The organization and function of medial rectus and inferior rectus non-twitch motoneurons in the oculomotor nucleus of monkey

摘要

The extraocular muscles in mammals, the effector organs of the oculomotor system, are fundamentally different from skeletal muscle. All extraocular muscles consist of two different layers, an orbital and a global layer. There are two basic categories of the muscle fibers: twitch or singly-innervated muscle fiber (SIF) and non-twitch or multiply-innervated muscle fiber (MIF). Previous studies in monkey revealed that SIF and MIF motoneurons are anatomically separated and have different premotor inputs. SIF and MIF motoneurons were identified by tracer injection into the belly, or the distal myotendinous junction, of the eye muscles. There are two groups of MIF motoneurons in the oculomotor nucleus, the C- and S-group. The C-group motoneurons innervate the medial rectus (MR) and inferior rectus (IR), while S-group motoneurons innervate the superior rectus and inferior oblique muscles. The motoneurons of C-group are located around the periphery of the oculomotor nucleus. We investigated the location of MR and IR MIF motoneurons in C-group, and the dendritic spread of MR compared with IR MIF motoneurons. We found that the MR and IR MIF motoneurons are two different populations of neurons in C-group. They lie relatively separated. The MR MIF motoneurons are located more dorsomedially than IR MIF motoneurons. The pattern of dendritic spread of these two MIF motoneurons is also different. The dendrites of IR MIF motoneurons spread into the supraoculomotor area bilaterally, but do not approach the Edinger-Westphal nucleus, in contrast, the dendrites of MR MIF motoneurons extend into the supraoculomotor area and the Edinger-Westphal nucleus unilaterally. The function of Edinger-Westphal nucleus is associated with the “near response”. In conclusion, the different location and different dendritic trees suggest that MR and IR MIF motoneurons have different functions. The IR MIF motoneurons may help to stabilize the eye position along with MIF motoneurons from other eye muscles, while the MR MIF motoneurons might also participate the vergence eye movements.