Interaction between motor axons from two different nerves reinnervating the pectoral muscle of Xenopus laevis.

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Abstract
An electrophysiological and morphological study of sprouting and regeneration of motor nerves was performed in the dually innervated pectoral muscle of X. laevis. Section of one of the nerves induced axon sprouting in the intact nerve. Synapse formation by the sprouting axons was slow since the intact nerve took > 3 mo. to increase its field of innervation by 70%. The rate of axon regeneration was faster than that of axon sprouting since the cut nerve reinnervated its former territory in < 1 mo. At early stages of synapse formation the sprouted or regenerated endings were poorly branched but any terminal branches were, as a rule, longer than normal. Signs of degeneration and replacement of endings were observed. Low levels of transmitter release persisted for several months in newly formed endings. This depression was more pronounced at the endings formed outside the normal field of innervation of the nerve. Poly-innervated muscle fibers were observed during reinnervation by regenerated or sprouted axons. Their number decreased gradually in the months that followed the beginning of reinnervation. Synaptic efficacy was lower at poly- than at mono-innervated muscle fibers. At doubly innervated spots and at separated spots on the same fiber average end-plate potential (EPP) amplitude was 1/3 and 2/3, respectively, of that recorded at singly innervated fibers. Electrophysiological and morphological data were compared at individual doubly innervated end-plate sites. EPP amplitude was positively correlated with the degree of ending development. Sprouted endings remain functional after periods of reinnervation of 30 mo., although signs of regression were observed. These were probably mediated by spontaneous degeneration of the terminals and replacement by endings from the regenerating nerve.