The superior rectus muscle is one of the extraocular muscles located in the orbit. It is innervated by the superior division of the oculomotor nerve (III). In the primary position (looking straight ahead), its primary function is elevation, although it also contributes to intorsion and adduction. It is associated with a number of medical conditions, and may be weak, paralysed, overreactive, or even congenitally absent in some people.
Structure
The superior rectus muscle originates from the annulus of Zinn. It inserts into the anterosuperior surface of the eye. This insertion has a width of around 11 mm. It is around 7.7 mm from the corneal limbus.
Relations
The superior rectus muscle is related to the other extraocular muscles, particularly to the medial rectus muscle and the lateral rectus muscle. The insertion of the superior rectus muscle is around 7.5 mm from the insertion of the medial rectus muscle, around 7.1 mm from the insertion of the lateral rectus muscle, and around 7.9 from the corneal limbus. It may rarely have two muscle bellies parallel to each other.
Function
The superior rectus muscle elevates, adducts, and helps intort (rotate the superior pole of the eye medially) the eye.
Clinical significance
Testing
The superior rectus muscle is the only muscle that is capable of elevating the eye when it is in a fully abducted position.
Exophthalmos
Much of the venous drainage of the orbit and the extraocular muscles passes close to the superior rectus muscle. Obstruction to this venous drainage can cause venous congestion in the eye, which may cause exophthalmos (bulging eye ball). This may be congenital, often with a familial genetic link, or acquired, most often caused by head injuries. Weakness of the inferior rectus muscle may strengthen the superior rectus muscle, causing it to be overreactive.