Variability of contractile properties and action potentials of motor unit in flexor digitorum brevis nad medial gastrocnemiusmuscles in the rat

The differences between motor unit (MU) properties in foot (fl exor digitorum brevis, FDB) and sural (medial gastrocnemius, MG) muscles in anaesthetized Wistar rats were studied. In experiments functional isolation of single MUs was achieved by stimulation of thin fi laments from the L5–L6 and L4–L5 ventral roots for FDB and MG muscles, respectively. The mean muscle mass amounted to 76 mg for FDB and 895 mg for MG. The force and the motor unit action potential (MUAP) properties were analysed. The three types of MUs (FF, FR and S) in the MG muscle were found. Sag phenomenon was visible in all fast units at 40 Hz stimulation. In FDB, the slow MUs were not noted. Sag was visible for only a part of fast MUs at lower frequencies of stimulation. In the MG muscle the fatigue index had bimodal distribution, whereas in FDB the distribution was continuous. The twitch and maximum forces were approximately 8 times lower in FBD then in MG, whereas the contraction time was signifi cantly longer. The force-frequency curves of all motor units in FDB were shifted towards lower frequencies and comparison to the same type of MUs in MG. The MUAP durations for FF and FR MUs were longer whereas the MUAP amplitudes were higher for FF units in MG in relation to FDB. In conclusion, the variability of MU properties in distal and proximal muscle concerns all studied contractile parameters and likely refl ects different motor control strategy.