[Localization of motoneurons of the vertical yoke muscles in a brainstem of the cat--a double labeling study].

Motor neurons innervating the vertical yoke muscles in cats were investigated using fluorescent dyes by the double labeling method. In 4 cats, the right superior rectus muscle (SR) and left inferior oblique muscle (IO) were injected with 3% fast blue (FB) and 5% diamidino yellow dihydrochloride (DY), respectively, for observing motoneurons innervating yoke muscles involving upward gaze. In 4 other cats, the left inferior rectus muscle (IR) and right superior oblique muscle (SO) were injected with 3% FB and 5% DY, respectively, to observe motoneurons innervating yoke muscles involving downward gaze. After SR and SO injections labeled cells were found in the bilateral oculomotor nucleus and the bilateral trochlear nucleus, respectively, predominantly on the contralateral side. Following IR and IO injections labeled cells were found only in the ipsilateral oculomotor nucleus. No double labeled cell was observed in any labeled neurons, which indicates that each of the yoke muscles was innervated by independent individual motoneurons. According to the present observation and past reports on rats, cats and monkeys, motoneurons innervating SR and SO have developed from lower mammals such as the rat into predominantly contralateral innervation in the monkey, from a phylogenetic point of view. On the other hand, compared to contralateral innervation in lower mammals, motoneurons innervating IR and IO have developed into ipsilateral innervation in the monkey.