Facial Nerve

Origin

The facial nerve fibers originate from nuclei in the brainstem at the level of the pons. The brainstem nuclei associated with the facial nerve are the nucleus solitarius, nucleus of the facial nerve, spinal nucleus of the trigeminal nerve, and superior salivatory nucleus.

The motor fibers found in the facial nerve proper originate in the facial nerve nuclei. The fibers conveying sensation from the skin of the ear have cell bodies located in the geniculate ganglion and terminate in the spinal nucleus of the trigeminal nerve.

The parasympathetic fibers of the intermediate nerve originate in the superior salivatory nucleus. The special sensory taste fiber cell bodies are located in the geniculate ganglion and terminate in the nucleus of the solitary tract.

The facial nerve, composed of both the main facial nerve root and the intermediate nerve, exits the pons at the ventrolateral pontomedullary junction, just ventral to the vestibulocochlear nerve.

Course

The facial nerve courses laterally to enter the facial canal of the temporal bone via the internal acoustic meatus. Once inside and embedded in the petrous temporal bone, it passes inferiorly in the facial canal, just posterior to the tympanic cavity, where it gives off several branches. At the inferior surface of the temporal bone, the facial nerve exits the facial canal via the stylomastoid foramen. Shortly thereafter, near the posterior edge of the parotid gland, it splits into its terminal branches (Myckatyn & Mackinnon, 2004).

Branches

The facial nerve is composed of two main roots, the facial nerve proper and the intermediate nerve.

The motor root of the facial nerve is primarily motor and gives rise to the following branches:

—nerve to stapedius muscle,

—digastric branch (to the posterior belly of digastricus),

—stylohyoid branch,

—posterior auricular nerve,

—parotid plexus,

—temporal branch,

—zygomatic branch,

—buccal branch,

—lingual branch,

—marginal mandibular branch,

—cervical branch.

An auricular branch of the facial nerve communicates with the auricular branch of the vagus nerve (CN X) and, thus, transmits somatic sensation from a portion of the skin of the external ear to the facial nerve.

A second communication from the facial nerve, at the level of the geniculate ganglion, to the lesser petrosal nerve (CN IX) also exists. It runs from the geniculate ganglion to the tympanic plexus, where facial and vagal nerve fibers join with the glossopharyngeal based tympanic plexus to form the lesser petrosal nerve. The function of the facial nerve fibers in this communication is unknown, but may be visceral sensory fibers from or preganglionic parasympathetic fibers to the parotid gland (Mavridis and Pyrgelis, 2016).

The intermediate nerve is sensory and parasympathetic and gives rise to the greater petrosal nerve and the chorda tympani.

Supplied Structures

The facial nerve is a mixed nerve supplying sensory, motor, and parasympathetic innervation to large portions of the face.

General sensory afferents from a small portion of the external ear and external acoustic meatus via a communication with the auricular nerve (vagus) convey sensation from the skin. This communication occurs close to the stylomastoid foramen as the auricular nerve passes by the facial nerve.

Special visceral afferent fibers from the anterior two thirds of the tongue send taste information back towards the brainstem by way of the chorda tympani nerve.

General visceral efferent fibers of the facial nerve send preganglionic parasympathetic innervation to targets throughout the face.

—The greater petrosal nerve innervates the nasal, lacrimal, and palatine glands. The preganglionic parasympathetic fibers of the greater petrosal nerve synapse with postganglionic neurons in the pterygopalatine ganglion.

—The chorda tympani nerve innervates the submandibular and sublingual glands. For both, the preganglionic parasympathetic fibers of the chorda tympani run with the lingual nerve (CN V3) before synapsing with postganglionic parasympathetic neurons in the submandibular ganglion.

—Special visceral efferent fibers of the facial nerve send branchial motor axons, via the posterior auricular nerve, via the posterior auricular nerve, to innervate muscles near the external ear and mastoid process. These include the occipitalis muscle and the posterior and superior auricular muscles. These are the only muscles of facial expression not innervated by the five major terminal roots of the facial nerve.

— Special visceral efferent fibers of the facial nerve are sent to the posterior belly of digastric via a digastric branch and to the stylohyoid muscle, via a stylohyoid branch.

—The bulk of the muscles of facial expression are innervated by the five terminal branches of the facial nerve. These branches are variable in course and often have lateral fibers that form web-like structure called the parotid plexus (Myckatyn and Mackinnon, 2004).

List of Clinical Correlates

—Bell’s palsy

—Loss of taste

—Reduced salivation

—Hyperacusis

—Dry eye

—Corneal reflex

References

Mavridis, I. N. and Pyrgelis, E. (2016) 'Clinical Anatomy of the Lesser Petrosal Nerve', Arch Neurosci., 3(2).

Myckatyn, T. M. & Mackinnon, S. E. (2004) A review of facial nerve anatomy. Semin Plast Surg, 18(1), 5-12.