[Contributions of histochemistry to the development of electromyography: pathological genesis of the subunit and heterogeneity of recruitment of the motor units].

Histochemical homogeneity of the motor unit and depletion of muscle unit glycogen by motoneurone repetitive-stimulation (Kugelberg and Edström) have offered the most suitable way of investigating how the fibres of the individual motor unit are distributed in skeletal muscles of animals. Revised analysis of the potential field of motor units action potentials in human beings (Buchthal and Rosenfalk) confirms that the fibres of a motor unit in normal human muscle lie solitary. Therefore the concept of the motor subunit in normal human muscle has been abandoned, while bundles of fibres belonging to the same motor unit are now known to occur only by reinnervation. Because of these findings, single fibre electromyography, as developed since 1963 by Ekstedt and Stalberg, appears as a main method to investigate the properties of different muscle fibres within the muscle unit. With regard to the heterogeneity of fibres and motor units, one must consider that statistical analysis of motor units and single fibre electrophysiological parameters have never shown any evidence of bi-or pluri-modal distribution in the histograms of duration and amplitude values. Consistent differences however have been found in the patterns of recruitment frequency of at least two types of motor units (Grimby and Kugelberg). It is suggested that these differences may represent the primary fact by which the structural, histochemical and other functional properties may be affected and maintained. The typization of muscle fibres should be conceived as the effect of a plastic process (in accordance with the continuous range of values) rather than a static one (in the absence of bimodal distribution).