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Alan Le, Vadims Poukens, Joseph L Demer; Also Compartmentalized: Innervation of the Inferioir Oblique (IO) Muscle in Primates. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):558.
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© ARVO (1962-2015); The Authors (2016-present)
Innervation of the superior oblique muscle is separated into minimally overlapping lateral and medial compartments in primates, and superior and inferior compartments in non-primate mammalian species. The horizontal rectus muscles are divided into superior and inferior zones innervating non-overlapping sets of muscle fibers. We explored the innervation pattern of the IO muscle and determined the possibility of a compartmentalized model.
Whole orbits were obtained from two adult humans and three rhesus monkeys. Each orbit was formalin fixed, embedded in paraffin, coronally sectioned at 10 μm thickness, and stained with Masson trichrome. In digital micrographs, oculomotor nerve (CN3) branches were traced in serial sections using Photoshop and reconstructed using ImageJ to create 3-D overlays of CN3 innervation in the IO muscle fibers.
CN3 travels from the deep orbit and innervates at the muscle belly of the IO. In both humans and monkeys, CN3 bifurcated into two major branches prior to entering the IO muscle. Arborization of the CN3 splits the IO muscle into two halves along its length. One branch enters immediately as it reaches the muscle belly and arborizes within muscle fibers from the posterior half. The other branch straddles externally along the muscle belly before it penetrates and arborizes in fibers of the anterior half.
Innervation of the IO muscle appears to follow a compartmental model with the arborization dividing the muscle along its length. Branching of the CN3 external to the muscle fibers allows for independent movement between the two compartments. One compartment could be more effective for elevation during adduction while the other might have a greater effect on extorsion in primary gaze.
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