Conrtactile propertiens and distribution of types of motor units innervated by axons from L.5 ventral roots in the rat medial gastrocnemius
There is a limited data concerning the rostro-caudal distribution of motoneurons within the motor nucleus. In present experiments single motor units in rat medial gastrocnemius were studied and their contractile properties as well as percentage participation of different types of motor units belonging to subpopulations innervated by axons in L4 and L5 ventral roots were analyzed. Experiments were based on functional isolation and electrical stimulation of as many as possible axons from L4 and L5 ventral roots. 35 motor units innervated by L4 and 90 units innervated by L5 ventral root were studied. We found that the composition of the three physiological types of motor units (fast fatigable, fast resistant and slow) in the two subpopulations of motor units was similar. However, the twitch time parameters (the contraction time and half-relaxation time) were slightly longer in L4 in comparison with L5 subpopulation (the difference was signifi cant for fast resistant units) although the difference has not been refl ected in expected shift of a steep part of force-frequency of stimulation curve towards lower frequencies in L4 subpopulation of motor units. Force parameters were similar for motor units belonging to two subpopulations. The small differences between L4 and L5 subpopulations of motor units can be due to possible differences in a spatial distribution of their muscle fi bres in the semipennate medial gastrocnemius.