Hey everyone! This is Nicole from Kenhub, and in today's tutorial we're going to talk about the oculomotor, trochlear and abducens nerves – three cranial nerves which are involved in the motor innervation of the eye. In this tutorial, we'll be looking at this image which is an image of the left eye as seen from a left lateral view. In addition to the nerves which are of course our main focus today, let's just point out a few structures that are of importance to us in this tutorial. Going right to left, we have the brainstem then the internal carotid artery followed by the optic nerve, the extraocular muscles of the eye and the eye and of course, in this tutorial, we'll be focusing mainly on these extraocular nerves here and the muscles that they innervate.
As we mentioned, today we'll be talking about the nerves that innervate the movements of the eye. These nerves include the oculomotor nerve which is cranial nerve III, the trochlear nerve which is cranial nerve IV, and the abducens nerve which is cranial nerve VI. For each of these nerves, we're also going to look at their nuclei, anatomical relationships and the structures that they innervate. In addition to these nerves, we'll also discuss the autonomic innervation of the eye which includes the branches of the ciliary ganglion. But first let's begin with the oculomotor nerve and its nuclei.
Although the oculomotor nerve is a fairly large nerve as you can see on the right, it actually arises from more than one nucleus – the oculomotor nuclei. The oculomotor nuclei are made up of two nuclei – the main oculomotor nucleus which is primarily involved in somatomotor innervation and the accessory oculomotor nucleus which is primarily involved in parasympathetic innervation. Let's, of course, begin with the main oculomotor nucleus.
So, as we can see, the main oculomotor nucleus is located within the midbrain at the level of the superior colliculus near the cerebral aqueduct. In this dorsal image of the midbrain and the cranial nerve nuclei, you can see the green oculomotor nucleus in the superior colliculus just here. Additionally, this nucleus communicates with the abducens and trochlear nucleus via the medial longitudinal fasciculus which runs along the midline of the brainstem around about here. The somatic efferent component of the oculomotor nerve originates from this main oculomotor nucleus which we'll talk a little more about in the coming slides.
The accessory oculomotor nucleus also known as the Edinger-Westphal nucleus is also found in the midbrain and as you can see in this image, it's located medial to the main oculomotor nucleus although in some references you may see it located laterally. This difference is due to the fact that there may be two cell groups that are considered to be the accessory oculomotor nucleus – one group lateral and one group medial to the main nucleus. However, for the purposes of this tutorial, we are going to assume that the medial cell group which you can see highlighted in green on our image is the Edinger-Westphal.
In contrast to the main oculomotor nucleus, the accessory oculomotor nucleus is made up of preganglionic parasympathetic motor neurons whose efferent fibers travel along with the oculomotor neuron and synapse on the ciliary ganglion to eventually supply the sphincter pupillae and the ciliary muscle. And of course after arising from the midbrain, the fibers from the accessory oculomotor nucleus join the fibers from the oculomotor nucleus to form two single nerves arising from either side of the brainstem – the oculomotor nerve.
After arising from the nucleus, the oculomotor nerve travels through the cranium and then through the orbit to reach the structures that it innervates. Therefore, in this section, we're going to start by talking about the intracranial relationships of the oculomotor nerve followed by the intraorbital relationships. In the image on the right of the oculomotor nerve, we can see it arising from the sulcus between the mammillary body and the cerebral peduncle which is located on the ventral aspect of the midbrain. As the nerve makes its way lateral to the dural wall of the cavernous sinus, we can see it also passing lateral to the dorsum sellae which also contains the pituitary gland. And after that, we can see it also passing laterally to the internal carotid artery, the trochlear nerve and the ophthalmic nerve. The oculomotor nerve enters the orbit via the superior orbital fissure. Once inside the orbit, it travels lateral to the optic nerve and soon divides into a superior and an inferior branch.
Now that we've finished talking about the relationships of the oculomotor nerve, let's talk about what the oculomotor nerve innervates.
As we just mentioned, the oculomotor nerve is primarily a somatic efferent nerve innervating most of the extraocular muscles of the eye. The oculomotor nerve can also be divided into two branches – a superior branch which as you can see is being pointed out by my arrow and an inferior branch. Let's talk through both of these branches individually starting with the superior branch.
The superior branch of the oculomotor nerve passes above the optic nerve before innervating the superior rectus muscle which is mainly responsible for the elevation of the eye. The superior branch then continues up to the longitudinal part of the levator palpebrae superioris which elevates the superior eyelid. The inferior branch, on the other hand, arises in the orbit and then travels below the optic nerve where it divides again into two branches. The first of these branches as you can see in this image innervates the inferior rectus. The inferior rectus helps move the eyeball downward.
The second structure this first branch of the inferior branch of the oculomotor muscle also innervates is the inferior oblique muscle. The inferior oblique muscle is responsible for the external rotation, the elevation and the abduction of the eyeball. The second branch of the inferior branch of the oculomotor nerve innervates the medial rectus which is responsible for adducting the eye. A branch of the second branch also provides parasympathetic innervation to the ciliary ganglion which I'm pointing out just here. But we'll talk about these a little bit more on our next slide.
One last thing I wanted to mention about the somatomotor supply of the oculomotor nerve is that the oculomotor nerve does not actually supply all of the extraocular muscles excluding the superior oblique which is innervated by the trochlear nerve and the lateral rectus muscle which is innervated by the abducens nerve.
Now that we've talked about the somatomotor nerve fibers of the oculomotor nerve, let's talk about the parasympathetic nerve fibers.
As we mentioned earlier, the accessory nucleus also provides fibers that travel along with the oculomotor nerve. These travel via the branch of the oculomotor nerve that innervates the medial rectus that is the second branch of the inferior branch of the oculomotor nerve. This branch supplies the parasympathetic root of the ciliary ganglion which we can see in this breakout here highlighted in green. The parasympathetic root of the ciliary ganglion in turn innervates the ciliary ganglion which you can see highlighted in green. From this ganglion, small parasympathetic fibers branch out to form the short ciliary nerves. The short ciliary nerves carry postsynaptic parasympathetic nerve fibers from two important muscles of the eye – the sphincter papillae muscles which constrict the pupil and the ciliary muscle which contracts the zonular fibers.
It's important to note that as one of the major parasympathetic ganglions of the head and neck, the ciliary ganglion is known to receive two types of fibers in addition to the parasympathetic fibers. These are the postsynaptic sympathetic fibers from the dilated pupillae muscle and the afferent sensory fibers from the iris and the cornea. However, only parasympathetic fibers synapse in the ganglion while the other two sets of fibers merely pass through.
Let's briefly summarize what we have seen so far about the oculomotor nerve. The oculomotor nerve has two nuclei – a main oculomotor nucleus and an accessory oculomotor nucleus. Additionally, there are two divisions of the oculomotor nerve – a somatomotor component and a parasympathetic component.
Now let's move on to talk about the trochlear nerve.
The nucleus of the trochlear nerve or cranial nerve IV is located in the midbrain. It's found just below the inferior colliculus and the oculomotor nucleus and we can see it highlighted in green on our image just here. It's important to note that fibers arising from the trochlear nuclei actually crossover the midline to form the contralateral trochlear nerve. In this image of the dorsal aspect of the brainstem, you can see this more clearly just here. Another important aspect of the trochlear nerves location is that it emerges on either side of the frenulum of the superior medullary vellum which is this structure just here.
Let's now move on to talk about the intracranial relationships of the trochlear nerve.
The trochlear nerve travels lateral and superior to the oculomotor nerve as it runs along the middle cranial fossa. It then enters the orbit via the superior orbital fissure. An easy way to remember the somatomotor innervation of the trochlear nerve is to remember that each trochlear nerve innervates the contralateral superior oblique muscle which is the only extraocular muscle to loop through and to use the trochlea in order to elevate the eye. The trochlea is located just here. The superior oblique muscle abducts, depresses and internally rotates the eye through this trochlea and it’s the only muscle innervated by the trochlear nerve.
Now that we've finished talking about the trochlear nerve, let's summarize a few important points. The trochlear nerve arises from the trochlear nucleus and crosses over the midline to innervate the contralateral superior oblique muscle which is the only extraocular muscle to pass through the trochlea.
Now that we've finished talking about the trochlear nerve, let's move on to the abducens nerve.
The nucleus of the abducens nerve or the sixth cranial nerve is located in the pons on the floor of the fourth ventricle and at the level of the facial colliculus. In this image of the dorsal aspect of the brainstem, you can see the abducens nerve nucleus highlighted in green being wrapped by fibers from the facial nucleus which is colored in purple. After leaving the nucleus, fibers travel downwards and anteriorly to emerge as the abducens nerve at the pontomedullary junction. You can see this more clearly on this sagittal section of the brainstem and cerebellum as viewed from the left. The abducens nerve having emerged from the pontomedullary junction is highlighted in green. Once it emerges from the pontomedullary junction, the abducens nerve travels upwards and courses medial to the trigeminal nerve. Once within the middle cranial fossa, the abducens nerve courses just lateral to the internal carotid artery and the ophthalmic nerve. In terms of its intraorbital relationships, the abducens nerve leaves the middle cranial fossa via the superior orbital fissure.
Let's talk now a little bit about the somatomotor function of the abducens nerve. Inside the orbit, it sends up to three to four small fibers that enter the body and innervate the lateral rectus muscle which is the only muscle innervated by the abducens nerve and which abducts the eyeball.
Now that we've finished the abducens nerve, let's briefly go over what we've discussed. The main takeaway is that the abducens nerve innervates the lateral rectus muscle. Let's finish up now with some clinical correlations.
The main clinical condition I want to talk about in this tutorial in relation to the oculomotor nerve is the condition called anisocoria. Anisocoria is a condition where the size of one pupil is unequal to that of the other. While this is actually normal for about 20% of people when the difference in size is not greater than 1 millimeter, when this difference does become greater than 1 millimeter or when it is observed in people with no prior anisocoria, the condition can be an indication of a life-threatening disease. The state usually arises when parasympathetic tone which normally maintains the pupil in a slight miosis – that is, the constriction of the eye – is overridden by sympathetic tone which causes the affected pupil to assume a mydriatic or dilated state. In the image on the right of a patient with anisocoria, you can see the dilated eye on the left with the constricted eye on the right.
Anisocoria can be caused by a range of conditions, however, the most severe are brain herniation, oculomotor nerve palsy and Horner's syndrome. In the case of brain herniation, the anisocoria is caused by strangulation of the oculomotor nerve due to increased pressure within the brain. In the case of oculomotor nerve palsy which can be caused by a subarachnoid hemorrhage in addition to anisocoria, the eye will most likely deviate down and out and ptosis may be observed. In the case of Horner's syndrome which can be caused by anything from a stroke to a tumor to a migraine, a classic triad of symptoms will usually present – miosis, partial ptosis and the loss of hemifacial sweating. Although Horner's syndrome is rare, it should not be confused with oculomotor nerve palsy as it arises from a lesion to sympathetic tone rather than parasympathetic tone. The anisocoria in this case arises from the severe constriction of one pupil compared to the other.
That's all for our tutorial today. Thanks for watching!
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