Lower Motor Neuron Peripheral Route of CN VII


The intracranial segment (within the angle) measures about 23 to 24 mm long and 1 to 2 mm wide and runs obliquely (anteriorly and laterally) from the pontomedullary sulcus to the internal auditory canal. y , y Both the motor root and the nervus intermedius emerge from the brain stem ventrolaterally near the posterior aspect of the pons, 9.5 to 14.5 mm from the midline and 0.5 to 2.0 mm medial to where CN VIII enters the brain stem. At times, the facial nerve axons pass through the transverse fibers of the middle cerebellar peduncle. y Within the subarachnoid space of the cerebellopontine angle, CN VII is joined by CN VIII, which travels lateral and slightly inferior to the facial nerve (.Fig 11-3 ). The nervus intermedius runs between CN VIII and the motor root of CN VII (hence its name). While in the cerebellopontine angle, the facial nerve has no epineurium and is covered only with pia mater. The trigeminal nerve is located anteriorly. The anterior

Figure 11-3 Relationship of the facial nerve to the cochlear and vestibular nerves from the cerebellopontine angle through the internal auditory ยก(From LaRouere MJ, Lundy LB: Anatomy and physiology of the facial nerve. In Jackler RK, Brackmann DE [eds]: Neurotology. St. Louis, Mosby-Year Book, 1994, pp 1271-1281.)

inferior cerebellar artery is associated with the nerves and usually is found ventrally, between the nerves and the pons. [8 INTERNAL AUDITORY MEATUS OF TEMPORAL BONE

The meatal segment (within the internal auditory canal) is about 7 to 8 mm long. The nervus intermedius joins the motor root to form a common trunk that occupies the area nervi facialis, which lies within the anterosuperior segment of the meatus. The motor fibers are more anterior, whereas the nervus intermedius fibers remain posteriorly. At the lateral end of the meatus, a horizontal partition, the transverse or falciform crest, separates the facial from the cochlear nerve inferiorly. In addition, a vertical crest (Bill's bar) separates the facial from the superior vestibular nerve located posteriorly. The blood supply to this region is the labyrinthine artery, a branch of the anterior inferior cerebellar artery.


Beginning at Bill's bar and ending at the stylomastoid foramen, this portion is comprised of three segments: (1) labyrinthine (3 to 4 mm long); (2) tympanic (12 to 13 mm long); and mastoid (15 to 20 mm long). The labyrinthine segment, which lies within the narrowest portion of the bony facial canal, extends from the anterosuperior region of the fundus to the geniculate ganglion. To reach the geniculate fossa, it passes between the ampulla of the superior semicircular canal and the cochlea and travels forward and downward. At the geniculate ganglion, the nerve turns 75 degrees posteriorly, thus creating the external genu. This marks the beginning of the tympanic segment. Exiting 90 degrees anteriorly from the geniculate is the first branch of CN VII, the greater superficial petrosal nerve.

The tympanic segment continues along the medial wall of the tympanic cavity, a few millimeters medial to the incus. It courses superior and posterior to the cochleariform process, along the upper edge of the oval window, and inferior to the lateral semicircular canal. At the origin of the stapedius tendon from the pyramidal process, the nerve turns 120 degrees, continuing inferiorly as the mastoid segment. The mastoid segment has two branches--the nerve to the stapedius and the chorda tympani. During its course through the mastoid along the posterior aspect of the external auditory canal, the nerve travels slightly posteriorly and usually passes lateral to the inferior aspect of the tympanic annulus. At times, however, the facial nerve may be lateral to the annulus during its entire descent, which puts the nerve at risk during mastoid surgery.y , y The major blood supply for the facial nerve within the facial canal is the superficial petrosal branch of the middle meningeal artery (proximally) and the stylomastoid artery (distally). y


CN VII exits the temporal bone via the stylomastoid foramen, which is between the mastoid and styloid processes and deep to the posterior belly of the digastric. Almost immediately, the nerve enters the parotid gland. After traveling anteriorly about 2 cm within the parotid, the nerve divides at the posterior border of the ramus of the mandible into an upper temporofacial and a lower (and smaller) cervicofacial nerve. Classically, these divide again at the pes anserinus into the temporal, zygomatic, buccal, mandibular, and cervical branches. Many anastomoses exist between these branches, and many variations in the branching patterns have been described.1^ After emerging from the superior, anterior, and inferior margins of the parotid gland, these branches enter the deep surface of their target muscle. Ultimately, all the facial muscles are innervated by CN VII, except for the levator palpebrae superioris (CN III). The extratemporal portion of the facial nerve derives its blood supply from the stylomastoid, posterior auricular, superficial temporal, and transverse facial arteries. y


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