Formation and regression of inappropriate nerve sprouts ing trochlear nerve regeneration in Xenopus laevis

After sectioning N IV in Xenopus laevis larvae, in addition to the regeneration of that never to the superior oblique muscle (SOM), sprouts form from N III, chiefly from the branch supplying the inferior oblique muscle (IOM). these were shown to innervate the SOM by methylene blue staining, silver and zinc iodide‐osmium impregnations, and electrical stimulation of N III. The competition thus established is resolved in favor of N IV, in a way suggesting a displacement of the inappropriate sprouts by the returning N IV fibers. This hypothesis received support by two anatomical findings: a delay in the decline of N III sprouts on the SOM when the return of N IV is delayed and a significant inverse relationship between the number of N III sprouts and regenerated N IV fibers on the SOM 39 days after N IV sectioning. Degenerating N III sprouts were detected while their number was declining. The physiological studies showed a significant reduction in the function of N III sprouts tested 30–34 days after N IV sectioning as compared with larvae tested 11–15 days after sectioning, which correlated with the anatomical observations We conclude that there is a withdrawal of inappropriate nerve fibers during the resolution of competition between the regenerating N IV fibers and these sprouts from N III.