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

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.