MOTOR UNIT POPULATIONS IN HEALTHY AND DISEASED MUSCLES

The numbers of functioning motor units can be estimated in proximal an d distal muscles of human limbs by an electrophysiological technique i n which the mean sizes of the motor unit potentials are compared with the maximum M-waves of the same muscles. Although manual methods of es timation have been used successfully in the past, the introduction of automated techniques has brought considerable advantages, including gr eater objectivity and reduced contamination of the results by ''altern ation.'' In healthy subjects, the intrinsic muscles of the band have a pproximately 100 motor units each, and the biceps brachii muscle has o nly slightly more. With advancing age, there is a loss of motor units, which appears to be more pronounced in distal muscles. The motor unit estimating methodology has been found to be of value in the diagnosis and assessment of patients suspected of having muscle denervation. In amyotrophic lateral sclerosis, the mean rate of motor unit loss is sw ift, whereas in late-onset cases of spinal muscular atrophy, the reduc tion in the motor unit population does not appear to progress. In only the most rapidly deteriorating cases of post-polio syndrome is it pos sible to demonstrate further loss of motor units In all of these dener vating disorders, and in peripheral neuropathies, the importance of co llateral reinnervation as a compensatory mechanism is emphasized.