Trochlear-oculomotor nerve interactions in Xenopus laevis tadpoles: a temporal study.

When the trochlear nerve (NIV), which innervates the superior oblique muscle (SOM), is crushed or cut at stages 48-49 in Xenopus tadpoles, fibers from the oculomotor nerve (NIII) sprout and invade the SOM. The maximal percentage of specimens having at least one oculomotor nerve fiber on the SOM on a given day increased from 9.1% following a single crushing of NIV to 84.2% following three successive severings of NIV and the average number of silver-impregnated NIII fibers per specimen increased from 0.23 +/- 0.16 (mean +/- S.E.M.) in the single-crush experiment to 7.35 +/- 1.33 in the triple-cut experiment. This increase directly reflects the delay in the return of NIV. As NIV returns to the SOM, a portion of the inappropriate innervation is lost; while another portion appears to be stable and is in evidence 90 days after a single sectioning of NIV. The more rapidly NIV returns to the SOM, the more complete is the displacement of the NIII fibers. This suggests that the association between NIII and the SOM changes with time so that easy displacement of the inappropriate innervation is likely only when the reinnervation by the appropriate nerve fibers is rapid.