Logan Steele
The extraocular muscles are six muscles that control eye movement and one muscle that controls eyelid elevation. They are located within the orbit, but are extrinsic and separate from the eyeball itself. These muscles are innervated by three cranial nerves: the abducens, the trochlear, and the oculomotor. The abducens nerve (CN VI) innervates the lateral rectus muscle, the trochlear nerve (IV) innervates the superior oblique muscle, and the oculomotor nerve (III) supplies all the rest of the extraocular muscles.
| Characteristics | Values |
|---|---|
| Innervation | Oculomotor nerve (CN III), Trochlear nerve (CN IV), Abducens nerve (CN VI) |
| Innervation of specific muscles | Trochlear nerve: Superior oblique muscle |
| Oculomotor nerve: All extraocular muscles except the superior oblique and lateral rectus muscles | |
| Abducens nerve: Lateral rectus muscle | |
| Medial rectus muscle: May receive innervation from three separate regions within the oculomotor nerve complex, labelled A, B, and C | |
| Levator palpebrae superioris: Receives innervation from the oculomotor nerve | |
| Inferior oblique muscle: Receives innervation from the oculomotor nerve | |
| Function | Control movements of the eyeball and upper eyelid |
| Control ocular rotations in horizontal, vertical, and torsional directions | |
| Control movements of the eye along three axes: horizontal, vertical, and torsional | |
| Responsible for binocular single vision | |
| Effects of damage | Damage to one of the cranial nerves will cause paralysis of its respective muscles and alter the resting gaze of the affected eye |
| Oculomotor nerve damage: Eye adopts a position known as 'down and out' | |
| Trochlear nerve damage: Patient will experience double vision and may develop a head tilt away from the site of the lesion | |
| Abducens nerve damage: Unopposed adduction of the eye, resulting in a convergent squint and horizontal double vision |
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What You'll Learn
The oculomotor nerve (CN III)
The oculomotor nerve, also known as cranial nerve III or CN III, is the largest of the ocular motor nerves, containing around 15,000 axons, including motor fibres and parasympathetic motor fibres. It exits from the rostral midbrain near the cerebral peduncle and supplies all the extraocular muscles except the superior oblique (innervated by the trochlear nerve or CN IV) and the lateral rectus (innervated by the abducens nerve or CN VI).
The oculomotor nerve governs several different muscles, and each receives its innervation from a separate group of lower motor neurons within the third nerve nucleus. A lesion of the oculomotor nerve affects most of the extraocular muscles. The affected eye is displaced laterally by the lateral rectus and inferiorly by the superior oblique, resulting in a ''down and out' position. This is known as oculomotor palsy or third nerve palsy.
Oculomotor nerve palsy can cause ptosis (drooping of the eyelid) due to a lack of innervation to the levator palpebrae superioris, and mydriasis (pupillary dilation) due to the loss of parasympathetic fibres responsible for innervating the sphincter pupillae muscle. The third nerve also carries axons from the Edinger-Westphal nucleus, which are responsible for pupillary constriction.
The oculomotor nerve also provides parasympathetic motor innervation to the intrinsic eye muscles, the ciliary muscle, and the iris sphincter muscle. Sympathetic nerve fibres from the internal carotid artery pass within the superior division of the oculomotor nerve to innervate the superior tarsal muscle.
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The trochlear nerve (CN IV)
The trochlear nerve exits from the caudal portion of the midbrain and crosses the midline to innervate the superior oblique muscle on the opposite side. This muscle is the longest of the extraocular muscles and is responsible for incyclotorsion, depression, and abduction.
A lesion of the trochlear nerve will paralyse the superior oblique muscle, resulting in vertical diplopia (double vision) when looking inferiorly. This is because the affected eye is pulled downwards by the superior oblique muscle. Patients may compensate for this by tilting their head forwards and tucking their chin in, which helps to minimise the double vision.
The trochlear nerve is distinct from the other cranial nerves as it exits from the dorsal surface of the brainstem. It is also unique in that it is the only nerve that innervates the superior oblique muscle.
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The abducens nerve (CN VI)
The abducens nerve, also known as the abducent nerve, is the sixth cranial nerve (CN VI) in humans and various other animals. It is responsible for the outward gaze of the eye and controls the movement of the lateral rectus muscle, one of the extraocular muscles. The abducens nerve originates from the abducens nucleus in the pons of the brainstem and exits the brainstem at the junction of the pons and the medulla. It then enters the subarachnoid space and travels through an area known as Dorello's canal.
The abducens nerve is examined in conjunction with the oculomotor and trochlear nerves by testing the movements of the eye. The patient is asked to follow a point, usually the tip of a pen, without moving their head. The target is moved in an 'H-shape', and the patient reports any blurring of vision or diplopia (double vision). This test is important because damage to the abducens nerve can cause diplopia, where the affected eye is pulled towards the midline. To compensate and avoid double vision, patients will rotate their heads so that both eyes are towards the temple.
Abducens nerve palsy is the most common acquired ocular motor nerve palsy. It can be caused by any structural pathology that leads to downward pressure on the brainstem, such as a space-occupying lesion, or anything that compresses or stretches the nerve, including tumours, aneurysms, fractures, strokes, infections, cavernous sinus diseases, and various neuropathies. The most common overall cause of sixth nerve impairment is diabetic neuropathy.
The abducens nerve has a long intracranial course, making it vulnerable to injury. It is a purely somatic motor nerve with no sensory function.
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The lateral rectus muscle
Damage to the abducens nerve can result in a neurological defect known as sixth nerve palsy or abducens nerve palsy. This defect can cause horizontal double vision and reduced lateral movement. The patient will be unable to abduct the eye and will be unable to look straight ahead, as the eye will be deviated medially towards the nose due to the medial rectus of the eye.
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The superior oblique muscle
The embryological development of the superior oblique muscle contributes to its unique pulley action. Initially, the muscle, tendon, and trochlear cartilage develop in a straight line. However, around 12 weeks into development, the tendon begins to loop around the trochlear cartilage, creating the distinctive pulley system. This mechanism allows the superior oblique muscle to depress the eyeball despite being inserted on its superior surface.
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Frequently asked questions
Extraocular muscles are the most specialized skeletal muscles in the human body. They are located within the orbit and are extrinsic and separate from the eyeball itself. They control the movements of the eyeball and the superior eyelid.
The extraocular muscles are innervated by three cranial nerves: the abducens, the trochlear, and the oculomotor.
Damage to the innervation of the extraocular muscles can cause altered ocular alignment or binocular double vision. Paralysis of the corresponding muscles may also occur.
