Hey everyone! It's Nicole from Kenhub, and today we're going to be talking about the ophthalmic nerve. During this tutorial, we'll be using this lateral view of the cranium and brainstem to look at the ophthalmic nerve with, of course, our brainstem over here on the right with the trigeminal nerve and the optic nerve emerging and then passing through to the orbit and the eyeball. In this tutorial, of course, although the focus is on the ophthalmic nerve, we're going to talk about a few different structures associated with it starting first with the trigeminal nerve which is also known as the fifth cranial nerve followed by the trigeminal ganglion and, finally, the ophthalmic nerve which is often referred to as V1 as it's the first branch of the trigeminal nerve. With regards to the ophthalmic nerve itself, we'll talk about its many branches before finishing by talking about some associated structures. But, for now, let's begin with the trigeminal nerve.
As I mentioned before, the trigeminal nerve is the fifth cranial nerve and is often referred to as CN V. In addition to being the largest of the twelve cranial nerves, it is often considered to be the most complex. After arising from its nucleus, it emerges from the midbrain at the junction of the pons and the middle cerebral peduncle as a large sensory root and a smaller motor root. It then merges with the trigeminal ganglion or the Gasseri ganglion and not like massive nerve cell bodies which we'll talk more about in the next slide.
The trigeminal nerve provides innervation for the sensation of the face, mucous membranes, and some other structures of the head; the motor innervation of the muscles of mastication which includes the masseter muscle shown in green on the right, as well as feedback for proprioception of the face and the lower jaw.
The trigeminal ganglion or the Gasseri ganglion is a flattened sensory ganglion located within the middle cranial fossa in a depression called the trigeminal cave or Meckel's cave. The ganglion gives rise to three branches of the trigeminal nerve – the ophthalmic nerve or V1, a sensory nerve; the maxillary nerve or V2, also a sensory nerve; and the mandibular nerve or V3 which carries both motor and sensory fibers. As the focus of today's tutorial is the ophthalmic nerve, we won't be discussing the maxillary and mandibular nerves. So, if you're looking for more information on the maxillary nerve and the mandibular nerve, please make sure you check out our website for the tutorials on these nerves. But, for now, let's talk a little bit about the ophthalmic nerve.
Let's now have a look at the ophthalmic nerve in a bit more detail. As I mentioned before, the ophthalmic nerve which is the first branch of the trigeminal nerve only carries sensory fibers. It is involved in sensation to the upper third of the face. The ophthalmic nerve is the primary branch arising from the trigeminal ganglion stemming from its upper part and then passing anteriorly for about 2.5 centimeters. At the cavernous sinus, it divides into three branches – the frontal nerve, the lacrimal nerve and the nasociliary nerve. We'll now have a look at each of these branches in more detail starting with the frontal nerve.
The frontal nerve is the most superior and largest branch of the ophthalmic nerve and is sometimes considered to be a continuation of the ophthalmic nerve. The frontal nerve runs along the superior part of the orbit just below the periosteum. The frontal nerve further divides into two branches – the supraorbital nerve and the supratrochlear nerve.
The supraorbital nerve is the thicker of the two branches and arises from this point here before passing through a hole in the skull known as the supraorbital foramen where it is further divided into smaller branches which you can see just here. The supraorbital nerve carries sensory information from the upper eyelid, the conjunctiva, the skin of the scalp and the frontal sinus.
The supratrochlear nerve which is also known as Arnold's nerve is the second branch of the frontal nerve. It is the more inferior branch of the frontal nerve and passes more anteriorly through the orbit before it exits the orbit and gives two secondary branches – a superior branch and an inferior one. The supratrochlear nerve carries sensory information from the skin of the upper eyelid, the conjunctiva and the skin at the lower forehead.
The lacrimal nerve is the smallest of the three branches and passes anteriorly below the frontal nerve close to the lacrimal gland where it is subdivided into inner and outer branches that you see here close to the lacrimal gland. It carries sensory information from the lacrimal gland, the conjunctiva and the lateral upper eyelids.
Now let's move on and have a look at the third main branch of the ophthalmic nerve – the nasociliary nerve. The nasociliary nerve is intermediate in size between the frontal and lacrimal nerves and runs deeper within the orbit. The nasociliary nerve on its root gives off five branches – the sensory root of the ciliary ganglion, the long ciliary nerves, the posterior ethmoidal nerve, the anterior ethmoidal nerve, and the infratrochlear nerve.
Let's now have a look at these branches in more detail starting with the sensory root of the ciliary ganglion. The sensory root of the ciliary ganglion is the first branch of the nasociliary nerve and is also referred to as the communicating branch to the ciliary ganglion. It receives sensory fibers from the short ciliary nerves which are these structures just here. The short ciliary nerves carry sensory fibers from the cornea, the ciliary body and the iris. These sensory fibers then pass through the ciliary ganglion without forming synapses. The sensory root of the ciliary ganglion then carries these fibers back to the nasociliary nerve.
Next we're going to look at the second main branch of the nasociliary nerve – the long ciliary nerves that you can see in this magnification in this image here. Like the short ciliary nerves, the long ciliary nerves carry sensory information from the cornea, the ciliary body and the iris. These nerves also carry fibers from the sympathetic nervous system to the dilator muscle of the iris which is this outer segment of the iris just here.
Moving a little bit further across the nasociliary nerve, we can see the third branch of it which is the posterior ethmoidal nerve. The posterior ethmoidal nerve leaves the orbit by a hole in the skull known as the posterior ethmoidal foramen and then enters the nasal cavity. It carries sensory information from the nasal septum, the medial walls of the nasal cavity which we can't see here but is essentially posterior of our view of this nasal septum and from the ethmoidal sinuses which in this image of the perpendicular plate of the ethmoid sinus includes these little sinuses along here, and the sphenoidal sinuses which you can see on the right here in this midsagittal section of the skull.
The anterior ethmoidal nerve arises just before its exit from the orbit via a foramen known as the anterior ethmoidal foramen and enters into the anterior and middle ethmoidal cells which it also provides nerve supply to. Although the following pathway is not visible on this image, it's worth talking about so we'll just talk briefly through what happens once the nerve continues. Once the anterior ethmoidal nerve passes through the anterior ethmoidal foramen, it then continues through the cribriform plate of the ethmoidal bone which is highlighted in green in this breakout image and here it gives off branches known as the anterior meningeal nerves to the meninges before finally entering the nasal cavity where it divides into two branches - the internal nasal branches of the anterior ethmoidal nerve and the external nasal branches of the anterior ethmoidal nerve. And the internal nasal branch of the anterior ethmoidal nerve again divides into branches which you can learn more about in our tutorial on the nerves of the nasal cavity. And as you can probably guess from its pathway above, altogether the anterior ethmoidal nerve innervates the anterior and middle ethmoidal cells, the anterior meninges, the mucous membrane of the anterior septum, the lateral wall of the nasal cavity and the tip of the nose and the ala.
Now let's have a look at the next branch of the nasociliary nerve – the infratrochlear nerve – which is considered to the continuation of the nasociliary nerve. After the anterior ethmoidal nerve branches off to enter the anterior ethmoidal foramen, the infratrochlear nerve then continues anteriorly along the medial wall of the orbit before exiting the orbit to provide sensory supply to the lacrimal sac, the lacrimal caruncle, the conjunctiva, the skin of the upper and lower eyelid and the bridge of the nose.
One other thing I'd like to mention before we move on to talk more about the ophthalmic nerve is that before the ophthalmic nerve gives off these three main branches, it gives off a very small filament which passes between the layers of the tentorium. This small branch is referred to as the recurrent tentorial branch of the ophthalmic nerve. This branch supplies the tentorium cerebelli and the supratentorial falx cerebri.
Now that we've described the ophthalmic nerve and its branches, let's talk about some of its surrounding structures that are related to it functionally or anatomically. These structures include the optic nerve, the internal carotid artery and the pterygopalatine ganglion. Let's begin with the optic nerve.
The optic nerve or CN II is another cranial nerve located within the orbit. The optic nerve carries visual information from the retina to the brain. We can see here that the nasociliary nerve and some of its branches crossover the optic nerve within the orbit.
Another structure that we'll look at briefly is the internal carotid artery. The internal carotid artery is a terminal branch of the common carotid artery that supplies the brain and is located medial to the ophthalmic nerve as it arises from the trigeminal ganglion. Here we can see it entering the skull in close proximity to the pituitary gland, the ophthalmic nerve, and the optic nerve. The third structure close to the ophthalmic nerve is the pterygopalatine ganglion which is mostly related to the maxillary nerve but also neighbors the structures of the ophthalmic nerve.
To finish off this tutorial, let's go over some clinical notes relevant to the ophthalmic nerve. If there's a loss of sensation in the upper third of the face, this may indicate that there's damage to the ophthalmic nerve. Damage to the ophthalmic nerve will result in loss in sensation to the same side of the lesion as the nerve fibers do not cross over. This is also referred to as ipsilateral sensory loss.
The corneal reflex occurs when the cornea is stimulated by either a foreign body or by touching it directly. This elicits an involuntary blinking response of the eyelids with possible lacrimation. This reflex is mediated by the nasociliary branch of the ophthalmic nerve which we can see here highlighted in green. This nerve carries sensory fibers to the brainstem resulting in activation of the facial nerve or cranial nerve seven which initiates the involuntary blinking response. It is important to stress that the stimulation of one cornea normally results in a consensual response which means that both eyelids will involuntary blink as a result. Damage to the nasociliary branch of the ophthalmic nerve can cause an absent corneal reflex. An important note is that there is a false absence of corneal reflex when someone is wearing contact lenses.
So thanks for sticking with me during this tutorial. I'm just going to spend the last few minutes talking about what we've covered today in summary. So going back to the start of the tutorial, we began with the trigeminal nerve which, via the trigeminal ganglion, gives off the ophthalmic nerve or V1. The ophthalmic nerve then gives off three branches, the first of which is the frontal nerve which gives off two further branches – the supraorbital nerve and the supratrochlear nerves. The second main branch is the lacrimal nerve, the smallest of the three branches. And the final main branch is the nasociliary nerve which gives off five further branches – the sensory root of the ciliary ganglion, the long ciliary nerves, the posterior ethmoidal nerve, the infratrochlear nerve and the anterior ethmoidal nerve.
So that concludes our tutorial on the ophthalmic nerve.
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Now, good luck everyone, and I will see you next time.