Anatomy clinical correlates: Trigeminal nerve (CN V)

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Introduction0:00–0:37

The trigeminal nerve is the fifth cranial nerve. It provides both sensory and motor functions, while also serving as a highway for parasympathetic innervation.
This is the nerve that allows you to feel wonderful things, like the kiss of your partner, or a summer breeze on your face, but also less pleasant things, like a dentist drilling into your teeth.
Luckily, understanding the anatomy and distribution of the trigeminal nerve can help us better diagnose and manage several conditions - and it even makes going to the dentist less painful!
First off, let’s remember some important anatomical features of the trigeminal nerve. The trigeminal nerve has three divisions: the ophthalmic or V1, the maxillary or V2, and the mandibular division or V3.

CN V0:37–2:05

Through its branches, the trigeminal nerve supplies sensory innervation to the skin of the entire face, the mucosa of sinuses, as well as the nasal, and oral cavities.
It also carries the sensory fibers for taste which go on to travel with the chorda tympani branch of cranial nerve VII, or the facial nerve.
The trigeminal nerve also carries parasympathetic innervation to the ciliary body and sphincter pupillae, the lacrimal gland, the nasal glands, palatal salivary glands, and the parotid, submandibular, and sublingual glands., though these parasympathetic fibres originally arise from other cranial nerves.Finally, the trigeminal nerve supplies motor innervation to the muscles of mastication.
And if all that wasn’t enough, the trigeminal nerve is also involved in the corneal reflex, also known as the blink reflex, which causes involuntary blinking when the cornea is stimulated to protect the eye from foreign bodies, like a pesky grain of sand getting in your eye.
In this reflex, the trigeminal nerve is the sensory or afferent pathway, while the facial nerve serves as the motor or efferent pathway.
Talk about a jack of all trades!So with that in mind, you can see why trigeminal nerve injuries can cause a lot of problems.

Trigeminal nerve injuries2:05–3:15

For starters, if the entire nerve is affected individuals might have decreased sensation, called hypoesthesia, or even complete loss of sensation, called anesthesia, of all areas supplied by the nerve.
Keep in mind that sometimes, only one or two of the divisions might be injured. In the case of a V1 injury for example, the corneal reflex would be absent.
You could test for this by stimulating the cornea by lightly touching it with a wisp of cotton. Normally, this would cause involuntary blinking, but if the reflex is absent there will be no blinking.
Without motor innervation from V3, the muscles of mastication become paralyzed. Usually, only the trigeminal nerve on one side is affected, so muscles of that side become paralyzed.
In this case, when the jaw is open, the healthy and properly innervated pterygoid muscles of the contralateral side function to move the jaw towards the affected side.
Without the normal opposition from the muscles of the affected side, the jaw deviates towards the paralyzed side when it is open.
Now let’s dive a bit deeper into the sensory function of the trigeminal nerve. Remember that the ophthalmic nerve transmits sensory fibers from the eyeball, the skin of the forehead and anterior scalp, the lining of the upper part of the nasal cavity and the air cells, the skin of the dorsal nose and its tip, and it even carries sensation from the meninges of the anterior cranial fossa.

Sensory innervation3:15–4:14

The maxillary nerve transmits sensory fibers from the skin of the face between the palpebral fissure and the mouth, the lateral external nose, the nasal cavity and the sinuses, and from the maxillary teeth.
The mandibular nerve transmits sensory fibers for the skin over the mandible, the side of the cheek and temple, the oral cavity including gross sensation from the anterior two thirds of the tongue, parts of the external ear and tympanic membrane, and the temporomandibular joint.

Trigeminal neuralgia4:14–4:48

With all of that in mind, let’s discuss trigeminal neuralgia, which is a common cause of facial pain. Trigeminal neuralgia produces excruciating unilateral shooting pain in the area supplied by one or more divisions of the trigeminal nerve, most often the mandibular nerve.
The pain can last from a couple of seconds to a couple of minutes and can occur multiple times a day. It can be set off by chewing, talking, shaving, washing the face or teeth, hot or cold air, or just by touching an area of the face.
Ok, it’s time for a break and a quiz. What is the corneal reflex?

Quiz4:48–5:01

How does an absent corneal reflex manifest?Great! Now let’s look at trigeminal herpes zoster, better known as shingles.

Trigeminal herpes zoster5:01–6:19

This condition is caused by the varicella-zoster virus, the same virus that causes chickenpox. See, if someone has had chickenpox at some point in their life, the virus can remain dormant in the trigeminal ganglion.
Later in life, the virus can reactivate and cause shingles in the distribution of any division of the trigeminal nerve on one side.
Clinically, trigeminal shingles presents with pain in the affected area. Later, a rash with tiny blisters, called a vesicular rash, will appear.
This condition is most dangerous if it affects the ophthalmic nerve, or V1, because the virus can spread to the eye and cause a major complication called herpes zoster ophthalmicus.
Symptoms of this complication include redness of the conjunctiva and sclera, and an inflamed swollen cornea. Corneal inflammation is called acute keratitis, and it’s the most common presentation of herpes zoster ophthalmicus.
Unfortunately, acute keratitis can ultimately cause vision loss. Sometimes this condition can also spread to other parts of the eye and cause uveitis, retinal necrosis, and optic neuritis which can all cause vision loss too.Now, another cause of trigeminal nerve injury is trauma, which usually affects one of the trigeminal nerve branches.

Traumatic injuries6:19–7:14

So let’s start by discussing trauma to the forehead! Remember that the ophthalmic nerve gives rise to the frontal nerve, which travels along the roof of the orbit and gives off two branches: the supraorbital nerve and the supratrochlear nerve.
These nerves pass from the orbit to the forehead through their corresponding supraorbital and the supratrochlear notches and supply sensation to the skin of the forehead.
Now, the supraorbital and the supratrochlear nerves can be injured as a result of injury to the supraorbital rim, which can occur when someone gets hit in the face by a ball.
This kind of injury to these nerves can cause paresthesia of the forehead and, in some cases, can progress to supraorbital neuralgia, meaning chronic forehead pain.Additionally, any blunt trauma to the face can cause a maxillary sinus fracture.The weakest point of the maxillary sinus is its roof, which separates it from the orbit.

Maxillary sinus fractures7:14–8:03

So with a maxillary sinus fracture, a branch of the maxillary nerve called the infraorbital nerve, that travels along the floor of the orbit, can be damaged.
This causes numbness in the cheek, upper lip, and upper gingiva. Some branches of the infraorbital nerve, called the superior alveolar nerves, travel through the maxillary sinus and provide sensory innervation to both the maxillary sinus and the upper teeth.
So when the maxillary sinus is inflamed either from trauma or another condition like maxillary sinusitis from an infection, this can put pressure on these nerves and cause referred pain to the maxillary teeth.Let’s take another break here, and see if you can remember - what is the major complication of trigeminal herpes zoster called.Okay, now let’s move on to the mandibular nerve and see how its branches can be injured.

Quiz8:03–8:15

Inferior alveolar nerve injuries8:15–9:14

An important branch here is the inferior alveolar nerve, which passes through the mandibular canal and provides innervation to the mandibular molars and premolar teeth, and the surrounding gingiva.
The inferior alveolar nerve then continues as the mental nerve, which innervates the lower anterior teeth and their surrounding gingiva, the skin over the chin, and the lower lip.
Because of its close relation to the teeth, the main trunk of the inferior alveolar nerve is most commonly injured during oral surgery.
So, whether it’s a third molar extraction, dental implant surgery, an osteotomy to remove defects, submandibular gland removal, or tumor removal, care must be taken to preserve this nerve.
Symptoms of inferior alveolar nerve injury usually include paresthesia, complete numbness, or pain in the teeth, gingiva, chin, and lower lip.
Another clinically important branch of the mandibular nerve is the auriculotemporal nerve. This nerve passes just above the parotid gland and gives branches that provide parasympathetic secretomotor innervation, originally from cranial nerve IX, to this gland.

Auriculotemporal nerve9:14–10:36

Secretomotor nerve activity at our salivary glands is what causes salivation to increase when we see food, think about eating or eat something very sour like a lemon.
The auriculotemporal nerve also provides sensory innervation to the external acoustic meatus and to the skin of the temporal region.
So, when the parotid gland becomes inflamed, this can cause referred pain to these areas. During surgery on or around the parotid gland, the auriculotemporal nerve requires special care.
If it gets damaged, then during its healing process, the parasympathetic fibers from the auriculotemporal nerve can mistakenly re-attach to other damaged nerve endings in the area, including the nerves connected to sweat glands and blood vessels in the skin.
Now instead of causing salivation like it should, the nerve will cause redness and sweating of the cheek whenever an individual is eating or just thinking about food.
This is clinically referred to as gustatory sweating and flushing, and the syndrome is called auriculotemporal or Frey syndrome.Alright, as a quick recap… Signs of trigeminal nerve injury include hypoesthesia or anesthesia of all areas supplied by the nerve, absent corneal reflex, paralysis of muscles of mastication, and jaw deviation towards the affected side.

Review10:36–12:48

Trigeminal neuralgia causes severe unilateral shooting pain in the area supplied by one or more divisions of the trigeminal nerve that lasts up to a couple of minutes and repeats throughout the day.
It can be triggered by chewing, talking, shaving, washing the face or teeth, hot or cold air on the skin, or just by touching an area of the face.Herpes zoster, or shingles, is caused by reactivated varicella-zoster virus that was dormant in the trigeminal ganglion.
It is most dangerous in the distribution of the ophthalmic nerve, as it can cause herpes zoster ophthalmicus, which can potentially cause vision loss.
Trauma can also damage branches of the trigeminal nerve. Supraorbital injury can damage the supraorbital and the supratrochlear nerves, and cause paresthesia of the forehead.
Maxillary sinus fractures can damage the infraorbital nerve and cause numbness in the cheek, upper lip, and upper gingiva.
Inflamed maxillary sinus mucosa can press on the superior alveolar nerves and cause referred pain in the maxillary teeth.
The inferior alveolar nerve can be injured during oral surgery. Symptoms include paresthesia, complete numbness, or pain in the teeth, gingiva, chin, and lower lip.
Finally, the auriculotemporal nerve can be injured during surgery of the parotid gland, or surgery nearby to it. If this injury happens, the nerve might not heal properly and can attach to other nerves such as the ones going to the sweat glands in the skin.
Instead of salivation, the nerve now causes a condition called Frey syndrome, whose main symptoms are sweating and redness of the cheek whenever an individual is eating or just thinking