FACTORS CAUSING DIFFERENCE IN FORCE OUTPUT AMONG MOTOR UNITS IN THE RAT MEDIAL GASTROCNEMIUS-MUSCLE

1. The isometric contractile properties and morphological characteristics of the muscle unit portion of motor units were investigated in the medial gastrocnemius (MG) muscle of Fischer 344 rats. Individual motor units were functionally isolated by stimulating single MG axons in finely dissected ventral root filaments. 2. To study the mechanical properties of the motor units in the rat MG muscle, ninety-six motor units in five animals were classified into three categories (type FF, FR and S units) using two physiological criteria: presence or absence of the 'sag' property and fatigability. The relative distribution of the different motor unit types in the sample was 35.4% for type FF, 47.9% for type FR, and 16.7% for type S units. 3. There was little overlap in the distribution of twitch contraction time between type F (including types FF and FR) and type S units. The mean value was 17.1 ms for type FF, 15.7 ms for type FR, and 28.0 ms for type S units. Type FF units produced the largest tetanic tension (mean +/- S.D.; 201 +/- 75 mN). Tension output of type S units was the smallest (15 +/- 6 mN), and that of type FR units was intermediate (100 +/- 45 mN). These values were significantly different. 4. A muscle unit portion of twenty-three motor units (8 FF, 6 FR, and 9 S units) was depleted of its glycogen through repetitive stimulation after characterization of its mechanical properties. Cross-sectional areas of unit fibres and innervation ratio were directly measured in sections stained for glycogen using a periodic and acid-Schiff (PAS) reaction. Specific tension of unit fibres was calculated by dividing the maximum tetanic tension of a unit by its total fibre area. 5. The number of unit fibres ranged from 44 to 77 for type S, 116 to 198 for type FR, and 221 to 356 for type FF units, and differences among their means (66, 154 and 271, respectively) were significant. Tetanic tension was correlated with innervation ratio for all of the twenty-three units, or units within a particular type. 6. Mean fibre area for type S units (1983-mu-m2) was significantly smaller than that for type FF units (3489-mu-m2). Fibres belonging to type FR units had an intermediate size (2648-mu-m2). Correlation between tetanic tension and fibre area was significant for either all units or units within a particular type. 7. Total cross-sectional area was significantly different among motor unit types, and was highly correlated to the maximum tetanic tension. 8. Specific tension was 16.7 +/- 2.9 N/cm2 for type S, 21.4 +/- 1.3 N/cm2 for type FR, and 25.1 +/- 2.9 N/cm2 for type FF units. These values were significantly different. 9. Stepwise multiple regression analysis revealed that variations in maximum tetanic tension among motor unit types could be explained principally by innervation ratio, whereas fibre size was the major factor to determine tetanic tension within a given motor unit type. 10. Unit fibre density for type S units was significantly smaller than that for either type FR or type FF units. There was no difference between type FR and type FF units. 11. Axonal conduction velocity was not correlated with innervation ratio for units within a particular type as well as for all twenty-three units of different types, suggesting that there was no or a weak, if any, correlation between axonal conduction velocity and the number of terminal branches for rat motoneurones.