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Alan Le, Vadims Poukens, Joseph L Demer; Evidence for Compartmental Innervation of the Mammalian Superior Oblique (SO) Muscle By The Trochlear Nerve. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2559.
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Innervation of human, monkey, and non-primate mammalian horizontal rectus muscles is divided into superior and inferior zones innervating non-overlapping sets of muscle fibers. We sought evidence of anatomical potential for compartmental SO innervation in mammals.
Whole orbits were obtained from 4 humans: two adults, a 17 mos old toddler, and a 33 wk fetus; 2 rhesus monkeys; 1 rabbit, and 1 cow. Each orbit was formalin fixed , embedded in paraffin, coronally sectioned at 10 μm thickness, and stained with Masson trichrome. In digital micrographs, trochlear nerve (CN4) branches were traced in serial sections using Photoshop and reconstructed using ImageJ to create 3-D overlays of CN4 innervation and muscle fibers. Gross dissection of fresh bovine specimens was used to trace muscle fibers through the trochlear to the scleral insertion.
In all four species, CN4 bifurccated prior to entering the SO muscle. In post-natal humans, CN4 arborized in non-overlapping superomedial and inferolateral muscle fibers zones. Fetal CN4 innervation showed more overlap with 4 major branches. Both monkey and the bovine orbits also displayed bifurcation of intramuscular CN4 with minimal overlap between resulting zones of innervated fibers. Gross dissection showed that the superior bovine SO compartment inserts posteriorly on the sclera for mainly vertical action, while the inferior portion inserts anteriorly for mainly torsional action. The rabbit SO had a double-headed origin unifying anteriorly but innervated by non-overlapping CN4 branches.
Compartmental innervation of the SO appears to be a general feature in mammals, perhaps refined during post-natal maturation in humans. Initial branching of CN4 external to the SO muscle would permit independent actions the two compartments, which at least in cow appears to be preferentially vertical vs. torsional. These neuroanatomical findings support MRI evidence of differential compartmental SO function in humans.
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