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

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Abstract
Extensor digitorum longus IV of X. 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. Motor units were stimulated separately and in pairs. Between many pairs, overlap of motor innervation was found (evidence of polyneuronal innvervation of muscle fibers). Evidence for overlap was a deficit in the tension developed when the 2 units were stimulated together compared with the sum of the tetanic tensions developed when the 2 units were stimulated separately. This provided a measure of the extent of overlap. Polyneuronal innervation was confirmed by intracellular recording from muscle fibers during stimulation of each motor axon. 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:tetanue ratio calculated. 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. Frequency distributions for the properties of motor units shown to overlap with others were not significantly different from those for total unit samples. Polyneuronal innervation of muscle fibers occurs throughout the motor unit population of this muscle. In several motor units, the isometric twitches of several individual muscle fibers were recorded, each fiber being stimulated by intracellular current injection. Contraction times of single fibers differed by up to 36% between muscle fibers in the same unit and twitch tension varied by up to 400%.