Anatomy of the facial nerve (CN VII)

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Introduction0:00–1:01

Humans can make thousands of expressions with their faces, and this is possible thanks to the 7th cranial nerve, also known as the facial nerve.
The facial nerve does much more than just control our facial expressions though, it also plays an important role in salivating, sensation for some parts of the skin, and it’s even involved in the perception of taste!
The facial nerve has many functions including somatic sensory, special sensory, branchial motor, and visceral or parasympathetic motor innervation.
Parasympathetic innervation travels with the facial nerve to glands such as the lacrimal glands, nasal glands, palatal mucosal, and submandibular and sublingual salivary glands.
It provides special sensory innervation to the anterior two thirds of the tongue; as well as somatic sensory innervation for a small portion of skin at the ear and external tympanic membrane.
Now, where did the facial nerve come from? The facial nerve actually originates from a structure that begins to appear when we are about the size of a poppy seed, known as the second pharyngeal arch.

Embryology1:01–1:49

Remember, the pharyngeal arches are 6 embryological structures, of which only 5 eventually develop into the muscles, arteries, bones and cartilage of the head and neck.
Many structures are derived from the second pharyngeal arch, such as the lesser horn of the hyoid bone, the styloid process, the stylohyoid ligament, and the stapes.
Importantly, the stylohyoid muscle, the posterior belly of the digastric muscle, the stapedius muscle and the muscles of facial expression are also derived from the second arch, which means they are all innervated by the facial nerve.Now, the upper motor neuron of the branchial motor pathway for the facial nerve starts from the motor cortex, and its axons travel through the internal capsule to eventually reach the facial motor nucleus located in the ventrolateral part of the pons, where it synapses with the pathway’s lower motor neuron.

CN VII1:49–2:42

The lower motor neuron has axons that pass dorsally, or posteriorly, in order to get around the abducens nucleus in the pons.
Then, they continue their journey ventrally, or towards the front, to exit the pons through its ventral side. These axons form the motor component of the facial nerve, which provides innervation for the muscles of facial expression - like that confused expression we all make when hearing about upper and lower motor neurons for the first time.
Let’s look at the course of the facial nerve and its branches, which begin intracranially and eventually emerge from the cranium to continue extracranially.

Course & branches2:42–6:34

So, let’s begin with the intracranial course! The facial nerve emerges from the junction of the pons and medulla, referred to as the pontomedullary junction, as two divisions: a large motor root as we mentioned before, and a smaller root called the nervus intermedius which carries taste, parasympathetic and somatic sensory fibers.
The two roots cross the posterior cranial fossa and then enter the petrous temporal bone via the internal acoustic meatus.
Almost immediately after entering the internal acoustic meatus, the nervus intermedius joins the motor root and together they take a sharp turn and enter the facial canal which is located along the medial wall of the tympanic cavity.
At this bend, there is an important structure known as the geniculate ganglion, which is where the cell bodies of the first order sensory fibers of the facial nerve are found.
Furthermore, instead of travelling through the facial canal, other fibers are given off forming the greater petrosal nerve which travels anteriorly to provide parasympathetic innervation to the nasal, palatine, pharyngeal mucous and lacrimal glands.Now, back to the facial nerve within the facial canal, it gives rise to two other branches; the nerve to stapedius which provides motor innervation to the stapedius muscle - a small muscle in the middle ear - and the chorda tympani, which branches off before the facial nerve exits the cranium at the stylomastoid foramen, and eventually exits the cranium and goes on to innervate the tongue.
Finally, the facial nerve exits the facial canal through the stylomastoid foramen and begins its extracranial journey. Once the facial nerve exits the stylomastoid foramen, it gives off the posterior auricular nerve which wraps around the posterior aspect of the ear and continues to provide motor innervation to the occipital belly of occipitofrontalis, stylohyoid, and posterior belly of the digastric muscle.
Additional cutaneous branches also provide somatic sensory innervation to a small area around the ear and external tympanic membrane.
The sensory nerve endings from this site take the somatic sensory information through the geniculate ganglion where its cell bodies are, and the axons continue along the nervus intermedius of the facial nerve.
From here, the information passes to the spinal trigeminal nucleus where it synapses with the second order neuron, then travels to the thalamus to synapse with the third order neuron which will bring the information to the cerebral cortex where it is processed.
Finally, the motor component of the facial nerve enters the parotid gland where it forms the parotid plexus. Remember that even though the facial nerve passes through the parotid gland, it doesn’t actually innervate it - instead, the parotid gland is innervated by the glossopharyngeal nerve, or ninth cranial nerve.
Within the parotid gland, the facial nerve wraps up its journey by splitting into five terminal branches: the temporal nerve, the zygomatic nerve, the buccal nerve, the marginal mandibular nerve and the cervical nerve, and these branches are responsible for innervating the muscles of facial expression.
A way to remember these branches is using the mnemonic “To Zanzibar By Motor Car!” Let’s take a quick break and try to identify all branches of the facial nerve.Now, the facial motor nucleus has a dorsal and a ventral part.

Quiz6:34–6:44

Motor neuron lesions6:44–8:50

The dorsal region of the facial motor nucleus controls the muscles of the upper face, so it’s hard at work when you’re paying attention and raising your forehead muscles.
This region receives innervation from upper motor neurons in both the right and left hemispheres of the brain, meaning it is under both contralateral and ipsilateral control.
The ventral region controls the muscles of the lower face, so it helps you smile. This region receives upper motor neuron innervation only from the opposite hemisphere, meaning it is under contralateral control.
This correlates with the clinical consequences of an upper or lower motor neuron lesion.In an upper motor neuron (UMN) lesion of the facial nerve, the upper facial muscles are mostly unaffected, because they’re under bilateral cortical control.
So, if the fibers from one side of the brain are lesioned, they still receive innervation from the upper motor neuron in the other hemisphere so that the muscles of the upper face won’t be affected.
However, this is not the case for the lower facial muscles which are only under contralateral control. This means that the muscles of the lower face on the contralateral side of the upper motor neuron lesion will be affected.
In a lower motor neuron lesion of the facial nerve, things are a bit different. That’s because the fibres that come from the upper motor neurons decussate, which is a fancy way to say they cross over to the other side, just before they synapse in the lower motor neurons.
From the lower motor neuron, these fibres travel on the ipsilateral side, meaning the right lower motor neuron controls all of the muscles on the right side of the face and vice versa.
So a lower motor neuron (LMN) lesion will lead to a complete LMN lesion of the facial nerve on the same side, regardless of which part of the facial nucleus was involved.
This lesion is known as a facial palsy. Now that we have discussed the motor pathway of the facial nerve, let's look at one of the reflex pathways it is involved in.

Corneal reflex8:50–9:58

Like any other reflex, we have an afferent sensory pathway which senses information, and an efferent motor pathway reacting to that information.
In the corneal reflex, also known as the blink reflex, the trigeminal nerve is the sensory or afferent pathway, while the facial nerve serves as the motor or efferent pathway.
The corneal reflex refers to involuntary blinking that happens when the cornea is stimulated by a foreign body in order to protect the eye, like when a grain of sand gets in your eye.
The stimulus is sensed by the nasociliary nerve where it then travels back to the ophthalmic nerve - CN V1 - and then to the spinal trigeminal nucleus located in the brainstem.
This nucleus sends signals to the facial motor nucleus on each side to begin the efferent portion of the reflex. From there, the facial nerve sends a signal to the orbicularis oculi muscle which contracts and causes the eyelid to close.

Parasympathetic innervation9:58–12:05

The facial nerve also provides parasympathetic innervation to the lacrimal glands, nasal glands, palatal mucosal, and submandibular and sublingual salivary glands.
All of these pathways begin as presynaptic or preganglionic fibres from the superior salivatory nucleus, which is located in the pons, and travel through the nervus intermedius.
From here, the pathway the parasympathetic nerves take to reach the different glands is a little bit different, so let’s first look at the pathway for parasympathetic innervation to the lacrimal gland.
From the nervus intermedius, these fibres travel through the greater petrosal nerve to synapse in the pterygopalatine ganglion, also known as the sphenopalatine ganglion, located in the pterygopalatine fossa.
The postsynaptic or postganglionic fibers then travel through the zygomatic nerve - a branch from CN V2 - to reach the lacrimal nerve - a branch from CN V1 - which innervates the lacrimal gland, initiating tear production.
Parasympathetic information to the nasal glands and palatal mucosal salivary glands also travel from the nervus intermedius to the greater petrosal nerve and synapse in the pterygopalatine ganglion.
However, the postganglionic fibres travel with the nasal and palatine nerves - branches from CN V2 - to innervate the nasal glands and the palatal mucosal salivary glands.Lastly, parasympathetic innervation to the submandibular and sublingual glands travels from the nervus intermedius to the chorda tympani, where these fibres then travel along with the lingual nerve, a branch from CN V3, and eventually synapse in the submandibular ganglion, which is located inferior to the lingual nerve.
Postganglionic axons innervate the submandibular and sublingual glands. Now don't forget that the facial nerve also provides special sensory information - specifically, taste to the anterior two thirds of the tongue and palate!

Special sensory innervation12:05–13:15

Taste information from the taste buds of the anterior two thirds of the tongue travels through the lingual nerve and then through the chorda tympani, which travels along with the lingual nerve in the infratemporal fossa, back up through the geniculate ganglion where their cell bodies are, and the axons continue to travel with the nervus intermedius of the facial nerve.
Eventually these fibres reach the nucleus of the solitary tract in the brainstem to synapse. On the other hand, the special sensory from the taste buds on the palate is a bit different, and it is carried through the palatine nerves.
These fibers pass through the pterygopalatine ganglion without synapsing. Then, they go through the pterygoid canal with the greater petrosal nerve and pass through the geniculate ganglion where their cell bodies are.
Then this information travels within the nervus intermedius to the nucleus of the solitary tract where the fibers will synapse.
Alright, as a quick recap. The facial nerve is the seventh cranial nerve and it has four functions: somatic sensory, special sensory, branchial motor, and parasympathetic.

Review13:15–14:24

It innervates the structures derived from the second pharyngeal arch, and it’s formed by two roots: a large motor root and a smaller root called nervus intermedius.
In its intracranial course it gives off three branches: the greater petrosal nerve, the nerve to stapedius and the chorda tympani.
In its extracranial course it gives off the posterior auricular nerve, various motor branches and five terminal branches: the temporal nerve, the zygomatic nerve, the buccal nerve, the marginal mandibular nerve and the cervical nerve, which innervate the muscles of facial expression.
The facial nerve also provides parasympathetic innervation to the lacrimal glands, nasal glands, palatal mucosal, submandibular, and sublingual salivary glands, as well as special sensory innervation to the anterior two-thirds of the tongue and palate.