A study of the motor neuron pool of the superior rectus muscle in albino rats by retrograde fluorescent double labeling technique.

Recent investigations revealed the localized distribution of the motoneuron subgroups within the mammalian oculomotor nucleus. In this study, we examined the motor neuron pool of the superior rectus muscle (SR) in 12 albino rats by injecting the retrograde fluorescent tracers Fluoro-Gold (FG) into one SR and Dil into the contralateral SR. We also examined the topographic and functional correlation between the neurons controlling the muscles of the upward gaze, i.e., SR and the inferior oblique muscle (IO) in another 5 albino rats by means of the same tracers. Our results demonstrated that: 1) the average total number of the neurons in the motor neuron pool of SR was 322.7 +/- 40.1; 2) topographically, about 94.65% of the motoneurons controlling SR were located in the contralateral side, while the remaining 5.35% lay ipsilaterally; 3) all the IO motoneurons were present on the same side of the muscle they innervate; 4) the motoneurons of SR and IO did not form separate subnuclei, but were intermingling with each other; 5) functionally, about 94.59%, 4.26% and 1.15% of the total motoneurons in the SR subnucleus of either oculomotor nucleus were projecting contralaterally, ipsilaterally and bilaterally, respectively; 6) although the distribution of the perikarya was not homogeneous, the bilaterally projecting (i.e., to both SR) cells were disposed centrally, and their double labeling indicated that their axons innervate the bilateral SR via axonal bifurcation and/or collateral branching; and 7) the motoneurons of SR and IO were functionally segregated from each other as no bilaterally projecting neurons (i.e., to SR and IO) were found.