Methods of studying neuromuscular function in humans: isometric myogram, excitation electromyogram, and topography of terminal innervation
Desmedt, J.E.
Acta Neurologica et Psychiatrica Belgica 58(12): 977-1017
1958
ISSN/ISBN: 0001-6284 PMID: 13626541 Document Number: 9354
Clinical neurophysiology as applied to the diagnosis and understanding of neurological diseases of the peripheral motor system has aroused increasing interest in recent years. This paper re - presents an attempt to standardize and perhaps improve the electrophysiological analysis of integrated responses evoked in human muscles by supramaximal electrical stimulation of peripheral nerves. The procedure used in the investigation of 4 muscle groups in the hand, forearm and face is described and discussed (fig. 1, 20 and 22). Attention is drawn to various technical details which may increase the reliability and scope of quantitative assessments of patholog'cal •function and/or drug action in the human neuromuscular apparatus. The intrα-muscular temperature should be checked with a deep thermocouple, since local artificial cooling produces important changes in the indirect responses (fig. 11). The ulnar nerve should be blocked with a local anesthetic at the elbow while the stimuli are delivered through a needle inserted close to the nerve trunk at the wrist. This prevents voluntary or reflex interference in the indirect responses and achieves stable experimental conditions. The ulnar block also suppresses the otherwise unbearable pain associated with high rate faradization of the nerve. The record'ng of the isometric myogram along with the electrogram of the adductor pollicis provides useful information on the contractile mechanism, a.o. twitch tension and duration, tetanus pattern as a function of frequency, maximal' tetanic tension, and possible failure of electro-mechanical coupling (Merton) (fig. 18, 19). Quantitative data on electrical and mechanical responses recorded in 28 normal adults of either sexes are presented (fig. 15 and 16; sections III, g, IV and V). The interpretation of the belly-tendon electromyogram recorded with skin electrodes is discussed in relation to the pattern of terminal innervation. The latter has been investigated on sections from adult muscles (post-mortem material) treated with the Koelle histochemical method which stains the cholinesterase concentrated in the subneural apparatus of the motor end-plate. In the hand and forearm muscles investigated, there is a critical location of the belly electrode for which the evoked response presents minimal latency, maximal size and simplest diphasic configuration which indicates that the potentials contributed by each activated muscle fibres are optimally integrated (fig. 3, B, 8, C, 15 and 21). This location has been found to coincide reasonably well with (1) the motor point of the corresponding muscle as determined by percutaneous stimulation, and (2) the junctional region of the superficial muscle bundles. As seen in figures 4 and 9, the motor end-plates are located near the middle portion of each muscle fibre but the innervation zone considered for the whole muscle presents a rather complex configuration which is obviously related to the pattern of tendon insertions for the various bundles. These observations can account for the fact that the duration of the belly-tendon electromyogram is larger than the duration of the fibre potential since the action potentials of the different muscle fibres are initiated at widely separated end-plates. The time of their arrival at the level of the belly electrode must consequently vary by a few milliseconds since muscle conduction velocity is only about 4 meters/second (Buchthal). It is also suggested that the pseudo-facilitation of the belly-tendon potentials which can be observed in normal subjects at stimulation rates above 10/second (fig. 12, 13) depends on increased synchronization of elementary potentials, possibly determined by a small increase in conduction velocity related to subthreshold membrane depolarization during the negative afterpotential. In the orbicularis oculi, the motor end-plates are not concentrated in a well-defined innervation zone, instead they are scattered in small groups over the length of the muscle (fig. 23). This may mean either that the muscle fibres running uninterrupted between the tendon insertions carry a number of motor end-plates (multiple innervation), or that shorter muscle fibres with single end-plates are arranged « in series ». Whatever the proportion of fibres of either type, it seems clear that the unusual pattern of terminal innervation thus disclosed can explain the well-known difficulty of recording simple and well-integrated motor unit potentials in facial muscles. It is also suggested that whereas in limb muscles the motor point generally overlies the innervation zone of superficial bundles, in most facial muscles the motor point corresponds to the entry of motor nerve twigs into the muscle.
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