Changes in contractile properties and action potentials of motor units in the rat medial gastrocnemius muscle during maturation

The development of motor unit (MU) properties after the elimination of polyneuronal innervation of muscle fibers has not been studied so far in contrast to the embryonic and neonatal development which are well known. Three groups of Wistar rats were investigated – 1 month old, 2 months old and the adult – 9 months old (1 m.o., 2 m.o. and 9 m.o. rats, respectively). The basic contractile properties and action potentials of MUs in the medial gastrocnemius muscle were analysed. In 1 mo rats the three main physiological types of MUs were already separated. It was visible that various MU’s contractile properties reached adult values at different stage of development. The twitch time parameters were similar for all studied age groups whereas the force was increasing, especially in the second month of life. The fatigue index for FF MUs as well as the twitch-to tetanus ratio for all three types of MUs in 1 and 2 m.o. rats were higher than for 9 m.o. rats. The amplitude and time parameters of MUAPs, except their latency, were not changing during the studied period of development.