The 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.
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.
- Muscles Innervated By The Oculomotor Nerve
- Blood Supply
- Clinical Implications
- Related Atlas Images
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.
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.
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.
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 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|
|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|
|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|
|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
|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.
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.
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.
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.
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 arteries 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.
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.
The symptoms of oculomotor nerve-related injury can differ based on the location of damage within the oculomotor and Edinger-Westphal nuclei in the midbrain, and whether it occurs inside or outside of the brainstem.
Damage Within the Midbrain Nuclei
Within the brainstem, organization of the oculomotor and Edinger-Westphal nuclei in the midbrain can mean quite specific localization of damage. More anterior lesions within the oculomotor nucleus would tend to damage motor supply to the ipsilateral inferior rectus, and the sphincter pupillae and ciliary muscles (Edinger-Westphal nucleus). Lesions occurring more posteriorly and laterally would affect the nerve supply to the ipsilateral medial rectus and inferior oblique muscles, whereas as more medially located lesions might affect the supply to the contralateral superior rectus muscle. The nerve supply to the superior rectus is the only case in which the input comes from the contralateral side of the oculomotor nucleus.
Damage WIthin the Brainstem
Injury within the midbrain at the level of the oculomotor nucleus can result in two different syndromes:
Moritz Benedikt Syndrome - Is a lesion of the oculomotor nerve fibers as they pass through the red nucleus. A lesion here will result in a contralateral tremor, due to damage to the superior rectus input, and the typical oculomotor nerve lesion symptoms:
- Deviation of the ipsilateral eye downward and outward (due to action of the intact superior oblique and lateral rectus muscles)
- A drooping of the ipsilateral eyelid (ptosis) due to a lack of levator palpabrae superioris action
- Diplopia (double vision)
- Ipsilateral loss of accommodation and light reflexes due to lack of sphincter pupillae and ciliary muscles
- Dilation of ipsilateral pupil (unopposed due to lack of sphincter pupillae action)
- Weber Syndrome - This syndrome results due to damage located more anteriorly than in Moritz Benedikt syndrome, just before the nerve fibers exit the brainstem. In this case, the typical oculomotor nerve lesion symptoms are present but the contralateral tremor progresses to a contralateral upper motor neuron paralysis affecting the superior rectus.
Damage Outside the Brainstem
Damage to the oculomotor nerve after it leaves the brainstem results in a collection of symptoms known as oculomotor nerve palsy. Symptoms include:
- Deviation of the ipsilateral eye out downward and outward
- Double vision
- Ipsilateral pupil dilation
- Unresponsive light and accommodation reflexes in the ipsilateral eye
Recall that as the oculomotor nerve fibers exit the brainstem they pass between the posterior cerebral and superior cerebellar arteries. This makes the oculomotor nerve susceptible to aneurysms that may press on the nerve, or aneurysm rupture, which will manifest as a sudden headache and symptoms of an oculomotor nerve lesion.
Interestingly, the distribution of visceral and somatic motor fibers, within the oculomotor nerve outside of the brainstem, can also be important clinically. The parasympathetic visceral motor fibers associated with the pupillary light reflex, tend to run superifically within the nerve. Thus they are more susceptible to compression from aneurysms in the posterior cerebral or posterior communicating arteries, as these arteries lie superior to the nerve as it exits the brainstem. On the other hand, in vascular ischemic disease, such as in diabetes, the more centrally located somatic motor fibers tend to be adversely affected. This is because the blood supply to the oculomotor nerve runs deep in the nerve, and interruptions to that blood supply will spare the more superficially located fibers related to the pupillary light reflex.
These details regarding the location of both the somatic motor and visceral motor components of the oculomotor nerve can be of great importance when assessing functioning of the nerve in patients. Oculomotor nerve lesion symptoms associated with visceral motor dysfunction accompanied by head pain would be indicative of an aneurysm. Painless dysfunction of the somatic motor functions of the nerve however, would be indicative of vascular ischemic disease, perhaps as a complication of diabetes.
Testing the proper functioning of the oculomotor nerve can be quite simple, but it is incredibly important to test both the somatic and visceral motor functioning of the nerve. In addition, testing must be combined with an understanding of the symptoms that would occur from damage to the oculomotor nerve structures within different regions of the brainstem, and also outside of the brainstem.
Somatic Motor Testing
As we know, the somatic motor function of the nerve supplies input to the majority of the muscles that move the eye. Often the proper functioning of these muscles is tested by getting the patient to follow an object, such as a pen, with their eyes in an ‘H’ pattern. For oculomotor nerve functioning, the practitioner needs to pay special attention to adduction, elevation, and depression of the eye to see if these movements occur. Also he or she must observe the eyelid to see if it is drooping or not.
Visceral Motor Testing
Testing of two functions needs to occur here: accommodation and the pupillary light reflex. However, testing the pupillary light reflex is perhaps more helpful in terms of isolating oculomotor nerve involvement.
To test accommodation, the practitioner typically has the patient follow the tip of their finger or a pen, as they move it from a distance to close to the patient’s face. Two things should happen: the eyes should both rotate medially and the pupils will constrict. Unfortunately, a lack of medial rotation could be due to damage to the somatic motor input to the medial rectus, and a lack of pupil constriction in both eyes might indicate damage to the optic nerve (CNII) rather than the oculomotor nerve. It is difficult to determine whether there has been damage to the input to the ciliary muscle that changes the shape of the lens with this type of testing.
However, also testing the pupillary light reflex will give a more complete picture of oculomotor nerve involvement. This is normally accomplished by shining a bright light into one eye and watching for pupil constriction in the eye being stimulated with light, and also the contralateral eye. Information that is coming into the brain from the eyes travels via the optic nerve (CNII). This information is sent to both the right and left Edinger-Westphal (EW) nuclei, regardless of which eye is being tested. If the practitioner shines a light in one eye and both pupils constrict, this indicates that both the optic and oculomotor nerves are intact. If the light is shone in one eye and neither pupil constricts, an absence of both the direct and consensual light reflexes, this indicates a lesion to the optic nerve in that eye. In this case the information doesn’t reach the EW nucleus to be passed to the oculomotor nerve in either eye.
If the light is shone is one eye and the pupil in that eye fails to constrict (an absence of the direct light reflex), this indicates an oculomotor nerve lesion to that eye. In this case the information has reached both EW nuclei because the optic nerve is intact, but the oculomotor nerve, to the tested eye, is lesioned, preventing pupil constriction.