title: Physiological properties of isolated motor units in normal and bilaterally innervated Xenopus gastrocnemius muscles.
abstract: 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.
authors.affiliation: Department of Physiology, University of Otago, Dunedin, New Zealand.
authors.forename: P W
authors.initials: PW
authors.lastname: Sheard
mesh_terms: D000125:Achilles Tendon / Q000502:physiology; D000818:Animals; D004558:Electric Stimulation; D006614:Hindlimb / Q000294:innervation; D007814:Larva; D009046:Motor Neurons / Q000502:physiology*; D009119:Muscle Contraction / Q000502:physiology; D009132:Muscles / Q000294:innervation* / Q000502:physiology; D009435:Synaptic Transmission / Q000502:physiology; D014982:Xenopus laevis