Abstract
Purpose: :
Since the LR is shaped as a broad vertical strap, segregated control of its inferior and superior zones could mediate potentially substantial torsional and vertical oculorotary LR actions. We supposed that if LR function is physiologically compartmental, abducens (CN6) innervation should also be demonstrably segregated in a wide range of mammals.
Methods: :
Intact, whole human, macaque monkey, rabbit, and cow orbits were fixed with formalin, embedded in paraffin, serially sectioned in coronal plane at 10 µm thickness and stained with Masson’s trichrome. Individual arborizations of CN6 were identified and outlined using distinct colors in serial high power digital micrographs for tridimensional nerve reconstruction using ImageJ.
Results: :
in all species, CN6 bifurcated into inferior and superior branches before entering the orbit or early in its orbital course. In human and monkey, the inferior CN6 branch enter the inferior portion of the LR muscle more proximally than the superior branch. In rabbit and cow, a branch from the inferior division of CN6 innervated the retractor bulbi muscle. Terminal nerve arborizations in the LR remained highly segregated, without overlap. In all species, subdivisions of each major trunk remain nearly parallel as they traverse the LR muscle from its global to orbital surface.
Conclusions: :
Compartmentalization of CN6 innervation into distinct superior and inferior zones proximal to LR muscle entry is a common feature in mammals. This finding is consistent with embryologic studies showing that the LR muscle has a bifid origin, with its superior and inferior segments deriving from different mesenchymal complexes. These findings support the notion that there could exist physiologically separate control of superior and inferior LR zones, potentially adding torsional and vertical oculorotary actions to supplement the abducting action of the LR.
Keywords: extraocular muscles: structure • eye movements