TRANSMISSION AND CONTRACTION FATIGUE OF RAT MOTOR UNITS IN RELATION TO SUCCINATE-DEHYDROGENASE ACTIVITY OF MOTOR UNIT FIBERS

Abstract

The fatigue in rat anterior tibial (a.t.) motor units was studied and related to microphotometric determinations of succinate dehydrogenase (SDH) activity of the motor unit muscle fibers. Anterior tibial contains fast-twitch type II fiber units with an average contraction time of 11 ms and about 5% slow-twitch type I fiber units with an average contraction time of 20 ms. In type II fibers stained for SDH, absorbance varied continuously from 0.046-0.569 and inversely to fiber size, except for the largest fibers. Resistance to fatigue of fast motor units to 100 Hz intermittent stimulation varied continuously within a wide range in near linear relations to absorbance for SDH of unit fibers and inversely to tetanic tension, except for motor units with the largest fibers and the largest tetanic tension. Neither resistance to fatigue nor SDH activity lent itself to any categorization of motor units or fibers into well demarcated functional or histochemical types, since both parameters varied continuously in the unit and fiber population of the muscle. The direct relation between resistance to fatigue of fast-twitch motor units and SDH activity of unit fibers appeared valid for fatigue resistance of: neuromuscular transmission, tested with 100 Hz intermittent stimulation which gave concomitant failure of electrical and mechanical response, excitation-contraction coupling, demonstrated by post-stimulatory depression of twitch tension with preserved maximum tetanus tension and action potential and contractile mechanism; excitation-contraction coupling, tested with low frequency stimulation which gave decline of twitch and maximum tetanus tension with preserved action potential. The endurance of each link in the chain of events leading to contraction, including neuromuscular junction and the excitation-contraction coupling system, is apparently under aerobic conditions matched to the contractile capacity of the fiber expressed by its oxidative enzyme activity.