Differences in the contractile properties and action potentials of motor units of the medical gastrocnemius muscle in young and adult male rats
The growing up of organism is connected with considerable changes in neuromuscular and other systems. Numerous studies concerning a development of muscles and nerves have been carried out, especially within the embrional and neonatal life period although postnatal development of motor unit properties has not been studied. The aim of the present study was to document changes in contractile properties of skeletal muscles during a life period when embryonic and neonatal isoforms of myosin disappear but muscle mass is dynamically increasing. The two groups of animals, the young Wistar rats - 40 days old and the adult ones - 270 days old were investigated. The basic contractile properties and action potential of motor units (MUs) in the medial gastrocnemius muscle were analyzed. The body weight of young rats was three times lower than of adults (146.9 g vs 461.3 g), but the muscle mass was four times lower (0.313 g vs 1.246 g, respectively). The contraction time and half-relaxation time of fast MUs in 40-day-old rats were slightly longer than in adults. The twitch forces for the three MU types were two to three times lower in young rats, where the tetanus forces for all MU types were three to five times lower for 40-day-old animals. The twitch-to-tetanus ratio for all three types of MUs in young animals was significantly higher than in adult ones (0.43 vs 0.29 for FF, 0.31 vs 0.21 for FR, and 0.15 vs 0.08 for S MUs for young and adult rats, respectively). Moreover, considerable difference in the proportion of MU types was also observed. In adult rats higher participation of fast MUs was observed (33.7% of FF MUs and 55.4% of FR MUs) than in young ones (30.1% of FF and 41.1% of FR MUs) whereas slow MUs constituted 10.8% and 28.8% of studied populations of MUs in adult and young rats respectively. No significant differences in motor unit action potential parameters between young and adult animals were noticed.
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