Facial Nerve (Seventh Cranial Nerve)
CN VII is responsible for the movement of the facial muscles. Voluntary facial movements, along with other motor activity, originate in the precentral gyrus. White-matter tracts pass through the internal capsule and cerebral peduncles along with the other corticobulbar fibers. The motoneurons destined for the upper face receive information from both sides (bilateral innervation), whereas the lower facial musculature receives information only from the contralateral cortex (Fig 1-36). The CN VII nucleus receives additional information from BG extrapyramidal connections, which are largely responsible for involuntary blinking. Abnormal blinking, as present in BG disorders such as Parkinson disease, is probably mediated through alterations in inhibition of the supranuclear control of the blink reflex.
The motor fibers of the CN VII nucleus originate in the tegmentum of the caudal pons, ventrolateral to the CN VI nucleus and medial to the spinal nucleus of CN V. The dorsal subnucleus receives bilateral innervation and supplies the upper face; the lateral subnucleus (contralateral innervation) supplies the lower face. The fascicles of CN VII pass dorsomedially to surround the CN VI nucleus, creating a bump on the floor of the fourth ventricle (the genu of CN VII intra-axially and the colliculus of CN VII on the floor of the fourth ventricle). CN VII exits the ventrolateral surface of the pons along with fascicles of the nervus intermedius, which contains the facial nerve sensory fibers and the visceral efferent fibers (see the section Parasympathetic Pathways later in this chapter). The subarachnoid portion of the nerve runs anteriorly and laterally to enter the internal auditory meatus along with CN VIII.
Figure 1-36 Supranuclear, nuclear, and infranuclear anatomy of the facial nerve (CN VII). A, The corticobulbar fibers travel through the internal capsule down into the medial one-third of the corticospinal tracts in the cerebral peduncles of the midbrain. The pathways for the upper one-third of facial function (brow and orbicularis muscles) run parallel to but apparently distinct from the pathways for the lower two-thirds along the pyramidal tracts. The corticobulbar fibers travel in the basis pontis; those that control the lower facial muscles decussate at the level of the pons to synapse on the contralateral CN VII nucleus. Corticobulbar fibers that control the upper facial muscles decussate to synapse on the contralateral CN VII nucleus, and some of the fibers do not cross, reaching the ipsilateral CN VII nucleus. B, CN VII is predominantly motor in function, with its nucleus located in the caudal pons. CN VII courses dorsomedially and encircles the nucleus of CN VI. After bending around the CN VI nucleus, CN VII exits the pons in the cerebellopontine angle close to CNs V, VI, and VIII. CN VIII, the motor root of CN VII, and the nervus intermedius (the sensory and parasympathetic root of CN VII) enter the internal auditory meatus. Sensory cells located in the geniculate ganglion continue distally as the chorda tympani nerve, which carries taste fibers. Peripheral fibers of the nervus intermedius portion of CN VII initiate salivary, lacrimal, and mucous secretion. C, After emerging from the parotid gland, CN VII innervates the muscles of facial expression via 5 peripheral branches.
(Part A reproduced with permission from Bhatti MT, Shiffman JS, Pass AF, Tang RA. Neuro-ophthalmologic complications and manifestations of upper and lower motor neuron facial paresis. Curr Neurol Neurosci Rep. 2010;10(6):448–458; part A illustration by Dave Peace; parts B, C illustrations by Christine Gralapp.)
Within the petrous bone, CN VII enters the fallopian canal and traverses 3 segments (the labyrinthine, the tympanic, and the mastoid) that run in close proximity to the semicircular canals. The parasympathetic fibers destined for the lacrimal gland separate from CN VII in the region of the geniculate ganglion to accompany the greater superficial petrosal nerve. The stapedial nerve exits to innervate the stapedius muscle, and the chorda tympani conducts parasympathetic innervation to the submaxillary gland and afferent fibers from the anterior two-thirds of the tongue. These special afferent fibers are responsible for taste in the anterior tongue and synapse in the geniculate ganglion.
The main branch of CN VII exits the stylomastoid foramen just behind the styloid process at the base of the mastoid. The extracranial trunk of the nerve passes between the superficial and deep lobes of the parotid gland, where it divides into 2 trunks: (1) the temporofacial superiorly and (2) the smaller cervicofacial inferiorly. These variably divide into 5 major sub-branches: (1) temporal, (2) zygomatic, (3) buccal, (4) mandibular, and (5) cervical. The temporal and zygomatic branches laterally innervate the orbicularis oculi muscles. The infraorbital and buccal branches may also variably contribute to the inferior orbicularis.
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.