Retrograde horseradish peroxidase transport after oculomotor nerve injury.

We studied the distribution of somatic motor neurons innervating the cat superior rectus 3-6 months after oculomotor nerve injury using intramuscular horseradish peroxidase (HRP). In normal cats, 98% or more of the labelled superior rectus motoneurons were in the contralateral oculomotor subnucleus. Two experimental cats who exhibited little or no evidence of recovery showed few labelled cells (4% of controls) which were distributed in both the ipsilateral and contralateral oculomotor nucleus. The other three experimental cats demonstrated definite signs of recovery, and HRP injections labelled more cells (20% of controls) also distributed in the ipsilateral and contralateral oculomotor subnuclei. This study shows that, after sectioning, the oculomotor nerve regenerates and anomalous connections develop between the somatic motoneurons of the ipsilateral oculomotor nucleus and the superior rectus. These findings support the hypothesis that acquired oculomotor synkinesis developing after third nerve injury results from misdirection of regenerating axons.