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The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a pair of nerves that innervates extraocular muscles, enabling eye movement and raising the eyelid. It is one of three cranial nerve pairs that play a role in eye movement, and it does the most work out of all three. The oculomotor nerve controls most of the eye muscles, and its branches connect to muscles on multiple sides of the eyeballs, delivering movement commands from the brain to those muscles.
| Characteristics | Values |
|---|---|
| Name | Oculomotor nerve (CN III) |
| Type | Cranial nerve |
| Function | Controls movement of the eye, including elevation of the upper eyelid, and coordination of eye muscles for visual tracking and gaze fixation. |
| Innervated Muscles | Levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, inferior oblique, sphincter pupillae, ciliary muscles, superior tarsal muscle |
| Sensory Information | Travels towards the spinal cord and parts of the brain for processing and identification |
| Motor Information | Originates in and travels from parts of the brain to target muscles |
| Nuclei Location | Brainstem (midbrain) |
| Cell Bodies Location | Two nuclei positioned close to one another, posteromedially in the midbrain |
| Afferent Information | Originates outside the brain |
| Efferent Information | Originates in and travels from parts of the brain |
| Parasympathetic Function | Constricts the pupil (miosis), innervates the ciliary muscles |
| Sympathetic Function | No direct function, but sympathetic fibres run with the oculomotor nerve to innervate the superior tarsal muscle |
| Clinical Significance | Damage to the oculomotor nerve can result in oculomotor nerve palsy, characterised by a "down and out" position of the eye and a dilated pupil |
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What You'll Learn
The oculomotor nerve (CN III)
The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a pair of cranial nerves that play a key role in eye movement and function. It controls the majority of the eye muscles, enabling various eye movements and functions.
Origin and Pathway
The oculomotor nerve originates from the midbrain, specifically from the oculomotor nucleus or Edinger-Westphal nucleus, located within the brainstem. It emerges from the anterior aspect of the midbrain, passing between the posterior cerebral and superior cerebellar arteries. The nerve then runs anteriorly in the subarachnoid space, lateral to the trigeminal nerve, and pierces the dura mater as it enters the cavernous sinus.
Functions and Innervation
The oculomotor nerve has both motor and parasympathetic functions. It innervates multiple muscles associated with eye movement and function, including:
- Levator palpebrae superioris: Raises the upper eyelid.
- Superior rectus: Elevates the eye when looking straight ahead.
- Inferior rectus: Moves the eye down from a primary position.
- Medial rectus: Adducts the eye from a primary position.
- Inferior oblique: Elevates the eye when adducted from a primary position.
- Sphincter pupillae: Constricts the pupil, regulating the amount of light entering the eye.
- Ciliary muscles: Contracts to adjust the lens shape for focusing on near objects.
Clinical Significance
Damage to the oculomotor nerve, known as oculomotor nerve palsy, can result in various clinical manifestations, including:
- Ptosis: Drooping of the upper eyelid due to paralysis of the levator palpebrae superioris muscle.
- "Down and out" position of the eye: Due to paralysis of the superior, inferior, and medial rectus, and the inferior oblique muscles.
- Dilated pupil: Caused by the unopposed action of the dilator pupillae muscle.
- Diplopia: Seeing double due to misalignment of the eyes.
- Abnormal swinging flashlight test: Shining a light into either eye does not result in pupillary constriction in the affected eye.
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Innervates extraocular muscles
The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or simply CN III, is a cranial nerve that enters the orbit through the superior orbital fissure. The nerve innervates the extraocular muscles that enable most movements of the eye and that raise the eyelid. The oculomotor nerve originates from the oculomotor nucleus, located within the midbrain of the brainstem, specifically the ventral aspect of the cerebral aqueduct.
The oculomotor nerve provides motor and parasympathetic innervation to some of the structures within the bony orbit. The motor functions of the oculomotor nerve include innervation of the majority of the extraocular muscles. These include the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, and inferior oblique muscles. The levator palpebrae superioris muscle is responsible for elevating the upper eyelid, while the other four muscles enable various eye movements.
The parasympathetic functions of the oculomotor nerve include supplying the sphincter pupillae and the ciliary muscles of the eye. The sphincter pupillae constricts the pupil, reducing the amount of light entering the eye. The ciliary muscles contract, causing the lens to become more spherical and adapted for short-range vision.
The oculomotor nerve also has sympathetic fibres that run with the superior branch to innervate the superior tarsal muscle, which helps to raise the eyelid. Additionally, the oculomotor nerve contains fibres that innervate the intrinsic eye muscles, enabling pupillary constriction and accommodation for near vision.
Damage to the oculomotor nerve, known as oculomotor nerve palsy, can result in clinical features such as ptosis (drooping of the upper eyelid), a "'down and out' position of the eye at rest", and a dilated pupil due to the unopposed action of the dilator pupillae muscle.
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Controls eye movement
The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or CN III, is a key nerve that controls eye movement. It carries signals that tell the eyes how and when to move, and it is responsible for innervating the muscles that move the eye and its components.
The oculomotor nerve is a cranial nerve pair that carries eye muscle command signals. These nerves handle most of the signals that tell the eyes to move in a certain direction. The nerve provides motor and parasympathetic innervation to structures within the bony orbit, including the eye muscles. The nerve originates from the oculomotor nucleus, located within the midbrain of the brainstem, and exits the anterior surface of the brainstem, passing between the posterior cerebral and superior cerebellar arteries.
The oculomotor nerve innervates the majority of the extraocular muscles, which are the muscles that move the eyeball and upper eyelid. These include the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, and inferior oblique muscles. The nerve also contains fibres that innervate the intrinsic eye muscles, which enable pupillary constriction and the ability to focus on near objects, such as when reading.
The somatic (voluntary) functions of the oculomotor nerve include the elevation of the upper eyelid through the innervation of the levator palpebrae superioris muscle. It also coordinates eye muscles for visual tracking and gaze fixation through the innervation of four eye muscles: the superior rectus muscle, medial rectus muscle, inferior rectus muscle, and inferior oblique muscle. These functions enable movements such as elevating the eye while looking straight ahead or moving the eye down from a primary position.
The oculomotor nerve also has autonomic parasympathetic (involuntary) functions, including constricting the pupil (miosis) by innervating the smooth muscle (sphincter pupillae) near the pupil and innervating the ciliary muscles. The ciliary muscles contract, causing the lens to become more spherical and adapted to short-range vision.
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Pupillary constriction and accommodation
The oculomotor nerve, also known as the third cranial nerve, cranial nerve III, or CN III, is a cranial nerve that plays a key role in eye movement and function. It innervates the extraocular muscles responsible for most movements of the eye, including the raising of the eyelid. CN III also contains fibres that innervate the intrinsic eye muscles, enabling pupillary constriction and accommodation.
Pupillary constriction refers to the process of the pupil reducing in size in response to changes in lighting conditions or as a result of accommodative efforts. This is a critical function to protect the retina from damage due to excessive light exposure and to control the amount of light entering the eye for optimal vision. The oculomotor nerve plays a vital role in pupillary constriction by supplying parasympathetic fibres to the eye via the ciliary ganglion. These fibres control the sphincter pupillae muscle, which constricts the pupil.
The "swinging-flashlight test" is a diagnostic tool used to assess the function of the oculomotor nerve in pupillary constriction. When light is shone into one eye, the opposite eye should demonstrate equal constriction. This phenomenon occurs due to the crossover of fibres from the optic nerves at the optic chiasm. Loss of pupillary constriction, as observed in conditions like oculomotor nerve palsy, can indicate a lesion on the oculomotor nerve or other underlying pathologies.
Accommodation refers to the ability to focus on objects at different distances, particularly near objects, such as during reading. The oculomotor nerve facilitates accommodation by innervating the ciliary muscle. When stimulated, the ciliary muscle contracts, altering the shape of the lens to adjust its refractive power. This process enables clear vision at varying distances.
The ciliary muscle plays a crucial role in accommodating near objects by contracting to increase the curvature of the lens, making it more spherical. This increase in lens curvature allows for the proper focusing of light rays from nearby objects onto the retina, ensuring clear vision. The contraction of the ciliary muscle is stimulated by the parasympathetic innervation provided by the oculomotor nerve.
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Ciliary muscles
The ciliary muscle is an intrinsic muscle of the eye, formed as a ring of smooth muscle in the eye's middle layer, the uvea (vascular layer). It is one of three intrinsic muscles of the eye, the others being the dilatator pupillae and sphincter pupillae muscles. The ciliary muscle occupies the biggest portion of the ciliary body, which lies between the anterior border of the choroid and the iris. The ciliary body forms a semi-transparent ring on the outer surface of the choroid and includes the ciliary muscle and the finger-like ciliary processes. The ciliary processes are attached to the lens via zonular fibres.
The ciliary muscle controls accommodation for viewing objects at varying distances and regulates the flow of aqueous humour into Schlemm's canal. It does this by changing the shape of the lens within the eye, but not the size of the pupil, which is carried out by the sphincter pupillae and dilator pupillae muscles. The ciliary muscle, along with the sphincter pupillae, is mainly instructed by the parasympathetic nerve fibres of the oculomotor nerve (CN III). The ciliary muscle receives parasympathetic innervation from the short ciliary nerves (postganglionic fibres) that arise from the ciliary ganglion. The ciliary ganglion receives preganglionic fibres via the oculomotor nerve (CN III) that originate from the accessory/Edinger-Westphal nucleus of the oculomotor nucleus in the midbrain.
When the ciliary muscle contracts, it pulls itself forward and moves the frontal region toward the axis of the eye. This releases the tension on the lens caused by the zonular fibres, causing the lens to become more spherical and adapted to short-range focus. Conversely, relaxation of the ciliary muscle causes the zonular fibres to become taut, flattening the lens, increasing the focal distance, and improving long-range focus.
The ciliary muscle is important in the treatment of glaucoma, a disease that damages the optic nerve and can lead to blindness. Muscarinic receptor agonists cause rapid miosis and contraction of the ciliary muscles, opening the trabecular meshwork, facilitating drainage of the aqueous humour into the canal of Schlemm, and ultimately decreasing intraocular pressure.
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Frequently asked questions
The oculomotor nerve, or CN III, is a cranial nerve that innervates extraocular muscles to enable eye movement and control of the eyelid.
The CN III nerve innervates the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus, and inferior oblique muscles.
The CN III nerve controls eye movement, pupil constriction, and the ability to focus on near objects.
