The extraocular muscles (EOM) are responsible for controlling the movements of the eyeball and upper eyelid. These muscles are also known as the extrinsic eye muscles, distinguishing them from intrinsic eye muscles which are responsible for controlling the movement of the iris. This article will discuss the anatomy of the EOM including their relevant attachments, innervation and actions.
It is useful to classify the extraocular muscles into two sub-groups; muscles that move the eye and muscles that move the upper eyelid. We discuss both groups below.
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Muscles responsible for the movement of the eye
This group contains six muscles; four muscles that run almost a straight course from origin to insertion and hence are called recti (Latin for straight), and two muscles that run a diagonal course, the oblique muscles.
There are four recti; superior, inferior, medial, and lateral. They all share a common origin, a fibrous ring of connective tissue located posteriorly at the apex of the orbit, called the tendinousring or the annulus of Zinn.
Origin: the superior aspect of the common tendinous ring
Insertion: the anterosuperior aspect of the sclera of the eye
Primary action: elevates the eye (directs the eye upwards)
Secondary action: assists with medial rotation and adduction
There are two oblique muscles, the superior and inferior oblique. They do not originate from the common tendinous ring but rather have bony origins within the orbital cavity.
Origin: the body of the sphenoid bone (posterior wall of the orbital cavity)
Insertion: it travels across the medial edge of the roof of the orbit and then hooks around the trochlea, a pully-like structure at the superior-medial corner of the orbital cavity. After hooking around the trochlea it turns posteriorly to be inserted into the posterior-lateral quadrant of the superior surface of the sclera.
A useful mnemonic for remembering the nerve supply to the extraocular muscles is:
LR = Lateral rectus: innervated by the sixth nerve
SO = Superior oblique: innervated by the fourth nerve
AO = All other extraocular muscles: innervated by the third nerve
Levator palpebrae superioris
The levator palpebrae superioris is a solitary triangular-shaped muscle responsible for elevation of the superior eyelid (striated muscle). Levator palpebrae superioris also contains smooth muscle fibres which are known as the superiortarsalmuscle.
Origin: the lesser wing of the sphenoid bone
Insertion: the superior tarsal plate of the upper eyelid
Sympathetic innervation: sympathetic nerve fibres from the internal carotid artery pass within the superior division of the oculomotor nerve (CN III) to innervate the superior tarsal muscle
Clinical relevance: cranial nerve palsy
Damage to any of the three cranial nerves innervating the extraocular muscles can result in paralysis of the corresponding muscles.
Oculomotor nerve palsy (CN III)
The oculomotor nerve supplies all extraocular muscles except the superior oblique (CNIV) and the lateral rectus (CNVI). Oculomotor palsy (a.k.a. ‘third nerve palsy’), therefore, results in the unopposed action of both the lateral rectus and superior oblique muscles, which pull the eye inferolaterally. As a result, patients typically present with a ‘down and out’ appearance of the affected eye.
Oculomotor nerve palsy can also cause ptosis (due to a lack of innervation to levator palpebrae superioris) as well as mydriasis due to the loss of parasympathetic fibres responsible for innervating to the sphincter pupillae muscle.
Trochlear nerve palsy (CN IV)
The only muscle the trochlear nerve innervates is the superior oblique muscle. As a result, trochlear nerve palsy (‘fourth nerve palsy’) typically results in vertical diplopia when looking inferiorly, due to loss of the superior oblique’s action of pulling the eye downwards. Patients often try to compensate for this by tilting their head forwards and tucking their chin in, which minimises vertical diplopia. Trochlear nerve palsy also causes torsional diplopia (as the superior oblique muscle assists with intorsion of the eye when the head tilts). To compensate for this, patients with trochlear nerve palsy tilt their head to the opposite side, in order to fuse the two images.
Abducens nerve palsy (CN VI)
The abducens nerve (CN VI) innervates the lateral rectus muscle. Abducens nerve palsy (‘sixth nerve palsy’) results in unopposed adduction of the eye (by the medial rectus muscle), resulting in a convergentsquint. Patients typically present with horizontal diplopia which is worsened when they attempt to look towards the affected side.
Dr James Nott
Anatomy Lecturer and Anatomy Lead for Geeky Medics
Dr Muhammad Shoaib
General Medicine Advanced Trainee
Moore, K. L., Dalley, A. F., & Agur, A. M. (2013). Clinically oriented anatomy. Lippincott Williams & Wilkins.
Snell, R. S. (2010). Clinical neuroanatomy. Lippincott Williams & Wilkins.
D.J. Bell and A. Manickam et al. Radiopaedia: Extraocular muscle nerve supply mnemonic. [LINK]
1. Adapted by Geeky Medics. Au.yousef. Extraocular muscle actions and innervation. Licence: [CC BY-SA].