Is the motor unit uniform?

Single motor units in the fast-twitch tibialis anterior muscle were functionally isolated by stimulation of microdissected ventral root filaments. The muscle fibres of 35 fast-twitch single motor units (motor unit fibres), 18 units from 3 to 6-month-old and 17 units from 20 to 24-month-old male rats, were identified by glycogen depletion. Measurements were made of staining intensities for intermyofibrillar succinate dehydrogenase, using microphotometric techniques, and of cross-sectional areas of the motor unit fibres. The average coefficients of variation for succinate dehydrogenase activity of the muscle fibres within each unit, between the different units and for repeated measurements of the same fibre in 10 consecutive cross-sections stained on different glasses were 26, 72 and 14%, respectively. The average coefficients of variation for cross-sectional fibre areas within each unit, between the different units and for repeated measurements were 21, 53 and 7%, respectively. Further, the succinate dehydrogenase activity of muscle fibres within the motor unit differed significantly depending on the position of the fibre along the superficial-deep axis of the muscle. The average succinate dehydrogenase activity in the 35 motor units was 21% lower (P less than 0.001) in the superficial as compared with the deep motor unit fibres. In order to eliminate the influence of altered motoneurone properties related to a transformation process or an age-related motoneurone dysfunction as indicated by a non-homogeneous myosin heavy chain composition in motor unit fibres, cross-sectional fibre areas and enzyme activities were also compared in motor units with a uniform myosin heavy chain composition in young animals. However, the same regional differences were observed in these 16 units as when all the 35 units were pooled together. Thus, in conformity with previous reports, the variability in enzyme activities within single motor unit fibres is small but too large for a motor unit homogeneity to be accepted. In addition, the present results demonstrate that the systematic differences in succinate dehydrogenase activities of motor unit fibres along the superficial-deep axis of the tibialis anterior muscle represent a biological rather than a methodological phenomenon.