Facial nerve

Question 1

Where are the motor controls of the facial nerve? (4)

Answer 1

controls the muscles of facial expression, posterior belly of the digastric, stylohyoid and stapedius muscles.

Question 2

Where is the sensory control of the facial nerve?

Answer 2

small area around the concha of the external ear.

Question 3

What is the special sensory innervation region of the facial nerve?

Answer 3

the anterior 2/3 of the tongue via the chords tympani

Question 4

Where is the parasympathetic innervation of the facial nerve? (3)

Answer 4

1. Submandibular/Sublingual salivary glands
2. Nasal, palatine and Pharyngeal mucous glands.
3. Lacrimal glands

Question 5

Describe the intracranial anatomical course of the facial nerve.

Answer 5

The nerve arises in the pons, an area of the brainstem. It begins as two roots; a large motor root, and a small sensory root (the part of the facial nerve that arises from the sensory root is sometimes known as the intermediate nerve).

The two roots travel through the internal acoustic meatus (IAM), a 1cm long opening in the petrous part of the temporal bone. Here, they are in very close proximity to the inner ear.

Still within the temporal bone, the roots leave the internal acoustic meatus, and enter into the facial canal. The canal is a ‘Z’ shaped structure. Within the facial canal, three important events occur:

1. The two roots fuse to form the facial nerve.
2. The nerve forms the geniculate ganglion (nerve cell bodies).
3. The nerve gives rise to:
   Greater petrosal nerve – parasympathetic fibres to mucous glands and lacrimal gland.
   Nerve to stapedius – motor fibres to stapedius muscle of the middle ear.
   Chorda tympani – special sensory fibres to the anterior 2/3 tongue and parasympathetic fibres to the submandibular and sublingual glands.
4. The facial nerve then exits the facial canal (and the cranium) via the stylomastoid foramen. This is an exit located just posterior to the styloid process of the temporal bone.

Question 6

Describe the Extracranial course of the facial nerve.

Answer 6

After exiting the skull, the facial nerve turns superiorly to run just anterior to the outer ear.

The first extracranial branch to arise is the posterior auricular nerve. It provides motor innervation to the some of the muscles around the ear. Immediately distal to this, motor branches are sent to the posterior belly of the digastric muscle and to the stylohyoid muscle.

The main trunk of the nerve, now termed the motor root of the facial nerve, continues anteriorly and inferiorly into the parotid gland (note – the facial nerve does not contribute towards the innervation of the parotid gland, which is innervated by the glossopharyngeal nerve).

Within the parotid gland, the nerve terminates by splitting into five branches:

• Temporal branch
• Zygomatic branch
• Buccal branch
• Marginal mandibular branch
• Cervical branch
  These branches are responsible for innervating the muscles of facial expression.

Question 7

What are the motor functions of the facial nerve?

Answer 7

The branches of the facial nerve innervate muscles of the head and neck. The muscles are all derivatives of the pharyngeal arch.

The first motor branch arises within the facial canal; the nerve to stapedius. The nerve passes through the pyramidal eminence to supply the stapedius muscle in the middle ear.

Between the stylomastoid foramen, and the parotid gland, three more motor branches are given off:

Posterior auricular nerve – Ascends in front of the mastoid process, and innervates the intrinsic and extrinsic muscles of the outer ear. It also supplies the occipital part of the occipitofrontalis muscle.
Nerve to the posterior belly of the digastric muscle – Innervates the posterior belly of the digastric muscle (a suprahyoid muscle of the neck). It is responsible for raising the hyoid bone.
Nerve to the stylohyoid muscle – Innervates the stylohyoid muscle (a suprahyoid muscle of the neck). It is responsible for raising the hyoid bone.
Within the parotid gland, the facial nerve terminates by bifurcating into five motor branches. These innervate the muscles of facial expression:

Temporal – Innervates the frontalis, orbicularis oculi and corrugator supercilii.
Zygomatic – Innervates the orbicularis oculi.
Buccal – Innervates the orbicularis oris, buccinator and zygomaticus.
Marginal mandibular – Innervates the depressor labii inferioris, depressor anguli oris and mentalis.
Cervical – Innervates the platysma.