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Oculomotor nerve: want to learn more about it?

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Oculomotor nerve

The oculomotor nerve is the third cranial nerve (CNIII), and one instance in which the name is a clear indication of the function of the nerve (Oculo = pertaining to the eye, motor = producing movement). Simply from the name then, it is easy to know that the oculomotor nerve will innervate muscles that move the eye itself or components of the eye. It is the movement producing functions of the nerve that make it a useful indicator of brain injury.

To understand the important clinical significance of the oculomotor nerve, this article will first discuss the origin and pathway of the nerve, its function, the muscles that are innervated by the nerve along with the movement they produce, and blood supply to the nerve.

Key facts
Function

Supplies 5 of 7 extrinsic muscles that move the eye, and two intrinsic muscles that control pupil constriction and lens accommodation.

Pathway Begins in the oculomotor and Edinger-Westphal nuclei in the midbrain, runs anterior through the cavernous sinus, through the superior orbital fissure into the orbit
Clinical implications Oculomotor nerve palsy, Weber and Mortiz Benedikt syndromes, diabetes mellitus, posterior cerebral and communicating artery aneurysms
Testing H pattern, ptosis, accommodation, and pupillary light reflex

Finally, the clinical manifestations of damage to the oculomotor nerve and possible brain injury implications will be discussed along with procedures that can be performed to test the proper functioning of all components of the oculomotor nerve.

Origin

It is first important to understand the distinction between the direction that motor and sensory information travel in the nervous system. Sensory information will be traveling towards the spinal cord and parts of the brain (afferent information) for processing and identification and thus will typically originate outside of the brain. Motor information on the other hand, will originate in, and then travel from parts of the brain out to target muscles (efferent information). The motor nerves will interact with the target muscles via the neuromuscular junction.

All cranial nerves with motor functions will originate from and thus have their nuclei located within either the brainstem (medulla, pons, or midbrain) or the spinal cord (the spinal accessory nerve/CNXI). The oculomotor nerve is no exception. The cell bodies of the oculomotor nerve are located within two nuclei positioned close to one another, posteromedially in the midbrain, the most superior component of the brainstem. The cell bodies and their somatic motor nerve fibers, or axons, that will innervate skeletal muscles associated with the eye, arise from the oculomotor nucleus. The cell bodies and their visceral motor nerve fibers, or axons, that innervate muscles within the eye itself, arise from the Edinger-Westphal nucleus.

Pathway

Both the somatic and visceral motor axons exit the anterior surface of the brainstem as the oculomotor nerve, appearing from between the midbrain and pons, passing between the posterior cerebral and superior cerebellar arteries. From here, the nerve runs anteriorly in the subarachnoid space, medial to the much larger trigeminal nerve (CNV) and its ganglion. It continues anteriorly to pierce the dura covering the cavernous sinus, passing through the superolateral aspect of the wall of the cavernous sinus, lateral to the internal carotid artery as it passes into the cranial cavity. The cavernous sinus, a plexus of veins, is located on either side of the sella turcica, which is a shallow depression on the superior aspect of the body of the sphenoid bone that houses the pituitary gland. In the anterior portion of the cavernous sinus, the oculomotor nerve divides into its superior and inferior branches.

Oculomotor, abducens, and trochlear nerves (lateral-left view)

Upon exiting the cavernous sinus, the oculomotor nerve branches run below the anterior clinoid process of the sphenoid bone to enter the orbit through the superior orbital fissure. Both branches will pass into the orbit within the boundaries of the common tendinous ring, a fibrous ring of tissue that surrounds the optic canal and part of the superior orbital fissure, in the posterior aspect of the orbit. From here the superior and inferior branches will pass anteriorly to supply extrinsic, or extra-ocular, muscles of the eye.

Once inside the orbit, the inferior branch of the oculomotor nerve will send a preganglionic branch to the ciliary ganglion (ganglion = a collection of nerve cell bodies) which is located just behind the eyeball. The preganglionic branch carries parasympathetic nerve fibers that will synapse with parasympathetic postganglionic fibers within the ganglion. These postganglionic fibers will then pass anteriorly to supply two intrinsic muscles of the eye.

Function

As mentioned previously, the oculomotor nerve is typically thought to be a purely motor nerve in its function. This is how the nerve will be discussed in this article, but note that there will be a small number of sensory fibers in the nerve also. These will carry proprioceptive information back from the eye regarding the location and movement of the eye.

Somatic motor function

These nerve axons will arise from the oculomotor nucleus and innervate skeletal muscles associated with the eye. There are seven extrinsic eye muscles (muscles that lay outside of the eye itself) that move the superior eyelid and the eyeball. Five of them are innervated by the oculomotor nerve and will be discussed in detail below.

Visceral motor function

The visceral motor axons of the oculomotor nerve are part of the autonomic nervous system, specifically the parasympathetic division. They will arise from the Edinger-Westphal nucleus and innervate two separate intrinsic muscles within the eye. These will constrict the pupil and cause accommodation of the lens of the eye respectively.

Muscles innervated by the oculomotor nerve

Extrinsic eye muscles (somatic motor function)

These muscles are located outside of the eye itself. There are seven in total but the oculomotor nerve supplies five of them. The first four mentioned here will move the eyeball; the last one will move the upper eyelid. They are the:

Superior rectus

Key facts
Origin

Superior part of the common tendinous ring

Insertion Sclera on the top of the eyeball, posterior to the corneoscleral junction
Action *Elevation, adduction, medial rotation of the eyeball

Inferior rectus

Key facts
Origin Inferior part of the common tendinous ring
Insertion Sclera on the bottom of the eyeball, posterior to the corneoscleral junction
Action *Depression, adduction, lateral rotation of the eyeball

Medial rectus

Key facts
Origin Medial part of the common tendinous ring
Insertion Sclera on the medial aspect of the eyeball, posterior to the corneoscleral junction
Action *Adduction of eyeball

Inferior oblique

Key facts
Origin Anterior aspect of the floor of the orbit
Insertion Sclera of the eyeball, deep to the insertion of the lateral rectus on the lateral aspect of the eyeball
Action Abduction, elevation, lateral rotation of the eyeball

* Bolded actions will be the only ones to occur when the line of sight is the same direction as the line of action of the muscle.

NOTE: The common tendinous ring is a fibrous ring of tissue that surrounds the optic canal in the posterior aspect of the orbit and provides a point of origin for all four recti muscles of the eye (superior, inferior, medial and lateral recti).

Levator palpabrae superioris

Key facts
Origin Anterior and superior to the optic canal on the lesser wing of the sphenoid bone
Insertion Superior tarsus and skin of the upper eyelid
Action Elevation of the upper eyelid

NOTE: #1 and 5 are supplied by the superior branch of the oculomotor nerve, the inferior branch supplies #2-4.

Intrinsic eye muscles (visceral motor function)

These muscles are located within the eye itself and are both supplied by parasympathetic fibers of the oculomotor nerve. They are actually the anterior extensions of the vascular layer of the eyeball. As such they don’t conform to the typical organization of other muscles with well defined origins and insertions. Moving from posterior to anterior within the vascular layer we have the choroid (the vascular component of the layer), ciliary body, and the iris.

Ciliary muscle

This muscle comprises part of the ciliary body, which lies between the anterior border of the choroid and the iris. The ciliary body includes the ciliary muscle and the ciliary processes, both of which form a complete ring around the eye. The muscular portion of the ciliary body is continuous with the ciliary processes, which are projections of the ciliary body that are in turn attached to the lens of the eye via fibers known as zonular fibers. This indirect attachment of the ciliary muscle to the lens of the eye, means that when this muscle contracts and relaxes the shape of the lens is altered allowing for accomodation. Accomodation simply refers to different strategies employed so that when viewing objects in front of us at different distances, our view of them can remain clear and focused.

Sphincter pupillae

Anterior to the ciliary body and muscle is the iris. The iris is also a circular structure that makes up the colored part of the eye. It surrounds a central opening or aperture known as the pupil. The muscle fibers of the sphincter pupillae are arranged in a circular pattern around the pupil so that when they are activated and contract the pupil is decreased in size or constricted.

Blood supply

The blood supply to the oculomotor nerve can be more easily understood if the nerve is broken down into intracranial and extracranial (i.e. in the orbit) segments.

Intracranial supply

The initial portion of the nerve is supplied by branches of the posterior cerebral artery, the thalamoperforating arteries. Arteries arising directly from the posterior cerebral, posterior communicating, superior cerebellar, and basilar artery will also supply blood to this segment of the nerve. The middle and distal portions of the nerve are typically supplied by a branch of the internal carotid artery as it passes through the cavernous sinus, the meningohypophyseal trunk.

Extracranial supply

Once the oculomotor nerve passes through the superior orbital fissure into the orbit, both the superior and inferior branches are supplied by arteries arising from the ophthalmic artery.

Oculomotor nerve: want to learn more about it?

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