Motor Unit Populations in Healthy and Diseased Muscles

The numbers of functioning motor units can be estimated in proximal and distal muscles of human limbs by an electrophysiological technique in which the mean sizes of the motor unit potentials are compared with the maximum M-waves of the same muscles. Although manual methods of estimation have been used successfully in the past, the introduction of automated techniques has brought considerable advantages, including greater objectivity and reduced contamination of the results by "alternation." In healthy subjects, the intrinsic muscles of the hand have approximately 100 motor units each, and the biceps brachii muscle has only 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 swift, whereas in late-onset cases of spinal muscular atrophy, the reduction in the motor unit population does not appear to progress. In only the most rapidly deteriorating cases of post-polio syndrome is it possible to demonstrate further loss of motor units. In all of these denervating disorders, and in peripheral neuropathies, the importance of collateral reinnervation as a compensatory mechanism is emphasized.