The facial nerve is the seventh of 12 paired cranial nerves. Thus it is also called cranial nerve VII. The facial nerves are mixed nerves, meaning they carry both motor and sensory information to the head and face. Thus, cranial nerve VII is involved with a variety of different functions in the head and face including facial movement and expression, transmission of taste sensation from the tongue, and some parasympathetic innervation to the head—which involves the autonomic nervous system. Specifically, the parasympathetic nerves stimulate the salivary (producing saliva) and lacrimal (producing tears) glands as well as the nasal mucosa. The nerve emerges from the brainstem at the junction of the pons and medulla oblongata. From its points of origin in nuclei in the pons, cranial nerve VII splits into two divisions: the motor root and the intermediate nerve. The larger motor root innervates many of the muscles in the face, and the smaller intermediate nerve carries taste, somatic sensory fibers, and parasympathetic fibers.
General Functional Components
The facial nerve has four components: branchial motor, visceral motor, general sensory, and special sensory. This entry will focus on the sensory portions of the nerve. The general sensory component brings information to the brain about the ear—particularly the skin near the ear, the wall of the acoustic meatus, and the external tympanic membrane. The special sensory portion brings sensory information to the brain regarding taste from the anterior two-thirds of the tongue as well as from the hard and soft palates of the mouth.
Anatomy and Physiology
Within the skull and inferior to the external ear, the facial nerve passes through an opening in the petrous portion of the temporal bone called the internal acoustic meatus. It travels a short distance, then turns sharply and runs along the wall of the tympanic cavity, where the bones of the middle ear are located. At the point where the nerve turns is the sensory ganglion of the facial nerve called the geniculate ganglion. The term genu is a Latin word for “knee” or “bend,” thus the geniculate ganglion is L-shaped and houses the neurons involved with motor, sensory, and parasympathetic activities.
The lingual nerve is part of the trigeminal nerve that sends sensory information of the tongue to the central nervous system. It also carries fibers from the facial nerve. Thus, the chorda tympani piggybacks with the lingual nerve on its way to the tongue and conveys taste sensation from the anterior two-thirds of the tongue and soft palate. Finally, parasympathetic innervation of the facial nerve also drops off branches to the sinuses and nasal cavity. A few motor fibers from the facial nerve innervate a small muscle, the stapedius, which stabilizes the stapes (one of the bones used for hearing) in the middle ear.
Clinical Symptoms, Diseases, and Treatments
The facial nerve is one of the most frequently damaged of the cranial nerves, especially during its passage through the temporal bone. Viral infections can cause inflammation and even swelling of the nerve. This may lead to paralysis of several muscles, but in most cases the paralysis disappears after the infection resolves.
Certain infections have been identified as causing acute swelling (inflammation) of the facial nerve such as the varicella-zoster and Epstein-Barr herpes viruses as well as Lyme disease. Reactivation of the latent (dormant) viruses can cause Bell’s palsy, particularly during cold weather, emotional stress, or trauma to the face. Bell’s palsy has a rapid onset: a person may go to bed without symptoms and wake with partial or complete paralysis. To reduce inflammation, corticosteroids have shown to be effective if provided near the onset of the palsy. Most acute cases of Bell’s palsy will recover completely after a month or so, but during the condition patients must protect the affected eye from drying out, as the eyelid will not be able to close.
Jennifer L. Hellier and Robin Michaels
See also: Bell’s Palsy; Cranial Nerves; Nerves; Trigeminal Nerve
Liang, Barbara. (2012). The 12 cranial nerves. Retrieved from http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP11504
Moore, Keith L., Anne M. R. Agur, & Arthur F. Dalley (Eds.). (2010). Essential clinical anatomy (4th ed.). Baltimore, MD: Williams and Wilkins.
Yale University School of Medicine. (1998). Cranial nerves. Retrieved from http://www.yale.edu/cnerves/