Polyneural innervation: mechanical properties of overlapping motor units in a small foot muscle of Xenopus laevis.

1. Extensor digitorum longus IV of Xenopus laevis was isolated together with its nerve and the 8th and 9th spinal roots. Motor units were functionally isolated, by splitting and stimulating ventral root filaments. 2. Motor units were stimulated separately and in pairs. Between many pairs, overlap of motor innervation was found (evidence of polyneuronal innervation of muscle fibres). Evidence for overlap was a deficit in the tension developed when the two units were stimulated together compared with the sum of the tetanic tensions developed when the two units were stimulated separately. This provided a measure of the extent of overlap. 3. Polyneuronal innervation was confirmed by intracellular recording from muscle fibres during stimulation of each motor axon. 4. For each motor unit, isometric twitch and tetanic tensions, twitch contraction time and the motor axon conduction time and distance were measured and axon conduction velocity and twitch:tetanus ratio were calculated. 5. The properties of the pairs of motor units found to overlap are reported and marked disparities between the properties of some units in overlapping pairs demonstrated. 6. Frequency distributions for the properties of motor units shown to overlap with others were not significantly different from those for total unit samples., Therefore polyneuronal innervation of muscle fibres occurs throughout the motor unit population of this muscle. 7. In several motor units the isometric twitches of several individual muscle fibres were recorded, each fibre being stimulated by intracellular current injection. It was found that contraction times of single fibres differed by up to 36% between muscle fibres in the same unit and that twitch tension varied by up to 400%.