Application of Cross-Sectional Single-Fiber Microchemistry to the Study of Motor-Unit Fatigability. In: Gantchev GN, Dimitrov B, Gatev P, editors. Motor Control. Boston: Springer US; 1987. pp. 23-7. [DOI: 10.1007/978-1-4615-7508-5_5]

Immobilization-induced changes in motor unit force and fatigability in the cat

The purpose of this study was to examine the effects of 3 weeks of immobilization on the mechanical properties of motor units in a cat hindlimb muscle. The muscle, tibialis posterior, was immobilized in a shortened position. Motor units were classified as type FF, F(int), FR, or S. Force, axonal conduction velocity, fatigability, and proportions of motor unit types were compared in control and immobilized muscles. All properties exhibited some change after immobilization, including slower axonal conduction velocities, greater twitch forces, slower twitch contraction times, and greater tetanic forces. In addition, most motor units were less fatigable after immobilization. The number of motor units that could not be included in one of the four classification categories increased significantly after immobilization; these units exhibited normal axon conductivity but failed to produce measurable force or associated EMG. Short-term immobilization induced a variety of physiological adaptations in neuromuscular processes that varied with motor unit type.

Association between biochemical and physiological properties in single motor units

Motor units from the cat tibialis posterior muscle were examined for an association between physiological and biochemical properties. Functionally isolated motor units were categorized on the basis of their physiological properties. This was followed by quantitative microbiochemical analysis of single muscle fibers from each unit, identified in cross sections using the glycogen-depletion method. The activities of malate dehydrogenase and beta-hydroxyacyl-CoA dehydrogenase distinguished between fatigable (type FF) and fatigue-resistant (types FR and S) units. The activities of both lactate dehydrogenase and adenylokinase were higher in fast- than in slow-contracting units. Cluster analyses, based on both physiological and biochemical properties or on biochemical properties alone, produced groupings identical to types FF, FR, and S. The association between physiological and biochemical properties substantiates the idea that biochemically distinct groups of motor units correspond to physiologically identifiable groups.