Comparison of the Superior and Inferior Rectus Muscles in Humans: An Anatomical Study with Notes on Morphology, Anatomical Variations, and Intramuscular Innervation Patterns

Abstract

A comparison of the superior and inferior rectus muscles was performed to determine whether they have similar structures and innervation attributable to their participation in the same type of, although antagonistic, eye movements. The study was conducted on 70 cadaveric hemiheads, and the anatomical variations in the superior and inferior rectus muscles were assessed. Sihler’s whole mount nerve staining technique was used on 20 isolated superior and 20 isolated inferior rectus muscle specimens to visualize the intramuscular distribution of the oculomotor nerve subbranches. In two cases (~2.8%), variant muscular slips were found that connected the superior and inferior rectus muscles. In 80% of cases, muscular branches arising directly from the inferior branch of the oculomotor nerve innervated the inferior rectus muscle, while in 20% of cases, the nerve to the inferior oblique muscle pierced the inferior rectus muscle and provided its innervation. In 15 of 70 specimens (21.4%), a branch to the levator palpebrae superioris muscle pierced the superior rectus muscle. The distance between the specific rectus muscle’s insertion and the anterior-most terminations of the nerves’ subbranches with reference to the muscle’s total length ranged from 26.9% to 47.2% for the inferior rectus and from 34.8% to 46.6% for the superior rectus, respectively. The superior rectus muscle is slightly longer and its insertion is farther from the limbus of the cornea than is the inferior rectus muscle. Both muscles share a common general pattern of intramuscular nerve subbranches’ arborization, with characteristic Y-shaped ramifications that form the terminal nerve plexus located near half of the muscles’ length. Unexpected anatomical variations of the extraocular muscles may be relevant during orbital imaging or surgical procedures.

Overall Distribution Pattern of the Intramuscular Nerves of the Extraocular Muscles and its Clinical Implications

“…The study also explored the compartment that would be of the greatest morphological significance for chemical denervation and surgical muscle transplantation to facilitate the correction of strabismus. Some studies have analyzed the intramuscular nerves of the four rectus muscles, as well as those of the IO and LPS muscles (Haladaj, 2020;Haladaj et al 2020a;Haladaj et al 2020b;Shin, 2020). These studies primarily described the distribution pattern of the intramuscular nerve branches, while only the location of the BTX-A injection site in the nerve dense region of the LR muscle was described.…”

Mass Spectrometric Profiling of Extraocular Muscle and Proteomic Adaptations in the mdx-4cv Model of Duchenne Muscular Dystrophy

“…Voluntary striated muscles differ considerably in their histological, anatomical, metabolic, biochemical, and physiological specialization between predominantly slow-twitching versus fast-twitching phenotypes [ 1 ]. A distinctly different subtype of skeletal muscles, as compared to non-craniofacial muscles, is presented by extraocular muscles (EOMs) [ 2 , 3 ], which control the movements of the eyes [ 4 ]. The finely tuned and highly coordinated actions of six EOMs, i.e., M. obliquus superior, M. obliquus inferior , M. rectus medialis, M. rectus lateralis, M. rectus superior , and M. rectus inferior [ 5 ], provide the eyeball with a wide range of complex movements about its horizontal and vertical axis.…”