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

Robert Haładaj,Michał Polguj,R Shane Tubbs

doi:10.1155/2020/9037693

Robert Haładaj, Michał Polguj + Show 1 more

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https://doi.org/10.1155/2020/9037693

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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.

Highlights

The four extraocular rectus muscles originate from the common tendinous ring and extend forward to attach to the eyeball by tendinous expansions that insert into the sclera [1, 2]
The superior rectus muscle is longer than other rectus muscles [3] and originates superior and lateral to the optic nerve
After the superior and inferior rectus muscles were visualized over their entire course, detailed measurements were performed, including the muscles’ length, width, thickness at the midlength, distance between the limbus and anterior-most end of the muscles’ insertion, and the distance at which the oculomotor nerve’s branches reached the muscles’ internal surface

Summary

Introduction

The four extraocular rectus muscles originate from the common tendinous ring and extend forward to attach to the eyeball by tendinous expansions that insert into the sclera [1, 2]. The superior rectus muscle is longer than other rectus muscles [3] and originates superior and lateral to the optic nerve. The origin of the inferior rectus muscle is located below the optic nerve Both the superior and inferior rectus muscles are thickest at their midlength and gradually become thinner at their distal ends [3]. The superior rectus directs the eye upward (elevates it), while the inferior rectus directs the eye downward (depresses it), and the two are each muscle’s primary action, respectively. Because of their location and primary action, these muscles are referred to as “the vertical rectus muscles” [4,5,6]

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