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The human eye has six muscles that control its movement and alignment. These are known as extraocular muscles and are separate from the eyeball itself. The four rectus muscles are the lateral rectus, the medial rectus, the inferior rectus, and the superior rectus. The superior rectus muscle is located at the top of the eye and is responsible for upward movement. The superior oblique muscle, on the other hand, is responsible for turning the eye inward.
| Characteristics | Values |
|---|---|
| Number of eye muscles | 6 |
| Types of eye muscles | Extrinsic, Intrinsic |
| Types of extrinsic eye muscles | Extraocular, Levator palpebrae superioris (LPS) |
| Function of extrinsic eye muscles | Control eye movement and eye alignment |
| Types of intrinsic eye muscles | Recti, Oblique |
| Function of intrinsic eye muscles | Enable the eye to focus on near objects and control how much light enters the eye |
| Number of recti muscles | 4 |
| Types of recti muscles | Superior rectus, Medial rectus, Lateral rectus, Inferior rectus |
| Function of superior rectus | Controls upward movement of the eye |
| Function of medial rectus | Moves the eye inward |
| Function of lateral rectus | Allows the eye to move outward |
| Function of inferior rectus | Allows the eye to move downward |
| Number of oblique muscles | 2 |
| Types of oblique muscles | Superior oblique, Inferior oblique |
| Function of superior oblique | Turns the eye inward |
| Function of inferior oblique | Extorts the eye (rotates the eye outward) |
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What You'll Learn
Superior rectus muscle
The superior rectus muscle is an extrinsic muscle of the eye, located outside the eyeball but within the orbit. It is one of the four rectus muscles, which also include the inferior rectus, the medial rectus, and the lateral rectus. The superior rectus is also one of the seven extraocular muscles, which are located within the orbit but are extrinsic and separate from the eyeball itself. These muscles serve to move the eyes within the orbit.
The superior rectus muscle originates from the annulus of Zinn, which is a common origin point for the rectus muscles. It spans from the common tendinous ring (annulus of Zinn) to the superior aspect of the eyeball, just posterior to the corneal limbus (corneoscleral junction). The muscle then courses anterolaterally, crossing over the eyeball’s equator to reach and insert onto the anterior half of the sclera superiorly. Its primary function is elevation, although it also contributes to intorsion and adduction. The superior rectus is the only muscle capable of elevating the eye when it is in a fully abducted position.
The superior rectus muscle is innervated by the superior branch of the oculomotor nerve (CN III). It is supplied by the superior division of the ipsilateral oculomotor nerve (CN III). Each superior rectus muscle is innervated by the contralateral oculomotor nucleus in the mesencephalon. The superior rectus muscle may be weakened or paralysed by problems with nerve conduction of the oculomotor nerve (CN III). This may be congenital, often with a familial genetic link, or acquired, most often caused by head injuries.
The superior rectus muscle is associated with a number of medical conditions and may be weak, paralysed, overreactive, or even congenitally absent in some people. Obstruction to the venous drainage of the orbit and the extraocular muscles can cause venous congestion in the eye, which may cause exophthalmos (bulging eyeball). Treatment for issues with the superior rectus muscle may involve eye surgery that weakens or repositions the muscle, generally with good outcomes.
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Superior oblique muscle
The superior oblique muscle, also known as obliquus oculi superior, is one of six extraocular muscles. These muscles are located within the orbit and are extrinsic to the eyeball, facilitating its movement in various directions. The superior oblique muscle is the longest in this group, spanning from the body of the sphenoid bone to the superolateral aspect of the eyeball.
The primary function of the superior oblique muscle is to produce eye movements that direct the gaze inferolaterally by abducting, depressing, and internally rotating the eye. This is achieved through its unique pulley action along the trochlea, a fibrous, cartilaginous pulley attached to the trochlear fovea of the frontal bone. The superior oblique tendon turns posterolaterally, crossing the eyeball to reach its insertion point on the outer posterior quadrant.
The superior oblique muscle is innervated by the trochlear nerve (CN IV), which is the only cranial nerve emerging from the posterior aspect of the brainstem. This nerve enters the orbit via the superior orbital fissure to innervate the muscle. The superior oblique is the only extraocular muscle with this innervation.
The muscle's development is not unique when compared to other extraocular muscles, but it differs from periocular tissues as it arises from paraxial mesoderm in the prechordal plate rather than neural crest cells. Embryologic studies reveal that at an early development stage, the muscle, tendon, and trochlear cartilage move together in a straight line. However, around 12 weeks into development, the tendon turns around the trochlear cartilage, creating the pulley action.
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Lateral rectus muscle
The lateral rectus is one of the seven extraocular muscles, which control every movement of the eye. These muscles are extrinsic and separate from the eyeball itself. The lateral rectus is one of the four straight muscles of the orbit, which are named by their position within the orbit relative to the eyeball. The primary action of the lateral rectus muscle is the abduction of the eyeball, allowing people to look to the right with the right eye and to the left with the left eye. It is supplied by the abducens nerve (CN VI), which enters its medial surface and provides general somatic efferent fibres.
The lateral rectus muscle originates in the bottom of the orbital cavity in the surrounding area of the optic canal, specifically in the lateral part of the common tendinous ring; the annulus of Zinn. The common tendinous ring is a ring of fibrous tissue, which surrounds the optic canal at the back of the orbit. The muscle then runs anteriorly and across the lateral part of the orbit to insert at the lateral side of the eyeball. The insertion of the lateral rectus muscle is around 8 mm from the insertion of the inferior rectus muscle, around 7 mm from the insertion of the superior rectus muscle, and around 10 mm from the corneal limbus.
The lateral rectus muscle is a flat strap-shaped muscle that is wider in its anterior part. It works in synergy or opposition with other extrinsic muscles of the eye to produce coordinated movements and direct the gaze. The blood vessels that supply the lateral rectus muscle also provide blood to half of the anterior segment of the eye, specifically, the nasal half of the anterior segment. Thus, during surgery, the surgeon must be aware of the vascular supply to the lateral rectus muscle.
Another disorder associated with the lateral rectus muscle is Duane Syndrome, which occurs when the sixth cranial nerve, which controls the lateral rectus muscle, does not develop properly. A sixth nerve palsy, also known as abducens nerve palsy, is a neurological defect that results from a damaged or impaired abducens nerve. This defect can result in horizontal double vision and reduced lateral movement.
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Oculomotor nerve
The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or CN III, is a cranial nerve that controls most eye movements and raises the eyelid. The nerve provides motor and parasympathetic innervation to several structures within the bony orbit, including the extraocular muscles. These muscles, located within the orbit but separate from the eyeball, control the movements of the eyeball and eyelid.
The oculomotor nerve has three main motor functions: innervation to the pupil, lens, and extraocular muscles. The nerve innervates the levator palpebrae superioris muscle, which raises the upper eyelid. It also innervates four eye muscles that coordinate eye movements: the superior rectus muscle, which elevates the eye when looking straight ahead; the medial rectus muscle, which adducts the eye from a primary position; the inferior rectus muscle, which moves the eye down from a primary position; and the inferior oblique muscle, which elevates the eye when the eye is adducted from a primary position.
The oculomotor nerve also contains autonomic (involuntary) and somatic (voluntary) nerve fibres. The autonomic fibres innervate the intrinsic eye muscles that enable pupillary constriction and accommodation, or the ability to focus on near objects. The somatic fibres, meanwhile, are responsible for the elevation of the upper eyelid and the coordination of eye muscles for visual tracking and gaze fixation.
The oculomotor nerve originates from the midbrain of the brainstem, specifically from the oculomotor nucleus located within the midbrain of the brainstem, ventral to the cerebral aqueduct. It exits the brainstem near the midline at the base of the midbrain and passes through the cavernous sinus before proceeding through the supraorbital fissure to reach the orbit of the eye. Within the orbit, the nerve divides into superior and inferior branches. The superior branch provides motor innervation to the superior rectus and levator palpebrae superioris, while the inferior branch provides motor innervation to the inferior rectus, medial rectus, and inferior oblique.
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Trochlear nerve
The trochlear nerve, also known as cranial nerve 4 or CN IV, is a motor nerve that controls eye movement. It is one of 12 sets of cranial nerves and the smallest of the cranial nerves, but it has the longest intracranial course. The nerve is responsible for supplying movement information to the superior oblique muscle, which is connected to the eye near the top of it. This muscle is the longest and thinnest muscle among the extraocular muscles, which are located within the orbit and are separate from the eyeball.
The trochlear nerve allows the superior oblique muscle to move, making it possible to look down and toward or away from the nose. The nerve gets its name from the Latin word "trochleae," which means "pulley," a device that helps lift and lower an object. The tendon of the superior oblique is tethered by a fibrous structure known as the trochlea, giving the nerve its name.
The trochlear nerve pair originates from a pair of symmetrical trochlear nuclei within the medial midbrain at the level of the inferior colliculus. The left and right nerves then travel dorsally, surrounded by the periaqueductal gray matter, before their exit in the dorsal midbrain. The nerve then moves along the lateral wall of the cavernous sinus before entering the orbit through the superior orbital fissure and continuing to extend anteriorly to the superior oblique muscle.
The trochlear nerve is examined in conjunction with the oculomotor and abducens nerves by testing the movements of the eye. The patient is asked to follow a point, commonly the tip of a pen, with their eyes without moving their head. Trochlear nerve palsy commonly presents with vertical diplopia, exacerbated when looking downwards and inwards, such as when reading or walking downstairs. Patients can also develop a head tilt away from the affected side.
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Frequently asked questions
The superior rectus muscle, located at the top of the eye, is responsible for upward eye movement.
The human eye has six muscles that control eye movement and alignment. These include four recti muscles (lateral rectus, medial rectus, inferior rectus, and superior rectus) and two oblique muscles (inferior oblique and superior oblique).
The recti muscles are straight muscles that attach to the front half of the eye. They work together with the oblique muscles to move the eye from side to side, up and down, and control its rotation.
Unlike the recti muscles, the oblique muscles do not attach directly to the eye. They take an angular approach to attach to the posterior surface of the sclera. The superior oblique muscle, for example, rotates the eye inward.
