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The extraocular muscles, also known as extrinsic ocular muscles, are the seven muscles surrounding the eyes of humans and other animals. Six of these muscles, the four recti muscles, and the superior and inferior oblique muscles, control the movement of the eye. The seventh muscle, the levator palpebrae superioris, controls eyelid elevation. The actions of the six muscles responsible for eye movement depend on the position of the eye at the time of muscle contraction. The superior oblique muscle pulls the eye downward and laterally, while the inferior oblique muscle pulls the eye upward and laterally. The recti muscles are named according to their relative positions of attachment, with the medial rectus being the muscle closest to the nose. The movements of the extraocular muscles are influenced by a system of muscle pulleys, which are essential for eye muscle movement and ensuring conformity to Listing's law.
| Characteristics | Values |
|---|---|
| Number of Extraocular Muscles | 7 |
| Number of Extraocular Muscles Responsible for Eye Movement | 6 |
| Names of the 6 Extraocular Muscles Responsible for Eye Movement | 4 recti muscles, superior oblique muscle, and inferior oblique muscle |
| Names of the 4 Recti Muscles | Superior rectus muscle, lateral rectus muscle, medial rectus muscle, and inferior rectus muscle |
| Function of the Levator Palpebrae Superioris | Controls eyelid elevation |
| Function of the Superior Oblique Muscle | Pulls the eye downward and laterally |
| Function of the Inferior Oblique Muscle | Pulls the eye upward and laterally |
| Function of the Superior and Inferior Recti Muscles | Pull slightly medially, causing the eye to roll with contraction |
| Function of the Lateral and Medial Recti Muscles | Contraction of one leads to inhibition of the other |
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What You'll Learn
The six extraocular muscles
The human eye has six muscles, which are split into two primary groups: the recti muscles and the oblique muscles. The recti muscles are the superior rectus, inferior rectus, medial rectus, and lateral rectus. The oblique muscles are the superior and inferior obliques.
The superior rectus comes from Latin roots, with "superior" meaning "above" and "rectus" meaning "straight". The superior rectus muscle is located on the top of the eye and helps the eye look up. The superior oblique muscle is on the upper medial side of the eye, meaning it is closer to the nose. Its primary job is to turn the eye inward, but it also contributes to other motions. The inferior oblique has a similar job to the inferior rectus, but it moves the eye upward when the eye is looking in toward the nose, rather than away.
The oblique muscles are also known as the "'opposite' muscles, as their actions are opposite to their names. For example, the inferior oblique moves the eye upward when looking toward the nose. The recti muscles, on the other hand, are "rightly fitting" to their names. Both superior muscles cause intorsion, and both inferior muscles cause extorsion.
The extraocular muscles are innervated by three cranial nerves. Damage to one of these nerves will cause paralysis of its respective muscles and alter the resting gaze of the affected eye.
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How the four recti muscles work
The four recti muscles are the superior rectus, inferior rectus, medial rectus, and lateral rectus. These muscles are involved in the control of the eyeball and superior eyelid. They are called recti muscles because they have a direct path from origin to attachment, which is in contrast with the oblique muscles, which have an angular approach to the eyeball. The recti muscles move the eyes about the longitudinal axis of the body, i.e. superiorly (upward) or inferiorly (downward), or about a vertical axis, i.e. laterally (backward) or medially (forward).
The superior rectus muscle is responsible for intorsion and elevation, or abduction, of the eyeball. It originates from the superior part of the common tendinous ring and attaches to the superior and anterior aspect of the sclera. The inferior rectus muscle causes extorsion and depression, or adduction, of the eyeball. It originates from the inferior part of the common tendinous ring and attaches to the inferior and anterior aspect of the sclera.
The medial rectus muscle is responsible for adduction of the eyeball. It originates from the medial part of the common tendinous ring and attaches to the medial and anterior aspect of the sclera. The lateral rectus muscle, on the other hand, is responsible for abduction of the eyeball. It originates from the lateral part of the common tendinous ring and attaches to the lateral and anterior aspect of the sclera.
The four recti muscles work together with the oblique muscles to control the movements of the eyeball and superior eyelid.
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The two oblique muscles
The superior oblique muscle is the longest muscle of all the extraocular muscles. It originates from the periosteum of the sphenoid bone, superomedial to the optic foramen, and the common tendinous ring. It then progresses anteriorly, parallel to the medial wall of the orbit, to reach the trochlea. The trochlea is a cartilaginous loop attached to the frontal bone's nasal part. After looping through the trochlea, the muscle becomes a tendon, turning posterolateral and passing beneath the superior rectus. It finally gets inserted posterior and lateral to the insertion of the superior rectus. The primary action of the superior oblique muscle is incyclotorsion, and it also performs depression and abduction. It is the only muscle that receives innervation from the trochlear nerve (CN IV). A lesion of the trochlear nerve will paralyse the superior oblique muscle, resulting in diplopia (double vision) and a head tilt away from the site of the lesion.
The inferior oblique muscle originates at the lower front of the nasal orbital wall, passes inferiorly over the inferior rectus muscle on its path laterally and posteriorly, and inserts under the lateral rectus muscle on the lateral, posterior part of the globe. Thus, the inferior oblique pulls the eye upward and laterally. The inferior oblique muscle, along with the inferior rectus muscle, also receives partial blood supply from the infraorbital artery, a branch of the maxillary artery.
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The levator palpebrae superioris
The muscle fibres of the levator palpebrae superioris penetrate the upper eyelid, inserting into its structures. The deep fibres attach to the anterior surface of the superior tarsus, while the superficial fibres radiate through the eyelid and orbicularis oculi, ultimately attaching to the skin of the superior eyelid. The most lateral fibres attach to the orbital tubercle, and the most medial fibres attach to the medial palpebral ligament.
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The three cranial nerves
The oculomotor nerve is a pair of nerves that carry commands from the brain to the muscles in and around the eyes, allowing for seamless eye movement and focus. It is involved in almost all eye movements, except for two: rotating the eyeball downward when the eye is pointed inward (controlled by the trochlear nerve) and rotating the eyeball outward toward the ear on the same side (controlled by the abducens nerve). The oculomotor nerve helps with eye functions such as accommodation, saccades, smooth pursuit, vestibulo-ocular reflex, and pupillary light reflex. Accommodation refers to the ability to continuously focus on an object as it moves toward or away from you. Saccades are quick back-and-forth eye movements that help scan the field of view. Smooth pursuit allows the eyes to smoothly follow moving objects. Vestibulo-ocular reflex combines the vestibular system, which controls balance, with head and eye muscle movements to maintain a steady gaze even during sharp movements or direction changes. Finally, the pupillary light reflex is the adjustment of the eyes to sudden light changes.
The trochlear nerve, or CN IV, is responsible for depressing the eye when the eye is adducted by stimulating the superior oblique muscle. If the eye fails to depress, it is an indication of paralysis. The head tilt test can be used to check for fourth nerve paralysis. The head is tilted to one shoulder and then the other, observing the amount of vertical separation of the eyes or diplopic images. The greatest separation occurs when the head is tilted to the same shoulder as the side of paralysis.
The abducens nerve, or CN VI, abducts the eye through stimulation of the lateral rectus muscle. If the eye fails to abduct, it indicates paralysis. The head position can provide a clue to a paretic ocular muscle. Lateral rectus paralysis is indicated when the head is turned away from the side of the paralysis.
Together, these three cranial nerves and their associated muscles enable the eyes to move and function properly, allowing for seamless vision and coordination.
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Frequently asked questions
The extraocular muscles, or extrinsic ocular muscles, are the seven extrinsic muscles of the eye in humans and other animals. Six of these muscles control the movement of the eye, while the seventh controls eyelid elevation.
The six extraocular muscles that control eye movement are the four recti muscles (superior rectus, lateral rectus, medial rectus, and inferior rectus) and the two oblique muscles (superior oblique and inferior oblique).
A good way to remember the names of the extraocular muscles is to use a mnemonic. For example, "Lateral Rectus, innervated by the 6th (LR6)" or "SO4: superior oblique, innervated by the 4th (trochlear) nerve". Another way to remember is to focus on the actions of the muscles, such as the lateral rectus muscle's role in coordinating the motion of the eye in conjunction with the medial rectus muscle.
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