Metabolic and contractile uniformity of isolated motor unit fibres of snake muscle.

Abstract

SUMMARY1. Motor units in the thin transversus abdominis muscle of the garter snake were identified and physiologically characterized in the living state. Motor unit fibres, and fibres chosen randomly to serve as controls, were subsequently excised and subjected to biochemical analyses.2. The metabolic capacity of fibres was assessed by measuring activities of three enzymes, each representing a different metabolic pathway. The microchemical enzyme assays were performed using enzyme extraction preparations of whole single fibres.3. Metabolic capacity ranged widely among the muscle's entire fibre population, even among fibres of the same type. In contrast, enzyme activities of twitch fibres belonging to individual motor units were, within analytical error, identical.4. Twitch contraction times of individual fibres within one motor unit were similar, compared to a wide range of contraction times observed among fibres of the same type but belonging to different motor units.5. When several motor units were studied in one muscle, a systematic relationship was observed among motor unit tension, enzymatic profile and contraction time. As motor unit tension increased, fibres exhibited greater capacities for glycolytic and high-energy phosphate metabolism, diminished capacity for oxidative metabolism, and faster twitch contraction times.6. Given the great diversity of metabolic and contractile properties exhibited within the fibre population, the uniformity of such properties within motor units indicates that neural influence dominates over other extrinsic factors present in the microenvironment of the muscle fibres.

Metabolic capacity and myosin expression in single muscle fibres of the garter snake.

“…The present results, taken together with a previous motor unit study (Nemeth et al 1991), indicate that the fibre diversity required in this small reptilian muscle results from a combination of two mechanisms: first, the primary division of the muscle into three intrinsic fibre types, and second, the subdivision by neural influence of one fibre type (slower twitch) into motor units with differing properties. One interpretation of these results is that fibre types play a similar role in larger mammalian muscles, excepting that fibres of all types are subdivided to achieve the required number of heterogeneous motor units.…”

“…The muscle's approximately twenty-five type S fibres comprise four to five motor units which vary systematically in size, similar to the motor unit arrangement of mammalian muscle (Lichtman & Wilkinson, 1987; see also Hammond & Ridge, 1978). Enzyme content and twitch time-to-peaks are similar among fibres belonging to one motor unit, and differ systematically among different size motor units in one muscle (Nemeth et al 1991). Thus, although the intrinsic functional capacity of S fibres is limited by the range of metabolic and contractile proteins expressed, properties of a particular fibre within this range are determined by the innervating motoneuron.…”

“…One muscle exhibits a broad and nearly continuous spectrum of metabolic (Lowry, Kimmey, Felder, Chi, Kaiser, Passonneau, Kirk & Lowry, 1978;Pette & Spamer, 1986) and contractile (Stuart, Binder & Enoka, 1983) fibre properties. Neural influence is at least partly responsible for this diversity, because fibres belonging to the same motor unit share virtually identical properties (Nemeth, Pette & Vrbova', 1981;Nemeth, Solanki, Gordon, Hamm, Reinking & Stuart, 1986;Nemeth, Rosser & Wilkinson, 1991), while those belonging to different motor units in one muscle differ systematically (Nemeth et al 1991). Similar precise organization at the level of fibre types, as opposed to motor units, has not been demonstrated.…”

Fatigue of rat hindlimb motor units: Biochemical– physiological associations

“…The variation in specific enzyme activity between fibers belonging to a single MU was no greater than the variation observed in duplicate samples from the same fiber. The validity of this restricted sampling technique was established rigorously 38, 39…”

“…Differences in fatigability across the three main types of mammalian motor units (MUs), type FF (fast‐contracting, fatigable), type FR (fast‐contracting, fatigue resistant), and type S (slow‐contracting, fatigue resistant), are associated with differences in the biochemical properties of their constituent muscle fibers (MFs),25, 36–39 in addition to the fibers' earlier established histochemical properties 8, 16, 17, 30. The protocol employed in many of these and related studies used a MU stimulation regimen consisting of 330‐ms, 40‐H Z trains applied at a rate of 1 H Z for 2 min.…”