A. R. M. Upton

THE DOUBLE CRUSH IN NERVE-ENTRAPMENT SYNDROMES

Abstract A comprehensive electromyographic study has been made of 115 patients with carpal-tunnel syndromes or lesions of the ulnar nerve at the elbow. In 81 cases there was electrophysiological evidence, often supported by clinical symptoms, of associated neural lesions in the neck. This association is not thought to be fortuitous, but rather the result of serial constraints of axoplasmic flow in nerve fibres.

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.

Cutaneous reflexes in small muscles of the hand

A study has been made of the responses of motoneurones innervating small muscles of the hand to electrical and mechanical stimulation of the skin. Both excitatory and inhibitory effects could be observed in the same muscle after a single stimulus to a given area of skin. The earliest excitatory and inhibitory responses are probably mediated by group III and the smaller group II afferent nerve fibres. A later inhibition results from activity in the larger group II fibres which are connected to cutaneous mechanoreceptors, especially those in the tips of the fingers and thumb. This late inhibitory reflex may operate through the fusimotor system. The possible roles of these reflexes are discussed in relation to previous investigations in man and the cat.

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.

Effect of strength training upon motoneuron excitability in man.

Two healthy females and twelve healthy males, aged 19-24 yr, underwent strength training for periods of 9-21 wk. The muscles trained included extensor digitorum brevis (N = 3), soleus (N = 7), brachioradialis (N = 4), and the hypothenar muscles (N = 3). The effect of training on motoneuron excitability was measured as the degree to which two reflex responses (V1 and V2) were potentiated by voluntary effort. Strength training was found to increase V1 and V2 potentiation by 49.7 and 38.9%, respectively, (P less than 0.01) for pooled muscle comparisons with the exception of the soleus V2 wave, which was rarely seen and excluded from this analysis. There was a positive correlation (r = 0.83, P less than 0.01) between the change in the V1 and V2 potentiation. It was argued that strength training may cause an increased ability to raise motoneuron excitability during voluntary effort.

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.

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.

SICK MOTONEURONS AND MUSCLE DISEASE

Is muscular dystrophy neurogenic? To have posed this question as recently as three years ago would have been merely to speculate without any hope of obtaining a satisfactory answer. Yet today, in the light of recent developments in neuromuscular disease, the question is not only valid but is felt by many to require an affirmative answer. It is neither the purpose of the present paper to survey the historical background that led to the concept of dystrophy as a primary myopathy, nor to point out the various features of the disease which should have prompted hesitation in accepting this explanation. Some of these features have already been touched upon in earlier publications,1~ 2 and they have been discussed at greater length in this volume by Rowland. Instead we shall present the results of a new type of electrophysiological investigation that has been applied to the study of a large number of patients with different types of muscular dystrophy. The aim of these studies has been to measure the numbers and sizes of the functioning motor units within a dystrophic muscle. A motor unit, as originally defined by Sherrington,3 comprised ;in alpha motoneuron, its axon, and all the muscle fibers that i t innervated. Very often, however, the term is used to include the muscle fibers only; this is the sense in which it will be employed in this paper.

Evidence for reversible motoneurone dysfunction in thyrotoxicosis

Motor unit estimating techniques have been employed as part of a comprehensive electrophysiological survey of peripheral nerve and muscle in 20 patients with thyrotoxicosis. In all patients there was evidence of a loss of operational motor units; the selective nature of this involvement suggested that the motoneurone soma was the site of the primary lesion. The reversible nature of the postulated motoneurone dysfunction was demonstrated by the increased motor unit counts in six patients who were studied again after treatment of their thyrotoxicosis.

Impaired potentiation of H-reflexes in patients with upper motoneurone lesions

The potentiation of H-reflexes during effort has been studied in 16 patients with mild hemiparesis. Impaired potentiation was present in all but one patient, and was still evident in some who had recovered fully from weakness. A tentative explanation has been proposed for these findings; it is suggested that there is loss of a background facilitatory pathway to motoneurones.