What is the anatomy of the facial nerves relevant to congenital facial paralysis?

Updated: Jan 07, 2019
  • Author: Alan D Bruns, MD, FACS; Chief Editor: Arlen D Meyers, MD, MBA  more...
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Answer

The facial nerve is a complex mixed nerve containing motor, parasympathetic, special sensory (taste), and sensory components.

The motor nucleus lies deep within the reticular formation of the pons, where it receives input from the precentral gyrus of the motor cortex. The motor fibers innervate the muscles of facial expression, posterior belly of the digastric muscle, stylohyoid muscle, and stapedius muscle. The upper motor neuron (supranuclear) tracts supplying the upper face cross once and then cross again in the pons; thus, bilateral innervation is present, whereas tracts to the lower face cross only once.

The parasympathetic fibers originate in the superior salivatory nucleus and are responsible for lacrimation and salivation via the greater superficial petrosal nerve and the chorda tympani, respectively.

Afferent taste fibers are carried from the anterior two thirds of the tongue to the nucleus tractus solitarius via the lingual nerve, chorda tympani, and nervus intermedius.

The facial nerve also provides some sensory innervation to the external auditory canal.

The intracranial segment of the facial nerve travels 23-24 mm from the brain stem at the level of the caudal pons to the internal auditory canal (IAC). The meatal segment includes 7-8 mm of the nerve between the fundus of the IAC and the meatal foramen. The facial nerve occupies the anterior-superior quadrant within the IAC. The labyrinthine segment is 3-5 mm in length and travels superior to the cochlea and vestibule to the geniculate ganglion.

The first branch of the facial nerve, the greater superficial petrosal nerve, is within this segment. The tympanic segment is 12-13 mm in length and begins at the geniculate ganglion, where the nerve turns 40-80° posteriorly (first genu) to enter the middle ear on the medial wall of the tympanic cavity superior to the oval window and inferior to the lateral semicircular canal and ends at the pyramidal eminence.

The nerve turns inferiorly (second genu) below the horizontal semicircular canal and continues as the mastoid (vertical) portion 15-20 mm and exits the stylomastoid foramen. The extratemporal portion of the facial nerve is distal to the stylomastoid foramen and supplies the muscles of facial expression. The facial nerve divides the parotid gland into superficial and deep lobes. Within the gland, branching of the nerve is variable. Most commonly, the nerve divides into an upper temporozygomatic and lower cervicofacial division. Five terminal branches innervate the mimetic musculature of the face, namely the temporal, zygomatic, buccal, marginal mandibular, and cervical branches.

Upper motor neuron lesions of the facial nerve occur at any point from the motor cortex proximal to the facial nucleus. Clinically, upper motor neuron lesions result in muscle sparing in the upper portion of the face but involvement of the lower two thirds of the facial mimetic musculature. Lower motor neuron lesions of the facial nerve occur at the level of the facial nucleus or distal to the nucleus. These lesions involve all the motor branches, which results in total hemiparesis. Lesions near the geniculate ganglion lead to paralysis, hyperacusis, and alteration of lacrimation, salivation, and taste. Lesions distal to the greater superficial petrosal branch cause paralysis associated with alteration in taste; however, lacrimation is normal. Extracranial injuries lead to individual deficits, depending on the involved branch. [34]


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