Medial rectus subgroups of the oculomotor nucleus and their abducens internuclear input in the monkey

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Abstract
Physiological experiments show that the abducens internuclear pathway is involved in the activation of only the medial rectus (MR) eye muscle. Previous anatomical experiments have shown that this pathway terminates in multiple foci within the oculomotor nucleus (OMN) of the monkey, and not only over the classical motoneuron subgroup. In this study the location of MR motoneurons in the monkey OMN is reinvestigated, and compared with the detailed pattern of terminations of the abducens internuclear pathway. The motoneurons were labelled by injections of retrograde tracer substances, HRP and [125I] wheat germ agglutinin (WGA), into extraocular muscles. Labelled MR motoneurons were found in three main divisions, called subgroup A, B, and C. Subgroup A corresponds mainly to the classical ventral MR subgroup. Subgroup B lies dorsal and caudal in OMN, occupying an area classically reserved for inferior rectus (IR). However, the representation of IR is shown to be further rostral in the dorsal OMN. Subgroup C is on the dorsomedial border of OMN. Its cells are significantly smaller than those of group A and B. In addition C could be labelled independently of the other subgroups by small injections into the outer (orbital) layer of MR muscle. This indicates a functional difference between the subgroups. It is suggested that subgroup C may be important for the tonic component of MR activity, possibly convergence. The location of abducens internuclear terminals, labelled by the injection of tritiated amino acids into the abudens nucleus, corresponds exactly to the position of MR motoneurons. These experiments provide a new picture of the internal OMN organization, and support the physiological findings that the abducens internuclear pathway activates only MR motoneurons.