What is the Facial Nerve?

The facial nerve is the seventh cranial nerve and is most well known for its importance in facial movement and expression. The facial nerve also carries nerve fibers that are involved in taste to the anterior 2/3 of the tongue and tearing (lacrimal gland). Furthermore, it has small branches involved in moderating our sensitivity to noise volume (stapedius muscle) and a few other muscles. Discussion herein is limited to nerves involved in facial motion. 

The motor neurons whose axons comprise the nerve reside in the brainstem. The nerve exits the skull by passing through the acoustic meatus with the eighth cranial nerve (auditory/balance nerve).  Thus, these nerves share a common path through the skull. The eighth nerve ends at the vestibular (balance) and auditory (hearing) end organs, the labyrinth and cochlea, respectively.  The seventh nerve, however, continues through the skull and exits at the stylomastoid foramen, which is in the skull base deep to the base of the ear and mastoid bone (the bone behind the ear). 

The facial nerve then immediately enters the parotid gland, within which it branches into five primary rami or divisions. If you place your right palm on your right each and spread your fingers out, this may roughly approximate the divisions of the nerve. These are (from top to bottom) the frontal (or temporal), the zygomatic, the buccal, the marginal mandibular, and the cervical. Each of these innervates a group of fine muscles of facial expression. The following is a rough guide to the areas each branch innervates. Note that there is some cross talk between branches.

Frontal (temporal)

The muscles of the forehead.

Zygomatic

The muscles involved in eye closure.

Buccal

The muscles involved in moving the nostril, upper lip and corner of mouth.

Marginal Mandibular

The muscles involved in depressing the lower lip/chin. 

Cervical

Lower chin muscles and superficial neck muscle (platysma).

So, it may be clear that impairment of the nerve, in any or all of its branches, can result in:

Frontal:  Paralysis of the forehead/inability to move the eyebrow. Usually, this means the eyebrow '˜hangs down' in front of the eye and can impair vision

Zygomatic

Difficulty with eye closure. Note that eye opening is NOT a problem. A completely different nerve innervates the muscles that lift the eyelid, and unless that is also injured, eye opening is not affected.

Buccal

Difficulty with smiling and motion of the mouth. This leads to problems with speech. The cheek tends to 'puff out' during speech on the affected side. In addition, there can be difficulty with nasal obstruction on the affected side as the muscles that help keep the nostril open are affected and the nose 'droops' on the affected side.

Marginal Mandibular

This is one of the longest of the branches and has the least 'cross-talk' with other nerves.  The muscles it innervates are involved with downward motion of the corner of the mouth. Injury here results in an asymmetric smile and problems with eating and drinking.

Cervical

This is arguably the least important of the branches. Injury of this nerve results in paralysis of the platysma muscle, a thin sheet that lies just deep to the skin.  This is the least likely to result in a functional impairment of visible deformity.

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