brief descriptions of the various methods presently available. A small muscle of the hand contains about 100 motor units and greater numbers are found in larger muscles; beyond 60 years the numbers begin to decline. In ALS approximately half the motor units cease to function within 6 months of the involvement of the motoneuron pool, while in adult spinal muscular atrophy further loss may not occur over several years. The reduction in MUNE values in myotonic dystrophy remains an enigma, but even more curious are the losses and subsequent recoveries occasionally observed in hyperthyroidism and chronic renal failure; possibly, nontransmitting ("silent") synapses are involved. MUNE may also be used to study CNS problems such as hemiplegia and congenital brachial palsy. The availability of more powerful computers for EMG should lead to advances in MUNE.

Adult↗
PubMed1995

The scientific contributions of Brenda Bigland-Ritchie.

Brenda Bigland-Ritchie has made seminal contributions to our understanding of skeletal muscle physiology--the energy cost of muscle when it shortens or is forcibly stretched, the relationship between EMG and force, the behavior of single motor units, and above all, the processes underlying neuromuscular fatigue. More than this, she has stimulated inquiry into the search for reflex mechanisms which may serve to balance the activity of the spinal cord with that of the fatiguing muscles. Her use of human volunteers for much of this work is extraordinary, and represents a major strength. Equally important are her well known and widely cited manuscripts. Not only are her findings clearly described and depicted, but every attempt is made to relate her results to the fatigue processes measured in animal or isolated tissue preparations.

Electromyography↗
PubMed1995

Fatigue brought on by malfunction of the central and peripheral nervous systems.

Increased fatigability necessarily occurs in every patient with muscle weakness, regardless of whether the latter is due to a central or peripheral neurological disorder. The tendency for disuse to increase fatigability, as a secondary phenomenon, must also be considered; disuse affects both motoneuron recruitment and the biochemical and physiological properties of the muscle fibers. In recent studies impaired recruitment has been observed in postpolio patients, while patients with multiple sclerosis or spinal cord injury have shown, in addition, altered neuromuscular function. Findings are also presented in ALS and the chronic fatigue syndrome. In general, the most dramatic increases in fatigability take place in disorders of the peripheral nervous system and almost any cell component can be incriminated. There is a need to study fatigability systematically in neurology and rehabilitation.

Central Nervous System Diseases↗
PubMed1995

Neurobiology of muscle fatigue. Advances and issues.

Throughout this epilogue, we have emphasized that rapid advances in understanding of neural and muscular aspects of fatigue have occurred since the 1980 London Symposium. However, in each instance of progress, from the single muscle fiber to the forebrain, the application of more precise techniques have raised important new questions. Neuroscientists and muscle physiologists have expanded opportunities for rigorous study of a topic of major scientific and social importance.

Animals↗