Axonal conduction velocity of motor units of rat's medial gastrocnemius muscle

Axonal conduction velocity and its relations to different contractile properties of motor units of medial gastrocnemius muscle were investigated in nine Wistar rats anaesthetized with pentobarbitone. Functionally isolated motor units were identified as slow (S), fast resistant (FR) and fast fatigable (FF). Axons of S motor units conducted significantly more slowly than of fast units, while there was considerable overlap between conduction velocities measured for FR and FF types. The mean values of conduction velocity were 50.9 m/s for S, 68.9 m/s for FR and 71.3 m/s for FF type motor units. Strong and significant negative correlation between conduction velocity and contraction time as well as half-relaxation time was demonstrated. However, only a weak correlation between conduction velocity and twitch tension, tetanic tension or fatigue index was found. The multiple regression analysis revealed that the major factor to determine conduction velocity was contraction time.