Physiological properties of isolated motor units in normal and bilaterally innervated Xenopus gastrocnemius muscles.

Physiological properties of isolated gastrocnemius motor units were measured in normal juvenile postmetamorphic Xenopus frogs and in a group of juvenile animals with a single bilaterally innervated hindlimb. The aim of this study was to evaluate changes to the organisation of the motor unit when the muscle is hyperinnervated. Animals with a single bilaterally innervated hindlimb have previously been shown to support up to twice the normal number of motoneurons projecting into a single hindlimb. Under these circumstances there was a lowering of average neuromuscular efficacy (as judged by motor unit twitch/tetanus ratio) in comparison with normal age-matched siblings. Motor units with neurons in the lateral motor column contralateral to the remaining hindlimb were indistinguishable from those originating ipsilaterally. There is a wide range of safety margins for neuromuscular transmission at the various terminals of individual frog motor units, and comparison of motor unit contraction times with twitch/tetanus ratios showed that under pressure of hyperinnervation, motoneurons tend to retain their safest terminals on muscle fibres with fast contraction times.