REGENERATION OF AXONS INTO THE TROCHLEAR ROOTLET AFTER ANTERIOR MEDULLARY LESIONS IN THE RAT IS SPECIFIC FOR IPSILATERAL IVTH NERVE MOTONEURONS

The fibre projection from the IVth nerve nucleus to the superior obliq ue muscle was determined quantitatively in the normal rat by defining fibre numbers in transverse sections of the IVth nerve, and neurone nu mbers after retrograde labelling by horseradish peroxidase (HRP) injec tion into the muscle. There were 183 +/- 27 (S.E.) labelled neurones i n the nucleus contralateral to the injected muscle and only 2 +/- 1 ip silateral. The ipsilateral fibre number was 234 +/- 7 and the cell/axo n ratio 0.8 +/- 0.1. Extensive analysis of all HRP retrogradely labell ed material revealed no central fibre contribution to the IVth nerve o ther than from neurones resident in the trochlear nucleus. The central portion of the trochlear nerve tract was severed at its point of decu ssation in the anterior medullary velum. Ninety days after lesion, 10 +/- 4 (6% of control) neurones were labelled in the ipsilateral trochl ear nucleus; none were labelled in the contralateral nucleus or in any other part of the midbrain, pens, medulla, or cerebellum. The number of myelinated fibres in the IVth nerve had decreased to 21 +/- 5 (9% o f control) so that the cell/axon ratio was 0.4 +/- 0.2, thus suggestin g that a single motoneurone has more fibres after lesion. In electron micrographs of the IVth nerve, larger than normal numbers of unmyelina ted fibres were seen. Many myelinated fibres displayed signs of abnorm al myelination. After regeneration, the projection was exclusively ips ilateral and not crossed as in the normal. These findings establish th at there is a high degree of specificity after regeneration since no m yelinated central nervous system axons other than trochlear fibres sel ect the IVth nerve root as a trajectory over which to regenerate. (C) 1991 Wiley-Liss, Inc.