Localization of motoneurons innervating the extraocular muscles in the chameleon (Chamaeleo chameleon)

The topography and localization of motoneurons innervating the six extraocular muscles in the chameleon (Chamaeleo chameleon) was studied following HRP injection in each of these individual muscles. Four muscles were innervated ipsilaterally: medial rectus, inferior rectus, inferior oblique and lateral rectus. The medial rectus muscle was innervated by the dorsomedial part of the oculomotor nucleus. The innervation to the inferior rectus muscle arose from the lateral part of the intermediate oculomotor subnucleus, which extended to the lateral part of the dorsal subdivision. The lateral rectus muscle was innervated by the abducens nucleus, which was composed by two subgroups of labeled cells, respectively observed in the principal and accessory abducens subnuclei, whereas efferents to the inferior oblique muscle originated from both the ventral and intermediate oculomotor subnuclei. The contralateral pattern consisted of motoneurons innervating the superior rectus and the superior oblique that were located respectively in the caudal portion of the ventral oculomotor nucleus and in the trochlear nucleus. These results confirmed data reported in most vertebrate species, and were discussed from a comparative and functional point of view.