The organization of the somatic cell nuclei within the oculomotor nuclear complex in rats

Although there is detailed knowledge on the organization of the motor neurons supplying the extraocular muscles, there are still some discrepancies concerning the results of different studies. This study is planned to reexamine the distribution of the motor neurons in the oculomotor nucleus of the rat. In the present study we used 20 young adult Sprague-Dawley rats in four groups which represent four extraocular muscle groups innervated by oculomotor nerve, namely medial rectus, inferior rectus, superior rectus and inferior oblique muscle groups. For each rat 1-2 μl of 30% Horseradish Peroxidase (HRP) is used as a tracer. Multiple injections were made to fully infiltrate the individual muscles. After determining the position of the medial rectus subgroup, injections were applied into more than one muscle in a single experiment in order to decide the proper localization of the motor neuron subgroups in relation to each other. For each of the muscle groups five rats were injected with HRP. The rostral end of the nucleus begins with a compact neuron group belonging to medial rectus subgroup. However, at the caudal end the neurons were scattered in a wider area gradually decreasing in number. Neurons innervating the medial rectus muscle were located ipsilaterally within the ventral and ventrolateral portions of the nucleus extending throghout its rostrocaudal length. Motoneurons belonging to the inferior rectus subgroup were observed ipsilaterally within the gap between the median raphe and medial end of the medial rectus subgroup. Motoneurons of the inferior oblique subgroup formed an ovoid cell mass ipsilaterally and slightly dorsal to the medial and inferior rectus subgroups. Neurons of the superior rectus subgroup were localized contralaterally within the caudal two thirds of the nucleus. The neurons of this subgroup were located at the ventral and ventromedial parts of the inferior oblique subgroup and dorsal to the inferior and the medial rectus subgroups.