Specificity of innervation among Xenopus twitch muscle fibers.

More than 50% of the Xenopus pectoralis twitch muscle fibers with two distant endplates are innervated at both sites by the same neuron. This study indicates that there are three separable twitch motor unit types in this muscle which show very little overlap in innervation. This study also shows that each motor unit is topographically localized and that similar type units are shifted relative to one another. It is concluded that these two factors may contribute importantly to the observed high incidence of mononeuronal innervation.