The facial nerve (the labyrinthine segment) is the seventh cranial nerve, or simply CN VII. It emerges from the pons of the brainstem, controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue. The nerves typically travels from the pons through the facial canal in the temporal bone and exits the skull at the stylomastoid foramen. It arises from the brainstem from an area posterior to the cranial nerve VI (abducens nerve) and anterior to cranial nerve VIII (vestibulocochlear nerve).

The facial nerve also supplies preganglionic parasympathetic fibers to several head and neck ganglia.

The facial and intermediate nerves can be collectively referred to as the nervus intermediofacialis.

Structure

The path of the facial nerve can be divided into six segments:

1. intracranial (cisternal) segment

2. meatal (canalicular) segment (within the internal auditory canal)

3. labyrinthine segment (internal auditory canal to geniculate ganglion)

4. tympanic segment (from geniculate ganglion to pyramidal eminence)

5. mastoid segment (from pyramidal eminence to stylomastoid foramen)

6. extratemporal segment (from stylomastoid foramen to post parotid branches)

The motor part of the facial nerve arises from the facial nerve nucleus in the pons, while the sensory and parasympathetic parts of the facial nerve arise from the intermediate nerve.

From the brain stem, the motor and sensory parts of the facial nerve join together and traverse the posterior cranial fossa before entering the petrous temporal bone via the internal auditory meatus. Upon exiting the internal auditory meatus, the nerve then runs a tortuous course through the facial canal, which is divided into the labyrinthine, tympanic, and mastoid segments.

The labyrinthine segment is very short, and ends where the facial nerve forms a bend known as the geniculum of the facial nerve (genu meaning knee), which contains the geniculate ganglion for sensory nerve bodies. The first branch of the facial nerve, the greater petrosal nerve, arises here from the geniculate ganglion. The greater petrosal nerve runs through the pterygoid canal and synapses at the pterygopalatine ganglion. Postsynaptic fibers of the greater petrosal nerve innervate the lacrimal gland.

The pyramidal eminence is the second bend in the facial nerve, where the nerve runs downward as the mastoid segment. In the temporal part of the facial canal, the nerve gives rise to the nerve to the stapedius muscle and chorda tympani. The chorda tympani supplies taste fibers to the anterior two thirds of the tongue, and also synapses with the submandibular ganglion. Postsynaptic fibers from the submandibular ganglion supply the sublingual and submandibular glands.

Upon emerging from the stylomastoid foramen, the facial nerve gives rise to the posterior auricular branch. The facial nerve then passes through the parotid gland, which it does not innervate, to form the parotid plexus, which splits into five branches (temporal, zygomatic, buccal, marginal mandibular, and cervical) innervating the muscles of facial expression.

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Regards
Aaron Province
Editorial Team
Journal of Clinical & Experimental Neuroimmunology