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Extraocular muscles, also known as extrinsic or orbit muscles, are the most specialised skeletal muscles in the human body. They are responsible for eye movement and eyelid elevation. There are seven extraocular muscles, including four rectus muscles, two oblique muscles, and the levator palpebrae superioris. These muscles work together to enable a range of gazes and eye positions. The primary blood supply for these muscles comes from the muscular branches of arteries such as the ophthalmic artery and its branches, the ciliary arteries, and the infraorbital artery. Understanding the function and anatomy of extraocular muscles is crucial for diagnosing various eye-related conditions and performing surgical procedures without causing damage to the nerves and blood vessels connected to these muscles.
| Characteristics | Values |
|---|---|
| Number of extraocular muscles | 6 or 7 |
| Names of extraocular muscles | Superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, inferior oblique, and levator palpebrae superioris |
| Function | Control eye movement and eyelid elevation |
| Direction of movement | North, east, south, west (or up, right, down, left) |
| Nerve supply | Third (oculomotor), fourth (trochlear), and sixth (abducent) nerves |
| Blood supply | Muscular branches of the ophthalmic artery, the lacrimal artery, and the infraorbital artery |
| Complications during surgery | Damage to blood vessels, unsatisfactory alignment, refractive changes, diplopia, perforation of the sclera, postoperative infections |
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What You'll Learn
There are seven extraocular muscles
The superior rectus muscle originates from the superior part of the annulus of Zinn and proceeds anteriorly along the orbital wall to insert itself into the sclera. The lateral rectus muscle, which is the thinnest of the extraocular muscles, also originates from the annulus of Zinn and moves forward to insert itself approximately 6.9 mm from the limbus.
The superior oblique muscle is the longest of the extraocular muscles. It originates from the periosteum of the sphenoid bone and 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 passing through the trochlea, the muscle becomes a tendon, turning posterolateral and passing below the superior rectus.
The oblique muscles include the superior and inferior obliques. The levator palpebrae superioris is a standalone muscle that does not contact the globe directly but is primarily responsible for eyelid elevation. The rectus muscles form the muscle cone within the orbit, with the apex at the annulus of Zinn, their common point of origin. Each muscle is enclosed by a fibrous capsule and is separated by an intermuscular septum, which divides the orbital fat pad.
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They control eye movement
Extraocular muscles, also known as extrinsic or orbit muscles, control eye movement. There are six extraocular muscles that control eye movement, and one that controls eyelid elevation. These muscles engage depending on the position of the eye at the time of contraction.
Of the six extraocular muscles, four control eye movement in the cardinal directions: superior rectus (up), inferior rectus (down), lateral rectus (right), and medial rectus (left). The superior rectus originates from the superior part of the annulus of Zinn and proceeds along the orbital wall to insert into the sclera. The lateral rectus originates from the lateral part of the annulus of Zinn and moves forward to insert into the sclera. The medial rectus arises from the medial part of the annulus of Zinn and inserts into the sclera. The inferior rectus arises from the inferior part of the annulus of Zinn and also inserts into the sclera.
The remaining two extraocular muscles are responsible for counteracting head movements and adjusting eye movement accordingly. These are the superior and inferior oblique muscles. The superior oblique originates from the sphenoid bone and progresses anteriorly, parallel to the medial wall of the orbit, to reach the trochlea. It then loops through the trochlea, becoming a tendon, and finally inserts behind and lateral to the insertion point of the superior rectus. The superior oblique is the longest extraocular muscle and is the only one innervated by the trochlear nerve. The oblique muscles also slightly rotate the eyes, turning them towards or away from the nose.
The extraocular muscles have a high nerve fibre to skeletal muscle fibre ratio compared to other skeletal muscles. They receive their primary blood supply from the muscular branches of the ophthalmic artery, the lacrimal artery, and the infraorbital artery. The blood supply to these muscles is important to consider during eye surgery to avoid complications such as anterior segment ischaemia.
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They have a high nerve fibre to skeletal muscle fibre ratio
The extraocular muscles are a group of muscles responsible for eye movements and are unique in their structure and function compared to other skeletal muscles in the body. One of their distinctive features is their high nerve fibre to skeletal muscle fibre ratio. This means that for every skeletal muscle fibre in these muscles, there are multiple nerve fibres supplying them. This ratio is much higher compared to other skeletal muscles, which typically have a lower nerve fibre to muscle fibre ratio.
This unique feature of the extraocular muscles is important for several reasons. Firstly, it allows for very precise control of eye movements. The high number of nerve fibres enables individual eye muscles to contract with a high degree of accuracy and subtlety, allowing for smooth and coordinated eye movements. This precision is crucial as even small misalignments in eye movement can result in double vision or blurred vision.
The high nerve fibre innervation also enables the extraocular muscles to respond quickly and generate rapid eye movements. The nerve supply to these muscles is adapted to provide rapid, high-frequency signals, ensuring quick and precise eye movements. This is essential for functions such as reading, where the eyes must move rapidly and accurately across a page, or in sports, where athletes need to track fast-moving objects.
Additionally, the high nerve fibre to muscle fibre ratio likely contributes to the extraocular muscles' endurance. These muscles are in near-constant use throughout the day, and the abundant nerve supply may help prevent fatigue, ensuring the muscles can sustain prolonged activity without tiring. This is in contrast to other skeletal muscles, which typically experience fatigue with extended or intense use.
The unique innervation of the extraocular muscles also has implications for their development and repair. During embryological development, the complex nerve supply to these muscles is carefully orchestrated to ensure precise eye movements from birth. Similarly, if the extraocular muscles are damaged, their high nerve fibre content may aid in repair and regeneration, potentially contributing to more successful recovery compared to other skeletal muscles.
In summary, the high nerve fibre to skeletal muscle fibre ratio in the extraocular muscles is a key feature that enables precise and rapid eye movements, likely contributes to muscle endurance, and may also influence development and repair processes. This unique innervation is specifically adapted to the critical role these muscles play in our visual system, ensuring we can accurately and quickly focus on and track objects in our environment. Understanding this feature of extraocular muscles is important in the fields of ophthalmology and neuroscience, providing insights into the intricate workings of the human body.
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The blood supply of EOM also supplies the anterior segment
The extraocular muscles (EOM) are located within the orbit but are extrinsic to the eyeball. There are seven extraocular muscles: the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, lateral rectus, inferior oblique, and superior oblique. These muscles control the movements of the eyeball and the superior eyelid.
The blood supply of the EOM is critical, as it also supplies most of the anterior segment of the eye. The primary blood supply for the EOM comes from the muscular branches of the ophthalmic artery, specifically the medial and lateral muscular branches. The medial muscular branch supplies the medial rectus, inferior rectus, and inferior oblique muscles. Meanwhile, the lateral muscular branch supplies the lateral rectus, superior rectus, and oblique muscles, as well as the levator palpebrae superioris.
The superior, inferior, and medial rectus muscles receive blood from two anterior ciliary arteries that communicate with the anterior circle of the ciliary body. Additionally, the lateral rectus muscle receives blood supply from a branch of the lacrimal artery, and the inferior rectus and inferior oblique muscles receive partial blood supply from the infraorbital artery, a branch of the maxillary artery.
The blood supply of the EOM is crucial during surgical procedures. Iatrogenic damage to the vortex veins, which are responsible for venous drainage, can occur during surgery. To prevent anterior segment ischemia, procedures that involve damage to the vascular supply should be limited to a maximum of two rectus muscles at a time. This consideration is essential because the blood supply of the EOM plays a significant role in supplying the anterior segment of the eye.
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The rectus muscles originate from the annulus of Zinn
Extraocular muscles are responsible for the eye's movement in different gazes. The rectus muscles, which are a group of four muscles (superior, medial, inferior, and lateral rectus), originate from the annulus of Zinn, a common tendinous ring lying in the back of the orbit. The annulus of Zinn is a dense fibrous band of connective tissue with firm attachments to the periosteum of the orbital apex and the optic nerve sheath. It surrounds the optic foramen at the orbital apex and divides the superior orbital fissure into intraconal and extraconal spaces.
The superior rectus muscle originates from the superior part of the annulus of Zinn and proceeds anteriorly along the orbital wall to insert into the sclera at a distance of approximately 7.7 mm from the limbus. It is the closest rectus muscle to the insertion point, followed by the medial rectus (around 5.5 mm), inferior rectus (6.5 mm), and lateral rectus (6.9 mm). The superior rectus is also connected to the levator palpebrae superioris muscle, which arises medial and superior to it but remains closely associated. The superior oblique muscle lies between the superior rectus and the sclera, and both muscles are connected by intermuscular septa.
The medial rectus, inferior rectus, and lateral rectus muscles also arise from the annulus of Zinn. They course anteriorly along the orbit's walls and insert into the sclera at varying distances. The four rectus muscles form the muscle cone within the orbit, with the apex of the cone being at the annulus of Zinn. The cone widens as the muscles progress anteriorly, forming its base at the points of muscle penetration into the tenon's sheath.
The lateral rectus muscle originates from the lateral part of the annulus of Zinn and is the thinnest of the rectus muscles, with a width of only 9.2 mm. It has the broadest arc of contact among all the rectus muscles, at 12 mm. The inferior oblique muscle arises just lateral to the nasolacrimal duct ostium in the anterior orbit and is the only muscle that must be retrieved from the tenons rather than directly from the scleral insertion point.
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Frequently asked questions
Extraocular muscles are also referred to as extrinsic muscles or muscles of the orbit. They are the muscles that control eye movement. There are 6 of these muscles in humans, 4 of which control movement in the cardinal directions: north, east, south, and west (or up, right, down, left). The other 2 muscles are responsible for counteracting head movements and adjusting eye movement accordingly.
The 6 extraocular muscles are the superior rectus, inferior rectus, lateral rectus, medial rectus, superior oblique, and inferior oblique. There is also a standalone muscle called the levator palpebrae superioris, which controls eyelid elevation.
The primary blood supply for the extraocular muscles is the muscular branches of the ophthalmic artery, the lacrimal artery, and the infraorbital artery.
