R. E. P. Sica

Electrophysiological estimation of the number of motor units within a human muscle

An electrophysiological method is described for estimating the numbers of motor units in the extensor digitorum brevis muscle in man. The results obtained are compared with counts of axons in the nerve to the muscle. The significance of the sizes of the evoked motor unit potentials is discussed.

Functional changes in motoneurones of hemiparetic patients

Forty-six patients have been studied after upper motor neurone lesions of cerebrovascular origin. The numbers of functioning motor units in extensor digitorum brevis muscles were reduced to approximately half between the second and sixth months after a hemiplegic episode. The surviving motor units tended to have slow twitches and appeared to increase their sizes after the lesions had been present for about 20 months. The findings are explained on the basis of transsynaptic changes in alpha-motoneurones after degeneration of corticospinal fibres.

Functional compensation in partially denervated muscles

In patients with various types of chronic motor denervation, the numbers of surviving motor units have been compared with the twitch tensions developed by the same muscle (extensor digitorum brevis). It was found that functional compensation in partially denervated muscles was often marked; in most patients abnormally small twitches occurred only when fewer than 10% of motor axons remained. The factors responsible for this compensation are considered. The twitch speeds of partially denervated muscles differed markedly, even among patients with the same disorder; there was evidence to suggest that the twitches of some motor units might become slower than those found in normal muscles.

SICK MOTONEURONES: A Unifying Concept of Muscle Disease

Abstract New electrophysiological techniques have demonstrated a loss of motor units in muscular dystrophy and myasthenia gravis; the sizes of the surviving motor units suggest that both diseases result from disordered function of motoneurones. Estimates have been made of the incidence of healthy, sick, and dead motoneurones in these conditions and, for comparison, in recognised denervating processes such as motor-neurone disease and the Kugelberg-Welander syndrome.

Fast and slow twitch units in a human muscle

A study has been made of the isometric twitches of single motor units in the extensor hallucis brevis (EHB) muscle in man. The twitch contraction times ranged from 35 to 98 msec and the pooled results indicated the presence of at least two types of unit. The recorded twitch tensions also varied considerably (from 2 to 14 g) but were not related to the twitch speeds. The twitch tension developed by the whole EHB muscle depended on the initial length of the muscle and on the age of the subject. The EHB muscle was estimated to contain approximately 56 motor units.

Motor unit estimations in small muscles of the hand

A quantal method has been employed to determine the numbers of functioning motor units in small muscles of the hand. In healthy subjects below the age of 60 years the mean number of hypothenar motor units was 380 ± 79 and the mean number of thenar units innervated by the median nerve was 340 ± 87. In older subjects there was a progressive reduction in the numbers of functioning units in both muscle groups; in some of these subjects the population of motor units in the extensor digitorum brevis was also estimated for comparison. A study was also performed on patients diagnosed as having generalized denervating diseases or with lesions of peripheral nerves and cervical roots; in every instance the number of functioning motor units was markedly reduced.

Electrophysiological study of dystrophia myotonica

A comprehensive electrophysiological study has been made of the extensor digitorum brevis muscle and its motor innervation in 17 patients with dystrophia myotonica. The mean contraction and half-relaxation times were prolonged in the isometric twitches of dystrophic muscles. Decremental responses to repetitive motor nerve stimulation were found in two patients. All the terminal latency measurements were normal and only one patient had a reduced nerve conduction velocity. As the patients aged their muscles became weaker, due to a progressive loss of motor units. This finding, and the normal sizes of many surviving motor units, suggested that the muscle changes resulted from a primary defect of motor innervation.

An electrophysiological study of Duchenne dystrophy.

An electrophysiological study has been made of extensor digitorum brevis muscles in 19 boys with Duchenne dystrophy. The isometric twitches of the dystrophic muscles developed less tension and were usually slower than those in controls. Impulse conduction velocity appeared to be reduced in distal regions of nerve axons but was normal proximally. The most interesting finding was a reduction in the number of functioning units in all but one of the patients. The sizes of the surviving units suggested that the results could be explained more easily in terms of a neurogenic process than a primary myopathic one.

An electrophysiological investigation of limb-girdle and facioscapulohumeral dystrophy

A detailed electrophysiological study has been made of the extensor digitorum brevis muscle and its motor innervation in 11 patients with limb-girdle or facioscapulohumeral dystrophy. In nine patients there were reductions in the populations of motor units and many surviving units appeared to be abnormally large. Most of the investigated muscles had slowed isometric twitches and decremental evoked potentials after repetitive nerve stimulation. The experimental observations have been interpreted in terms of a neuropathic process.

MOTONEURONE DISEASE AND AGEING

Abstract New methods for estimating numbers of functioning motor units in human muscles have shown that muscle denervation is part of the normal ageing process and that it becomes more striking beyond the age of 60. The similarity of this age to the peak age-incidence of motoneurone disease is not regarded as fortuitous—rather it is argued that the disease is an accelerated form of normal ageing. It is thought that motoneurone ageing is determined genetically, but that secondary factors may influence its course.