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The extraocular muscles are a group of six muscles that work together to control eye position and movement. They are innervated by three cranial nerves: the oculomotor nerve (CN III), the trochlear nerve (CN IV), and the abducens nerve (CN VI). These nerves arise from their respective motor nuclei in the midbrain-brainstem and travel through the skull to innervate specific somites, the EOMs. Damage to one of these cranial nerves will cause paralysis of its respective muscles and alter the resting gaze of the affected eye. The oculomotor nerve supplies multiple extraocular muscles, while the trochlear and abducens nerves supply individual extraocular muscles.
| Characteristics | Values |
|---|---|
| Number of extraocular muscles | 6 |
| Function | Control eye position and movement |
| Innervation | Oculomotor nerve (CN III), Trochlear nerve (CN IV), Abducens nerve (CN VI) |
| Blood supply | Branches of the ophthalmic artery, including the ciliary arteries |
| Controlled movements | North, east, south, west (or up, right, down, left) |
| Number of controlled movements | 4 out of 6 muscles control movement in the cardinal directions |
| Other functions | Counteract head movements and adjust eye movement |
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What You'll Learn
Oculomotor nerve (CN III)
The oculomotor nerve, also known as cranial nerve III or CN III, is one of three cranial nerves that innervate the extraocular muscles. The other two are the trochlear nerve (CN IV) and the abducens nerve (CN VI). CN III originates in the midbrain and innervates the ipsilateral orbit, except for the superior rectus and the levator palpebrae, which are contralaterally innervated.
The extraocular muscles control eye movements, and damage to one of the cranial nerves will cause paralysis of its respective muscles. This will alter the resting gaze of the affected eye. A lesion of the oculomotor nerve will affect most of the extraocular muscles. The affected eye will be displaced laterally by the lateral rectus and inferiorly by the superior oblique, resulting in a position known as 'down and out'.
In addition to the superior rectus and the levator palpebrae, the oculomotor nerve also controls the inferior rectus, medial rectus, and inferior oblique muscles. These muscles work together to control eye position and movement. For example, to elevate the eye while looking straight ahead, the superior rectus and inferior oblique contract together as the inferior rectus and superior oblique relax.
Oculomotor nerve palsy is characterised by a 'down and out' dilated pupil with ipsilateral ptosis. It can also cause Horner's syndrome, which is a triad of symptoms including partial ptosis (drooping of the upper eyelid), miosis (pupillary constriction), and anhidrosis (absence of sweating) on the ipsilateral side of the face.
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Trochlear nerve (CN IV)
The trochlear nerve, also known as the cranial nerve 4 or CN IV, is a motor nerve that facilitates eye movement. It is one of 12 sets of cranial nerves and is the smallest of the cranial nerves. It has the longest intracranial course among the cranial nerves, originating in the midbrain and extending to the superior oblique muscle near the top of the eyeball.
The trochlear nerve is responsible for supplying movement information to the superior oblique muscle, which allows us to look down and towards our nose. It controls the abduction and intorsion of the eye. The nerve fibres from the trochlear nucleus cross in the midbrain before exiting, innervating the contralateral superior oblique muscle. The tendon of the superior oblique muscle is held by a fibrous structure called the trochlea, which means "pulley" in Latin. This structure aptly describes the function of the tendon, which houses the tendon of the superior oblique muscle.
Damage to the trochlear nerve can result in paralysis of the superior oblique muscle, leading to diplopia (double vision) and a head tilt away from the lesion side. This condition is known as fourth nerve palsy or trochlear nerve palsy. Other symptoms of trochlear nerve palsy include blurry vision or minor vision problems when looking down, such as when reading a book or descending stairs. The diplopia is typically vertical or diagonal and is more pronounced with a downward gaze.
The vulnerability of the trochlear nerve to injury is relatively higher than that of other cranial nerves due to its long path through the head. Head trauma from vehicle accidents, boxing, or even mild head injuries can cause damage to the nerve. Other causes of trochlear nerve palsy include microvascular disease, increased intracranial pressure, and pressure from a nearby tumour.
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Abducens nerve (CN VI)
The abducens nerve (CN VI) is the sixth cranial nerve and is responsible for the abduction of the eye. It controls the lateral rectus muscle, which is one of the extraocular muscles. The lateral rectus muscle originates from the lateral part of the common tendinous ring and attaches to the anterolateral aspect of the sclera. It acts to abduct the eyeball, or in other words, to rotate the gaze away from the midline.
The abducens nerve arises from the abducens nucleus, located in the dorsal pons, and travels to the cavernous sinus via a long cisternal segment. This nerve has a long intracranial course and is vulnerable to direct and indirect injury. It is also secondarily involved in the innervation of the contralateral rectus muscle, ensuring that both eyes move laterally in a coordinated manner.
Abducens nerve palsy is a common condition that can be caused by structural pathologies that lead to downward pressure on the brainstem, such as tumours, aneurysms, fractures, or increased intracranial pressure. Patients with abducens nerve palsy typically experience binocular horizontal diplopia (double vision), which is more severe in the distance and results in an esotropia in primary gaze. They may also exhibit a head-turn to maintain binocularity and minimise double vision.
To examine the abducens nerve, eye movement tests are conducted in conjunction with the oculomotor and trochlear nerves. The patient is asked to follow a point, often the tip of a pen, without moving their head. The target is moved in an 'H-shape' pattern, and the patient reports any blurring of vision or double vision.
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Superior oblique muscle
The superior oblique muscle is one of the six extrinsic muscles of the eye, known as the extraocular muscles. These muscles are located within the orbit and surround the eyeball, facilitating its movement in various directions. The superior oblique muscle is the longest muscle in this group.
The superior oblique muscle originates from the body of the sphenoid bone, medial to the origin of the levator palpebrae superioris muscle and superomedial to the optic canal. Unlike the recti group of muscles, the superior oblique muscle does not originate from the common tendinous ring. Instead, it originates externally, superomedially to the tendinous ring. From its sphenoid attachment, the muscle runs anteriorly, parallel to the medial wall of the orbit.
The primary action of the superior oblique muscle is intorsion (internal rotation), the secondary action is depression (primarily in the adducted position), and the tertiary action is abduction (lateral rotation). It is the only extraocular muscle innervated by the trochlear nerve (the fourth cranial nerve). The trochlear nerve is the only cranial nerve that emerges from the posterior aspect of the brainstem. It takes a long path through the endocranium and enters the orbit via the superior orbital fissure to innervate the superior oblique muscle.
The superior oblique muscle loops through a pulley-like structure (the trochlea of the superior oblique) and inserts into the sclera on the posterotemporal surface of the eyeball. It is the pulley system that gives the superior oblique muscle its actions, causing depression of the eyeball despite being inserted on the superior surface. The tendon of the superior oblique muscle turns posterolaterally to cross the eyeball and continues to reach its insertion point on the outer posterior quadrant of the eyeball.
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Lateral rectus muscle
The lateral rectus muscle is one of the four straight muscles of the orbit responsible for eye movement in the cardinal directions. It is the only muscle supplied by the abducens nerve (CN VI). The lateral rectus muscle abducts the eye, turning it laterally in the orbit. It works in synergy or opposition with other extrinsic muscles of the eye to produce coordinated movements and direct the gaze.
The lateral rectus muscle is a flat strap-shaped muscle that is wider in its anterior part. It arises from the common tendinous ring and runs anteriorly and across the lateral part of the orbit to insert at the lateral side of the eyeball. The muscle inserts anterior to the equator of the eyeball, about 7 mm from the limbus of the cornea. The insertion of the lateral rectus muscle is also 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 neuron cell bodies of the abducens nerve are located in the abducens nucleus in the pons. These neurons project axons as the abducens nerve, which exit from the pontomedullary junction of the brainstem, travel through the cavernous sinus, and enter the orbit through the superior orbital fissure. It then enters the medial surface of the lateral rectus to innervate it.
The lateral rectus muscle is supplied by branches of the abducens nerve (CN VI), which enter its medial surface and provide general somatic efferent fibres. The lacrimary artery runs along the superior border of the lateral rectus muscle to supply the lacrimal gland. The ophthalmic artery, which stems from the internal carotid artery, supplies the lateral rectus muscle either directly or through its lacrimal branch.
A lesion of CN VI will paralyse the lateral rectus muscle. The affected eye will be adducted by the resting tone of the medial rectus. This defect can result in horizontal double vision and reduced lateral movement. The patient will be unable to abduct the eye and will experience diplopia (double vision). The proper function of the lateral rectus is tested clinically by asking the patient to look laterally.
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Frequently asked questions
The extraocular muscles are supplied by three cranial nerves: the oculomotor nerve (CN III), the trochlear nerve (CN IV), and the abducens nerve (CN VI).
The oculomotor nerve supplies multiple extraocular muscles, including the superior rectus, inferior rectus, medial rectus, and inferior oblique muscles. It also controls the levator palpebrae superioris, which is the only muscle involved in raising the superior eyelid.
Damage to the oculomotor nerve will result in paralysis of most of the extraocular muscles. The affected eye will be displaced laterally and inferiorly, adopting a position known as 'down and out'. The patient will be unable to elevate or adduct the eye ipsilateral to the damaged oculomotor nerve.
