- The extraocular muscles are a group of six skeletal muscles that control the precise and coordinated movements of the eyeball, allowing it to move in various directions. These muscles are essential for functions such as tracking moving objects, shifting gaze, and maintaining binocular vision—the ability to use both eyes together to perceive depth and a single, unified visual field.
- Unlike most skeletal muscles in the body, extraocular muscles are characterized by rapid contraction speed, fine motor control, and continuous low-level activity, even during fixation, to maintain ocular alignment.
- The six extraocular muscles include four rectus muscles—superior rectus, inferior rectus, medial rectus, and lateral rectus—and two oblique muscles—superior oblique and inferior oblique. These muscles originate from the annulus of Zinn, a fibrous ring located at the back of the orbit near the optic canal, with the exception of the inferior oblique, which originates from the orbital floor. They insert into the sclera, the white outer surface of the eye, allowing them to rotate the globe in various directions.
- Each extraocular muscle has a specific primary action, though most contribute to secondary and tertiary movements as well:
- Medial rectus: moves the eye inward (adduction)
- Lateral rectus: moves the eye outward (abduction)
- Superior rectus: moves the eye upward (elevation) and slightly inwards (adduction and intorsion)
- Inferior rectus: moves the eye downward (depression) and slightly inwards (adduction and extorsion)
- Superior oblique: rotates the eye downward and outward (depression and intorsion)
- Inferior oblique: rotates the eye upward and outward (elevation and extorsion)
- These muscles are innervated by three cranial nerves:
- Oculomotor nerve (CN III): controls the superior, medial, and inferior rectus muscles and the inferior oblique
- Trochlear nerve (CN IV): innervates the superior oblique
- Abducens nerve (CN VI): innervates the lateral rectus
- The coordination of extraocular muscles between both eyes is critical for conjugate gaze—the ability to move both eyes in the same direction simultaneously. This coordination is managed by neural circuits in the brainstem and vestibular systems that synchronize movement and adapt to changes in head position, allowing for stable vision even during motion.
- Disorders affecting the extraocular muscles or their innervation can result in strabismus (misalignment of the eyes), diplopia (double vision), or limited eye movement. For example, damage to the abducens nerve can cause the eye to deviate inward due to unopposed action of the medial rectus. Conditions such as thyroid eye disease, myasthenia gravis, cranial nerve palsies, or orbital trauma may also impair extraocular muscle function.
- Assessment of these muscles is a fundamental part of a neurological or ophthalmic exam, often involving tracking tests, cover/uncover tests, and H-pattern gaze testing, which evaluates eye movement in the six cardinal directions.
