1998 ISEK Congress Keynote Lecture: Motor units: how many, how large, what kind?

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Abstract

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.

Introduction

It was Sherrington [1]who, in 1929, introduced the term ‘motor unit’ to describe a single motor axon and the colony of muscle fibers which it innervated. Sherrington recognized that the strength of a voluntary or reflex contraction would depend on the numbers of participating motor units. In the same year Adrian and Bronk [2]described the coaxial needle electrodes which they had used, in combination with a capillary electrometer, to record the discharges of individual motor units during voluntary contractions of human muscles. Following this auspicious start, research on motor units proceeded only slowly and intermittently, until the experiments reported by Henneman and his colleagues in 1965. These workers were the first to dissect and stimulate single motor axons [3], and were able to show that the contractile properties of motor units differed considerably, even within the same muscle [4]. These important pioneering studies can be said to have ushered in the modern era of motor unit anatomy and physiology, an era which has seen its share of successes and surprises. The present review will examine selected aspects of this field and, for obvious reasons, will include contributions from our own laboratory.

Section snippets

Motor units: how many?

In animals the number of motor units in a muscle is easily found by counting the large-diameter axons which remain in the motor nerve after the muscle afferents have degenerated following dorsal rhizotomy (distal to the dorsal root ganglia). Alternately the muscle or motor nerve can be injected with a suitable tracer, such as horseradish peroxide; after allowing time for the tracer to be transported retrogradely in the motor axons, the number of labelled motoneurons can be counted in serial

Motor units: how large?

The sizes of motor units, that is, the numbers of muscle fibers supplied by single motor axons, were first estimated by dividing the total number of muscle fibers in the muscle by the number of motor axons. These simple calculations were sufficient to show that the human external ocular muscles and the platysma had very much smaller motor units than the limb muscles [20]. In the arms and legs there was a correlation between the respective sizes of the muscles and motor units, such that the

Motor units: what kind?

The fact that there are types of motor units which differ from each other in their contractile, histochemical and immunochemical properties, as well as in their sizes and fatigability, has already been referred to. The definitive animal studies in this field were started by Henneman in 1965 [3]and then developed by others, notably Burke (see Burke [42]for review) and Stuart (see Stephens and Stuart [57]). A more recent view is that, although certain motor unit characteristics tend to occur

Conclusions

Motor unit number estimation, and the ability to record twitches of single motor units, have increased our knowledge of human muscle in a number of important respects. Nevertheless significant questions remain to be answered and care should be exercised before the results of animal experiments are applied to the human situation.

Acknowledgements

I am grateful to all my colleagues, and particularly Dr. Victoria Galea, who have studied motor units with me over so many years. I am also indebted to Jane Butler, Rebecca Newberry and Heidi Scarfone for technical and secretarial assistance, and to the Natural Sciences and Engineering Research Council for financial support. Dr. K. Arasaki very kindly allowed me to include some of his unpublished MUNEs.

Dr. Alan McComas is Emeritus Professor of Biomedical Sciences and of Medicine at McMaster University. He trained in physiology and medicine at the University of Durham (UK) and then undertook postdoctoral research in basic and clinical neurophysiology at the University of Newcastle Upon Tyne (UK). He moved to Canada in 1971, joining the staff of the new Faculty of Health Sciences at McMaster. He is responsible for pioneering the electrophysiological estimation of motor unit numbers, and he has

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    Dr. Alan McComas is Emeritus Professor of Biomedical Sciences and of Medicine at McMaster University. He trained in physiology and medicine at the University of Durham (UK) and then undertook postdoctoral research in basic and clinical neurophysiology at the University of Newcastle Upon Tyne (UK). He moved to Canada in 1971, joining the staff of the new Faculty of Health Sciences at McMaster. He is responsible for pioneering the electrophysiological estimation of motor unit numbers, and he has drawn attention to the importance of electrogenic sodium pumping in exercise. He has maintained these and other research activities since taking early retirement in 1996; his most recent book is Skeletal Muscle: Form and Function.

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