Table of Contents
Extraocular muscles are present in the retrobulbar space (behind the eyeball) and are responsible for coordinated movements of the eyeball.
Extraocular muscles: Rectus & Oblique group of muscles
1. Rectus muscles 2. Oblique muscles
- Medial rectus a. Superior oblique (CN IV)
- Lateral rectus (CN VI) b. Inferior oblique
- Superior rectus
- Inferior rectus
3. Levator palpebrae superioris (not an extraocular muscle)
Origin and insertion of the extraocular muscles
The superior rectus originates from the annulus of Zinn (common tendinous ring) and is attached to the top of the eye 7.7 mm from the limbus.
The inferior rectus is attached to the bottom of the eye 6.5 mm from the limbus. It also originates from the annulus of Zinn.
The medial rectus is attached to the side of the eye near the nose 5.5 mm from the limbus.
The lateral rectus is attached to the side of the eye near the temple 6.9 mm from the limbus.
The superior oblique originates from the apex of the orbit. The apex of the orbit is just above the annulus of Zinn. The tendon of the muscle moves from behind the orbit through a pulley-like structure called the trochlea and comes anteriorly and is attached to the top of the eye at the superior and lateral aspect of the eyeball. Its tendon crosses beneath the superior rectus muscle before attachment.
The inferior oblique starts from the front of the orbit near the nose from the maxillary bone just behind the lacrimal space. It travels backward posteriorly and laterally in the orbit before attaching to the inferior of the globe.
Innervation of the extraocular muscles
|Muscle||M. rectus superior||M. rectus inferior||M. rectus lateralis||M. rectus medialis||M. obliquus inferior||M. obliquus superior|
|Innervation||N. oculomotorius (III), R. superior||N. oculomotorius (III), R. superior||N. abducent||N. oculomotorius (III), R. inferior||N. oculomotorius (III), R. inferior||N. trochlearis (IV)|
The extraocular eye muscles are innervated by three cranial nerves. The third cranial nerve, the oculomotor nerve, innervates all the extraocular muscles except the lateral rectus and the superior oblique muscles. The lateral rectus is innervated by the abducens nerve (CN VI). The superior oblique muscle is innervated by the trochlear nerve (CN IV). The levator palpebrae superior muscle is also innervated by the oculomotor nerve.
The oculomotor nerve is divided into two divisions: upper and lower division. The upper-division supplies the superior rectus muscle and the levator palpebrae superioris muscle. The lower division supplies the inferior rectus, medial rectus, and inferior oblique.
Cranial nerve IV (trochlear nerve) enters the middle of superior oblique muscle between the origin and the trochlea. Nerves of all the other muscles enter their respective muscles at the junction of the anterior two-thirds and posterior one-third.
Function of the extraocular muscles
The superior rectus moves the eye upwards and medially in the vertical plane. Both the superior recti pull up the eyeball and allow the individual to look in an upward direction. Upward gaze is because of the function of the two superior recti.
The inferior rectus moves the eye downwards and medially in the vertical plane. Both the inferior recti pull the eyeball downwards and allow the individual to see in a downward direction. Downward gaze is because of the function of the two inferior recti.
The lateral rectus moves the eye outward laterally in the horizontal plane. And the medial rectus moves the eye medially toward the nose in the horizontal plane. Lateral rectus of one eye and medial rectus of the other eye contract simultaneously when an individual is looking one side.
The superior oblique rotates the eyeball inward, downwards and nasally along the long axis of the eye.
The inferior oblique rotates the eye outward and upward.
Levator palpebrae superior is the muscle of the eyelid. It holds the eyelid up, keeping the eyes open. The blinking reflex involves this muscle through a reflex arc.
Assessment of the extraocular muscles
Medial and lateral recti have simple actions – they move the eye in the horizontal plane (left and right). To test the extraocular eye muscles, the patient is asked to follow the examiner’s finger who moves it in the air making the figure of an H. The superior oblique muscle points the eye down when it is looking medially and the inferior oblique muscle points the eye up when it is looking medially.
So, if a patient is looking up and to his right, he is using his:
- Right eye – lateral and superior rectus muscles
- Left eye – medial rectus and superior oblique muscles
Knowing the function and innervation of the extraocular muscles is extremely helpful for diagnosing neurological disorders such as brain herniation, and eye motility disorders like strabismus.
Extraocular muscles are also affected in various diseases affecting the eye, such as Graves disease, orbital metastasis, orbital trauma, and cellulitis, etc.